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Lettuce transplant root and shoot growth and development in relation to nitrogen, phosphorus, potassium, and water management

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Lettuce transplant root and shoot growth and development in relation to nitrogen, phosphorus, potassium, and water management
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Soundy, Puffy, 1962-
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English
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viii, 305 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Fertigation ( jstor )
Leaf area ( jstor )
Lettuce ( jstor )
Nitrogen ( jstor )
Nutrition ( jstor )
Plant roots ( jstor )
Plants ( jstor )
Root growth ( jstor )
Sowing ( jstor )
Tissue transplantation ( jstor )
Dissertations, Academic -- Horticultural Sciences -- UF ( lcsh )
Growth (Plants) ( lcsh )
Horticultural Sciences thesis, Ph. D ( lcsh )
Lettuce -- Planting ( lcsh )
City of Weston ( local )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 297-303).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Puffy Soundy.

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LETTUCE TRANSPLANT ROOT AND SHOOT GROWTH AND DEVELOPMENT IN
RELATION TO NITROGEN, PHOSPHORUS, POTASSIUM, AND WATER MANAGEMENT














By

PUFFY SOUNDY


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1996


























In loving memory of my eldest brother, Paul Walter Soundy, who departed from us March 31, 1995. Walter has been, and will continue to be my role model.















ACKNOWLEDGMENTS


I wish to express my thanks to Dr. D.J. Cantliffe, committee chairman, for his patience, guidance, and assistance in the research project and for his suggestions in the preparation of this dissertation.

Thanks are conveyed to Drs G.J. Hochmuth, R.T. Nagata, P.J. Stoffella, and E.A. Hanlon for their interest and helpfulness in the research project.

Thanks to everyone in the Seed Physiology Lab for all the help, friendship, and encouragement.

Everyone at the Horticultural Unit is thanked for the assistance with field experiments.

Financial support from Fulbright Scholarship, a grant from the Foundation for Research and Development, financial assistance from Dr. D.J. Cantliffe, and gift donations by Speedling, Inc., are gratefully acknowledged.

Special thanks and appreciation are extended to my friend Deborah Franklin, for her constant assistance and support.

Thanks to my parents, Keystone and Flora, and my brothers, Walter, Trevor, Victor, and Harvey, for


iii








encouraging me to study. The sacrifice has been a great one for us all, but time and their love has seen me through.


iv















TABLE OF CONTENTS


page

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . iii

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . vii

CHAPTERS

1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1

2 REVIEW OF LITERATURE . . . . . . . . . . . . . . . . 5

Introduction . . . . . . . . . . . . . . . . . . . . 5
Lettuce Transplant Nutrition and Water Requirements 6 Effect of Light on Lettuce Transplant Growth . . . . 14 Effect of Temperature on Lettuce Transplant Growth . 19 Conclusions . . . . . . . . . . . . . . . . . . . . 23

3 PHOSPHORUS REQUIREMENTS FOR LETTUCE TRANSPLANT GROWTH
USING A FLOATATION IRRIGATION SYSTEM . . . . . . . . 25

Introduction . . . . . . . . . . . . . . . . . . . . 25
Materials and Methods . . . . . . . . . . . . . . . 27
Results and Discussion . . . . . . . . . . . . . . . 33
Summary . . . . . . . . . . . . . . . . . . . . . . 61

4 NEED FOR SUPPLEMENTAL POTASSIUM FOR LETTUCE TRANSPLANT PRODUCTION . . . . . . . . . . . . . . . . . . . . . 64

Introduction . . . . . . . . . . . . . . . . . . . . 64
Materials and Methods . . . . . . . . . . . . . . . 67
Results and Discussion . . . . . . . . . . . . . . . 74
Summary . . . . . . . . . . . . . . . . . . . . . 103

5 ROOT AND SHOOT GROWTH RESPONDS TO NITROGEN NUTRITION OF LETTUCE TRANSPLANTS . . . . . . . . . . . . . . . 105

Introduction . . . . . . . . . . . . . . . . . . . 105
Materials and Methods . . . . . . . . . . . . . . 108
Results and Discussion . . . . . . . . . . . . . . 114


v









Summary . . . . . . . . . . . . . . . . . . . . . 141

6 PROMOTION OF LETTUCE TRANSPLANT ROOT DEVELOPMENT BY
PROPER MANAGEMENT OF NITROGEN AND IRRIGATION . . . 144


Introduction . . . . . . . . . . . .
Materials and Methods . . . . . . .
Results and Discussion . . . . . . .
Summary . . . . . . . . . . . . . .

7 SUMARY . . . . . . . . . . . . . .

APPENDICES

A PHOSPHORUS EXPERIMENTS . . . . . . .

B POTASSIUM EXPERIMENTS . . . . . . .

C NITROGEN EXPERIMENTS . . . . . . . .

D NITROGEN AND IRRIGATION EXPERIMENTS LIST OF REFERENCES . . - - - . . . . . .

BIOGRAPHICAL SKETCH . . . - - . . . . . .


. . . . . . . 144 . . . . . . . 146 . . . . . . . 154 . . . . . . . 245 . . . . . . . 247




. . . . . . . 256 . . . . . . . 265 . . . . . . . 275 . . . . . . . 286 . . . . . . . 297 . . . . . . . 304


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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

LETTUCE TRANSPLANT ROOT AND SHOOT GROWTH AND DEVELOPMENT IN RELATION TO NITROGEN, PHOSPHORUS, POTASSIUM, AND WATER MANAGEMENT

By

PUFFY SOUNDY

December 1996

Chairperson: Daniel J. Cantliffe Major Department: Horticultural Sciences

Lettuce (Lactuca sativa L.) transplants grown with floatation irrigation often show limited root growth, resulting in root systems not pulling out completely from the transplant flat, and poor establishment in the field. 'South Bay' lettuce transplants grown in a peat+vermiculite media in the greenhouse were fertilized with varying concentrations of N, P, and K, via floatation irrigation at selected frequencies, to determine optimum nutrient and water management for production of high quality transplants, with sufficient roots to fill a 11 cm3 tray cell, and for rapid field establishment.

Phosphorus at 0, 15, 30, 45, or 60 mg-L~1 applied every two to four days, increased fresh and dry shoot and root


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mass, root length and area, leaf area, pulling success, leaf tissue P, relative growth rate (RGR), specific leaf area (SLA), leaf area ratio (LAR), leaf mass ratio (LMR), but reduced root:shoot ratio (RSR), net assimilation rate (NAR), and root mass ratio (RMR). Quality transplants and the earliest and greatest head mass were obtained by fertigating every two days with 15 mg-L-1 P.

Floatation fertigation with K at 0, 15, 30, 45, or 60 mg-L-1 applied every two to four days, increased fresh and dry root mass only when the concentration of water extractable K in the media was less than 15 mg-kg~1, but when higher (24 mg-kg-1), root mass was unaffected. Fresh and dry shoot mass, leaf area, RSR, RGR, LMR, and RMR were unaffected by applied K, regardless of the initial K concentration in the media. Lettuce growth and yield in the field was not affected by pretransplant K.

To determine the optimum N concentration and

fertigation frequency, transplants were fertigated every day or every second, third, or fourth day with N at 0, 30, 60, 90, or 120 mg-L-1. Nitrogen at 30 mg-L-1 (summer) or 60 mg-L-1 (fall, winter, or spring) maximized root growth, provided that fertigation frequency was daily or every second day. Therefore, N concentration and fertigation frequency must be considered together. Pretransplant N improved lettuce head mass and reduced time to maturity.

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CHAPTER 1

INTRODUCTION



Unsatisfactory results in stand establishment of

direct-seeded lettuce crops using both pelleted and raw seed, particularly during conditions of environmental stress, has led to the use of transplants as a means of establishing economically viable plant stands (Cliffe, 1989). Guzman et al. (1989) found that superior plant stand was the major factor resulting in increased marketable yields from transplanted crisphead and romaine lettuce. They concluded that perhaps growers in south Florida, with harsh and unreliable weather, could minimize economic losses and become more reliable suppliers of lettuce if a portion of the lettuce crop was transplanted. According to Klassen (1986), other reasons growers were transplanting rather than direct-seeding included better plant-to-plant uniformity especially for a once-over harvested crop such as lettuce, early season weed control, more precise spacing of plants, and elimination of the need to thin densely seeded rows.

The environmental conditions to which vegetable

transplants are exposed to during their early growth play an


1








2

important role in final crop yield (Masson et al., 1991b). The early growing environment of transplants can be manipulated in ways that are not possible with direct-seeded crops (Wurr & Fellows, 1982). Several factors that are known to affect vegetable transplant size, quality, and growth in the field are root container size (Nicklow and Minges, 1962; Knavel, 1965; Dufault and Waters, 1985; Weston and Zandstra, 1986; Weston, 1988; Hall, 1989; Kemble et al., 1994; Liptay and Edwards, 1994; Maynard et al., 1996; Nicola and Cantliffe, 1996), seedling nutrition before transplanting (Jaworski and Webb, 1966; Jaworski et al., 1967; Kratky and Mishima, 1981; Dufault and Waters, 1985; Tremblay and Senecal, 1988; Weston and Zandstra, 1989; Garton and Widders, 1990; Masson et al., 1991a, b; Melton and Dufault, 1991; Dufault and Schultheis, 1994), transplant age (Chipman, 1961; Leskovar et al., 1991), and transplant storage (Dufault and Melton, 1990; Leskovar and Cantliffe, 1991). Irrigation systems could also influence transplant growth and development both in the greenhouse and subsequently during the field production cycle (Leskovar and Cantliffe, 1993; Leskovar and Heineman, 1994).

Containerized vegetable transplants grown in greenhouses can either be overhead irrigated or subirrigated. A floatation or subirrigation system was constructed by Speedling, Inc. as an alternative method to








3

the conventional overhead irrigation (Thomas, 1993). While disease control is the major benefit of the floatation system because leaves are maintained dry, according to Anon. (1986) there are other advantages. There are no variations in plant growth due to uneven watering and fertilization from an overhead irrigation system. The end result is more uniform plant growth. Furthermore, because there is no overhead irrigation, pesticides remain on the plant longer, making re-application less frequent and reducing both pesticide material and labor costs. According to Anon., part of the success of the system is the direct result of the transplant flat. The expanded polystyrene flat floats, making the approach to this type of bottom irrigation possible.

Using the floatation system, Leskovar and Cantliffe (1993) improved uniformity and quality of pepper transplants, compared to using overhead irrigation. When drought stress and root pruning methods were used to harden and prevent stem elongation in fresh-market tomato transplants grown with a floatation system, an increase in lateral root elongation and a decrease in shoot:root ratio were reported (Leskovar et al., 1994). A reduction in shoot:root ratio and an improvement in water-use efficiency of pepper transplants were also reported by Leskovar and








4

Heineman (1994), when plants were produced via the floatation system of irrigation.

However, growers have not been able to produce the highest quality lettuce transplants on a seasonal basis using the floatation system. A well developed root system is essential so that transplants can be easily pulled from the transplant flat, or pushed out utilizing a mechanical transplanter. If shoots are too long, the plants will tend to fall over, resulting in easily damaged plants and scorched leaves especially when transplanted onto plasticmulched beds. If shoots are too short, they cannot be easily handled and can be trapped under plastic mulch. When using the floatation system of irrigation, careful management of fertilization is important since large amounts of fertilizers, especially N, can greatly increase lettuce transplant shoot growth at the expense of root growth (Tremblay et al., 1987; Tremblay and Senecal, 1988; Masson et al., 1991a).

The overall objective of this research was to optimize fertilizer and irrigation programs to produce an ideal lettuce transplant, with optimum shoot and root development for rapid field establishment and high quality yields, under the floatation system of irrigation.














CHAPTER 2

REVIEW OF LITERATURE



Introduction



Approximately 4,000 ha of crisphead lettuce were grown in Florida during the 1993-94 production season, mostly on the Histosols around Lake Okeechobee and Zellwood (Anon., 1995). However, decline of the Histosols due to oxidation and competition with other lettuce production areas such as California have limited lettuce production on the organic soils. Cantliffe (1990) suggested that the expansion of lettuce production into the abundant sandy soils of Florida could greatly increase lettuce production potential in Florida. Commercially acceptable yields of high quality from sandy soils require new production systems such as plastic mulch and transplants instead of the traditional directseeding used on Histosols. Florida growers have, however, been unable to produce lettuce transplants with suitable root development especially under a desirable floatation or subirrigation system, for transplanting into sandy soils. Knowledge of the factors which influence transplant growth

5








6

such as plant nutrition, irrigation, supplementary lighting, and temperature, is therefore important to produce quality transplants.


Lettuce Transplant Nutrition and Water Requirements



Vegetable transplants grown in plug cells require

careful management of fertilizers (Dufault and Waters, 1985; Weston, 1988) due to limited volume in the cell and high seedling densities. Concentrations of essential plant nutrient elements within media are frequently insufficient to sustain plant growth for an extended period (Garton and Widders, 1990). Production of quality transplants is a prerequisite to a successful crop, especially in lettuce where the period of containerized transplant growth comprises up to 30 % of the total crop production time (Karchi et al., 1992).

Kratky and Mishima (1981) grew lettuce transplants by misting them with either 0, 200, 600, or 1800 mg-L~1 of a water soluble 13N-11P-21K fertilizer. Misting was performed twice daily with an application rate of 3.8 mm-day-1. Plants were transplanted to the field and grown to maturity. A foliar application of 200 to 600 mg-L-1 13N-11P-21K plus 4 to 8 g of 8N-14P-7K preplant fertilizer per liter of media for the 200 mg-L-1 foliar fertilizer and 0 to 4 g-L'1 for the








7

600 mg-L-1 rate was recommended. No foliar fertilization was found undesirable since transplant mass, head firmness, and

head mass were reduced. The 1800 mg-L- foliar rate with added preplant fertilizer was also undesirable since it caused production of excessively tender transplants, fewer saleable heads, and smaller head size. Differences at the time of transplanting were larger than 15-fold among treatments. However, at crop harvest, differences were less than 30 % for average head mass.

Tremblay and Senecal (1988) grew lettuce, broccoli, pepper, and celery transplants in a growth chamber maintained at 95 % RH and a day/night temperature of 23/18 'C. Plants were watered every morning with distilled water. Fertilization treatments were initiated at emergence and were done to runoff every afternoon. Treatments were factorial combinations of 150 or 350 mg-L-1 N and 50, 200, or 350 mg-L-1 K. Growth measurements were made at 18, 20, 31, and 38 days after sowing for lettuce, broccoli, pepper,

and celery, respectively. Nitrogen at 350 mg-L~', compared to 150 mg-L-1 N, increased leaf area and shoot dry mass of celery, broccoli, pepper, and lettuce, but reduced the percentage of shoot dry matter for all plant species except celery, which was not affected. Broccoli and pepper specific leaf area (SLA) was enhanced by increased N concentration while celery SLA was reduced and lettuce was unaffected.








8

Root dry mass was reduced with 350 mg-L~1 N, compared to 150 mg-L~1 N, for all species except for pepper, which was not affected. The root:shoot ratio of all species was reduced by 350 compared to 150 mg-L-1 N. Tremblay and Sen6cal (1988) concluded that 150 mg-L-1 N, compared to 350 mg-L-1 N, led to production of high quality transplants.

For K, Tremblay and Senecal found that celery leaf area increased linearly with K concentration but the increase for broccoli was curvilinear. Leaf area of lettuce increased with K at 350 mg-L-1 N, but there was no response detectable with 150 mg-L-1 N. They also reported that there were indications that the expansion of lettuce leaves was driven primarily by shoot dry-mass accumulation. They supported this statement by two observations: 1) increases in leaf area followed increases in shoot dry matter accumulation; and 2) SLA was not significantly modified by N and K treatments, indicating that leaf expansion was matched by a concomittant increase in shoot dry matter. Root growth characteristics and root:shoot ratio for broccoli, celery, and lettuce were not affected by K fertilization. The percentage of pepper root dry matter, however, decreased linearly with increasing K concentration.

Masson et al. (1991a) increased shoot dry mass for all plant species tested by high concentrations of N fertilization. Nutrient solutions with N at 400 mg-L-1








9

increased celery, lettuce, broccoli, and tomato shoot dry mass by 37 %, 38 %, 61 %, and 38 %, respectively, compared with 100 mg-L-1 N. Overhead fertigation was performed twice daily to partial runoff. Increasing the N concentration from 100 to 400 mg-L-' decreased the percentage of shoot dry matter in all species. A similar response was previously reported for celery, lettuce, broccoli, and pepper (Tremblay at al., 1987; Tremblay and Senecal, 1988). Leaf area ratio (LAR) of broccoli and tomato increased in a curvilinear fashion with N concentration. The LAR and specific leaf area (SLA) of broccoli and tomato changed in a similar way in response to lighting and fertilization treatments.

Increasing N fertilization decreased celery, lettuce, and broccoli root dry mass (Masson et al., 1991a). Tomato dry root mass increased in a linear fashion to N fertilization as noted by Weston and Zandstra (1989). Tomato root dry mass was 16 % higher with 400 than with 100 mg-L-1 N. Root:shoot dry mass ratio decreased in a curvilinear fashion in relation to N concentration in celery, lettuce, and broccoli but in a linear fashion for tomato. For celery and lettuce, this decrease was more evident under increased light intensity.

Masson et al. (1991b) reported that increasing the

supply of N to the transplant, resulted in a linear increase in total and marketable yield of celery, with the highest








10

yields obtained with 300 mg-L-1 N. Compared with N at 100 mg-L~1, total and marketable yields obtained from celery transplants fertilized with 300 mg-L-1 were increased by 16 % and 15 %, respectively. There was an increase of 16 % in the marketable head mass of lettuce when transplants were

fertilized with 400 mg-L~1 N, compared with 100 mg-L-1 N. The use of high concentrations of N in transplant production not only increased head mass at harvest, but also promoted earlier maturity. Marketable mass and diameter of inflorescence of broccoli increased linearly with increasing concentrations of N fertilization. Increases of marketable mass of broccoli were measured for transplants fertilized with 400 mg-L-1 N, rather than those fertilized with 100 mg-L-1. In general, Masson et al. found that tomato yields were negligibly affected by lighting and N treatments.

Guzman (1993) compared two tray cell sizes and three formulas of soluble fertilizers on quality of crisphead lettuce transplants. The transplants received four nutrient applications in four weeks. Flats were floated in nutrient solution containing 60 g-L-1 fertilizer. Irrigation was by means of daily overhead misting, except for days when fertilizers were applied. During transplant production, more growth occurred with high N (20N-8.6P-16.7K) and least with high P (9N-19.4P-12.5K), regardless of the season. Guzman also reported that lettuce transplants grown under high N








11

were larger than desired and bruised more during transplanting, resulting in slower recovery from transplant shock. Lettuce transplants produced with high P were the smallest, and according to Guzman, this probably indicated an improper ratio of N and P. Transplants produced with medium P (15N-14.2P-15K) had the best quality. In the field, however, yields were not significantly different due to pretransplant fertilizer treatments.

Karchi et al. (1992) also investigated the response of lettuce transplants to varying concentrations of N and P. Nutrient solutions were prepared from liquid phosphoric acid and granular ammonium nitrate (33 % N) to give nutrient

solutions portioned to 175 mg-L~1 N:75 mg-L-1 P; 292 mg-L-1 N:25 mg-L-1 P; 58 mg-L-1 N:126 mg-L-1 P and 32 mg-L-1 N:137 mg-L-1 P. They also compared these nutrient solutions to a water only treatment and to a water treatment supplemented by 175 mg-L~1 N:75 mg-L-1 P 18 days after seedling emergence. They found that the least dry leaf mass resulted from the water treatment and the greatest, resulted from transplants

produced with 175 mg-L~1 N:75 mg-L-1 P treatment. Root development, however, was found to be promoted by high P and low to equal N concentration. The 292 mg-L~1 N:25 mg-L-1 P solution led to a significant decrease in leaf mass, plant

mass and leaf area compared to the 175 mg-L-1 N:75 mg-L-1 P treatment. Karchi et al. concluded, therefore, that high N








12

with correspondingly low P levels had a negative effect on transplant growth.

Costigan and Mead (1987) reported that K concentrations in plants increased rapidly during the first few weeks of growth, and this made it very difficult to determine the critical level of K required for maximum growth rates. The percentage K in lettuce dry matter typically increases from

1 to 5 % within two weeks of sowing. Costigan and Mead performed sand culture experiments in the glasshouse to determine the internal K concentrations required by lettuce and cabbage transplants. They repeated the experiments with and without Na, since Na might affect K uptake by the plant. They grew lettuce and cabbage transplants in 14-cm diameter polypropylene plant pots containing 1 kg of sand, and irrigated with nutrient solutions. The solution K concentrations were varied by addition of different amounts

of K2SO4. Before sowing, they wet the sand to saturation with nutrient solution. Once the plants emerged, they watered the pots daily with 150 mL of nutrient solution applied as a spray, followed by a short spray with water to rinse the leaves. They found that the critical levels for a 10 % reduction in plant growth rate were 2.2 % K for cabbage and 4.3 % K for lettuce. In the presence of Na, the corresponding critical levels were 0 and 1.0 % K. Costigan and Mead demonstrated that cabbage was more able to








13

substitute Na for K than was lettuce. They, however, concluded that in most practical situations, it was unlikely that plants would have access to large amounts of Na when K was limiting.

To summarize transplant fertilization research, N nutrition appears to be the driving force in lettuce transplant shoot growth (Tremblay and Sen6cal, 1988; Masson et al., 1991a). However, optimum amounts of N for shoots were not necessarily optimum for root growth. Increasing N increased shoot growth, but decreased root growth. Tremblay and Sen6cal (1988) obtained the largest lettuce transplant

shoot mass with 350 compared with 150 mg-L1 N, while Masson et al. (1991a) produced the largest shoots with 400 compared with 100 mg-L~1 N. In both cases, these amounts were the highest levels of N tested, and they were applied daily through overhead fertigation. Karchi et al. (1992), however, found that a proper combination of N and P was required to enhance lettuce transplant root growth. They produced the

best transplants with either 175 mg-L-1 N and 75 mg-L~1 P or 58 mg-L-2 N and 126 mg-L~1 P. Potassium nutrition, on the other hand, did not appear to have any impact on lettuce transplant shoot and root growth.

Fertilizers can either be applied to transplants

independent of irrigation, or they could be applied with the irrigation water (fertigation). When fertigation is








14

employed, careful management of fertilization is important since large amounts of fertilizers, especially N, could be applied when irrigation demands are high, especially where floatation irrigation is employed. If overfertilization occurs with floatation irrigation, there is no method to leach excessive salts.



Effect of Light on Lettuce Transplant Growth



Supplemental lighting of greenhouse-grown crops is not currently widely practiced in the United States (Decoteau and Friend, 1991). Only 5 % of the commercial greenhouse space in the United States is fitted with supplemental lighting systems (Thomas, 1990). Greenhouses with supplemental lighting systems are primarily used in ornamental crop production for prolonging the natural photoperiod during short days, supplemental light on overcast days, and night period interruption. Supplementary lighting has not been traditionally used in the production of vegetable transplants in the United States, and research on the effects of supplemental lighting on transplant development and subsequent yield performance is limited (Decoteau and Friend, 1991).

Sodium lamps were reported to be ideal for plant

growing because of their durability, their favorable light








15

spectrum, and their high coefficient of conversion of electric energy into the energy of photosynthetically active radiation (Dullforce, 1971; Dennis and Dullforce, 1975; Tibbits et al., 1983).

Research has been reported on the effects of artificial lighting on the growth and morphology of the lettuce crop. However, according to Wurr et al. (1986), these have largely been observed on lettuce grown in a controlled environment (Soffe et al., 1977; Krizek and Ormond, 1980; Craker and Siebert, 1983), or under winter glasshouse conditions (Dennis and Dullforce, 1975) with butterhead lettuce. Wurr et al. (1986), therefore, conducted greenhouse and field experiments to determine the effects of supplementary lighting applied during transplant production, on lettuce transplant growth and maturity characters. They reported that in 1984, but not in 1985, tungsten lighting produced transplants with greater dry mass than the control. Highpressure sodium lighting had no effect on transplant mass in either year. Furthermore, in 1984 both lamp types gave rise to longer leaves than the control plants, but in 1985 this was only true for high-pressure sodium lighting.

Wurr et al. (1986) reported that in the field, inspite of lighting effects on transplant morphology, there were no effects in either year on lettuce mean head mass at maturity or the time from sowing to maturity. They concluded that








16

there was unlikely to be any benefit to growers in terms of increased head mass from providing supplementary lighting during transplant production, though it could be used early in spring to boost plant growth. However, they did not measure the effect of supplementary lighting on root growth.

Masson et al. (1991a) reported that supplementary

lighting, 100 yUol-s-1-m2 PAR, increased shoot dry mass of celery, lettuce, broccoli, and tomato transplants by 22 %, 40 %, 19 %, and 24 %, respectively. Supplementary lighting also improved the percentage of shoot dry matter for broccoli, tomato, and lettuce but not for celery. Tesi and Tallarico (1984) reported that an increase in the percentage of shoot dry matter improved cold resistance and that a quality tomato transplant should have > 10 % dry matter. Masson et al. (1991a) reported that leaf area for lettuce and broccoli transplants was increased under supplementary lighting, but no effect was detected for celery and tomato. Supplementary lighting lowered the specific leaf area (SLA) of celery, broccoli, and tomato, but not that of lettuce. Apparently, a low SLA is desirable since it was associated with greater leaf thickness. According to Masson et al. (1991a), under high photosynthetic photon flux density, the palisade layer cells generally elongated so that the leaves were thicker and a decrease in SLA was observed.








17

Supplementary lighting also reduced leaf area ratio (LAR) in celery, broccoli, and tomato transplants.

Masson et al. reported that transplant root dry mass of all plant species increased with 100 gmol-s-'-m2 PAR supplementary lighting by 97 %, 42 %, 38 %, and 21 % for celery, lettuce, broccoli, and tomato, respectively. The root:shoot dry mass ratio (RSDMR) of celery and broccoli was increased by supplementary lighting. Lighting, however, did not affect this relationship for lettuce and tomato. Decreases in RSDMR caused by high N concentrations have been reported for several species (Dufault, 1985; Tremblay et al., 1987; Tremblay and Senecal, 1988; Weston and Zandstra, 1989). According to Masson et al. (1991a), this decrease was more evident for celery and lettuce under supplementary lighting.

Supplementary light at the transplant stage had no

long-term effect on yield of celery or broccoli (Masson et al., 1991b). Supplementary lighting also did not influence lettuce yield or quality. Early yields of tomato transplants treated with supplementary lighting were higher on average than transplants produced under natural light alone. Cumulative tomato yields were, however, not affected by transplant lighting. Boivin et al., (1986) also obtained an increase of 31 % in the mass of marketable fruits in the first 3 weeks of harvest from greenhouse-grown tomato








18

transplants that had received supplementary light energy of

100 umol-s-1-m- (PAR)

Basoccu and Nicola (1990), working with lettuce

transplants, reported that, when natural light was decreased by 50 %, there was a decrease in percentage dry matter, fresh and dry mass, as well as number of leaves. In the field, lettuce head mass was increased by 18 % in plants which received natural light as opposed to those which received 50 % of the light during transplant production. A similar response was found with head diameter. According to Basoccu and Nicola, head diameter is influenced by the number of leaves present during transplanting.

Poniedzialek et al. (1988) studied the effect of controlled temperatures and light intensities on the shortening of the period of time in which lettuce transplants of good quality could be obtained. They found that supplementary light was decisive for shortening this time. The higher intensity of light, i.e. 40 Wn--2, shortened the period of production by 5 to 9 days compared with a lower intensity (20 W-m-2) . A light intensity of 20 W-M-2 was found not be sufficient for adequate growth of plants and accounted for a pronounced prolongation of the period of production and an increased number of days with supplementary illumination. On the other hand, an increase in light intensity of 40 W-m-2 brought about a faster








19

increase in the area and number of leaves and in the content of dry matter and chlorophylls a and b.

To summarize, supplementary light seems to affect lettuce transplant quality only during greenhouse production. According to Poniedzialek et al. (1988) light intensity of 40 W-m2 shortened the period of time in which lettuce transplants could be obtained. Shortening the period of lettuce transplant production could lead to lower transplant production costs. Furthermore, 100 gmol-s- -m2 PAR supplementary lighting increased lettuce transplant root mass (Masson et al., 1991a). An improvement in root growth is essential especially for lettuce transplants produced via floatation irrigation. Growers have had difficulty in producing lettuce transplants with sufficient root systems under floatation irrigation system. Perhaps the use of supplementary light could lead to faster production of lettuce transplants which could be more easily pulled from the transplant flats.



Effect of Temperature on Lettuce Transplant Growth



Lettuce is a cool season crop. According to Guzman

(1990), mean temperatures between 11 and 19 'C enhance yield and quality in 'Great Lakes' types in California. In Florida, mean temperatures below 21 0C are conducive for








20

good yield and head quality. Furthermore, mean temperatures below 16 0C tend to delay maturity and, although internal quality of head is excellent, head size is reduced. Guzman further reported that for Florida cultivars such as 'South Bay' and 'Raleigh', temperatures of 4 0C practically caused growth to cease. Mean temperatures above 21 0C, on the other hand, tend to reduce lettuce yield and quality. According to Guzman, lettuce quality is affected by high temperature of long duration at any stage of growth, but that early exposure appears to have the most pronounced effect.

According to Sadler and Cantliffe (1990), lettuce grown from transplants is susceptible to premature flowering (bolting) when stressed by high temperature conditions in the greenhouse. Bolting can lead to total loss of crop or loss of lettuce quality due to elongated cores (stems), ribbiness of leaves, and loss of head compactness (density). Therefore, heat stress related problems, such as bolting, could offset the possible gains from transplanting lettuce. They grew 'Vanguard', a California cultivar known to bolt readily under high temperature conditions, and 'South Bay', a Florida cultivar highly resistant to bolting in growth chambers at 35/30, 30/25, 25/20, 20/20 'C day/night temperatures. They found that 'Vanguard' bolted about two weeks earlier than 'South Bay' when grown under high temperature conditions. Though the cultivars bolted at








21

different dates, those plants of each cultivar grown at the highest transplant temperatures bolted first, followed sequentially by those grown at the lower temperatures. They concluded that temperature at which the transplants are raised directly influenced the onset of bolting, regardless of temperatures in the field.

Guzman (1990) studied the effect of greenhouse temperature on lettuce transplant quality and field performance. Lettuce transplants were either grown in a cool, air-conditioned greenhouse, or in a warm greenhouse exposed to natural conditions. Day and night temperatures were not stated. In the cool greenhouse, mean temperatures were kept below 27 'C with relatively small fluctuations, while in the warm greenhouse, fall temperatures approached 38 'C for several hours each day. Guzman found that transplants grown for four weeks in a warm greenhouse were larger and more tender than those grown for four weeks in a cool greenhouse.

Guzman (1990) reported that there were significant reductions in plant stand in the fall season in Florida compared to winter, and indicated that high temperatures during transplanting in the fall were stressful to transplant growth. Yields in the fall were found to be, in general, lower than in winter. But in both fall and winter, lower yields and quality were more pronounced in treatments








22

exposed for longer periods to high temperatures during transplant production. The most obvious quality disorder was excessive head core length in the fall. Only transplants kept for four weeks in a cool greenhouse had acceptable core lengths. Excessive core length was due to high temperatures and long days in the warm greenhouse, but similar conditions appeared to have minimal effect on the winter crop. Guzman concluded that lower field temperatures following transplanting possibly nullified the high temperature effect during the transplant stage.

Poniedzialek et al. (1988) studied the effect of controlled temperatures and light intensities on the shortening of the production time for high quality lettuce transplants. They reported that an increase in day temperature from 15 to 22 0C accounted for a shortening of the period of growth of lettuce transplants. No significant differences were found in transplant fresh mass regardless of temperature. An interaction of light intensity and temperature was also observed. At higher intensity of supplementary illumination (40 W-2) the content of dry mass increased at a higher temperature, i.e. 22 0C. At a lower intensity of light (20 W-m-) an increase of

temperature from 15 to 22 'C was insignificant. Also, in plants grown at a higher temperature of 22 'C without








23

supplementary lighting, the content of chlorophylls a and b was reduced considerably.

To summarize, temperature has a major effect on lettuce growth, both in the greenhouse and field conditions. Transplants produced under temperatures of 30 0C or higher, tend to bolt readily in the field (Sadler and Cantliffe, 1990). Furthermore, such transplants produced lower yields and poor internal quality in the field (Guzman, 1990). However, according to Guzman (1990) and Sadler and Cantliffe (1990), if transplants are produced at temperatures below 27 C, they can to a certain extent, overcome the problem of premature bolting of lettuce plants. Improved growth of lettuce transplants was also reported when plants were produced under 22 0C than under 15 0C (Poniedzialek et al. (1988). The higher temperature in combination with supplementary light led to shortening of the time needed to produce lettuce transplants of good quality.


Conclusions



Any beneficial effects of N, P, and K nutrition,

irrigation, light, and temperature on lettuce transplant growth should be judged according to a predetermined standard for lettuce transplant quality. High quality or ideal transplants have enough roots to fill a tray cell to








24

enable plants to be easily pulled from the transplant flat, and maximize water and nutrient absorption. Shoots which are too large and stretched are not ideal since transplants could easily be damaged during transplanting.

Fertilizers can either be applied to plants independent of irrigation, or they could be applied with the irrigation water (fertigation), such as is the case with floatation (sub-) irrigation. Nitrogen has been found to be the element with the largest impact on lettuce transplant shoot growth. In studies where floatation irrigation was used, it was important to manage both N and fertigation frequency. During periods of high irrigation demands, frequent fertigation with low concentrations of N would be required to minimize excessive shoot growth.

Extension of photoperiod and increasing the light

intensity with supplementary light could be beneficial to lettuce transplants by improving root growth. Similarly for temperature, production of quality transplants could be ensured by cooling or warming the greenhouse to optimize growing conditions.














CHAPTER 3

PHOSPHORUS REQUIREMENTS FOR LETTUCE TRANSPLANT GROWTH USING A FLOATATION IRRIGATION SYSTEM



Introduction



Vegetable transplants grown in plug cells require

careful management of fertilizers (Dufault and Waters, 1985; Weston, 1988) due to limited volume in the cell and high seedling densities. Concentrations of essential plant nutrient elements within media are frequently insufficient to sustain plant growth for an extended period due to frequent irrigation requirements (Garton and Widders, 1990). Production of vigorous seedlings is a prerequisite to a successful crop, especially in lettuce where the period of containerized transplant growth comprises up to 30 % of the cropping time (Karchi and Cantliffe, 1992). Improved nutrient regimes would contribute to efficient development of quality transplants (Tremblay and Sen6cal, 1988).

The role of P in transplant growth has been

investigated in a number of vegetable crops. In celery, increasing the P concentration from 5 to 125 mg-L'


25








26

increased transplant diameter and height, shoot and root mass, and leaf area (Dufault, 1985). In tomato, increasing P from 5 to 45 mg-L-' increased transplant height, stem diameter, leaf number, leaf area, and fresh shoot mass, but not dry shoot or root mass (Melton and Dufault, 1991). Dufault and Schultheis (1994) reported that increasing P from 5 to 45 mg-L- increased fresh and dry shoot mass, leaf area, and leaf count in combination with 75 or 225 mg-L1 N, but not with 25 mg-L1 N. Phosphorus at 5, 15, or 45 mg-L-' did not influence dry root mass.

Data are lacking on the response of lettuce transplant roots and shoots to frequent P applications using a floatation irrigation system. In this system, nutrients are supplied with each irrigation by floating flats directly in nutrient solution. Growers using this system have been unable to produce lettuce transplants with sufficient roots in a tray cell to enable easy removal of transplants from the transplant flat (Robles, personal communication). Perhaps, optimizing P fertilization practices could lead to improved root development in lettuce transplants.

In the present investigation, a range of P

concentrations were supplied via floatation irrigation to determine the P requirements for production of easy-to-pull transplants, which would rapidly establish in the field.








27

Materials and Methods



Greenhouse Experiments


'South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of

1.9 x 1.9 x 6.3 cm; 10.9 cm3 (length x width x depth; volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v), with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg-m-3, was used for media. Three experiments were conducted (Table 3-1). The plants were grown with natural photoperiod


Table 3-1. Sowing schedule and initial media test (Hanlon et
al., 1994) for Experiments 1 to 3.

Expt Sowing date Media test"
pH EC N03-N P K Ca Mg
(dS - m-') (mg - kg-1)
1 17 Jun 1993 4.7 0.9 1.3 12.4 14.6 14.2 11.6 2 18 Sep 1995 4.5 0.6 0 0.6 46.2 6.3 22.2 3 31 Jan 1996 5.2 0.2 0.3 0.4 24.4 0.6 5.8 ZConcentrations in the saturated paste extract.


extended to 16 h by 1000-W, high-pressure sodium lamps (250 ymol-M-2.- photosynthetic photon flux) . A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just above the plant canopy, and media temperatures were recorded by a








28

Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR). The flats were seeded, then covered with a thin layer of vermiculite, overhead irrigated enough to moisten the vermiculite, and transferred to a cooler at 20 'C for germination. After 48 h, flats were returned to the greenhouse.

Plants in Experiments 1 and 2 were irrigated every two to four days, depending on water needs, by floating flats in nutrient solution containing P at 0, 15, 30, 45, or 60 mg-L' as Na.HPO4. Other nutrients were supplied at equivalent rates to all plants and consisted of (in mg-L-1) 100 N, 30 K, 100 Ca, and half-strength Hoagland's solution for micronutrients only (Hoagland and Arnon, 1950), which was comprised of Mg, S, B, Cu, Cl, Mo, and Zn. In Experiment 2, the Ca level was reduced from 100 to 30 mg-L-.

Plants in Experiment 3 were irrigated every second day by floating flats in nutrient solution containing P at 0, 15, 30, 60, or 90 mg-L-1 in factorial combination with N at 60 or 100 mg-L-1. Phosphorus was supplied from Na2HPO4, while N was supplied from NH4NO3. Other nutrients were supplied as described above.

Experiments 1 and 2 were arranged in a randomized

complete-block design with 5 treatments and 4 replications. Experiment 3 was a randomized complete-block design with 10








29

treatments consisting of a factorial combination of 5 levels of P and 2 levels of N, replicated four times.

Plant samples, 5 per treatment, were taken at 14, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE). Growth variables calculated were: root:shoot ratio (RSR = dry root mass + dry shoot mass), relative growth rate (RGR = [ln (final total dry mass) - ln (initial total dry mass) - (final time - initial time)]), net assimilation rate (NAR = [(final total dry mass - initial total dry mass) - (final time - initial time) x {(ln (final leaf area) - ln (initial leaf area)} + (final leaf area initial leaf area)]), specific leaf area (SLA = leaf area + dry shoot mass), leaf area ratio (LAR = leaf area total dry mass), leaf mass ratio (LMR = dry shoot mass + total dry mass), and root mass ratio (RMR = dry root mass + total dry mass) (Hunt, 1978; 1982; Dubik et al., 1992).

At the last sampling date in Experiments 2 and 3, fresh roots were scanned with a Hewlett Packard desktop scanner and analyzed with MacRHIZO software (Regent Instruments Inc., Quebec, Canada) at 300 dpi for length, area, and diameter. Additionally, pull force, the force required to pull a lettuce transplant out of a flat using Model DPP Dial Push-Pull Gauge (John Chatillon and Sons, Kew Gardens, NY)









30


attached to a binder clip, was measured. Pulling success was calculated as the percentage of 5 plants per treatment that could be pulled out of the flats without any breakage.

Dry shoot samples from the last sampling dates were

ground to pass a 20-mesh screen and dry-ashed for P or aciddigested for total Kjeldahl N according to Wolf (1982). For total P determination, 0.5 g subsamples were weighed into 10 mL beakers. The samples were then dry-ashed in a muffle furnance at 500 0C for 10 h. The ash was moistened with 1 N HCl, poured into 50 mL volumetric flasks, and brought to volume with 1 N HCl. The solutions were filtered through 'Q8' filter papers (Fisher brand), with a particle retention of > 10 gim, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and analyzed with Model 61-E Inductively Coupled Plasma Spectrometry (Thermo Jarrell Ash Corporation, Franklin, MA).

The acid digestion procedure consisted of weighing 0.25 g subsamples into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes that were

then heated on a digestion block at 375 'C. After the digestion process was completed (a total of 2.5 h), the samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through 'P8' filter papers (Fisher brand), with a particle








31


retention of > 25 pm, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR).

Data were subjected to analysis of variance using the

Statistical Analysis System (SAS Institute, Inc., Cary, NC). Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts. Field Experiments


Plants from each treatment in Greenhouse Experiments 2 and 3 were transplanted into an Arredondo fine sandy soil (loamy, siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on-black polyethylene-mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville (Table 3-2). Experiment 1 was a randomized complete-block design with 5 treatments and 4 replications. Experiment 2 was a randomized complete-block design, with 10 treatments consisting of a factorial combination of 5 levels of P and 2 levels of N, replicated 4 times. Preplant fertilizer (13N-OP-10.8K) was applied broadcast and incorporated in the bed at 230 kg-ha-1. Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. There were 30








32

Table 3-2. Transplanting schedule and initial soil test
(Hanlon et al., 1994) for Experiments 1 and 2. Experiment Transplanting Soil test=
date pH EC P K Ca Mg
( dS -m-) -(mg -kg-1)
1 17 Oct 1995 5.9 0.1 185 30 733 54
2 29 Feb 1996 5.8 0.0 247 37 695 43
ZpH and EC determined on 2:1 water to soil ratio procedure, while elements are from a Mehlich-1 extractant.


plants per plot planted on double offset rows with a spacing of 0.3 m between plants and between rows on the bed (equivalent to 54,000 plants per ha).

Just after transplanting, 100 mL of nutrient solution (150 mg-L-1 20N-8.6P-16.7K) was applied to each transplant hole as a starter fertilizer. Water was applied twice daily for 20 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark (1991). Starting one week after transplanting, fertilizer at a rate of 15 kg-ha-' N and 16 kg-ha' K, supplied from NH4NO3 and KNO3, was injected once weekly using a venturi pump (Netafim Irrigation, Altamonte Springs, FL), with the last application one week before harvest to give a total amount of 150 kg-ha1 N and 180








33

kg-ha-1 K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al., 1988).

At head maturity, the center 20 plants in a plot were cut, weighed individually, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements. Wrapper leaves were sampled at harvest and analyzed for total P and Kjeldahl N as previously described for Greenhouse Experiments. Field data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc, Cary, NC). Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts.


Results and Discussion



Greenhouse Exreriments


Experiment 1 was conducted during the summer, under

greenhouse temperatures ranging from 21 to 37 OC (Fig. 3-1) The average daily maximum media temperature was 31 OC, while the average daily minimum media temperature was 22 OC. During the course of the trial, there were totals of 6 cloudy and 23 sunny days. Six of the sunny days were followed with rain in the afternoon.








34


40 3530
0

cc cc w w





21 301 16
GROWING PERIOD (JUN 21 - JUL 16)

-s-- air max -4- media max -+- air min -- media min


Fig. 3-1. Maximum and minimum air and media temperature
during transplant production for Experiment 1, Jun/Jul
1993.








35

Fresh shoot and root mass, and leaf area, were not

determined at 15 days after sowing (DAS). For plants sampled 15 DAS, there was a positive linear response of dry shoot mass to applied P (Table 3-3). The major increase in dry shoot mass to applied P occurred between 0 and 15 mg-L. For plants sampled 21 and 29 DAS, fresh and dry shoot mass increased in quadratic fashion to applied P. At any level of applied P, fresh and dry shoot mass were improved compared to 0 P. For plants sampled 15 DAS, dry root mass responded in quadratic fashion to applied P, and was greatest with 0 P. However, for plants sampled 21 and 29 DAS, applied P did not influence fresh and dry root mass. For plants sampled 21 and 29 DAS, leaf area increased in quadratic fashion to applied P, and was least with 0 P. Leaf tissue P increased in quadratic fashion, implying that P did not affect root growth.

Root:shoot ratios decreased in quadratic fashion in

response to P, regardless of sampling date. The greatest RSR values were obtained with 0 P, while there were similar RSR values in plants grown with 15 to 60 mg-L-1 P. Plants grown with 0 P had the greatest RSR values because shoots were smaller compared to plants grown with any level of P, while root growths were similar among all plants.









36


Table 3-3. Root and shoot characteristics of lettuce transplants
as affected by P nutrition for Experiment 1, June/July 1993.

Phosphorus Fresh Dry Fresh Dry Leaf Leaf Root:
applied shoot shoot root root area tissue shoot
mass mass mass mass P ratio
(mg-L1) (mg) (mg) (mg) (mg) (cm2) (g-kg-1)
15 Days After Sowing
0 12.4 3.8 0.31
15 16.5 3.2 0.20
30 15.6 3.5 0.22
45 15.3 2.9 0.19
60 16.5 3.2 0.20
Response L** Q** L*
21 Days After Sowing
0 355 28.0 155 12.0 12.9 0.43
15 688 40.6 169 11.9 26.1 0.29
30 781 45.6 176 12.4 29.3 0.27
45 741 43.4 179 12.7 28.6 0.30
60 736 45.1 189 13.2 30.2 0.30
Response Q** Q** NS NS Q** Q**
29 Days After Sowing
0 685 58.0 304 25.3 25.0 1.2 0.44
15 1268 85.4 307 23.8 46.8 3.0 0.29
30 1297 85.6 301 23.8 48.1 4.2 0.28
45 1401 92.3 320 24.7 50.3 4.6 0.27
60 1297 89.8 341 26.6 48.5 4.6 0.30
Response Q** Q** NS NS Q** Q** Q**


0.05 (*),


Linear (L) or quadratic (Q) effects significant at P =
0.01 (**), or nonsignificant (NS).








37

For plants grown to 21 DAS, there was a positive linear increase in RGR values in response to applied P (Table 3-4). For plants grown to 29 DAS, RGR values were not influenced by P and were lower than for plants grown to 21 DAS, implying that P was more important earlier in growth. Greater RGR values for plants grown to 21 compared to 29 DAS meant that younger plants had higher efficiency for growth than older ones. For plants grown to 29 DAS, NAR decreased in quadratic fashion in response to applied P. The production of dry matter per unit leaf area (NAR) was greater in plants grown with 0 P, but the total production of dry matter over the same period was greater with any level of P.

For plants sampled 21 and 29 DAS, SLA and LAR increased in quadratic fashion in response to applied P. Lowest SLA and LAR values were obtained with 0 P, while there were similar values with any other level of P. The reduction in SLA and LAR values for plants grown with 0 P reflects the reduction in both leaf size and assimilate production (Dubik et al., 1990).

For plants sampled 15 DAS, both LMR and RMR values were not affected by P. For plants sampled 21 and 29 DAS, LMR values increased in quadratic fashion, while RMR values decreased in quadratic fashion in response to applied P. For plants grown to 29 DAS, approximately 70 % of the dry matter









Table 3-4. Influence of P nutrition on growth characteristics of
Experiment 1, June/July 1993.


lettuce transplants for


Phosphorus Relative Net Specific Leaf Leaf Root
applied growth assimilation leaf area mass mass
rate rate area ratio ratio ratio
(mg -L ) (mg - mg-1 - wk-') (mg - cm-2- wk-1) (cm2-mg') (cm2 -mg-1)
15 Days After Sowing
0 0.76 0.24
15 0.84 0.16
30 0.82 0.18
45 0.84 0.16
60 0.83 0.17
Response NS NS 21 Days After Sowing
0 0.89 0.46 0.32 0.70 0.30
15 0.97 0.64 0.49 0.77 0.23
30 1.11 0.64 0.51 0.79 0.21
45 1.13 0.66 0.51 0.77 0.23
60 1.08 0.67 0.52 0.77 0.23
Response L** Q**
29 Days After Sowing
0 0.73 2.36 0.43 0.30 0.70 0.30
15 0.74 1.65 0.55 0.43 0.78 0.22
30 0.64 1.36 0.56 0.44 0.78 0.22
45 0.74 1.59 0.55 0.43 0.79 0.21
60 0.69 1.53 0.54 0.42 0.77 0.23
Response NS Q* Q**
Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS).


w
m








39

was allocated to shoots and 30 % allocated to roots in lettuce transplants grown with 0 P. Plants grown with 15 to 60 mg-L-1 P allocated about 78 % of dry matter to shoots, with only 22 % to roots. With added P, more dry matter was, therefore, partitioned to shoots rather than to roots.

The results of Experiment 1 indicated that high quality transplants could be produced without added P, when the peat+vermiculite media had at least 12 mg-kg1 P (water extractable) before any fertilizer applications.

In order to further test this conclusion, Experiment 2 was conducted during the fall, instead of summer, under greenhouse temperatures ranging from 18 to 46 0C (Fig. 3-2). The average daily maximum media temperature was 33 0C, while the average daily minimum media temperature was 26 0C. During the course of the experiment, there were 12 sunny days with two of the days resulting in afternoon showers, and 16 cloudy days with rain during four of the days.

For plants sampled 13, 21, and 28 DAS, fresh and dry shoot mass increased in quadratic fashion in response to applied P (Table 3-5). The major responses of shoot mass to applied P occurred between 0 and 15 mg-L-1, regardless of sampling date. For plants sampled 13 DAS, fresh and dry root mass were unaffected by P. For plants sampled 21 and 28 DAS, there was a positive linear increase in fresh and dry root mass in response to applied P. For plants grown to 28 DAS,









40


0
LUJ




(L LUJ


5)v

45403530252015-


105-


19


13


GROWING PERIOD (SEP 19 - OCT 13)


+ IE air max --+- media max -+- air min -- media min


Fig. 3-2. Maximum and minimum air and media temperature
during transplant production for Experiment 2, Sep/Oct
1995.


I I


I I I I I









Table 3-5. Root and
for Experiment 2,


shoot characteristics of lettuce transplants as affected by September/October 1995.


P nutrition


Phosphorus Fresh Dry Fresh Dry Root Root Root Leaf Pull Pulling
applied shoot shoot root root length area diameter area force success
mass mass mass mass
(mg-L~1) (mg) (mg) (mg) (mg) (cm) (cm2) (mm) (cm2) (N) (%)
13 Days After Sowing
0 118 7.4 52 2.9 5.0
15 259 11.9 53 2.9 11.0
30 281 12.6 57 3.1 12.1
45 288 12.7 56 3.2 12.2
60 279 12.7 55 3.1 11.9
Response Q** Q** NS NS Q**
21 Days After Sowing
0 465 28.1 176 11.9 15.2
15 1276 60.2 222 14.0 41.8
30 1385 59.8 215 12.5 43.7
45 1353 61.2 242 15.2 43.1
60 1365 63.3 243 14.8 43.0
Response Q** Q** L** L* Q**
28 Days After Sowing
0 762 58.9 245 19.1 246 24.4 0.32 25.2 0.019 30
15 2080 113.9 293 21.0 271 27.9 0.33 66.3 0.021 95
30 2076 111.7 298 20.8 275 27.3 0.32 66.4 0.019 90
45 2067 116.9 329 22.8 305 30.6 0.32 65.5 0.019 85
60 2115 117.9 344 23.7 310 31.5 0.32 67.1 0.020 90
Response Q** Q** L** L* L** L** NS Q** NS Q*
Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS).








42

root length and area increased in linear fashion in response to applied P, but root diameter was unaffected by P. Leaf area increased in a quadratic fashion to applied P, regardless of sampling date. Phosphorus application to the media did not affect pull force, but improved pulling success from 30 % to approximately 90 %. Most of the P effect occurred between 0 and 15 mg-L~1 P. In Chapter 6, pull force was related to pulling success, but this was not so in the present work probably because there were smaller differences in root mass among the treatments in the present investigation.

For plants sampled 28 DAS, leaf tissue P increased in quadratic fashion to applied P, from about 1 to 6 g-kg-1 (Table 3-6). Root:shoot ratios decreased in quadratic fashion in response to applied P, regardless of sampling date. The largest RSR values were obtained with 0 P. Root:shoot ratios were similar with all P treatments within sampling date.

For plants grown to 21 DAS, RGR increased in linear

fashion to applied P, while for plants grown to 28 DAS, RGR was not affected by P. Therefore, P appears to once again be more important earlier in plant growth than later on. For plants grown to 21 or 28 DAS, NAR decreased in a quadratic fashion to applied P. Net assimilation rate was greatest with 0 P regardless of sampling date, but the total









Table 3-6. Influence of P nutrition on growth characteristics of lettuce
Experiment 2, September/October 1993.


transplants for


Phosphorus Leaf Root: Relative Net Specific Leaf Leaf Root
applied tissue shoot growth assimilation leaf area mass mass
P ratio rate rate area ratio ratio ratio
(mg-L-) (g-kg1) (mg-mg-wk') (mg-cm-2-wk1) (cMr2-mg-) (cm2-mg-1)
13 Days After Sowing
0 0.40 0.68 0.49 0.72 0.28
15 0.25 0.92 0.74 0.80 0.20
30 0.25 0.96 0.77 0.80 0.20
45 0.25 0.97 0.77 0.80 0.20
60 0.25 0.94 0.75 0.80 0.20
Response Q* Q** Q** Q* Q*
21 Days After Sowing
0 0.43 1.36 3.25 0.54 0.38 0.70 0.30
15 0.23 1.61 2.58 0.70 0.56 0.81 0.19
30 0.21 1.52 2.31 0.73 0.61 0.83 0.17
45 0.25 1.57 2.47 0.71 0.56 0.80 0.20
60 0.23 1.59 2.57 0.68 0.55 0.81 0.19
Response Q** L* Q** Q** Q** Q** Q**
28 Days After Sowing
0 1.2 0.33 0.67 1.92 0.43 0.32 0.75 0.25
15 3.5 0.18 0.60 1.14 0.58 0.49 0.85 0.15
30 5.5 0.19 0.60 1.11 0.60 0.50 0.84 0.16
45 5.8 0.19 0.60 1.17 0.56 0.47 0.84 0.16
60 6.2 0.20 0.60 1.18 0.57 0.48 0.83 0.17
Response Q** Q** NS Q** Q** Q** Q** Q**
Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS).


w








44

production of dry matter over the same period was greater with any level of P.

For plants sampled 13, 21, and 29 DAS, SLA and LAR

increased in quadratic fashion to applied P. Lowest SLA and LAR values were obtained with 0 P, while there were similar values with any level of P. The reduction in SLA and LAR values for plants grown with 0 P reflects the reduction in both leaf size and assimilate production (Dubik et al., 1990).

For plants sampled 13, 21, and 29 DAS, LMR values

increased in quadratic fashion, while RMR values decreased in quadratic fashion in response to applied P. For plants grown to 28 DAS, approximately 75 % of the dry matter was allocated to shoots and 25 % allocated to roots in lettuce transplants grown with 0 P. Plants grown with 15 to 60 mg P-L' allocated about 84 % of dry matter to shoots, with only 16 % to roots. Once again, added P caused more dry matter to be partitioned to shoots rather than to roots, and more so for transplants in the present experiment than in the summer grown ones.

In Experiment 2, high quality transplants were produced with 15 to 60 mg-L' P. Although transplants grown with 0 P had greater RSR, NAR, and RMR values, they were inferior to transplants grown with any other level of P because they could not be easily pulled from the transplant flat. A








45

reason why plant roots responded more to applied P in this experiment but not in Experiment 1, might be due to lower initial P levels in the media in this experiment (0.6 mg-kg-2) compared to Experiment 1 (12.4 mg-kg-').

In the previous two experiments, 100 mg-L-1 N was used when growing transplants at various levels of P. Subsequent studies with N in Chapter 6, however, revealed that optimum N for lettuce transplant root growth might be in the 60 mg-L-1 range or less, supplied every second day through floatation irrigation. Therefore, in Experiment 3, N was included as a variable to compare 60 versus 100 mg-L-1 N concentration at selected levels of P. Furthermore, the highest level of P was increased from 60 to 90 mg-L-', since in Experiment 2 root mass may not have reached greatest level with an application rate of 60 mg-L-1 P.

Experiment 3 was conducted during the winter, under

greenhouse temperatures ranging from 14 to 38 OC (Fig. 3-3). The average daily maximum media temperature was 29 0C, while the average daily minimum media temperature was 21 OC. During the course of the experiment, there were a total of 17 sunny and 9 cloudy days.

For plants sampled 15 and 22 DAS, there were no P and N interactions for dry shoot mass (Table 3-7). By both sampling dates, dry shoot mass increased in quadratic fashion in response to applied P. The major increase in dry








46


0
w
D


w

w


40


353025

2015105-


GROWING PERIOD (FEB 5 - FEB 28)


--- air max -4- media max -+- air min -- media min


Fig. 3-3. Maximum and minimum air and media temperature
during transplant production for Experiment 3, February
1996.


--









47


Table 3-7. Root and shoot characteristics of lettuce transplants
as affected by P and N nutrition for Experiment 3, Feb 1996.

Nutrient Dry Dry Root Root Root Leaf
applied shoot root length area diameter area
mass' mass
(mg-L-1) (mg) (mg) (cm) (cm2) (mm) (cm2)
15 Days After Sowing
P


0 15 30 60 90
Response
N
60
100
Response
P x N

P
0 15 30 60 90
Response
N
60 100
Response
P x N

P
0 15 30 60 90
Response
N


4.0 9.0 9.6
10.2 10.2 Q**

8.4 8.8 NS NS


9.8
42.3 46.1 47.1 46.7 Q**

35.8
41.0
**
NS

N1 N2 12.7 12.1 82.1 104.7 83.5 105.6 82.8 104.4 81.0 101.6 Q** Q**


2.8 2.8 3.0 3.0 2.9 NS


3.0 2.7
*
NS
22 Days


After Sowing


6.6
14.7 15.3 15.5 15.5 Q**


14.0 13.0
*
NS
28 Days

8.6
24.5 23.9
24.4 25.4 Q**


2.3 6.9
7.4 8.0 8.1 Q**


6.4 6.7 NS NS

N, N2
3.6 3.4 26.1 32.7 27.6 36.4 28.3 38.3 27.6 37.9 Q** Q**


**


After Sowing


94
282 276 306 292 Q**


8.4 26.7 26.0 29.5 27.0 Q**


0.28 0.30 0.30 0.31 0.29 Q**


N1 N2 4.2 4.2 48.4 68.1 50.3 70.6 55.0 69.9 49.0 70.6 Q** Q**


60 23.2 255 24.2 0.30
100 19.5 245 22.9 0.30
Response ** NS NS NS
P x N ** NS NS NS NS **
ZN, = 60 mg-L-1; N2 = 100 mg-L-1. Quadratic (Q).
NS, *, *Nonsignificant (NS) or significant at 5% (*), 1% (**) levels.









48

shoot mass to P occurred between 0 and 15 mg-L-1. Dry shoot mass was not influenced by N for plants sampled 15 DAS. For plants sampled 22 DAS, dry shoot mass was greater in plants grown with 100 than with 60 mg-L-1 N. For plants sampled 28 DAS, dry shoot mass increased in quadratic fashion to applied P at both levels of N. Nitrogen had no influence on dry shoot mass, but dry shoot mass was increased with all

levels of applied P. With 100 mg-L-1 N, the response of dry shoot mass to P was greater than with 60 mg-L-' N.

Nitrogen did not interact with P to influence dry root mass, regardless of sampling date (Table 3-7). For plants sampled 15 DAS, applied P did not influence dry root mass. Root mass was less in plants grown with 100 than 60 mg-L-1 N. For plants sampled 22 and 28 DAS, dry root mass increased in quadratic fashion in response to applied P. The major root response to P was between 0 and 15 mg-L-1. Root mass accumulation was adversely affected by increased N by both sampling dates. For plants grown to 28 DAS, root length, area, and diameter increased in quadratic fashion in response to applied P. The smallest root length, area, and diameter were obtained with 0 P. Applied N did not influence any of the measured root parameters.

For plants sampled 15 DAS, there were no P by N

interactions for leaf area which increased in quadratic fashion in response to applied P. Applied N did not








49

influence leaf area by this sampling date. For plants sampled 22 and 28 DAS, leaf area increased in quadratic fashion to applied P, regardless of N concentration applied. Nitrogen had no influence on leaf area, but leaf area was

increased at all levels of applied P. With 100 mg-L- N, the response of leaf area to P was greater than with 60 mg-L-1 N.

For plants grown to 28 DAS, there were no P by N

interactions for leaf tissue N (Table 3-8). Leaf tissue N decreased in quadratic fashion in response to applied P. The response was probably a dilution effect since transplants were larger at any level of P compared to 0 P. Plants grown with 100 mg-L-' N had more N concentration in the leaves than those grown with 60 mg-L-' N. Leaf tissue P increased in quadratic fashion to applied P, regardless of N applied. Nitrogen had no influence on leaf tissue P at 0 or 15 mg-L-1 P, but it was increased with all other levels of applied P. With 100 mg-L~' N, the response of leaf tissue P to applied P was greater than with 60 mg-L~1 N.

For plants sampled 15 and 28 DAS, there were no P by N interactions for RSR. Root shoot ratios decreased in quadratic fashion in response to applied P. The largest RSR values were obtained with 0 P, while the smallest RSR values were obtained with all levels of applied P. Plants grown with 60 mg-L-1 N had larger RSR values than those grown with










50


Table 3-8. Influence of P and N nutrition on growth characteristics of
lettuce transplants for Experiment 3, February 1996.

Nutrient Leaf Leaf Root: Relative Net
applied tissue tissue shoot growth assimilation
N PZ ratio rate rate
(mg L-1) (g-kg1) (g-kg1) (mg-mg-l-wk-') (mg-cm~2-wk')
15 Days After Sowing
P


0.69 0.31 0.31 0.29 0.29


0.40 0.36

NS
22 Days After Sowing
N1 N2
0.66 0.69 0.37 0.32 0.39 0.28 0.36 0.30 0.37 0.29
Q** Q**




*


0 15
30 60 90
Response
N
60 100
Response P x N

P
0 15 30 60 90
Response
N
60 100
Response P x N

P
0 15
30 60 90
Response
N


60 24.7
100 31.8
Response **
P x N NS
ZN, = 60 mg-L-1; N2 = Quadratic (Q).
NS, *, "Nonsignificant


28 Days
N, N2
0.9 1.0 3.8 4.0 4.8 5.9 5.1 7.4 5.8 8.6 Q** Q**


**
100 mg-L-4.


After Sowing

0.70 0.27 0.26 0.27
0.29
Q**


0.40 0.31
**
NS


0.90 1.58 1.59 1.56 1.56 Q**

1.41 1.46 NS NS


3.38 2.91 2.92 2.83 2.80 Q*

3.09 2.85
*
NS


0.25 0.73 0.66
0.64 0.63
Q**

0.56 0.61 NS NS


1.23
1.46 1.30
1.21 1.23 NS

1.33 1.25 NS
NS


(NS) or significant at 5% (*), 1% (**) levels.


48.5 24.6 23.5 22.7
22.1 Q**








51

100 mg N-L-. For plants sampled 22 DAS, N had no influence on RSR values, but RSR values were decreased with all levels of applied P. With 100 mg-L-1 N, the response of RSR to P was greater than with 60 mg-L-' N, because added N favored shoot growth rather than root growth.

For plants grown to 22 or 28 DAS, there were no P by N interactions for RGR and NAR (Table 3-8). By both sampling dates, RGR values increased in quadratic fashion in response to applied P. Nitrogen did not influence RGR values. For plants grown to 22 DAS, NAR values decreased in quadratic fashion in response to applied P. The greatest NAR values

were obtained with 0 P, and the least with 90 mg-L-1 P. Net assimilation rate was greater with 60 than 100 mg-L-1 N. By 28 DAS, P and N did not influence NAR values.

For plants sampled 15 and 28 DAS, there were no P by N interactions for SLA (Table 3-9). By both sampling dates, SLA values increased in quadratic fashion in response to applied P. Most of the response of SLA to applied P occurred between 0 and 15 mg-L'. For plants sampled 15 DAS, applied N did not influence SLA, while for plants sampled 28 DAS,

SLA was improved by 100 compared to 60 mg-L- N. For plants sampled 22 DAS, SLA values increased in quadratic fashion in response to applied P, regardless of N added. Specific leaf area increased in plants fertilized with 60 mg-L- N when P









52


Table 3-9. Influence of P and N nutrition on growth
characteristics of lettuce transplants for Experiment
February 1996.


Nutrient Specific Leaf Leaf Root
applied leaf area mass mass
area ratio ratio ratio
(mg-L-1) (cm2-mg~1) (cm2-mg-1)
15 Days After Sowing
P


0 15 30 60 90
Response
N
60
100
Response
P x N

P
0 15 30 60 90
Response
N
60
100
Response
P x N

P
0 15 30 60 90
Response
N


0.58 0.77 0.77 0.78 0.80 Q**


0.74 0.74
NS NS

N, N2
0.35 0.37 0.67 0.72 0.66 0.72 0.64 0.77 0.63 0.76 Q** Q**




**



0.34 0.62 0.63 0.67 0.65
Q**


0.34 0.59 0.59 0.60 0.62
Q**


0.54 0
0.56 0
NS *
NS N
22 Days After Sowing
N, N2 N,
0.21 0.22 0.60 0.49 0.54 0.73 0.48 0.56 0.72 0.47 0.59 0.73 0.46 0.59 0.73
Q** Q** Q**




**


0.59 0.76 0.76 0.77 0.78 Q**


.72 .75
*
S


N2
0.59 0.76 0.78 0.77 0.77 Q**


28 Days After Sowing
N, N2 N, N2
0.19 0.21 0.58 0.60 0.45 0.53 0.76 0.82 0.46 0.55 0.76 0.83 0.50 0.55 0.76 0.83 0.45 0.57 0.74 0.82
Q** Q** Q** Q**


0.41 0.24 0.24 0.23
0.22 Q**

0.28 0.25
**
NS


N,
0.40 0.27
0.28 0.27 0.27 Q**


N2
0.41 0.24 0.22 0.23 0.23 Q**


**

N1 N2
0.42 0.40 0.24 0.18 0.24 0.17 0.24 0.17 0.26 0.18 Q** Q**


60 0.56
100 0.61
Response **
P x N NS * ** **
zN, = 60 mg-L-1; N2 = 100 mg-L~1. Quadratic (Q). NS, **Nonsignificant (NS) or significant at 5% (*), 1% (**) levels.


3,


**








53

was applied. At 100 mg-L- N, SLA increased with all levels of applied P.

For plants sampled 15 DAS, there were no P by N

interactions for LAR (Table 3-9). Leaf area ratios increased in quadratic fashion in response to applied P. Most of the P

effect occurred between 0 and 15 mg-L- P. Applied N did not influence LAR values. For plants sampled 22 and 28 DAS, N had no influence on LAR, but LAR was increased with all

levels of applied P. With 100 mg-L-1 N, the response of LAR to P was greater than with 60 mg-L1 N.

For plants sampled 15 DAS, there were no P by N

interactions for LMR and RMR. Both LMR and RMR increased in quadratic fashion in response to applied P. Leaf mass ratio was least, while RMR was greatest with 0 P. Nitrogen at 100 mg-L1 increased LMR, but reduced RMR compared to N at 60 mg-L-1. For plants sampled 22 and 28 DAS, LMR values increased in quadratic fashion, while RMR values decreased in a quadratic fashion in response to applied P. Nitrogen had no influence on LMR or RMR, but LMR was increased, while RMR was decreased with all levels of applied P.

Fertigation frequency was every second day in Experiment 3 compared to Experiments 1 and 2 where fertigation frequency ranged from fertigating every two days to fertigating every four days. When fertigation was every two days, fresh and dry root mass increased in response to








54

15 mg-L-1, with no further increases in root mass at higher P concentrations up to 90 mg-L-1 even though the initial P concentration in the media was low (0.4 mg-kg-1). In Experiment 2, root mass was increased with each level of fertilizer P because the initial P concentration in the media was low (0.6 mg-kg1), indicating that perhaps 60 mg-L-1 P was not adequate with the irrigation programs used. Therefore, in a media with less than 0.5 mg-kg1 water extractable P, frequent fertigation is desirable.

These experiments have revealed that P applied via the floatation irrigation system improved growth of both roots and shoots of lettuce transplants, especially when P in the media was low. Melton and Dufault (1991) reported that 5 to 45 mg-L- P did not influence tomato transplant shoot and root growth. Tremblay et al. (1987) reported that increasing P from 100 to 200 mg-L-1 did not influence celery transplant root growth. Their studies did not have a 0 P treatment to compare growth responses with. Lorenz and Vittum (1980) reported that the critical tissue P concentration for most vegetable species is about 3.0 g-kg' of dry mass. This value corresponded to an application of 15 mg-L-1 P in the present work. In all the three experiments conducted, tissue P in plants produced with 0 P was approximately 1.0 g-kg-1, while a range of tissue P concentration from 3.0 to 8.6 g-kg1 produced lettuce shoots with similar mass. Based on








55

these results, a range of 3.0 to 4.0 g-kg' P can be considered adequate tissue P concentration for production of high quality lettuce transplants. It is not clear, however, why increased N led to more tissue P concentrations. Perhaps bigger plants due to N had greater energy requirements for growth processes and therefore took in more P.

Regardless of season grown, average daily maximum media temperatures were similar, i.e. 31, 33 and 29 0C, while average daily minimum media temperatures were also similar

at 22, 26 and 21 IC for Experiments 1, 2, and 3, respectively. Improved shoot growth in Experiment 2 (Sep/Oct) compared to Experiments 1 (Jun/Jul) and 2 (Feb), was probably related to higher temperatures inside the greenhouse during the fall. In Experiment 3, transplants produced with 0 P were very small compared with the previous experiments, probably due to low P (0.4 mg-kg) in the peat+vermiculite mix as well frequent fertigations, without P, that might have leached any available P in the media. Transplants produced with 0 P in Experiment 3 had similar poor growth, regardless of N concentration. (In Chapters 5 and 6, transplants did not respond to either P or K with 0 N). There was similar shoot and root growth with any level of applied P. Nitrogen at 100 mg-L1 improved shoot growth especially in response to applied P, but additional N adversely affected root growth compared to N at 60 mg-L-.








56

Results in Chapters 4 and 6 also indicated that applying more than 60 mg-L-1 N improved transplant shoot growth, but not root growth.

In general, RGR values were improved, while NAR values were reduced with any level of P. Values of RGR and NAR were larger at 21 than at 28 DAS, indicating that younger plants had greater growth efficiency than older ones. With added P, RSR values were similar in Experiment 1 and 3, but lower in Experiment 2. Higher temperatures in Experiment 2 caused more shoot growth at the expense of root growth. Weston and

Zandstra (1989) reported that P from 15 to 60 mg-L-1 had no effect on RSR values of tomato transplants. In Chapter 5, lower RSR values were obtained at low temperatures (average

daily minimum media temperature of 11 OC), indicating that extreme temperatures adversely affected RSR values. In all the experiments, LMR and RMR were similar regardless of sampling date, implying that there was no shift in dry matter allocation between shoots and roots with time. The same was true for K in Chapter 4, but not for N in Chapters 5 and 6. As N increased, more dry matter became allocated to shoots than to roots, with time.

Results from scanning the roots, revealed that the response of root length and root area paralleled the response of root mass to applied P, regardless of time of transplant production. Quality transplants had total root








57

lengths were between 276 and 306 cm, and total root area between 26 and 30 cm2. With any level of applied P, pulling success was improved tremendously compared to 0 P, but pull force was unaffected. Only 30 % of the plants produced with

0 P could be pulled from the transplant flats, compared to approximately 90 % pulling success with added P. In Chapter 6, pull force was related to pulling success, but this was not so in the present work probably because there were smaller differences in root mass among the treatments in the present investigation.


Field Experiments


Plants from Greenhouse Experiment 2 (fall) and

Experiment 3 (winter) were grown to maturity to evaluate the effects of pretransplant P on earliness, yield and lettuce head quality.

Plants of all treatments in the fall crop of Experiment

1 were harvested in December, 64 days after transplanting (DAT). Lettuce head mass increased in quadratic fashion in response to pretransplant P (Table 3-10). Head mass was greater from plants receiving P as a pretransplant treatment compared to those plants not receiving P.

Firmness, head height, stem width, and core length increased in a quadratic fashion with pretransplant P. Firmness, and head height ratings were improved by








Table 3-10. Effects of P nutrition during transplant production on lettuce head mass
and head quality characteristics for Experiment 1, harvested 20 December 1995.

Phosphorus Head Firm Head Head Stem Core Leaf
applied mass ratingz height diameter width length tissue
P
(mg -L ) (g) (1-5) (mm) (mm) (mm) (mm) (g - kg-')
0 601 4.3 112 120 25 38 2.9
15 743 4.8 124 122 30 54 3.1
30 711 4.9 123 124 31 58 3.0
45 721 4.9 130 129 30 57 2.8
60 738 4.8 127 121 31 63 2.7
Response Q* Q** Q* NS Q** Q* NS
'Lettuce head firmness on a scale of 1 = loose, 5 = compact. Quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant
(NS).


(-n









59


transplant P, while stem width and core length were enlarged. Heads were less developed with 0 P, while heads

from transplants produced with 15 to 60 mg-L-' pretransplant P were more developed, indicating greater earliness. At harvest, tissue P levels were equal regardless of pretransplant P because plants were grown in a field with soil high in available P.

Plants of all treatments in the spring crop of

Experiment 2 were harvested in May, 64 DAT. There were no P by N interactions for head mass or head quality characteristics. There was a positive linear response of head mass to pretransplant P. Head mass was improved at harvest with all pretransplant P fertilization treatments, but was unaffected by pretransplant N fertilization (Table 3-11).

Stem width increased in quadratic fashion, while core length increased in linear fashion in response to pretransplant P. Stem width and core length were enlarged by transplant P, indicating greater earliness, but were unaffected by pretransplant N. Lettuce head firmness, height, and diameter were unaffected by pretransplant P or N. At harvest, tissue N and P levels were equal regardless of pretransplant P or N applied.

In the field, lettuce head mass was influenced by pretransplant P, regardless of time of production. All








Table 3-11. Effects of P and N nutrition during transplant production on lettuce head
mass and head quality characteristics for Experiment 2, harvested 2 May 1996.

Nutrient Head Firm Head Head Stem Core Leaf Leaf
applied mass rating' height diameter width length tissue tissue
N P
(mg - L-') (g) (1-5) (mm) (mm) (mm) (mm) (g - kg-') (g - kg-1)
P
0 451 4.6 124 103 22 34 36.6 2.1
15 533 4.9 124 107 24 36 35.0 2.0
30 517 4.8 124 104 24 36 34.9 2.1
60 524 4.8 123 108 24 38 36.8 2.1
90 552 4.8 124 108 24 38 34.3 2.2
Response L** NS NS NS Q* L* NS NS
N
60 528 4.8 125 106 24 36 34.7 2.0
100 502 4.7 123 106 23 37 36.3 2.2
Response NS NS NS NS NS NS NS NS
P x N NS NS NS NS NS NS NS NS
'Lettuce head firmness on a scale of 1 = loose, 5 = compact. Linear (L) or quadratic (Q).
NS, **Nonsignificant (NS) or significant at 5% (*), 1% (**) levels.








61

pretransplant P treatments had a similar effect of increasing head mass at harvest. Tissue P levels were equal at harvest regardless of pretransplant P applied. Hochmuth et al. (1991) reported values of 25 to 50 g-kg~1 P (soil type not reported) to be indicative of an adequate P range for crisphead lettuce. Values of tissue P were slightly less than this in Experiment 2, but plants looked healthy with tissue P of 21 g-kg-1. Stem width and core length were improved by pretransplant P, indicating greater earliness due to P fertilization, thus adequate plant size at transplanting. Earliness is of particular significance in north Florida where the growing period is shortened by either low temperatures in fall plantings or high temperatures in spring plantings. Low temperatures could result in lettuce heads freezing, while high temperatures could cause premature bolting. At transplanting, plants produced with pretransplant P were larger than those produced with no P. Therefore, larger plants at transplanting led to earliness and larger head size at harvest.


Summarv



'South Bay' lettuce transplants were produced with

different levels of P supplied via floatation irrigation, to








62

determine the optimum P concentration necessary for production of high quality transplants, and subsequent high quality crop in the field. A quality transplant has sufficient roots to fill a tray cell to facilitate ease of pulling from the transplant flat. Plants were propagated by floating flats in a nutrient solution containing either 0, 15, 30, 45, or 60 mg-L-1 P in summer and fall experiments,

and either 0, 15, 30, 60, or 90 mg-L-' P in factorial combination with 60 or 100 mg N-L-1 in a winter experiment. Photoperiod was extended to 16 h in all experiments.

Phosphorus applied at frequent rates via the floatation irrigation system affected growth of both roots and shoots of lettuce transplants. However, after the initial P addition of 15 mg-L-1, further P additions resulted in a minimal growth response. Transplants produced with 0 P had similar poor growth, regardless of N applied. Nitrogen at 100 mg-L-1 improved the response of shoot growth to any level of P, but adversely affected root growth compared to N at 60 mg-L-1.

In general, RGR values were improved, while NAR values were reduced with any level of P. Values of RGR and NAR were larger by 21 DAS than by 28 DAS, indicating that younger plants had greater growth efficiency than older ones. Quality transplants had RSR of approximately 0.25, total root lengths between 276 and 306 cm, and total root area








63

between 26 and 30 cm- in a 10.9 cm' cell volume. Only 30 % of the plants produced with 0 P could be pulled from the transplant flats, compared to approximately 90 % pulling

success with added P. At least 15 mg-L1 P, supplied every two days via floatation irrigation, is recommended for production of high quality lettuce transplants in a peat+vermiculite media containing low concentrations of water extractable P.

All pretransplant P treatments had a similar effect of increasing head mass at harvest time, and in reducing time to maturity regardless of production season. At transplanting, plants produced with transplant P were larger than those produced with no transplant P. Phosphorus fertilization in the transplant cell, led to improved earliness and yields.

This work demonstrated that at least 15 mg-L- P,

supplied via floatation irrigation to a peat+vermiculite mix, was required to build an ideal transplant with sufficient roots to fill a tray cell for ease of pulling out of transplant flats. Phosphorus fertilization also resulted in larger transplants for rapid field establishment, leading to earlier lettuce harvest.














CHAPTER 4

NEED FOR SUPPLEMENTAL POTASSIUM FOR LETTUCE TRANSPLANT PRODUCTION



Introduction



The environmental conditions to which vegetable transplants are exposed during early growth play an important role in final crop yield (Masson et al., 1991b). The early growing environment of transplants can be manipulated in ways that are not possible with direct-seeded crops (Wurr and Fellows, 1982). Several factors that are known to affect vegetable transplant size, quality, and growth in the field include nutritional conditioning before transplanting (Jaworski and Webb, 1966; Jaworski et al., 1967; Kratky and Mishima, 1981; Weston and Zandstra, 1989; Garton and Widders, 1990; Masson et al., 1991a, 1991b; Melton and Dufault, 1991; Dufault and Schultheis, 1994).

The role of fertilizer K in vegetable transplant growth has been investigated. Dufault (1985) produced celery transplants and gave them weekly applications of various N, P, and K solutions. The treatments were factorial


64








65

combinations of N at 10, 50, or 250 mg-L-, P at 5, 25, or 125 mg-L-1, and K at 10, 50, or 250 mg-L-. Potassium did not affect celery growth. The media contained 40 mg-L1 hydrochloric acid extractable K and may have supplied all the necessary K requirements. Melton and Dufault (1991) grew tomato transplants with either 25, 75, or 225 mg-L-1 K applied three times per week. They found that K did not influence transplant height, stem diameter, leaf number, leaf area, total chlorophyll, fresh shoot mass, or dry shoot and root mass. Tomato transplant growth did not respond to fertilizer K probably because the media Melton and Dufault used already contained 103 mg-L~1 K (extraction method not reported).

Tremblay and Senecal (1988) grew lettuce, broccoli,

pepper, and celery transplants with 150 or 350 mg-L-1 N, as well as 50, 200, or 350 mg-L-1 K, applied daily. Growth measurements were made at 18, 20, 31, and 38 days after sowing for lettuce, broccoli, pepper, and celery, respectively. They reported that celery and broccoli leaf area increased by adding 50 to 350 mg-L~' K. Leaf area of lettuce was increased with added K only with 350 mg-L-1 N, but not with 150 mg-L-' N. Increasing the K concentration in conjunction with 150 mg-L-' N decreased pepper leaf area while, with 350 mg-L-' N, the inverse pattern was true.








66

Broccoli dry shoot mass increased in response to K, with a maximum at 200 mg-L-1 K. Lettuce dry shoot mass was increased more sharply by increasing K when grown with 350 than with 150 mg-L-1 N. Celery dry shoot mass was maximized with 200 mg-L-1 K when N concentration was 350 mg-L-1, but minimum at this K concentration when added N was only 150 mg-L-1. The percentage of shoot dry matter in lettuce and pepper increased with K concentration when N was 150 mg-L-',

but decreased when N was 350 mg-L-1. Root growth characteristics as well as root:shoot ratio for broccoli, celery, and lettuce were not affected by K fertilization.

Most of the previously described experiments were

conducted with weekly applications of fertilizer. Data are lacking on the growth response of lettuce roots and shoots to frequent K applications such as practiced in the floatation system of irrigation. In this system, nutrients are supplied with every irrigation by floating flats directly in nutrient solution. Growers using this system have been unable to produce lettuce transplants with sufficient roots in a tray cell to enable easy removal of transplants from the transplant flat (Robles, personal communication). Perhaps optimizing K could improve root development in lettuce transplants.

The present investigation was conducted to determine the optimum K concentration, supplied via floatation








67

irrigation, that could produce quality transplants with sufficient roots in a tray cell to facilitate easy removal of transplants from the transplant flat, and lead to rapid field establishment. In previous experiments (Chapter 3), photoperiod was extended to 16 h. In order to determine if supplementary light would be of greater benefit in promoting lettuce growth than extended photoperiod during periods of low light intensity, both systems were compared.


Materials and Methods



Greenhouse Experiments


'South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of

1.9 x 1.9 x 6.3 cm; 10.9 cm3 (length x width x depth; volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v), with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg-m~3, was used for media. Three experiments were conducted (Tables 4-1 and 4-2). The plants were grown with natural photoperiod extended to 16 h by 1000-W, high-pressure sodium lamps (250 pmol-m-2.S-1 photosynthetic photon flux). A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just









68

Table 4-1. Sowing schedule and initial media test (Hanlon et
al., 1994) for Experiments 1 and 3.

Expt Sowing date Media test
pH EC N03-N P K Ca Mg
(dS - m-') (mg - kg-1)
1 14 Jul 1993 4.7 0.9 1.3 12.4 14.6 14.2 11.6 3 31 Jan 1996 5.2 0.2 0.3 0.4 24.4 0.6 5.8 concentrations in the saturated paste extract.


Table 4-2. Initial media test (Hanlon et al., 1994) for
Experiment 2, sown 28 January 1994.

Media type Media testpH EC N03-N P K Ca Mg
(dS - m-1) (mg - kg-1)
Peat+vermiculite 4.9 0.1 0 0.7 10.9 0.9 1.8
Peat+rockwooly 5.3 0.1 0 0.3 2.5 0.8 0.8
Peat 4.0 0.1 0 0.6 2.0 0.8 1.2
ZConcentrations in the saturated paste extract. YForty % hydrofile and 10 % hydrorepellent rockwool.


above the plant canopy, and media temperatures were recorded by a Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR) . Separate temperature measurements were made for the treatments under extended photoperiod and those under supplementary light in Experiment 2. Photosynthetically active radiation (PAR) during the plant growing period was measured with a light meter just above the plant canopy. For consistency, measurements were taken at 10:00 h every morning.

The flats were seeded then covered with a thin layer of vermiculite, overhead irrigated enough to moisten the vermiculite, then transferred to a cooler at 20 0C for








69

germination. After 48 h, flats were returned to the greenhouse.

Plants in Experiment 1 were irrigated every two to four days by floating flats in nutrient solution containing K at 0, 15, 30, 45, or 60 mg-L-1 as KCl. Other nutrients were supplied at equivalent rates to all plants and consisted of (in mg-L2) 100 N, 30 P, 100 Ca, and half-strength Hoagland's solution for micronutrients only (Hoagland and Arnon, 1950) that was comprised of Mg, S, B, Cu, Mo, and Zn. The experiment was a randomized complete-block design with 5 treatments and 4 replications.

Plants in Experiment 2 were grown with either the

natural photoperiod extended to 16 h or with supplementary lighting for the entire 16 h photoperiod from 1000-W, highpressure sodium lamps (250 ymol-m-2. S1 photosynthetic photon flux). Plants were irrigated once every two to four days, by floating flats in nutrient solution containing K at 0 or 60 mg*L' as KCl. Other nutrients were applied as described for Experiment 1. Peat+vermiculite (1:2, v/v), peat+rockwool (1:1, v/v), and peat media, were used for the experiment. The factorial experiment was arranged in a split-block design. There were 3 replications within each light treatment consisting of 2 levels of K and 3 media.

Plants in Experiment 3 were irrigated every other day by floating flats in nutrient solution containing K at 0,









70

15, 30, 45, or 60 mg-L-' in combination with N at 60 or 100 mg-L-1. Potassium was supplied from KCl, while N was

supplied from NH4NO3. Other nutrients were applied as described for Experiment 1. The experiment was a randomized complete-block design with 10 treatments consisting of a factorial combination of 5 levels of K and 2 levels of N, replicated four times.

Plant samples, 5 per treatment, were taken at

approximately 14, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE). Growth variables calculated were: root:shoot ratio (RSR = dry root mass + dry shoot mass), relative growth rate (RGR = [ln (final total dry mass) - In (initial total dry mass) - (final time initial time)]), net assimilation rate (NAR = [(final total dry mass - initial total dry mass) + (final time - initial time) x {(ln (final leaf area) - ln (initial leaf area)} + (final leaf area - initial leaf area)]), specific leaf area (SLA = leaf area + dry shoot mass), leaf area ratio (LAR = leaf area total dry mass), leaf mass ratio (LMR = dry shoot mass + total dry mass), and root mass ratio (RMR = dry root mass + total dry mass) (Hunt, 1978; 1982; Dubik et al., 1992).








71

Leaf petioles were collected at 23 and 30 DAS in

Experiment 2, and at the last sampling dates in Experiments

1 and 3 for sap testing. The sap was squeezed from collected petiole pieces using a hydraulic sap press onto sampling sheets according to Hochmuth (1992). A CARDY meter (Spectrum Technologies, Inc., Plainfield, IL) was used to measure K' concentrations in the petiole sap.

Dry shoot samples from the last sampling dates were

ground to pass a 20-mesh screen and dry-ashed for K or aciddigested for total Kjeldahl N according to Wolf (1982). For total K determination, 0.5 g subsamples were weighed into 10 mL beakers. The samples were then dry-ashed in a muffle furnance at 500 0C for 10 h. The ash was moistened with 1 N HCl and poured into 50 mL volumetric flasks, and brought to volume with 1 N HC. The solutions were filtered through 'Q8' filter papers (Fisher brand), with a particle retention of > 10 gm, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and analyzed with Model 61-E Inductively Coupled Plasma Spectrometry (Thermo Jarrell Ash Corporation, Franklin, MA).

The acid digestion procedure consisted of weighing 0.25 g subsamples into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes, which were

then heated on a digestion block at 375 OC. After the









72

digestion process was completed (a total of 2.5 h), the samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through 'P8' filter papers (Fisher brand), with a particle retention of > 25 gm, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR).

Data were subjected to analysis of variance using PROC GLM and/or PROC MIXED (SAS Institute, Inc., Cary, NC). Treatment sums of squares were partitioned into linear or quadratic polynomial contrasts in Experiments 1 and 3. Plants in the peat mix in Experiment 2 did not grow, perhaps due to poor aeration, therefore the treatment was eliminated from data analysis.


Field Experiment


Plants from each treatment in Greenhouse Experiment 3 were transplanted into an Arredondo fine sandy soil (loamy, siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on black polyethylene-mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville, on 29 February 1996. The soil had a water pH of

5.8, with 0 dS-m1 for electrical conductivity, and a








73

nutrient content (Hanlon et al., 1994) of (in mg-kg,) 247 P, 37 K, 695 Ca, and 43 Mg (Mehlich-1 extractant). The experiment was a randomized complete-block design with 10 treatments consisting of a factorial combination of 5 levels of K and 2 levels of N, with each treatment replicated four times. Preplant fertilizer (13N-0P-10.8K) was applied broadcast and incorporated in the bed at 230 kg-ha1. Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. There were 30 plants per plot planted on double offset rows with a spacing of 0.3 m between plants and between rows on the bed, equivalent to 54,000 plants/ha.

Just after transplanting, 100 mL of nutrient solution (150 mg-L1 20N-8.6P-16.7K) was applied to each transplant hole as a starter fertilizer. Water was applied daily for 45 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark (1991). Starting one week after transplanting, fertilizer at a rate of 15 kg-ha' N and 16 kg-ha- K as NH4NO3 and KNO3, was injected once weekly using a venturi pump (Netafim Irrigation, Altamonte Springs, FL), with the last application one week before harvest to give a








74

total amount of 150 kg-ha-1 N and 180 kg-ha1 K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al., 1988).

At lettuce head maturity, the center 20 plants in a

plot were cut, individually weighed, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements. Wrapper leaves were sampled at harvest for analysis of tissue K and N according to Wolf (1982) as described for Greenhouse Experiments. Field data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc., Cary, NC). Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts.


Results and Discussion



Greenhouse Experiments


Experiment 1 was conducted during the summer under

greenhouse temperatures ranging from 22 to 36 OC (Fig. 4-1). The average daily maximum media temperature was 32 OC, while the average daily minimum media temperature was 23 OC. During the course of the experiment, there were totals of 25 sunny days of which two were followed with rain in the afternoon, and 2 cloudy days.








75


40,


w

F
a:
w
(L
w


201510

5

0
17


311


GROWING PERIOD (JUL 17 - AUG 11)


-H- air max -+- media max -+- air min -E- media min


Fig. 4-1. Maximum and minimum air and media temperature
during transplant production for Experiment 1, Jul/Aug
1993.


- -


OB


353025-


11









76

For plants sampled 15, 21, and 28 days after sowing

(DAS), applied K did not influence fresh and dry shoot mass (Table 4-3). For plants sampled 15 and 21 DAS, applied K did not influence fresh and dry root mass. However, by 28 DAS, there was a positive linear response of fresh and dry root mass to applied K. For plants sampled 15 DAS, leaf area increased linearly to applied K, but by later sampling dates, applied K did not influence leaf area.

For plants grown to 21 or 28 DAS, there was a positive linear increase in petiole sap K in response to applied K. Leaf tissue K also increased linearly to applied K. Therefore, increased root growth was associated with increased tissue K.

For plants sampled 15, 21, and 28 DAS, RSR values were not influenced by applied K, since shoots were unaffected while there was little increase in root growth due to added K (Table 4-4). For plants grown to 21 DAS, RGR, and NAR values responded in quadratic fashion in response to applied K. Relative growth rates and NAR values were least with 30 mg-L-1 K and greatest with 60 mg-L-1 K. By 28 DAS, RGR and NAR were not influenced by K. For plants sampled 15 DAS, SLA and LAR increased linearly in response to applied K. For plants sampled 21 and 28 DAS, SLA, and LAR were not influenced by K.









77


Table 4-3. Root and shoot characteristics of lettuce transplants
as affected by K nutrition for Experiment 1, July/August 1993.

Potassium Fresh Dry Fresh Dry Leaf Leaf Leaf
applied shoot shoot root root area petiole tissue
mass mass mass mass sap K K
(mg-L-1) (mg) (mg) (mg) (mg) (cm2) (mg-L-1) (g-kg~1)
15 Days After Sowing
0 152 9.7 56 3.6 6.4
15 177 10.5 57 3.9 7.2
30 183 11.2 57 3.8 7.7
45 172 10.5 54 3.6 7.4
60 175 10.2 50 3.5 7.4
Response NS NS NS NS L*
21 Days After Sowing
0 704 40.2 167 10.4 26.4 2725
15 699 41.1 182 11.3 26.3 2800
30 721 39.4 183 10.8 27.1 2900
45 734 40.2 182 11.1 28.1 2950
60 726 41.2 214 13.0 26.8 3075
Response NS NS NS NS NS L**
28 Days After Sowing
0 1366 80.0 298 19.9 46.3 2300 41.9
15 1347 82.4 296 20.8 46.5 2125 42.8
30 1395 80.7 302 20.8 47.6 2325 47.6
45 1454 88.3 324 22.4 49.6 2825 48.0
60 1474 85.6 333 22.8 50.2 2450 48.7
Response NS NS L* L** NS L** L**
Linear (L) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS).








Table 4-4. Influence of K nutrition on growth characteristics of lettuce transplants,
Experiment 1, July/August 1993.

Potassium Root: Relative Net Specific Leaf Leaf Root
applied shoot growth assimilation leaf area mass mass
ratio rate rate area ratio ratio ratio
(mg-L-1) (mg-mg--wk-') (mg-cm-2-wk-') (cm2-mg') (cm2-mg-1)
15 Days After Sowing
0 0.37 0.66 0.48 0.73 0.27
15 0.38 0.69 0.50 0.73 0.27
30 0.34 0.68 0.51 0.75 0.25
45 0.34 0.70 0.52 0.75 0.25
60 0.34 0.73 0.54 0.75 0.25
Response NS L* L** NS NS
21 Days After Sowing
0 0.26 1.33 2.66 0.66 0.52 0.79 0.21
15 0.28 1.30 2.59 0.64 0.50 0.78 0.22
30 0.28 1.21 2.30 0.69 0.54 0.78 0.22
45 0.32 1.29 2.40 0.70 0.55 0.78 0.22
60 0.32 1.38 2.70 0.65 0.49 0.76 0.24
Response NS Q** Q* NS NS NS NS
28 Days After Sowing
0 0.25 0.68 1.40 0.58 0.46 0.80 0.20
15 0.25 0.68 1.43' 0.56 0.45 0.80 0.20
30 0.26 0.70 1.41 0.59 0.47 0.80 0.20
45 0.25 0.77 1.57 0.56 0.45 0.80 0.20
60 0.27 0.69 1.45 0.59 0.46 0.79 0.21
Response NS NS NS NS NS NS NS
Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS).


-_j








79

Applied K did not influence LMR or RMR values,

regardless of sampling date (Table 4-4). For plants sampled 15 DAS, approximately 75 % of the lettuce transplant dry matter was allocated to shoots and 25 % allocated to roots. As the plants grew older, by 28 DAS, proportionally more dry matter became allocated to shoots (80 %) compared to roots (20 %).

Results of Experiment 1 indicated that K applied to a

peat+vermiculite media with 15 mg-L-1 water extractable K, increased transplant root growth, but not shoot growth. Sufficient K for shoot growth may have been available or released from the media during the growing cycle.

In Experiment 2, transplant growth response to K was compared among three media types, peat, peat+rockwool, and peat+vermiculite mixes. Peat and rockwool (molten, spun basalt rock fibers) have inherently lower K levels than vermiculite (Table 4-2). Since the experiment was conducted in February when light intensities are normally low, the benefit of supplementary light for 16 h was compared with an extension of the photoperiod to 16 h.

Experiment 2 was conducted during the winter, under greenhouse temperatures ranging from 8 to 37 0C (Figs 4-2 and 4-3). The average daily maximum media temperatures were

24 and 26 'C under extended photoperiod and under supplementary light, respectively. Average daily minimum








80


0
w
a: a:
w
a.
w


30


25201510-


5



2 26
GROWING PERIOD (FEB 2 - FEB 26)


+E air max -4- media max + air min -- media min


Fig. 4-2. Maximum and minimum air and media temperature
during transplant production under extended photoperiod for Experiment 2, February 1994.


'A FN .








81


35


30


025
0 w
a20
I

15
w
CL
W 10F

5


0
3 26
GROWING PERIOD (FEB 3 - FEB 26)

-a- air max -+- media max -*- air min a media min


Fig. 4-3. Maximum and minimum air and media temperature
during transplant production under supplementary
lighting for Experiment 2, February 1994.








82

media temperature was 14 'C with both light treatments. During the course of the experiment, there were totals of 13 sunny and 16 cloudy days. Supplementary light contributed more than natural light to the light integral (PAR) received in the greenhouse by the lettuce transplants (Fig. 4-4).

For plants sampled 23 DAS, light, K, and media did not interact to influence fresh shoot mass (Table 4-5). Light and media treatments did not influence fresh shoot mass, while more fresh shoot mass occurred in transplants grown with 60 mg-L-1 than with no K. Supplementary light for 16 h led to increased dry shoot mass compared to extending the photoperiod to 16 h, particularly in peat+vermiculite compared to peat+rockwool mix.

For plants sampled 30 DAS, both K and media influenced fresh and dry shoot mass (Table 4-6). When produced with 60 mg-L- K, plants grown in peat+rockwool mix had more fresh shoot mass than plants grown in the peat+vermiculite mix. There was no response in dry shoot mass to applied K in the peat+vermiculite mix. In peat+rockwool mix, applied K resulted in an increase in dry shoot mass. Plants grown with supplementary light for 16 h had greater shoot mass than those grown with the photoperiod extended to 16 h.

For plants sampled 23 DAS, only the media used

influenced fresh root mass; there was no effect of added K or light (Table 4-5). Root mass of plants grown in








83


900800700600
0
E 50.* 400a: 300200100

0G(
1 27
GROWING PERIOD (FEB 1 -FEB 27)


-6- natural -- supplementary


Fig. 4-4. Photosynthetically active radiation (PAR) under
natural or supplementary lighting for Experiment 2,
February 1994.










84


Table 4-5. Root and shoot characteristics of lettuce transplants 23 days
after sowing as affected by light, potassium, and media for Experiment
2, February 1994.


Mix


Lights (h) K (mg-L-')
4 16 Response 0 60 Response


Peat+vermiculite 10 Peat+rockwool 10
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite 2 Peat+rockwool 1
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite 21 Peat+rockwool 5
Response

Peat+vermiculite Peat+rockwool Response

Peat+vermiculite Peat+rockwool Response

Peat+vermiculite Peat+rockwool Response

Peat+vermiculite Peat+rockwool Response

Peat+vermiculite Peat+rockwool Response
zNatural photoperiod the entire 16 h.
1, *, ''Nonsignificant levels.


03
96 NS

61.4 59.0 NS

22 95 NS

11.1
9.4 NS

36.7 36.9 NS


83 88


**


Fresh shoot mass (mg)
1014 NS 1005


1057 NS
NS
Dry shoot mass
83.6 **
69.6 *
**


(mg)


Fresh root mass (mg)
294 *
229 NS
**
Dry root mass (mg)
16.8 NS 15.3 NS
NS
Leaf area (cm2)
35.8 NS 35.6 NS
NS


999 NS

72.8
54.7
**

257 199
**

14.1 11.9 NS

36.9 31.1
**


Leaf petiole sap K (mg-L')
1967 NS 1950
498 NS 101


**


Roc


0.18 0.16 NS
Specif
0.60 0.62 NS
Leaf
0.51
0.54 NS

0.85 0.86 NS

0.15
0.14 NS


ic


ot:shoot ratio
0.20 NS 0
0.23 NS 0
NS N
leaf area (cm2 -mg')


0.43 ** 0.52 *
**


area ratio (cn?-mg-')
0.36 ** 0.42 *
**
Leaf mass ratio
0.84 NS 0.82 NS
NS
Root mass ratio
0.16 NS 0.18 NS
NS


.19
.22
S


0.52 0.57
**

0.44 0.47 NS

0.84 0.82 NS

0.16 0.18 NS


1012 NS
1154 **
*

72.2 NS 73.9 **
NS

260 NS 226 NS
*

13.8 NS 12.8 NS
NS

35.6 NS
41.4 **
**

2200 * 985 **
**


0.19 0.17 NS

0.51 0.57
**

0.43 0.49
**

0.84 0.86 NS

0.16
0.14 NS


NS NS


NS NS


NS NS


NS NS


NS NS


extended by 4 h to 16 h or supplementary light for

(NS) or significant t-test at 5% (*), 1% (**)









85


Table 4-6. Root and shoot characteristics of lettuce transplants
30 days after sowing as affected by light, potassium, and media for Experiment 2, February 1994.

Mix Lightz (h) K (mg - L-1)
4 16 Response 0 60 Response


Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool
Response

Peat+vermiculite Peat+rockwool Response


1450 1575
NS E


Fresh shoot mass (mg)
1427 NS 1456
1598 NS 1329
* NS


Dry shoot mass (mg)
110.6 134.3 * 103.7 131.4 * NS NS


348 289
**

26.
20.
**


Fresh root mass (mg)
450 *
327 NS
**
Dry root mass (mg)
8 36.6 **
8 25.9 *
**


Leaf area (c2)
52.3 50.1 NS
52.6 50.7 NS
NS NS
Leaf petiole sap K ( 2467 2083 **
593 585 NS
** **


Leaf tissue K (g-kg-')
37.8 30.1 *
11.0 9.0 NS
** **


123.9 93.1
**

378 238
**

29.8 17.7
**

52.0 38.5
**


ng-L~')
1917 78
**


30.3 2.7
**


1422 NS 1844 **
**

121.0 NS 142.0 **
**

420 * 377 **
*

33.6 ** 28.9 **
**

50.4 NS 64.8 **
**

2633 ** 1100 **
**

37.7 ** 17.3 **
**


zNatural photoperiod extended by 4 h to 16 h or supplementary light for the entire 16 h. NS * **Nonsignificant (NS) or significant t-test at 5% (*), 1%
(**) levels.








86

peat+vermiculite mix was greater than for plants grown in peat+rockwool mix. Dry root mass of plants sampled 23 DAS was not affected by any treatment. By 30 DAS, plants grown in peat+vermiculite mix had greater fresh and dry root mass compared to those in peat+rockwool mix under both light treatments (Table 4-6). Fresh root mass was increased by supplementary light only when peat+vermiculite mix was used. For both media types, fresh and dry root mass were greater with 60 mg-L-1 K compared to no K. The greatest dry root mass was 36.6 mg obtained from plants grown in peat+vermiculite mix with supplementary light. The greatest dry root mass from the peat+rockwool mix was 28.9 mg, obtained from plants grown with 60 mg-L- K.

Regardless of sampling date, applied K did not influence leaf area when plants were grown in peat+vermiculite mix, indicating sufficient K in the media (Tables 4-5 and 4-6). The plants had greater leaf area when 60 mg-L1 K compared to no K was added to peat+rockwool mix. With no K, plants in peat+vermiculite mix had greater leaf area than plants in peat+rockwool mix. The opposite was true

with 60 mg-L- K.

For plants grown to 23 DAS (Table 4-5), petiole sap K concentration was greater when plants were grown in peat+vermiculite mix instead of peat+rockwool mix. Petiole sap K increased when K was applied to either media type. By








87

30 DAS (Table 4-6), plants grown with 60 mg-L- K had more petiole sap K than those grown with no K. Plants grown in peat+vermiculite mix had greater concentrations of petiole sap K than those grown in peat+rockwool mix. It is not clear why in peat+vermiculite mix, supplementary light resulted in lower petiole sap K, while petiole sap K values were not influenced by light treatment for plants grown in peat+rockwool mix.

Plants grown in peat+vermiculite mix had greater total leaf tissue K concentration than plants grown in peat+rockwool mix due to inherently higher levels of K in vermiculite (Table 4-6). It is unclear why supplementary light, compared with an extension of the photoperiod, resulted in lower leaf K concentration in plants grown in peat+vermiculite mix but not in peat+rockwool mix. Plants grown with 60 mg-L-1 K had greater K concentration in the leaves compared to plants grown with no K, especially those plants grown in peat+rockwool mix.

For plants sampled 23 DAS, neither of the treatments influenced RSR values (Table 4-5). For plants sampled 30 DAS, RSR was affected by both K and media (Table 4-7). Plants grown with 60 mg-L' K in peat+vermiculite mix had greater RSR values than those grown with no K, because added K increased dry root mass but not dry shoot mass. Plants grown in peat+vermiculite mix also had greater RSRs than









88


Table 4-7. Influence of light, potassium, and media on growth
characteristics of lettuce transplants 30 days after sowing
for Experiment 2, February 1994.

Mix Lightz (h) K (mg-L-)
4 16 Response 0 60 Response
Root:shoot ratio
Peat+vermiculite 0.25 0.27 NS 0.24 0.28 **
Peat+rockwool 0.20 0.20 NS 0.19 0.20 NS
Response ** ** ** **
Relative growth rate (mg-mg-'-wk-')
Peat+vermiculite 0.64 0.54 NS 0.58 0.60 NS
Peat+rockwool 0.59 0.60 NS 0.50 0.68 **
Response NS NS NS NS
Net assimilation rate (mg-cm-wk~')
Peat+vermiculite 1.48 1.68 NS 1.53 1.63 NS
Peat+rockwool 1.24 1.66 NS 1.29 1.61 NS
Response NS NS NS NS
Specific leaf area (cm2mg-') Peat+vermiculite 0.48 0.37 * 0.42 0.43 NS
Peat+rockwool 0.50 0.38 ** 0.42 0.46 NS
Response NS NS NS NS
Leaf area ratio (cm2-mg')
Peat+vermiculite 0.38 0.29 * 0.34 0.33 NS
Peat+rockwool 0.42 0.32 ** 0.36 0.39 NS
Response NS NS NS *
Leaf mass ratio
Peat+vermiculite 0.80 0.79 NS 0.81 0.78 *
Peat+rockwool 0.83 0.84 NS 0.84 0.83 NS
Response ** ** ** **
Root mass ratio
Peat+vermiculite 0.20 0.21 NS 0.19 0.22 *
Peat+rockwool 0.17 0.16 NS 0.16 0.17 NS
Response ** ** ** **
Natural photoperiod extended by 4 h to 16 h or supplementary light for the entire 16 h.
NS, *Nonsignificant (NS) or significant t-test at 5% (*), 1%
(**) levels.








89

plants grown in peat+rockwool mix. Added K did not influence RSRs for plants grown in peat+rockwool mix. For plants grown to 30 DAS (Table 4-7), RGR values were lower in

peat+rockwool mix with no K than with 60 mg-L- K, while in the peat+vermiculite mix, RGR was not affected by K. Neither treatment influenced NAR. In Experiment 1, RGR and NAR were also not influenced by added K in a peat+vermiculite mix.

When photoperiod was extended to 16 h, plants grown with no K in either mix had similar SLA values by 23 DAS, but when grown with 60 mg-L< K, those plants grown in peat+rockwool mix had greater SLA values than plants in peat+vermiculite mix (Table 4-8). Application of 60 mg-L1 K led to smaller SLA values compared to no K when plants were grown in peat+vermiculite compared to peat+rockwool mix. Under 16 h supplementary light, SLA values were not influenced by applied K in either media. With no K, SLA values were greater in peat+rockwool than in peat+vermiculite mix. For plants sampled 23 DAS (Table 4-5), 16 h supplementary light reduced SLA compared to extended photoperiod, particularly when plants were grown in peat+vermiculite than in peat+rockwool mix.

For plants sampled 30 DAS, supplementary light for 16 h led to decreased SLA and LAR compared to simply extending the photoperiod to 16 h (Table 4-7). A low SLA is desirable, though, because it is associated with a thicker leaf.









90


Table 4-8. Influence of light, potassium, and media on SLA of
lettuce transplants 23 days after sowing for Experiment 2,
February 1994.

Mix Lightz (h)
4 16
K (mg-L-') K (mg-L-1)
0 60 Response 0 60 Response
Specific leaf area (cn2 -mg-') Peat+vermiculite 0.63 0.57 ** 0.42 0.44 NS
Peat+rockwool 0.61 0.64 NS 0.53 0.50 NS
Response NS ** ** NS
zNatural photoperiod extended by 4 h to 16 h or supplementary light for the entire 16 h. S, ,-Nonsignificant (NS) or significant t-test at 5% (*), 1%
(**) levels.









91

According to Masson et al. (1991a), under high photosynthetic photon flux density, the palisade layer cells generally elongate so that the leaves are thicker, resulting in a decrease in SLA. Greater leaf area ratios for plants grown in peat+rockwool than in peat+vermiculite was associated with greater leaf areas in comparison to dry shoot mass in these plants.

Neither treatment affected LMR or RMR values for plants sampled 23 DAS (Table 4-5). By 30 DAS, plants grown with 60 mg-L- K had smaller LMR values compared with plants grown without K. Similarly, plants grown in peat+vermiculite mix had smaller LMR values than plants grown in peat+rockwool mix. The opposite response to applied K and to media type occurred for RMR. Once again, added K increased root growth more than shoot growth.

In Experiment 2, plants grown in peat+rockwool mix (3 mg-kg' water extractable K) responded more to applied K compared with plants grown in peat+vermiculite mix (11 mg-kg- water extractable K) because, unlike vermiculite, rockwool is inherently low in K. In the peat+rockwool mix, 60 mg-L1 K compared to no K led to increases in shoot, root, and leaf growth. Potassium fertilization also led to increased K concentrations in transplant leaves. Root mass responded more to K and media treatments at 30 than at 23








92

DAS, indicating that treatment effects on root growth became more apparent with time.

In general, RSR, RGR, NAR, SLA, and LAR were not affected by K. Even though these transplant growth characteristics were not affected by K, transplants grown with no K in the peat+rockwool mix had inferior quality since they could not be easily removed from the transplant flat (data not provided). Stems broke during removal in plants grown with no K, rather than breaking at the rootshoot interface as with plants without N (Chapters 5 and 6) or P (Chapter 3).

In the previous two experiments, 100 mg-L- N was used when growing transplants at various levels of K. Subsequent studies with N in Chapter 6, however, revealed that optimum N for lettuce transplant root growth might be in the 60 mg-L- range or less, supplied every second day through floatation irrigation. Therefore, in Experiment 3, N was included as a variable to compare 60 versus 100 mg-L1 N concentration at selected levels of K.

In order to further test the conclusion reached in

Experiment 1 that supplemental K may not be necessary for production of high quality transplants in a peat+vermiculite mix, Experiment 3 was conducted during the winter, instead of summer, under greenhouse temperatures ranging from 14 to 38 0C (Fig. 4-5). The average daily maximum media




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LETTUCE TRANSPLANT ROOT AND SHOOT GROWTH AND DEVELOPMENT RELATION TO NITROGEN, PHOSPHORUS, POTASSIUM, AND WATER MANAGEMENT By PUFFY SOUNDY A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1996

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In loving memory of my eldest brother, Paul Walter Soundy, who departed from us March 31, 1995. Walter has been, and will continue to be my role model.

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ACKNOWLEDGMENTS I wish to express my thanks to Dr. D.J. Cantliffe, committee chairman, for his patience, guidance, and assistance in the research project and for his suggestions in the preparation of this dissertation. Thanks are conveyed to Drs G.J. Hochmuth, R.T. Nagata, P.J. Stoffella, and E.A. Hanlon for their interest and helpfulness in the research project. Thanks to everyone in the Seed Physiology Lab for all the help, friendship, and encouragement. Everyone at the Horticultural Unit is thanked for the assistance with field experiments. Financial support from Fulbright Scholarship, a grant from the Foundation for Research and Development, financial assistance from Dr. D.J. Cantliffe, and gift donations by Speedling, Inc., are gratefully acknowledged. Special thanks and appreciation are extended to my friend Deborah Franklin, for her constant assistance and support . Thanks to my parents. Keystone and Flora, and my brothers, Walter, Trevor, Victor, and Harvey, for iii

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encouraging me to study. The sacrifice has been a great one for us all, but time and their love has seen me through. iv

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TABLE OF CONTENTS ACKNOWLEDGMENTS iii ABSTRACT vii CHAPTERS 1 INTRODUCTION 1 2 REVIEW OF LITERATURE 5 Introduction 5 Lettuce Transplant Nutrition and Water Requirements 6 Effect of Light on Lettuce Transplant Growth .... 14 Effect of Temperature on Lettuce Transplant Growth . 19 Conclusions 23 3 PHOSPHORUS REQUIREMENTS FOR LETTUCE TRANSPLANT GROWTH USING A FLOATATION IRRIGATION SYSTEM 25 Introduction 25 Materials and Methods .21 Results and Discussion 33 Summary ' * 61 4 NEED FOR SUPPLEMENTAL POTASSIUM FOR LETTUCE TRANSPLANT PRODUCTION g4 Introduction Materials and Methods 61 Results and Discussion ! ! ! ! 74 Summary ! ' * *103 5 ROOT AND SHOOT GROWTH RESPONDS TO NITROGEN NUTRITION OF LETTUCE TRANSPLANTS 105 Introduction 205 Materials and Methods . . . 108 Results and Discussion * * V

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Summary 141 6 PROMOTION OF LETTUCE TRANSPLANT ROOT DEVELOPMENT BY PROPER MANAGEMENT OF NITROGEN AND IRRIGATION ... 144 Introduction 144 Materials and Methods 146 Results and Discussion 154 Summary 245 7 SUMMARY 247 APPENDICES A PHOSPHORUS EXPERIMENTS 256 B POTASSIUM EXPERIMENTS 265 C NITROGEN EXPERIMENTS 275 D NITROGEN AND IRRIGATION EXPERIMENTS 286 LIST OF REFERENCES 297 BIOGRAPHICAL SKETCH 304 vi

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy LETTUCE TRANSPLANT ROOT AND SHOOT GROWTH AND DEVELOPMENT IN RELATION TO NITROGEN, PHOSPHORUS, POTASSIUM, AND WATER MANAGEMENT By PUFFY SOUND Y December 1996 Chairperson: Daniel J. Cantliffe Major Department: Horticultural Sciences Lettuce (Lactuca sativa L.) transplants grown with floatation irrigation often show limited root growth, resulting in root systems not pulling out completely from the transplant flat, and poor establishment in the field. 'South Bay' lettuce transplants grown in a peat+vermiculite media in the greenhouse were fertilized with varying concentrations of N, P, and K, via floatation irrigation at selected frequencies, to determine optimum nutrient and water management for production of high quality transplants, with sufficient roots to fill a 11 cm^ tray cell, and for rapid field establishment. Phosphorus at 0, 15, 30, 45, or 60 mg-L"^ applied every two to four days, increased fresh and dry shoot and root vii

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ma J iss, root length and area, leaf area, pulling success, leaf tissue P, relative growth rate (RGR) , specific leaf area (SLA), leaf area ratio (LAR) , leaf mass ratio (LMR) , but reduced root: shoot ratio (RSR) , net assimilation rate (NAR) , and root mass ratio (RMR) . Quality transplants and the earliest and greatest head mass were obtained by fertigating every two days with 15 mg-L"^ P. Floatation fertigation with K at 0, 15, 30, 45, or 60 mg-L-^ applied every two to four days, increased fresh and dry root mass only when the concentration of water extractable K in the media was less than 15 mg•kg-^ but when higher (24 mg-kg-^, root mass was unaffected. Fresh and dry shoot mass, leaf area, RSR, RGR, LMR, and RMR were unaffected by applied K, regardless of the initial K concentration in the media. Lettuce growth and yield in the field was not affected by pretransplant K. To determine the optimum N concentration and fertigation frequency, transplants were fertigated every day or every second, third, or fourth day with N at 0, 30, 60, 90, or 120 mg-L-^ Nitrogen at 30 mg-L"^ (summer) or 60 mg-L'^ (fall, winter, or spring) maximized root growth, provided that fertigation frequency was daily or every second day. Therefore, N concentration and fertigation frequency must be considered together. Pretransplant N improved lettuce head mass and reduced time to maturity. viii

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CHAPTER 1 INTRODUCTION Unsatisfactory results in stand establishment of direct-seeded lettuce crops using both pelleted and raw seed, particularly during conditions of environmental stress, has led to the use of transplants as a means of establishing economically viable plant stands (Cliffe, 1989). Guzman et al. (1989) found that superior plant stand was the major factor resulting in increased marketable yields from transplanted crisphead and romaine lettuce. They concluded that perhaps growers in south Florida, with harsh and unreliable weather, could minimize economic losses and become more reliable suppliers of lettuce if a portion of the lettuce crop was transplanted. According to Klassen (1986), other reasons growers were transplanting rather than direct-seeding included better plant-to-plant uniformity especially for a once-over harvested crop such as lettuce, early season weed control, more precise spacing of plants, and elimination of the need to thin densely seeded rows. The environmental conditions to which vegetable transplants are exposed to during their early growth play an 1

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2 important role in final crop yield (Masson et al., 1991b). The early growing environment of transplants can be manipulated in ways that are not possible with direct-seeded crops (Wurr & Fellows, 1982) . Several factors that are known to affect vegetable transplant size, quality, and growth in the field are root container size (Nicklow and Minges, 1962; Knavel, 1965; Dufault and Waters, 1985; Weston and Zandstra, 1986; Weston, 1988; Hall, 1989; Kemble et al., 1994; Liptay and Edwards, 1994; Maynard et al., 1996; Nicola and Cantliffe, 1996), seedling nutrition before transplanting (Jaworski and Webb, 1966; Jaworski et al., 1967; Kratky and Mishima, 1981; Dufault and Waters, 1985; Tremblay and Senecal, 1988; Weston and Zandstra, 1989; Garton and Widders, 1990; Masson et al., 1991a, b; Melton and Dufault, 1991; Dufault and Schultheis, 1994), transplant age (Chipman, 1961; Leskovar et al., 1991), and transplant storage (Dufault and Melton, 1990; Leskovar and Cantliffe, 1991). Irrigation systems could also influence transplant growth and development both in the greenhouse and subsequently during the field production cycle (Leskovar and Cantliffe, 1993; Leskovar and Heineman, 1994) . Containerized vegetable transplants grown in greenhouses can either be overhead irrigated or subirrigated. A floatation or subirrigation system was constructed by Speedling, Inc. as an alternative method to

PAGE 11

3 the conventional overhead irrigation (Thomas, 1993) . While disease control is the major benefit of the floatation system because leaves are maintained dry, according to Anon. (1986) there are other advantages. There are no variations in plant growth due to uneven watering and fertilization from an overhead irrigation system. The end result is more uniform plant growth. Furthermore, because there is no overhead irrigation, pesticides remain on the plant longer, making re-application less frequent and reducing both pesticide material and labor costs. According to Anon., part of the success of the system is the direct result of the transplant flat. The expanded polystyrene flat floats, making the approach to this type of bottom irrigation possible. Using the floatation system, Leskovar and Cantliffe (1993) improved uniformity and quality of pepper transplants, compared to using overhead irrigation. When drought stress and root pruning methods were used to harden and prevent stem elongation in fresh-market tomato transplants grown with a floatation system, an increase in lateral root elongation and a decrease in shoot: root ratio were reported (Leskovar et al., 1994). A reduction in shoot: root ratio and an improvement in water-use efficiency of pepper transplants were also reported by Leskovar and

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4 Heineman (1994), when plants were produced via the floatation system of irrigation. However, growers have not been able to produce the highest quality lettuce transplants on a seasonal basis using the floatation system. A well developed root system is essential so that transplants can be easily pulled from the transplant flat, or pushed out utilizing a mechanical transplanter. If shoots are too long, the plants will tend to fall over, resulting in easily damaged plants and scorched leaves especially when transplanted onto plasticmulched beds. If shoots are too short, they cannot be easily handled and can be trapped under plastic mulch. When using the floatation system of irrigation, careful management of fertilization is important since large amounts of fertilizers, especially N, can greatly increase lettuce transplant shoot growth at the expense of root growth (Tremblay et al., 1987; Tremblay and Senecal, 1988; Masson et al., 1991a) . The overall objective of this research was to optimize fertilizer and irrigation programs to produce an ideal lettuce transplant, with optimum shoot and root development for rapid field establishment and high quality yields, under the floatation system of irrigation.

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CHAPTER 2 REVIEW OF LITERATURE Introduction Approximately 4,000 ha of crisphead lettuce were grown in Florida during the 1993-94 production season, mostly on the Histosols around Lake Okeechobee and Zellwood (Anon., 1995) . However, decline of the Histosols due to oxidation and competition with other lettuce production areas such as California have limited lettuce production on the organic soils. Cantliffe (1990) suggested that the expansion of lettuce production into the abundant sandy soils of Florida could greatly increase lettuce production potential in Florida. Commercially acceptable yields of high quality from sandy soils require new production systems such as plastic mulch and transplants instead of the traditional directseeding used on Histosols. Florida growers have, however, been unable to produce lettuce transplants with suitable root development especially under a desirable floatation or subirrigation system, for transplanting into sandy soils. Knowledge of the factors which influence transplant growth 5

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6 such as plant nutrition, irrigation, supplementary lighting, and temperature, is therefore important to produce quality transplants . Lettuce Transplant Nutrition and Water Reani rpmenf .s Vegetable transplants grown in plug cells require careful management of fertilizers (Dufault and Waters, 1985; Weston, 1988) due to limited volume in the cell and high seedling densities. Concentrations of essential plant nutrient elements within media are frequently insufficient to sustain plant growth for an extended period (Garton and Widders, 1990) . Production of quality transplants is a prerequisite to a successful crop, especially in lettuce where the period of containerized transplant growth comprises up to 30 % of the total crop production time (Karchi et al. , 1992) . Kratky and Mishima (1981) grew lettuce transplants by misting them with either 0, 200, 600, or 1800 mg-L'^ of a water soluble 13N-11P-21K fertilizer. Misting was performed twice daily with an application rate of 3,8 mm-day-^ Plants were transplanted to the field and grown to maturity. A foliar application of 200 to 600 mg-L"^ 13N-11P-21K plus 4 to 8 g of 8N-14P-7K preplant fertilizer per liter of media for the 200 mg-L"^ foliar fertilizer and 0 to 4 g-L'^ for the

PAGE 15

7 600 mg-L"^ rate was recommended. No foliar fertilization was found undesirable since transplant mass, head firmness, and head mass were reduced. The 1800 mg-L'^ foliar rate with added preplant fertilizer was also undesirable since it caused production of excessively tender transplants, fewer saleable heads, and smaller head size. Differences at the time of transplanting were larger than 15-fold among treatments. However, at crop harvest, differences were less than 30 % for average head mass. Tremblay and Senecal (1988) grew lettuce, broccoli, pepper, and celery transplants in a growth chamber maintained at 95 % RH and a day/night temperature of 23/18 °C. Plants were watered every morning with distilled water. Fertilization treatments were initiated at emergence and were done to runoff every afternoon. Treatments were factorial combinations of 150 or 350 mg-L"^ N and 50, 200, or 350 mg-L"^ K. Growth measurements were made at 18, 20, 31, and 38 days after sowing for lettuce, broccoli, pepper, and celery, respectively. Nitrogen at 350 mg-L-\ compared to 150 mg-L'^ N, increased leaf area and shoot dry mass of celery, broccoli, pepper, and lettuce, but reduced the percentage of shoot dry matter for all plant species except celery, which was not affected. Broccoli and pepper specific leaf area (SLA) was enhanced by increased N concentration while celery SLA was reduced and lettuce was unaffected.

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8 Root dry mass was reduced with 350 mg-L*^ N, compared to 150 mg'L"^ N, for all species except for pepper, which was not affected. The root: shoot ratio of all species was reduced by 350 compared to 150 mg-L"^ N. Tremblay and Senecal (1988) concluded that 150 mg-L"^ N, compared to 350 mg-L'^ N, led to production of high quality transplants. For K, Tremblay and Senecal found that celery leaf area increased linearly with K concentration but the increase for broccoli was curvilinear. Leaf area of lettuce increased with K at 350 mg-L'^ N, but there was no response detectable with 150 mg-L"^ N. They also reported that there were indications that the expansion of lettuce leaves was driven primarily by shoot dry-mass accumulation. They supported this statement by two observations: 1) increases in leaf area followed increases in shoot dry matter accumulation; and 2) SLA was not significantly modified by N and K treatments, indicating that leaf expansion was matched by a concomittant increase in shoot dry matter. Root growth characteristics and root: shoot ratio for broccoli, celery, and lettuce were not affected by K fertilization. The percentage of pepper root dry matter, however, decreased linearly with increasing K concentration. Masson et al. (1991a) increased shoot dry mass for all plant species tested by high concentrations of N fertilization. Nutrient solutions with N at 400 mg-L"^

PAGE 17

9 increased celery, lettuce, broccoli, and tomato shoot dry mass by 37 %, 38 %, 61 %, and 38 %, respectively, compared with 100 mg-L"^ N. Overhead fertigation was performed twice daily to partial runoff. Increasing the N concentration from 100 to 400 mg-L"^ decreased the percentage of shoot dry matter in all species. A similar response was previously reported for celery, lettuce, broccoli, and pepper (Tremblay at al., 1987; Tremblay and Senecal, 1988). Leaf area ratio (LAR) of broccoli and tomato increased in a curvilinear fashion with N concentration. The LAR and specific leaf area (SLA) of broccoli and tomato changed in a similar way in response to lighting and fertilization treatments. Increasing N fertilization decreased celery, lettuce, and broccoli root dry mass (Masson et al., 1991a). Tomato dry root mass increased in a linear fashion to N fertilization as noted by Weston and Zandstra (1989) . Tomato root dry mass was 16 % higher with 400 than with 100 mg-L'^ N. Root: shoot dry mass ratio decreased in a curvilinear fashion in relation to N concentration in celery, lettuce, and broccoli but in a linear fashion for tomato. For celery and lettuce, this decrease was more evident under increased light intensity. Masson et al. (1991b) reported that increasing the supply of N to the transplant, resulted in a linear increase in total and marketable yield of celery, with the highest

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10 yields obtained with 300 mg-L"^ N. Compared with N at 100 mg-L'^, total and marketable yields obtained from celery transplants fertilized with 300 mg-L'^ were increased by 16 % and 15 %, respectively. There was an increase of 16 % in the marketable head mass of lettuce when transplants were fertilized with 400 mg-L"^ N, compared with 100 mg-L'^ N. The use of high concentrations of N in transplant production not only increased head mass at harvest, but also promoted earlier maturity. Marketable mass and diameter of inflorescence of broccoli increased linearly with increasing concentrations of N fertilization. Increases of marketable mass of broccoli were measured for transplants fertilized with 400 mg-L-^ N, rather than those fertilized with 100 mg-L-^ In general, Masson et al. found that tomato yields were negligibly affected by lighting and N treatments. Guzman (1993) compared two tray cell sizes and three formulas of soluble fertilizers on quality of crisphead lettuce transplants. The transplants received four nutrient applications in four weeks. Flats were floated in nutrient solution containing 60 g-L'^ fertilizer. Irrigation was by means of daily overhead misting, except for days when fertilizers were applied. During transplant production, more growth occurred with high N (20N-8 . 6P-16 . 7K) and least with high P (9N-19.4P-12.5K) , regardless of the season. Guzman also reported that lettuce transplants grown under high N

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11 were larger than desired and bruised more during transplanting, resulting in slower recovery from transplant shock. Lettuce transplants produced with high P were the smallest, and according to Guzman, this probably indicated an improper ratio of N and P. Transplants produced with medium P ( 15N-14 . 2P-15K) had the best quality. In the field, however, yields were not significantly different due to pretransplant fertilizer treatments. Karchi et al. (1992) also investigated the response of lettuce transplants to varying concentrations of N and P. Nutrient solutions were prepared from liquid phosphoric acid and granular ammonium nitrate (33 % N) to give nutrient solutions portioned to 175 mg-L'^ N:75 mg-L'^ P; 292 mg-L"^ N:25 mg-L-i P; 58 mg-L'^ N:126 mg-L"^ P and 32 mg-L"^ N:137 mg-L"^ P. They also compared these nutrient solutions to a water only treatment and to a water treatment supplemented by 175 mg-L-^ N:75 mg-L'^ P 18 days after seedling emergence. They found that the least dry leaf mass resulted from the water treatment and the greatest, resulted from transplants produced with 175 mg-L"^ N:75 mg-L*^ P treatment. Root development, however, was found to be promoted by high P and low to equal N concentration. The 292 mg-L'^ N:25 mg-L"^ P solution led to a significant decrease in leaf mass, plant mass and leaf area compared to the 175 mg-L"^ N:75 mg-L"^ P treatment. Karchi et al. concluded, therefore, that high N

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12 with correspondingly low P levels had a negative effect on transplant growth. Costigan and Mead (1987) reported that K concentrations in plants increased rapidly during the first few weeks of growth, and this made it very difficult to determine the critical level of K required for maximum growth rates. The percentage K in lettuce dry matter typically increases from 1 to 5 % within two weeks of sowing. Costigan and Mead performed sand culture experiments in the glasshouse to determine the internal K concentrations required by lettuce and cabbage transplants. They repeated the experiments with and without Na, since Na might affect K uptake by the plant. They grew lettuce and cabbage transplants in 14-cm diameter polypropylene plant pots containing 1 kg of sand, and irrigated with nutrient solutions. The solution K concentrations were varied by addition of different amounts of K2SO4. Before sowing, they wet the sand to saturation with nutrient solution. Once the plants emerged, they watered the pots daily with 150 mL of nutrient solution applied as a spray, followed by a short spray with water to rinse the leaves. They found that the critical levels for a 10 % reduction in plant growth rate were 2.2 % K for cabbage and 4.3 % K for lettuce. In the presence of Na, the corresponding critical levels were 0 and 1.0 % K. Costigan and Mead demonstrated that cabbage was more able to

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13 substitute Na for K than was lettuce. They, however, concluded that in most practical situations, it was unlikely that plants would have access to large amounts of Na when K was limiting. To summarize transplant fertilization research, N nutrition appears to be the driving force in lettuce transplant shoot growth (Tremblay and Senecal, 1988; Masson et al., 1991a). However, optimum amounts of N for shoots were not necessarily optimum for root growth. Increasing N increased shoot growth, but decreased root growth. Tremblay and Senecal (1988) obtained the largest lettuce transplant shoot mass with 350 compared with 150 mg-L"^ N, while Masson et al. (1991a) produced the largest shoots with 400 compared with 100 mg-L-i N. In both cases, these amounts were the highest levels of N tested, and they were applied daily through overhead fertigation. Karchi et al. (1992), however, found that a proper combination of N and P was required to enhance lettuce transplant root growth. They produced the best transplants with either 175 mg-L"^ N and 75 mg-L'^ p or 58 mg-L-^ N and 126 mg-L"^ P. Potassium nutrition, on the other hand, did not appear to have any impact on lettuce transplant shoot and root growth. Fertilizers can either be applied to transplants independent of irrigation, or they could be applied with the irrigation water (fertigation) . When fertigation is

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14 employed, careful management of fertilization is important since large amounts of fertilizers, especially N, could be applied when irrigation demands are high, especially where floatation irrigation is employed. If overf ertilization occurs with floatation irrigation, there is no method to leach excessive salts. Effect of Light on LettncP Transpla nt Growth Supplemental lighting of greenhouse-grown crops is not currently widely practiced in the United States (Decoteau and Friend, 1991). Only 5 % of the commercial greenhouse space in the United States is fitted with supplemental lighting systems (Thomas, 1990) . Greenhouses with supplemental lighting systems are primarily used in ornamental crop production for prolonging the natural photoperiod during short days, supplemental light on overcast days, and night period interruption. Supplementary lighting has not been traditionally used in the production of vegetable transplants in the United States, and research on the effects of supplemental lighting on transplant development and subsequent yield performance is limited (Decoteau and Friend, 1991) . Sodium lamps were reported to be ideal for plant growing because of their durability, their favorable light

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15 spectrum, and their high coefficient of conversion of electric energy into the energy of photosynthetically active radiation (Dullforce, 1971; Dennis and Dullforce, 1975; Tibbits et al . , 1983) . Research has been reported on the effects of artificial lighting on the growth and morphology of the lettuce crop. However, according to Wurr et al. (1986), these have largely been observed on lettuce grown in a controlled environment (Soffe et al., 1977; Krizek and Ormond, 1980; Craker and Siebert, 1983), or under winter glasshouse conditions (Dennis and Dullforce, 1975) with butterhead lettuce. Wurr et al. (1986), therefore, conducted greenhouse and field experiments to determine the effects of supplementary lighting applied during transplant production, on lettuce transplant growth and maturity characters. They reported that in 1984, but not in 1985, tungsten lighting produced transplants with greater dry mass than the control. Highpressure sodium lighting had no effect on transplant mass in either year. Furthermore, in 1984 both lamp types gave rise to longer leaves than the control plants, but in 1985 this was only true for high-pressure sodium lighting. Wurr et al. (1986) reported that in the field, inspite of lighting effects on transplant morphology, there were no effects in either year on lettuce mean head mass at maturity or the time from sowing to maturity. They concluded that

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16 there was unlikely to be any benefit to growers in terms of increased head mass from providing supplementary lighting during transplant production, though it could be used early in spring to boost plant growth. However, they did not measure the effect of supplementary lighting on root growth. Masson et al. (1991a) reported that supplementary lighting, 100 //mol • s'^ -m"^ PAR, increased shoot dry mass of celery, lettuce, broccoli, and tomato transplants by 22 %, 40 %, 19 %, and 24 %, respectively. Supplementary lighting also improved the percentage of shoot dry matter for broccoli, tomato, and lettuce but not for celery. Tesi and Tallarico (1984) reported that an increase in the percentage of shoot dry matter improved cold resistance and that a quality tomato transplant should have > 10 % dry matter. Masson et al. (1991a) reported that leaf area for lettuce and broccoli transplants was increased under supplementary lighting, but no effect was detected for celery and tomato. Supplementary lighting lowered the specific leaf area (SLA) of celery, broccoli, and tomato, but not that of lettuce. Apparently, a low SLA is desirable since it was associated with greater leaf thickness. According to Masson et al. (1991a), under high photosynthetic photon flux density, the palisade layer cells generally elongated so that the leaves were thicker and a decrease in SLA was observed.

PAGE 25

17 Supplementary lighting also reduced leaf area ratio (LAR) in celery, broccoli, and tomato transplants. Masson et al . reported that transplant root dry mass of all plant species increased with 100 //mol • s'-" -m"^ PAR supplementary lighting by 97 %, 42 %, 38 %, and 21 % for celery, lettuce, broccoli, and tomato, respectively. The root: shoot dry mass ratio (RSDMR) of celery and broccoli was increased by supplementary lighting. Lighting, however, did not affect this relationship for lettuce and tomato. Decreases in RSDMR caused by high N concentrations have been reported for several species (Dufault, 1985; Tremblay et al., 1987; Tremblay and Senecal, 1988; Weston and Zandstra, 1989). According to Masson et al. (1991a), this decrease was more evident for celery and lettuce under supplementary lighting. Supplementary light at the transplant stage had no long-term effect on yield of celery or broccoli (Masson et al., 1991b). Supplementary lighting also did not influence lettuce yield or quality. Early yields of tomato transplants treated with supplementary lighting were higher on average than transplants produced under natural light alone. Ciomulative tomato yields were, however, not affected by transplant lighting. Boivin et al . , (1986) also obtained an increase of 31 % in the mass of marketable fruits in the first 3 weeks of harvest from greenhouse-grown tomato

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18 transplants that had received supplementary light energy of 100 A^mol-s'^ -111-2 (PAR). Basoccu and Nicola (1990), working with lettuce transplants, reported that, when natural light was decreased by 50 %, there was a decrease in percentage dry matter, fresh and dry mass, as well as number of leaves. In the field, lettuce head mass was increased by 18 % in plants which received natural light as opposed to those which received 50 % of the light during transplant production. A similar response was found with head diameter. According to Basoccu and Nicola, head diameter is influenced by the number of leaves present during transplanting. Poniedzialek et al. (1988) studied the effect of controlled temperatures and light intensities on the shortening of the period of time in which lettuce transplants of good quality could be obtained. They found that supplementary light was decisive for shortening this time. The higher intensity of light, i.e. 40 W-iar^, shortened the period of production by 5 to 9 days compared with a lower intensity (20 W-m'^) . A light intensity of 20 W-m-2 was found not be sufficient for adequate growth of plants and accounted for a pronounced prolongation of the period of production and an increased number of days with supplementary illiamination. On the other hand, an increase in light intensity of 40 W-m'^ brought about a faster

PAGE 27

19 increase in the area and number of leaves and in the content of dry matter and chlorophylls a and b. To summarize, supplementary light seems to affect lettuce transplant quality only during greenhouse production. According to Poniedzialek et al. (1988) light intensity of 40 W-m'^ shortened the period of time in which lettuce transplants could be obtained. Shortening the period of lettuce transplant production could lead to lower transplant production costs. Furthermore, 100 /umol • s"^ -m'^ PAR supplementary lighting increased lettuce transplant root mass (Masson et al . , 1991a). An improvement in root growth is essential especially for lettuce transplants produced via floatation irrigation. Growers have had difficulty in producing lettuce transplants with sufficient root systems under floatation irrigation system. Perhaps the use of supplementary light could lead to faster production of lettuce transplants which could be more easily pulled from the transplant flats. Effect of Temperature on Lettuce Transplant Growth Lettuce is a cool season crop. According to Guzman (1990), mean temperatures between 11 and 19 °C enhance yield and quality in 'Great Lakes' types in California. In Florida, mean temperatures below 21 °C are conducive for

PAGE 28

20 good yield and head quality. Furthermore, mean temperatures below 16 °C tend to delay maturity and, although internal quality of head is excellent, head size is reduced. Guzman further reported that for Florida cultivars such as 'South Bay' and 'Raleigh' , temperatures of 4 °C practically caused growth to cease. Mean temperatures above 21 °C, on the other hand, tend to reduce lettuce yield and quality. According to Guzman, lettuce quality is affected by high temperature of long duration at any stage of growth, but that early exposure appears to have the most pronounced effect. According to Sadler and Cantliffe (1990), lettuce grown from transplants is susceptible to premature flowering (bolting) when stressed by high temperature conditions in the greenhouse. Bolting can lead to total loss of crop or loss of lettuce quality due to elongated cores (stems), ribbiness of leaves, and loss of head compactness (density) . Therefore, heat stress related problems, such as bolting, could offset the possible gains from transplanting lettuce. They grew 'Vanguard', a California cultivar known to bolt readily under high temperature conditions, and 'South Bay', a Florida cultivar highly resistant to bolting in growth chambers at 35/30, 30/25, 25/20, 20/20 °C day/night temperatures. They found that 'Vanguard' bolted about two weeks earlier than 'South Bay' when grown under high temperature conditions. Though the cultivars bolted at

PAGE 29

21 different dates, those plants of each cultivar grown at the highest transplant temperatures bolted first, followed sequentially by those grown at the lower temperatures. They concluded that temperature at which the transplants are raised directly influenced the onset of bolting, regardless of temperatures in the field. Guzman (1990) studied the effect of greenhouse temperature on lettuce transplant quality and field performance. Lettuce transplants were either grown in a cool, air-conditioned greenhouse, or in a warm greenhouse exposed to natural conditions. Day and night temperatures were not stated. In the cool greenhouse, mean temperatures were kept below 27 °C with relatively small fluctuations, while in the warm greenhouse, fall temperatures approached 38 °C for several hours each day. Guzman found that transplants grown for four weeks in a warm greenhouse were larger and more tender than those grown for four weeks in a cool greenhouse. Guzman (1990) reported that there were significant reductions in plant stand in the fall season in Florida compared to winter, and indicated that high temperatures during transplanting in the fall were stressful to transplant growth. Yields in the fall were found to be, in general, lower than in winter. But in both fall and winter, lower yields and quality were more pronounced in treatments

PAGE 30

22 exposed for longer periods to high temperatures during transplant production. The most obvious quality disorder was excessive head core length in the fall. Only transplants kept for four weeks in a cool greenhouse had acceptable core lengths. Excessive core length was due to high temperatures and long days in the warm greenhouse, but similar conditions appeared to have minimal effect on the winter crop. Guzman concluded that lower field temperatures following transplanting possibly nullified the high temperature effect during the transplant stage. Poniedzialek et al. (1988) studied the effect of controlled temperatures and light intensities on the shortening of the production time for high quality lettuce transplants. They reported that an increase in day temperature from 15 to 22 °C accounted for a shortening of the period of growth of lettuce transplants. No significant differences were found in transplant fresh mass regardless of temperature. An interaction of light intensity and temperature was also observed. At higher intensity of supplementary illumination (40 W-m"^) the content of dry mass increased at a higher temperature, i.e. 22 °C . At a lower intensity of light (20 W-m"^) an increase of temperature from 15 to 22 °C was insignificant. Also, in plants grown at a higher temperature of 22 "C without

PAGE 31

23 supplementary lighting, the content of chlorophylls a and b was reduced considerably. To summarize, temperature has a major effect on lettuce growth, both in the greenhouse and field conditions. Transplants produced under temperatures of 30 °C or higher, tend to bolt readily in the field (Sadler and Cantliffe, 1990) . Furthermore, such transplants produced lower yields and poor internal quality in the field (Guzman, 1990) . However, according to Guzman (1990) and Sadler and Cantliffe (1990), if transplants are produced at temperatures below 27 °C, they can to a certain extent, overcome the problem of premature bolting of lettuce plants. Improved growth of lettuce transplants was also reported when plants were produced under 22 °C than under 15 °C (Poniedzialek et al. (1988) , The higher temperature in combination with supplementary light led to shortening of the time needed to produce lettuce transplants of good quality. Conclusions Any beneficial effects of N, P, and K nutrition, irrigation, light, and temperature on lettuce transplant growth should be judged according to a predetermined standard for lettuce transplant quality. High quality or ideal transplants have enough roots to fill a tray cell to

PAGE 32

24 enable plants to be easily pulled from the transplant flat, and maximize water and nutrient absorption. Shoots which are too large and stretched are not ideal since transplants could easily be damaged during transplanting. Fertilizers can either be applied to plants independent of irrigation, or they could be applied with the irrigation water (fertigation) , such as is the case with floatation (sub-) irrigation. Nitrogen has been found to be the element with the largest impact on lettuce transplant shoot growth. In studies where floatation irrigation was used, it was important to manage both N and fertigation frequency. During periods of high irrigation demands, frequent fertigation with low concentrations of N would be required to minimize excessive shoot growth. Extension of photoperiod and increasing the light intensity with supplementary light could be beneficial to lettuce transplants by improving root growth. Similarly for temperature, production of quality transplants could be ensured by cooling or warming the greenhouse to optimize growing conditions. 1

PAGE 33

CHAPTER 3 PHOSPHORUS REQUIREMENTS FOR LETTUCE TRT^SPLANT GROWTH USING A FLOATATION IRRIGATION SYSTEM Introduction Vegetable transplants grown in plug cells require careful management of fertilizers (Dufault and Waters, 1985; Weston, 1988) due to limited volume in the cell and high seedling densities. Concentrations of essential plant nutrient elements within media are frequently insufficient to sustain plant growth for an extended period due to frequent irrigation requirements (Garton and Widders, 1990) . Production of vigorous seedlings is a prerequisite to a successful crop, especially in lettuce where the period of containerized transplant growth comprises up to 30 % of the cropping time (Karchi and Gantliffe, 1992). Improved nutrient regimes would contribute to efficient development of quality transplants (Tremblay and Senecal, 1988) . The role of P in transplant growth has been investigated in a number of vegetable crops. In celery, increasing the P concentration from 5 to 125 mg-L"^ 25

PAGE 34

increased transplant diameter and height, shoot and root mass, and leaf area (Dufault, 1985) . In tomato, increasing P from 5 to 45 mg-L"' increased transplant height, stem diameter, leaf number, leaf area, and fresh shoot mass, but not dry shoot or root mass (Melton and Dufault, 1991) . Dufault and Schultheis (1994) reported that increasing P from 5 to 45 mg-L'^ increased fresh and dry shoot mass, leaf area, and leaf count in combination with 75 or 225 mg-L'^ N, but not with 25 mg-L""' N. Phosphorus at 5, 15, or 45 mg-L'' did not influence dry root mass. Data are lacking on the response of lettuce transplant roots and shoots to frequent P applications using a floatation irrigation system. In this system, nutrients are supplied with each irrigation by floating flats directly in nutrient solution. Growers using this system have been unable to produce lettuce transplants with sufficient roots in a tray cell to enable easy removal of transplants from the transplant flat (Robles, personal communication) . Perhaps, optimizing P fertilization practices could lead to improved root development in lettuce transplants. In the present investigation, a range of P concentrations were supplied via floatation irrigation to determine the P requirements for production of easy-to-pull transplants, which would rapidly establish in the field.

PAGE 35

27 Materials and Methods Greenhouse Experiments 'South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of 1.9 X 1.9 X 6.3 cm; 10.9 cm-' (length x width x depth; volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v) , with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg•m"^ was used for media. Three experiments were conducted (Table 3-1) . The plants were grown with natural photoperiod Table 3-1. Sowing schedule and initial media test (Hanlon et al., 1994) for Experiments 1 to 3 . Expt Sowing date Media test' pH EC (dS-m-^) NO3-N P K (mg • kg" Ca Mg 1 17 Jun 1993 4.7 0.9 1.3 12 .4 14 . 6 14.2 11.6 2 18 Sep 1995 4.5 0.6 0 0 . 6 46.2 6.3 22.2 3 31 Jan 1996 5.2 0.2 0.3 0 .4 24.4 0.6 5.8 ^Concentrations in the saturated paste extract. extended to 16 h by 1000-W, high-pressure sodium lamps (250 /imol-m-2-s-i photosynthetic photon flux). A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just above the plant canopy, and media temperatures were recorded by a

PAGE 36

28 Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR) . The flats were seeded, then covered with a thin layer of vermiculite, overhead irrigated enough to moisten the vermiculite, and transferred to a cooler at 20 °C for germination. After 48 h, flats were returned to the greenhouse . Plants in Experiments 1 and 2 were irrigated every two to four days, depending on water needs, by floating flats in nutrient solution containing P at 0, 15, 30, 45, or 60 mg-L'^ as Na.HPO^ . Other nutrients were supplied at equivalent rates to all plants and consisted of (in mg-L'^ 100 N, 30 K, 100 Ca, and half-strength Hoagland' s solution for micronutrients only (Hoagland and Arnon, 1950), which was comprised of Mg, S, B, Cu, CI, Mo, and Zn. In Experiment 2, the Ca level was reduced from 100 to 30 mg-L"\ Plants in Experiment 3 were irrigated every second day by floating flats in nutrient solution containing P at 0, 15, 30, 60, or 90 mg-L"^ in factorial combination with N at 60 or 100 mg-L-^ Phosphorus was supplied from Na2HP04, while N was supplied from NH4NO3. Other nutrients were supplied as described above. Experiments 1 and 2 were arranged in a randomized complete-block design with 5 treatments and 4 replications. Experiment 3 was a randomized complete-block design with 10

PAGE 37

29 treatments consisting of a factorial combination of 5 levels of P and 2 levels of N, replicated four times. Plant samples, 5 per treatment, were taken at 14, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE) . Growth variables calculated were: root: shoot ratio (RSR = dry root mass ^ dry shoot mass), relative growth rate (RGR = [In (final total dry mass) In (initial total dry mass) ^ (final time initial time)]), net assimilation rate (NAR = [(final total dry mass initial total dry mass) h(final time initial time) x { (In (final leaf area) In (initial leaf area) } ^ (final leaf area initial leaf area) ] ) , specific leaf area (SLA = leaf area dry shoot mass), leaf area ratio (LAR = leaf area total dry mass), leaf mass ratio (LMR = dry shoot mass ^ total dry mass) , and root mass ratio (RMR = dry root mass ^ total dry mass) (Hunt, 1978; 1982; Dubik et al . , 1992). At the last sampling date in Experiments 2 and 3, fresh roots were scanned with a Hewlett Packard desktop scanner and analyzed with MacRHIZO software (Regent Instruments Inc., Quebec, Canada) at 300 dpi for length, area, and diameter. Additionally, pull force, the force required to pull a lettuce transplant out of a flat using Model DPP Dial Push-Pull Gauge (John Chatillon and Sons, Kew Gardens, NY)

PAGE 38

30 attached to a binder clip, was measured. Pulling success was calculated as the percentage of 5 plants per treatment that could be pulled out of the flats without any breakage. Dry shoot samples from the last sampling dates were ground to pass a 20-mesh screen and dry-ashed for P or aciddigested for total Kjeldahl N according to Wolf (1982) , For total P determination, 0.5 g subsamples were weighed into 10 mL beakers. The samples were then dry-ashed in a muffle furnance at 500 °C for 10 h. The ash was moistened with 1 N HCl, poured into 50 mL voliometric flasks, and brought to volume with 1 N HCl. The solutions were filtered through 'Q8' filter papers (Fisher brand), with a particle retention of > 10 //m, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and analyzed with Model 61-E Inductively Coupled Plasma Spectrometry (Thermo Jarrell Ash Corporation, Franklin, MA) . The acid digestion procedure consisted of weighing 0.25 g subsamples into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes that were then heated on a digestion block at 375 °C. After the digestion process was completed (a total of 2.5 h) , the samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through ^P8' filter papers (Fisher brand), with a particle

PAGE 39

31 retention of > 25 //m, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR) . Data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc., Gary, NC) . Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts. Field Experiments Plants from each treatment in Greenhouse Experiments 2 and 3 were transplanted into an Arredondo fine sandy soil (loamy, siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on-black polyethylene-mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville (Table 3-2) . Experiment 1 was a randomized complete-block design with 5 treatments and 4 replications. Experiment 2 was a randomized complete-block design, with 10 treatments consisting of a factorial combination of 5 levels of P and 2 levels of N, replicated 4 times. Preplant fertilizer ( 13N-0P-10 . 8K) was applied broadcast and incorporated in the bed at 230 kg-ha'^ Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. There were 30

PAGE 40

32 Table 3-2. Transplanting schedule and initial soil test (Hanlon et al., 1994) for Experiments 1 and 2. Experiment Transplanting Soil test^ date PH EC P K Ca Mg (dS-m"' — (mgkg-') — 1 17 Oct 1995 5.9 0.1 185 30 733 54 2 29 Feb 1996 5.8 0.0 247 37 695 43 _t — — ^ >— f while elements are from a Mehlich-1 extractant. plants per plot planted on double offset rows with a spacing of 0.3 m between plants and between rows on the bed (equivalent to 54,000 plants per ha). Just after transplanting, 100 mL of nutrient solution (150 mg-L"^ 20N-8 . 6P-16 . 7K) was applied to each transplant hole as a starter fertilizer. Water was applied twice daily for 20 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark (1991). Starting one week after transplanting, fertilizer at a rate of 15 kg -ha"' N and 16 kg -ha-' K, supplied from NH.NOj and KNO3, was injected once weekly using a venturi pump (Netafim Irrigation, Altamonte Springs, FL) , with the last application one week before harvest to give a total amount of 150 kg-ha"' N and 180

PAGE 41

33 kg-ha"^ K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al., 1988). At head maturity, the center 20 plants in a plot were cut, weighed individually, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements . Wrapper leaves were sampled at harvest and analyzed for total P and Kjeldahl N as previously described for Greenhouse Experiments. Field data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc, Gary, NG) . Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts. Results and Discuss inn Greenho use Experiments Experiment 1 was conducted during the summer, under greenhouse temperatures ranging from 21 to 37 °C (Fig. 3-1) . The average daily maximum media temperature was 31 °G, while the average daily minimum media temperature was 22 °G. During the course of the trial, there were totals of 6 cloudy and 23 sunny days. Six of the sunny days were followed with rain in the afternoon.

PAGE 42

34 40 GROWING PERIOD (JUN 21 JUL 1 6) -sair max media max air min -bmedia min Fig. 3-1. Maximum and minimum air and media temperature during transplant production for Experiment 1, Jun/Jul

PAGE 43

35 Fresh shoot and root mass, and leaf area, were not determined at 15 days after sowing (DAS) . For plants sampled 15 DAS, there was a positive linear response of dry shoot mass to applied P (Table 3-3) . The major increase in dry shoot mass to applied P occurred between 0 and 15 mg-L"''. For plants sampled 21 and 29 DAS, fresh and dry shoot mass increased in quadratic fashion to applied P. At any level of applied P, fresh and dry shoot mass were improved compared to 0 P, For plants sampled 15 DAS, dry root mass responded in quadratic fashion to applied P, and was greatest with 0 P. However, for plants sampled 21 and 2 9 DAS, applied P did not influence fresh and dry root mass. For plants sampled 21 and 29 DAS, leaf area increased in quadratic fashion to applied P, and was least with 0 P. Leaf tissue P increased in quadratic fashion, implying that P did not affect root growth. Root: shoot ratios decreased in quadratic fashion in response to P, regardless of sampling date. The greatest RSR values were obtained with 0 P, while there were similar RSR values in plants grown with 15 to 60 mg-L'^ P. Plants grown with 0 P had the greatest RSR values because shoots were smaller compared to plants grown with any level of P, while root growths were similar among all plants.

PAGE 44

36 Table 3-3. Root and shoot characteristics of lettuce transplants as affected by P nutrition for Experiment 1, June/ July 1993. Phosphorus Fresh Dry Fresh Dry Leaf Leaf Root : applied shoot shoot root root area tissue shoot (mg-L-M mass mass mass mass P ratio (mg) / Tn rt ^ v"ig I (mg) (mg) (cm") (g-kg-i) Days After Sowing 0 12.4 3 . 8 0 . 31 15 3.2 0.20 30 3.5 0.22 45 15.3 2.9 0.19 60 16.5 3.2 0.20 Response T * * Ij Q** L* 21 Days After Sowing 0 355 28 . 0 155 12.0 12. 9 0.43 15 688 40.6 169 11.9 26.1 0.29 30 781 45. 6 176 12.4 29.3 0.27 45 741 43.4 179 12.7 28.6 0.30 60 736 45,1 189 13,2 30.2 0.30 Response Q** Q** NS NS Q** Q** 0 29 Days After Sowing 685 58,0 304 25.3 25 . 0 1.2 0.44 15 1268 85.4 307 23. 8 46.8 3.0 0.29 30 1297 85.6 301 23.8 48.1 4.2 0.28 45 1401 92.3 320 24.7 50.3 4.6 0.27 60 1297 89.8 341 26.6 48.5 4.6 0.30 Response Q** Q** NS NS Q** Q** Q** lainear (i.; or quadratic (Q) effects significant at P = 0, 0.01 (**), or nonsignificant (NS) . 05 (*),

PAGE 45

37 For plants grown to 21 DAS, there was a positive linear increase in RGR values in response to applied P (Table 3-4) . For plants grown to 29 DAS, RGR values were not influenced by P and were lower than for plants grown to 21 DAS, implying that P was more important earlier in growth. Greater RGR values for plants grown to 21 compared to 29 DAS meant that younger plants had higher efficiency for growth than older ones. For plants grown to 29 DAS, NAR decreased in quadratic fashion in response to applied P. The production of dry matter per unit leaf area (NAR) was greater in plants grown with 0 P, but the total production of dry matter over the same period was greater with any level of P. For plants sampled 21 and 29 DAS, SLA and LAR increased in quadratic fashion in response to applied P. Lowest SLA and LAR values were obtained with 0 P, while there were similar values with any other level of P. The reduction in SLA and LAR values for plants grown with 0 P reflects the reduction in both leaf size and assimilate production (Dubik et al. , 1990) . For plants sampled 15 DAS, both LMR and RMR values were not affected by P. For plants sampled 21 and 29 DAS, LMR values increased in quadratic fashion, while RMR values decreased in quadratic fashion in response to applied P. For plants grown to 29 DAS, approximately 70 % of the dry matter

PAGE 46

38 u 0 0 -p to -rH IX) 03 IX) o rH n o CM CM rH n 0 m -U CM rH rH tH rH (M eg CM CM + n CM CM CM * o fD w * E U O O O o o 2 o o o o o a o o o O o o to (0 4-1 0) to (C )^ E M MH to tC (U IX) ^ CM ^ n r00 CD 03 00 o o o o o CO Z o ren r-~ r~rrrr~r~ o o o o o u ifi mg" u IM ro (Nl a; Q) E a Q) ^^ u CO rH (d c o •H -p to J-l (U to H E -H Q) to -U to 2; to n o (U > -p -p o 0) P ID ? U E nt tu to 1 E -H ru PO M o •D (U x: (U (U a a -H ^^ rH to rH u o a to x: a Eh cu to tr. c; •I o ^ u to >, to Q <-H CM CTi rH rH CvJ n 'S" in in IT) + + o o o o o o O CD 00 CTl rrt-r~r+ + o o o o o o o n m CM CO ^ T -x + o o o o o oi t3^ c; 3 o to , IX) ^ IX) r~ Qj ^ VD VX) IX) >X) 4J i^ o o o o o '< 01 >, IB Q 0 to 10 U3 in >3> '3' in in in in 00000 01 Q vD in VD cn po 00 c; 0 a; in O rH o 00 00 00 rH O + 0) to q 0 a; 00 ^ ^ T 1^ vo rCTl 00000 in in o ^ IX) + p c (0 u •H 1-1 •H C •H n CO -P + u 0 (U
PAGE 47

39 was allocated to shoots and 30 % allocated to roots in lettuce transplants grown with 0 P. Plants grown with 15 to 60 mg-L"^ P allocated about 78 % of dry matter to shoots, with only 22 % to roots. With added P, more dry matter was, therefore, partitioned to shoots rather than to roots. The results of Experiment 1 indicated that high quality transplants could be produced without added P, when the peat+vermiculite media had at least 12 mg-kg'^ P (water extractable) before any fertilizer applications. In order to further test this conclusion, Experiment 2 was conducted during the fall, instead of summer, under greenhouse temperatures ranging from 18 to 4 6 °C (Fig. 3-2) . The average daily maximum media temperature was 33 °C, while the average daily minimum media temperature was 2 6 °C. During the course of the experiment, there were 12 sunny days with two of the days resulting in afternoon showers, and 16 cloudy days with rain during four of the days. For plants sampled 13, 21, and 28 DAS, fresh and dry shoot mass increased in quadratic fashion in response to applied P (Table 3-5). The major responses of shoot mass to applied P occurred between 0 and 15 mg•L-^ regardless of sampling date. For plants sampled 13 DAS, fresh and dry root mass were unaffected by P. For plants sampled 21 and 28 DAS, there was a positive linear increase in fresh and dry root mass in response to applied?. For plants grown to 28 DAS,

PAGE 48

40 GROWING PERIOD (SEP 1 9 OCT 1 3) air max media max —»— airmin -bmedia min Fig. 3-2. Maximum and minimum air and media temperature during transplant production for Experiment 2, Sep/Oct 1995.

PAGE 49

c 0 H P cri m •H C CO U -H Q) P ^ u 3 •H U C D D Oi to b >i 0) Si H U P T) D 0 (U Oi it-i 4J U (U — n c u p ro eg 0 Q) E 0 P u 0) ro u P x: P 0) 4-1 Cn ^ iH O C E O 0) u "4-1 oc; H 0 • in to i O (0 p o ro E Q p E m u H 0) U X) x: Q) O to 4-1 to •P -p 0) 0 to u u p 0 ro ro O p e U \ ro p x: (u " •§ 4-1 b o to -P 0) >-i o to 0 +J p x: ro c 0 Qj Q to E x: (I) m w T5 «. C CM x: 4-) ro to o to p H ro c»i n o c; •M o to p D 4J

1 ro O O rH in o in o n VD o in o in o n cn CTi CD cn cn r-i cn CM ro CM CM CM ro ro ro ro ro CO 000002 r^ cn ro ID in -It •1:5 * ^ rr~ o .-H iJl Q CNj CM CM ro ro to , ro Q 00 tN CM CM CM ro in in o ro <-i o tn 00 t~i-t o CM ro CM CM CM CM CO CM ro r~ VD in 2 in in in in in ^ cn ID r~ * * riH CM CM CM 01 * tr) cn rH CD cn OI iH in (X) CD riH CM CM CM CM 0) o a; -K VD CNJ in ro in ro 00 a\ CM iH cn cn CM CM CM CM CM CM CN CM ro ro rH CM 00 CM ro K cn cn r~cn cn + 00 0 cn rH ro a 00 ro rH VD rCM VD in IX) ID in rH rH rH rH rH rH rH rH in VD VD * in ro in O 00 in VD CN ro ro ro rH rH rH rH CM o VD t~ in VD 00 VD rH I~ O O O rH CM CM CM CM in O rH o in o ro ^ VD 0) «Q O en O rH in o in o ro ^ VD cu ro -p c ro u H M-l H C t3i H to to 4J U 0) <4H IH 0) + 5 K tic ro + p * CO 0 ro 2 tr 4-) p C 0 ca 5" fi OJ •H to c c: p 0 ro •H (D CO c c H 0 a; , c

PAGE 50

42 root length and area increased in linear fashion in response to applied P, but root diameter was unaffected by P. Leaf area increased in a quadratic fashion to applied P, regardless of sampling date. Phosphorus application to the media did not affect pull force, but improved pulling success from 30 % to approximately 90 %. Most of the P effect occurred between 0 and 15 mg-L'^ P. In Chapter 6, pull force was related to pulling success, but this was not so in the present work probably because there were smaller differences in root mass among the treatments in the present investigation. For plants sampled 28 DAS, leaf tissue P increased in quadratic fashion to applied P, from about 1 to 6 g-kg-' (Table 3-6). Root: shoot ratios decreased in quadratic fashion in response to applied P, regardless of sampling date. The largest RSR values were obtained with 0 P. Root: shoot ratios were similar with all P treatments within sampling date. For plants grown to 21 DAS, RGR increased in linear fashion to applied P, while for plants grown to 28 DAS, RGR was not affected by P. Therefore, P appears to once again be more important earlier in plant growth than later on. For plants grown to 21 or 28 DAS, NAR decreased in a quadratic fashion to applied P. Net assimilation rate was greatest with 0 P regardless of sampling date, but the total

PAGE 51

43 o to ro 0 nj to -p (U ro (0 o 14-1 (0 -H (t) Q) 4J J to ^^ u H H U <4-i (0 QJ 10 CO f-i c O •H -P 10 2 e CD o o o o OJ CM OJ CN CM -f O O O O O Ql CM O O O O CX5 03 CC 00 o o o o o * (Ti ^ IT) •a* rrr~r+ * o o o o o c o cTi o CTi n rH iH CNJ + + o o o o o c o iH m o iH r~(X) <30 00 CD + * o o o o o c c» iH in n in U3 IT) in + + o o o o o o LT) in «p rCNJ iH r^ iH rH o o o o o c in in ^ ^ n r00 00 00 00 (< -K o o o o o csj en o r~c» n ^ in ^ ^ + * o o o o o o dJ W to E 2 to M — > 4-1 4-> o to Cn P o o s: a: CO o H (0 tt) D >H to 10 to J 4-1 Qj 0) to a a ID 00 C\J WD cn CTi cn (J, o o o o C -I 12 O Co (U 4J to >, fD Q T O r-l in '3< CTl * OCtj^OOOOO 0 JO u to Q in 00 iH rreg in ^ in * CM CVJ CM CM O tn c; 0 Co ^ to ' >, to Q 00 m 00 o <£) ^ in in in * * o o o o o o ^ .H 00 ^ T-{
PAGE 52

44 production of dry matter over the same period was greater with any level of P. For plants sampled 13, 21, and 2 9 DAS, SLA and LAR increased in quadratic fashion to applied P. Lowest SLA and LAR values were obtained with 0 P, while there were similar values with any level of P. The reduction in SLA and LAR values for plants grown with 0 P reflects the reduction in both leaf size and assimilate production (Dubik et al . , 1990) . For plants sampled 13, 21, and 2 9 DAS, LMR values increased in quadratic fashion, while RMR values decreased in quadratic fashion in response to applied P. For plants grown to 28 DAS, approximately 75 % of the dry matter was allocated to shoots and 25 % allocated to roots in lettuce transplants grown with 0 P. Plants grown with 15 to 60 mg P-L-i allocated about 84 % of dry matter to shoots, with only 16 % to roots. Once again, added P caused more dry matter to be partitioned to shoots rather than to roots, and more so for transplants in the present experiment than in the summer grown ones. In Experiment 2, high quality transplants were produced with 15 to 60 mg-L"^ P. Although transplants grown with 0 P had greater RSR, NAR, and RMR values, they were inferior to transplants grown with any other level of P because they could not be easily pulled from the transplant flat. A

PAGE 53

reason why plant roots responded more to applied P in this experiment but not in Experiment 1, might be due to lower initial P levels in the media in this experiment (0,6 mg-kg"M compared to Experiment 1 (12.4 mg-kg"^. In the previous two experiments, 100 mg-L"^ N was used when growing transplants at various levels of P. Subsequent studies with N in Chapter 6, however, revealed that optimum N for lettuce transplant root growth might be in the 60 mg-L"^ range or less, supplied every second day through floatation irrigation. Therefore, in Experiment 3, N was included as a variable to compare 60 versus 100 mg-L"^ N concentration at selected levels of P. Furthermore, the highest level of P was increased from 60 to 90 mg•L-^ since in Experiment 2 root mass may not have reached greatest level with an application rate of 60 mg-L"^ P. Experiment 3 was conducted during the winter, under greenhouse temperatures ranging from 14 to 38 °C (Fig. 3-3) . The average daily maximum media temperature was 29 °C, while the average daily minimum media temperature was 21 °C. During the course of the experiment, there were a total of 17 sunny and 9 cloudy days. For plants sampled 15 and 22 DAS, there were no P and N interactions for dry shoot mass (Table 3-7) . By both sampling dates, dry shoot mass increased in quadratic fashion in response to applied P. The major increase in dry

PAGE 54

46 01 I I I I I — I — I — I — I — I — I — I — I — r "T — I — I — r T — I — r28 GROWING PERIOD (FEB 5 FEB 28) air max media max —*— air min -bmedia min Fig, 3-3. Maximum and minimum air and media temperature during transplant production for Experiment 3, February 1996. ^

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47 Table 3-7. Root and shoot characteristics of lettuce transplants as affected by P and N nutrition for Experiment 3, Feb 1996. Nutrient Dry Dry Root Root Root Leaf ann 1 "i pH UJL^ L.^ ^ «L ^ \Ji shoot root ±engun area diameter area mass^ mass (mg-L-M (mg) (mg) (cm) (cm^) ( TTlTn ) \ ilUll J \ cm ; P 15 Days After Sowing 0 4.0 2.8 o o 15 9.0 2 . 8 30 9.6 3.0 1 . 4 60 10.2 3.0 o . U 90 10.2 2 . 9 Response N Q** NS Q** 60 8.4 3.0 6. 4 100 R 8 6 . 7 Response NS * NS P x N NS NS NS 22 Days After Sowing P Ni N2 0 9.8 6.6 3.6 3.4 15 42.3 14.7 26.1 32.7 30 46.1 15.3 27.6 36.4 60 47.1 15.5 28.3 38.3 90 46.7 15.5 27.6 37.9 J?espojr3se N Q** Q** Q** Q** 60 35.8 14 . 0 100 41.0 13.0 Response * * * P " N NS NS * * 0 15 30 60 90 Response N 60 100 Response P X N 28 Days After Sowing N2 N, N2 12.7 12 .1 8.6 94 8.4 0.28 4 .2 4.2 82.1 104 .7 24.5 282 26.7 0.30 48 .4 68.1 83.5 105 , 6 23.9 276 26.0 0,30 50 .3 70. 6 82.8 104 .4 24.4 306 29.5 0.31 55 .0 69. 9 81.0 101 . 6 25.4 292 27.0 0.29 49 .0 70.6 Q** Q * * Q** Q** Q** Q** Q * * Q** 23.2 255 24,2 0.30 19.5 245 22. 9 0.30 ** NS NS NS ** NS NS NS NS ** 'Ni = 60 mg-L-i; Nj = 100 mg-L'^. Quadratic (Q) . ' "Nonsignificant (NS) or significant at 5% levels . (*), 1% (**)

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48 shoot mass to P occurred between 0 and 15 mg-L"^. Dry shoot mass was not influenced by N for plants sampled 15 DAS. For plants sampled 22 DAS, dry shoot mass was greater in plants grown with 100 than with 60 mg-L'^ N. For plants sampled 28 DAS, dry shoot mass increased in quadratic fashion to applied P at both levels of N. Nitrogen had no influence on dry shoot mass, but dry shoot mass was increased with all levels of applied P. With 100 mg-L"^ N, the response of dry shoot mass to P was greater than with 60 mg-L"^ N. Nitrogen did not interact with P to influence dry root mass, regardless of sampling date (Table 3-7) , For plants sampled 15 DAS, applied P did not influence dry root mass. Root mass was less in plants grown with 100 than 60 mg-L'^ N. For plants sampled 22 and 28 DAS, dry root mass increased in quadratic fashion in response to applied P. The major root response to P was between 0 and 15 mg-L"^. Root mass accumulation was adversely affected by increased N by both sampling dates. For plants grown to 28 DAS, root length, area, and diameter increased in quadratic fashion in response to applied P. The smallest root length, area, and diameter were obtained with 0 P. Applied N did not influence any of the measured root parameters. For plants sampled 15 DAS, there were no P by N interactions for leaf area which increased in quadratic fashion in response to applied P. Applied N did not

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49 influence leaf area by this sampling date. For plants sampled 22 and 28 DAS, leaf area increased in quadratic fashion to applied P, regardless of N concentration applied. Nitrogen had no influence on leaf area, but leaf area was increased at all levels of applied P. With 100 mg-L'^ N, the response of leaf area to P was greater than with 60 mg-L"^ N. For plants grown to 28 DAS, there were no P by N interactions for leaf tissue N (Table 3-8) . Leaf tissue N decreased in quadratic fashion in response to applied P. The response was probably a dilution effect since transplants were larger at any level of P compared to 0 P. Plants grown with 100 mg-L"^ N had more N concentration in the leaves than those grown with 60 mg-L"^ N. Leaf tissue P increased in quadratic fashion to applied P, regardless of N applied. Nitrogen had no influence on leaf tissue P at 0 or 15 mg-L"^ P, but it was increased with all other levels of applied P. With 100 mg-L"^ N, the response of leaf tissue P to applied P was greater than with 60 mg-L"^ N. For plants sampled 15 and 28 DAS, there were no P by N interactions for RSR. Root shoot ratios decreased in quadratic fashion in response to applied P. The largest RSR values were obtained with 0 P, while the smallest RSR values were obtained with all levels of applied P. Plants grown with 60 mg-L"^ N had larger RSR values than those grown with

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50 Table 3-8. Influence of P and N nutrition on growth characteristics of lettuce transplants for Experiment 3, February 1996. Nutrient applied (mg-L-') Leaf tissue N (g-kg-') Leaf Root: Relative tissue shoot growth ratio rate (gkg'') (mg-mg-^ • wk"' ) Net assimilation rate (mgcm"^ • wk'' ) 0 15 30 60 90 Response N 60 100 Response P X N 0 15 30 60 90 Response N 60 100 Response P X N 0 15 30 60 90 Response N 60 100 Response P X N ^N, 15 Days After Sowing 0.69 0.31 0.31 0.29 0.29 Q** 0.40 0.36 * NS 22 Days After Sowing 0. 69 0.32 0.28 0.30 0.29 Q** TT7 0. 66 0, 0, 0, 37 39 36 0.37 Q** 0. 90 1. 58 1.59 1.56 1.56 Q** 1.41 1.46 NS * NS 28 Days After Sowing N: 48.5 0.9 1.0 0.70 0,25 24.6 3.8 4.0 0.27 0.73 23.5 4.8 5.9 0.26 0.66 22.7 5.1 7.4 0.27 0.64 22.1 5.8 8.6 0.29 0.63 Q** Q** Q** Q** Q** 24.7 0.40 0.56 31.8 0.31 0.61 * * ** NS NS * * NS NS Nj = 100 mgL"' 3. 38 2.91 2.92 2.83 2.80 Q* 3.09 2.85 * NS 1.23 1.46 1.30 1,21 1,23 NS 1,33 1.25 NS NS I 60 mg-L"' Quadratic (Q) . Nonsignificant NS (NS) or significant at 5% (*), i% (**) levels.

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51 100 mg N-L"'. For plants sampled 22 DAS, N had no influence on RSR values, but RSR values were decreased with all levels of applied P. With 100 mg-L"^ N, the response of RSR to P was greater than with 60 mg-L'^ N, because added N favored shoot growth rather than root growth. For plants grown to 22 or 28 DAS, there were no P by N interactions for RGR and NAR (Table 3-8) . By both sampling dates, RGR values increased in quadratic fashion in response to applied P. Nitrogen did not influence RGR values. For plants grown to 22 DAS, NAR values decreased in quadratic fashion in response to applied P. The greatest NAR values were obtained with 0 P, and the least with 90 mg-L"^ P. Net assimilation rate was greater with 60 than 100 mg-L"^ N, By 28 DAS, P and N did not influence NAR values. For plants sampled 15 and 28 DAS, there were no P by N interactions for SLA (Table 3-9) . By both sampling dates, SLA values increased in quadratic fashion in response to applied P. Most of the response of SLA to applied P occurred between 0 and 15 mg-L'^ For plants sampled 15 DAS, applied N did not influence SLA, while for plants sampled 28 DAS, SLA was improved by 100 compared to 60 mg-L'^ N. For plants sampled 22 DAS, SLA values increased in quadratic fashion in response to applied P, regardless of N added. Specific leaf area increased in plants fertilized with 60 mg-L'^ N when P

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52 Table 3-9. Influence of P and N nutrition on growth characteristics of lettuce transplants for Experiment 3, February 1996. Nutrient Snecif ic Leaf T.ea f applied leaf area mass mass area^ ratio ratio ratio (mg-L-M ( cm^ •mg"'' ) ( cm^ -mg"^ ) P 15 Days After Sowing 0 0.58 0.34 0.59 0.41 15 0.77 0.59 0.76 0.24 30 0.77 0.59 0.76 0.24 60 0. 78 0.60 0.77 0.23 90 0.80 0. 62 0.78 0.22 Response N Q** Q** Q** Q** 60 0.74 0.54 0.72 0.28 100 0.74 0.56 0.75 0.25 Response NS NS * ** P X N NS NS NS NS 0 15 30 60 90 Response N 60 100 Response P X N 0 15 30 60 90 Response N 60 100 Response P X N 0.35 0.67 0.66 0.64 0.63 Q** N2 0.37 0.72 0.72 0.77 0.76 Q** 22 Days After Sowing Ni 0.21 0.49 0.48 0.47 0.46 Q** N2 0.22 0.54 0.56 0.59 0.59 Q** Ni 0. 60 0.73 0.72 0.73 0,73 Q** N2 0.59 0.76 0.78 0.77 0.77 Q** 0.56 0. 61 ** NS Ni 0.40 0.27 0.28 0.27 0.27 N2 0.41 0.24 0.22 0.23 0.23 Q** 28 Days After Sowing 0.34 Ni N2 Ni N2 Ni N2 0.19 0.21 0.58 0. 60 0.42 0.40 0. 62 0.45 0.53 0.76 0.82 0.24 0.18 0. 63 0.46 0.55 0.76 0.83 0.24 0.17 0. 67 0.50 0.55 0.76 0.83 0.24 0.17 0. 65 0.45 0.57 0.74 0.82 0.26 0.18 Q** Q** Q** Q** Q** Q** Q** % = 60 mg-L-V Quadratic (Q) . *' "Nonsignificant levels. N2 = 100 mg-L-^ (NS) or significant at 5% {*), 1% (**)

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53 was applied. At 100 mg-L"N, SLA increased with all levels of applied P. For plants sampled 15 DAS, there were no P by N interactions for LAR (Table 3-9) . Leaf area ratios increased in quadratic fashion in response to applied P. Most of the P effect occurred between 0 and 15 mg-L"^ P. Applied N did not influence LAR values. For plants sampled 22 and 28 DAS, N had no influence on LAR, but LAR was increased with all levels of applied P. With 100 mg-L"^ N, the response of LAR to P was greater than with 60 mg-L"^ N. For plants sampled 15 DAS, there were no P by N interactions for LMR and RMR. Both LMR and RMR increased in quadratic fashion in response to applied P. Leaf mass ratio was least, while RMR was greatest with 0 P. Nitrogen at 100 mg-L"^ increased LMR, but reduced RMR compared to N at 60 mg-L-^ For plants sampled 22 and 28 DAS, LMR values increased in quadratic fashion, while RMR values decreased in a quadratic fashion in response to applied P. Nitrogen had no influence on LMR or RMR, but LMR was increased, while RMR was decreased with all levels of applied P. Fertigation frequency was every second day in Experiment 3 compared to Experiments 1 and 2 where fertigation frequency ranged from fertigating every two days to fertigating every four days. When fertigation was every two days, fresh and dry root mass increased in response to

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54 15 itig-L'^, with no further increases in root mass at higher P concentrations up to 90 mg-L"^ even though the initial P concentration in the media was low (0.4 mg-kg"^. In Experiment 2, root mass was increased with each level of fertilizer P because the initial P concentration in the media was low (0.6 mg-kg"^), indicating that perhaps 60 mg-L"^ P was not adequate with the irrigation programs used. Therefore, in a media with less than 0.5 mg-kg'^ water extractable P, frequent fertigation is desirable. These experiments have revealed that P applied via the floatation irrigation system improved growth of both roots and shoots of lettuce transplants, especially when P in the media was low. Melton and Dufault (1991) reported that 5 to 45 mg-L-^ P did not influence tomato transplant shoot and root growth. Tremblay et al. (1987) reported that increasing P from 100 to 200 mg-L"^ did not influence celery transplant root growth. Their studies did not have a 0 P treatment to compare growth responses with. Lorenz and Vittum (1980) reported that the critical tissue P concentration for most vegetable species is about 3.0 g-kg-^ of dry mass. This value corresponded to an application of 15 mg-L'^ P in the present work. In all the three experiments conducted, tissue P in plants produced with 0 P was approximately 1.0 q-kq-\ while a range of tissue P concentration from 3.0 to 8.6 g-kg-i produced lettuce shoots with similar mass. Based on

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55 these results, a range of 3.0 to 4.0 g-kg"^ P can be considered adequate tissue P concentration for production of high quality lettuce transplants. It is not clear, however, why increased N led to more tissue P concentrations. Perhaps bigger plants due to N had greater energy requirements for growth processes and therefore took in more P. Regardless of season grown, average daily maximum media temperatures were similar, i.e. 31, 33 and 29 °C, while average daily minimum media temperatures were also similar at 22, 26 and 21 °C for Experiments 1, 2, and 3, respectively. Improved shoot growth in Experiment 2 (Sep/Oct) compared to Experiments 1 (Jun/Jul) and 2 (Feb), was probably related to higher temperatures inside the greenhouse during the fall. In Experiment 3, transplants produced with 0 P were very small compared with the previous experiments, probably due to low P (0.4 mg-kg-^) in the peat+vermiculite mix as well frequent f ertigations, without P, that might have leached any available P in the media. Transplants produced with 0 P in Experiment 3 had similar poor growth, regardless of N concentration. (In Chapters 5 and 6, transplants did not respond to either P or K with 0 N) . There was similar shoot and root growth with any level of applied P. Nitrogen at 100 mg-L"^ improved shoot growth especially in response to applied P, but additional N adversely affected root growth compared to N at 60 mg-L'^

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56 Results in Chapters 4 and 6 also indicated that applying more than 60 mg-L"^ N improved transplant shoot growth, but not root growth. In general, RGR values were improved, while NAR values were reduced with any level of P. Values of RGR and NAR were larger at 21 than at 28 DAS, indicating that younger plants had greater growth efficiency than older ones. With added P, RSR values were similar in Experiment 1 and 3, but lower in Experiment 2, Higher temperatures in Experiment 2 caused more shoot growth at the expense of root growth. Weston and Zandstra (1989) reported that P from 15 to 60 mg-L'^ had no effect on RSR values of tomato transplants. In Chapter 5, lower RSR values were obtained at low temperatures (average daily minimum media temperature of 11 °C) , indicating that extreme temperatures adversely affected RSR values. In all the experiments, LMR and RMR were similar regardless of sampling date, implying that there was no shift in dry matter allocation between shoots and roots with time. The same was true for K in Chapter 4, but not for N in Chapters 5 and 6. As N increased, more dry matter became allocated to shoots than to roots, with time. Results from scanning the roots, revealed that the response of root length and root area paralleled the response of root mass to applied P, regardless of time of transplant production. Quality transplants had total root

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57 lengths were between 276 and 306 cm, and total root area between 26 and 30 cm'. With any level of applied P, pulling success was improved tremendously compared to 0 P, but pull force was unaffected. Only 30 % of the plants produced with 0 P could be pulled from the transplant flats, compared to approximately 90 % pulling success with added P. In Chapter 6, pull force was related to pulling success, but this was not so in the present work probably because there were smaller differences in root mass among the treatments in the present investigation. Field Experiments Plants from Greenhouse Experiment 2 (fall) and Experiment 3 (winter) were grown to maturity to evaluate the effects of pretransplant P on earliness, yield and lettuce head quality. Plants of all treatments in the fall crop of Experiment 1 were harvested in December, 64 days after transplanting (DAT) . Lettuce head mass increased in quadratic fashion in response to pretransplant P (Table 3-10). Head mass was greater from plants receiving P as a pretransplant treatment compared to those plants not receiving P. Firmness, head height, stem width, and core length increased in a quadratic fashion with pretransplant P. Firmness, and head height ratings were improved by

PAGE 66

58 to CO (0 LO CTi 0) (Ti xi i-H >-l u n 4-) 4-1 0) u H 0) Q c 0 o CM c o T3 •H (U 4-1 4-1 u W T3 > O ra 4J c (0 4-1 a c m 0) C E (13 H M 4-1 01 Oh X c •H >-i :3 O 73 "+-1 to O u -H H 4-1 4J -H to M • H 4-1 3 C 4-1 u 04 m >-i o u to 4J >, U 4-1 0) -H H-l rH •W (0 U 3 tr I 73 rt) (U 0) 73 £1 ttj (0 0) to to •H 4-1 Oi 73 to n3 to 0) (D O -l 4-1 to O 73 a -H to .H o a CU (X3 (T^ iH o oo r~c/o CM n m CNj c\j s 00 ^ CO m O m LD IT) LO ^ I en iT) O iH O rH CM n ro m n o CM cn i-H CM CM rsj CM CM CM ^ en o r~CM CM n CM K a n CO cr> (Ti 00 * • -K ^ T O n iH iH 00 o -^r ,H CM n -ft O rH ID O in 0) to c o a to 0) oc; 4J (0 u -H m •H d r4 to c: o c >-i o 4-J U + (T3 a om' TO u o II If) + LO to o o lo o a. m 4-1 c to u H M-l i-l C Cn •H to to 4-1 U 0) 73 to d) Si o 4-1 4-J tl) u •H 4-1 (0 U • •d m CO 3 2 oi

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59 transplant P, while stem width and core length were enlarged. Heads were less developed with 0 P, while heads from transplants produced with 15 to 60 mg-L'^ pretransplant P were more developed, indicating greater earliness. At harvest, tissue P levels were equal regardless of pretransplant P because plants were grown in a field with soil high in available P. Plants of all treatments in the spring crop of Experiment 2 were harvested in May, 64 DAT. There were no P by N interactions for head mass or head quality characteristics. There was a positive linear response of head mass to pretransplant P. Head mass was improved at harvest with all pretransplant P fertilization treatments, but was unaffected by pretransplant N fertilization (Table 3-11) . Stem width increased in quadratic fashion, while core length increased in linear fashion in response to pretransplant P. Stem width and core length were enlarged by transplant P, indicating greater earliness, but were unaffected by pretransplant N. Lettuce head firmness, height, and diameter were unaffected by pretransplant P or N. At harvest, tissue N and P levels were equal regardless of pretransplant P or N applied. In the field, lettuce head mass was influenced by pretransplant P, regardless of time of production. All

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vj /ri lU /It si u 13 J-J 4-) 0^ (U I— I iH >i C (0 0 c (NJ o H T) P Q) u 4-1 :3 W T3 0) o > a (0 4-) c CM w c C (t >-i H -u Di a C •H w d u o o c o m -H u 4-) -H -H 4-J ^ (0 4-» -H C (U 4-1 S U (0 T) ^4 C (d u Oi IW 4-) 0 -H -H w (C 4-) =J u cr M-i T) <*-! (X) .-I G 1 (C n w 0) m M (0 (0 H 60 w ^ ^ ^ 0) M-l (0 (0 W QJ -H 4_> o cu e 4-1 4-J -H u 4-1 QJ 0) -H 4-> c J-l 4-) H (fl T3 W (t) to Q) (0 C T3 0) OJ (0 in I rH O T-l .-I CM C/2 CM CM CM CM CM 2 lt) ^ s po n n 00 (>o •>;T ^I? I^D 00 CD 00 n m n n CM ^ ^ ^ ^ O r\J CM (NJ CM C\J 00 rCO CO s o o o o o CO ^ ^ ^ 00 ^ S CM CM CM CM CM >X) CT> 00 00 00 CO ^ ^ ^ 2 K ^ 00 r'3' CM 1-^ 00 ^ CM lD lT) IT) iT) If) m o o o t-H 00 vD cn O CM • • CO CO CM CM 2 S 00 ^ IX) 00 00 00 00 CO CO CO CO 2 2 CO CO ^ 00 s 2: CM CM CO CO «x) 2 2 o o CO CO IT) 00 2 2 CM CM 00 CO CO 2 2 00 CM cvg o UO ID CO CO 2 2 0) 01 c; o QJ ft; 2 o o o QJ to CO X Q) ft; 4-J O rt) a g o u in 4-1 o QJ H (0 u to n3 c; o to (0 Q) c e •a QJ u ^ 4-) " o £ QJ QJ o 03 * QJ .

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61 pretransplant P treatments had a similar effect of increasing head mass at harvest. Tissue P levels were equal at harvest regardless of pretransplant P applied. Hochmuth et al. (1991) reported values of 25 to 50 g-kg"^ P (soil type not reported) to be indicative of an adequate P range for crisphead lettuce. Values of tissue P were slightly less than this in Experiment 2, but plants looked healthy with tissue P of 21 g-kg-^ Stem width and core length were improved by pretransplant P, indicating greater earliness due to P fertilization, thus adequate plant size at transplanting. Earliness is of particular significance in north Florida where the growing period is shortened by either low temperatures in fall plantings or high temperatures in spring plantings. Low temperatures could result in lettuce heads freezing, while high temperatures could cause premature bolting. At transplanting, plants produced with pretransplant P were larger than those produced with no P. Therefore, larger plants at transplanting led to earliness and larger head size at harvest . Summary ^South Bay' lettuce transplants were produced with different levels of P supplied via floatation irrigation, to

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62 determine the optimum P concentration necessary for production of high quality transplants, and subsequent high quality crop in the field. A quality transplant has sufficient roots to fill a tray cell to facilitate ease of pulling from the transplant flat. Plants were propagated by floating flats in a nutrient solution containing either 0, 15, 30, 45, or 60 mg-L"^ P in summer and fall experiments, and either 0, 15, 30, 60, or 90 mg-L"^ P in factorial combination with 60 or 100 mg N-L"^ in a winter experiment. Photoperiod was extended to 16 h in all experiments. Phosphorus applied at frequent rates via the floatation irrigation system affected growth of both roots and shoots of lettuce transplants. However, after the initial P addition of 15 mg-L'^ further P additions resulted in a minimal growth response. Transplants produced with 0 P had similar poor growth, regardless of N applied. Nitrogen at 100 mg-L-^ improved the response of shoot growth to any level of P, but adversely affected root growth compared to N at 60 mg-L-^ In general, RGR values were improved, while NAR values were reduced with any level of P. Values of RGR and NAR were larger by 21 DAS than by 28 DAS, indicating that younger plants had greater growth efficiency than older ones. Quality transplants had RSR of approximately 0.25, total root lengths between 276 and 306 cm, and total root area

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between 26 and 30 cm^ in a 10.9 cm-' cell volume. Only 30 % of the plants produced with 0 P could be pulled from the transplant flats, compared to approximately 90 % pulling success with added P. At least 15 mg-L"^ P, supplied every two days via floatation irrigation, is recommended for production of high quality lettuce transplants in a peat+vermiculite media containing low concentrations of water extractable P. All pretransplant P treatments had a similar effect of increasing head mass at harvest time, and in reducing time to maturity regardless of production season. At transplanting, plants produced with transplant P were larger than those produced with no transplant P. Phosphorus fertilization in the transplant cell, led to improved earliness and yields. This work demonstrated that at least 15 mg-L"^ P, supplied via floatation irrigation to a peat+vermiculite mix, was required to build an ideal transplant with sufficient roots to fill a tray cell for ease of pulling out of transplant flats. Phosphorus fertilization also resulted in larger transplants for rapid field establishment, leading to earlier lettuce harvest.

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CHAPTER 4 NEED FOR SUPPLEMENTAL POTASSIUM FOR LETTUCE TRANSPLANT PRODUCTION Introduction The environmental conditions to which vegetable transplants are exposed during early growth play an important role in final crop yield (Masson et al . , 1991b). The early growing environment of transplants can be manipulated in ways that are not possible with direct-seeded crops (Wurr and Fellows, 1982) . Several factors that are known to affect vegetable transplant size, quality, and growth in the field include nutritional conditioning before transplanting (Jaworski and Webb, 1966; Jaworski et al., 1967; Kratky and Mishima, 1981; Weston and Zandstra, 1989; Garton and Widders, 1990; Masson et al., 1991a, 1991b; Melton and Dufault, 1991; Dufault and Schultheis, 1994). The role of fertilizer K in vegetable transplant growth has been investigated. Dufault (1985) produced celery transplants and gave them weekly applications of various N, P, and K solutions. The treatments were factorial 64

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65 combinations of N at 10, 50, or 250 mg-L"', P at 5, 25, or 125 mg-L'^ and K at 10, 50, or 250 mg-L'\ Potassium did not affect celery growth. The media contained 40 mg-L"^ hydrochloric acid extractable K and may have supplied all the necessary K requirements. Melton and Dufault (1991) grew tomato transplants with either 25, 75, or 225 mg-L'^ K applied three times per week. They found that K did not influence transplant height, stem diameter, leaf number, leaf area, total chlorophyll, fresh shoot mass, or dry shoot and root mass. Tomato transplant growth did not respond to fertilizer K probably because the media Melton and Dufault used already contained 103 mg-L"^ K (extraction method not reported) . Tremblay and Senecal (1988) grew lettuce, broccoli, pepper, and celery transplants with 150 or 350 mg-L"^ N, as well as 50, 200, or 350 mg-L"^ K, applied daily. Growth measurements were made at 18, 20, 31, and 38 days after sowing for lettuce, broccoli, pepper, and celery, respectively. They reported that celery and broccoli leaf area increased by adding 50 to 350 mg-L"^ K. Leaf area of lettuce was increased with added K only with 350 mg-L"^ N, but not with 150 mg-L"^ N. Increasing the K concentration in conjunction with 150 mg-L"^ N decreased pepper leaf area while, with 350 mg-L"^ N, the inverse pattern was true.

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66 Broccoli dry shoot mass increased in response to K, with a maximum at 200 mg-L"'' K. Lettuce dry shoot mass was increased more sharply by increasing K when grown with 350 than with 150 mg-L"^ N. Celery dry shoot mass was maximized with 200 mg-L'^ K when N concentration was 350 mg-L"'', but minimum at this K concentration when added N was only 150 mg-L"^. The percentage of shoot dry matter in lettuce and pepper increased with K concentration when N was 150 mg-L"^, but decreased when N was 350 mg-L"\ Root growth characteristics as well as root: shoot ratio for broccoli, celery, and lettuce were not affected by K fertilization. Most of the previously described experiments were conducted with weekly applications of fertilizer. Data are lacking on the growth response of lettuce roots and shoots to frequent K applications such as practiced in the floatation system of irrigation. In this system, nutrients are supplied with every irrigation by floating flats directly in nutrient solution. Growers using this system have been unable to produce lettuce transplants with sufficient roots in a tray cell to enable easy removal of transplants from the transplant flat (Robles, personal communication) . Perhaps optimizing K could improve root development in lettuce transplants. The present investigation was conducted to determine the optimum K concentration, supplied via floatation

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irrigation, that could produce quality transplants with sufficient roots in a tray cell to facilitate easy removal of transplants from the transplant flat, and lead to rapid field establishment. In previous experiments (Chapter 3), photoperiod was extended to 16 h. In order to determine if supplementary light would be of greater benefit in promoting lettuce growth than extended photoperiod during periods of low light intensity, both systems were compared. Materials and Methods Greenhouse Experiments ^South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of 1.9 X 1.9 X 6.3 cm; 10.9 cm^ (length x width x depth; volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v) , with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg-m'\ was used for media. Three experiments were conducted (Tables 4-1 and 4-2) . The plants were grown with natural photoperiod extended to 16 h by 1000-W, high-pressure sodium lamps (250 /^ol-m"^-s"^ photosynthetic photon flux). A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just

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68 Table 4-1. Sowing schedule and initial media test (Hanlon et al., 1994) for Experiments 1 and 3. Expt Sowing date Media test^ pH EC -N P K Ca Mg (dS-m"' ) (mgkg-^ ) 1 14 Jul 1993 4.7 0.9 1 . 3 12. 4 14 . 6 14 . 2 11 . 6 3 31 Jan 1996 5.2 0.2 0. 3 0. 4 24 .4 0. 6 5. 8 ^Concentrations in the saturated paste extract. Table 4-2. Initial media test (Hanlon et al., 1994) for Experiment 2, sown 28 January 1994. Media type Media test^ pH EC NO3-N P K Ca Mg (dS-m'M (mg-kg-^ Peat+vermiculite 4.9 0.1 0 0.7 10.9 0.9 1.8 Peat+rockwooiy 5.3 0.1 0 0.3 2.5 0.8 0.*8 Peat 4.0 0.1 0 0.6 2.0 0.8 1.2 ^Concentrations in the saturated paste extract. ^Forty % hydrofile and 10 % hydrorepellent rockwool. above the plant canopy, and media temperatures were recorded by a Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR). Separate temperature measurements were made for the treatments under extended photoperiod and those under supplementary light in Experiment 2. Photosynthetically active radiation (PAR) during the plant growing period was measured with a light meter just above the plant canopy. For consistency, measurements were taken at 10:00 h every morning. The flats were seeded then covered with a thin layer of vermiculite, overhead irrigated enough to moisten the vermiculite, then transferred to a cooler at 20 °c for

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69 germination. After 48 h, flats were returned to the greenhouse . Plants in Experiment 1 were irrigated every two to four days by floating flats in nutrient solution containing K at 0, 15, 30, 45, or 60 mg-L"^ as KCl , Other nutrients were supplied at equivalent rates to all plants and consisted of (in mg-L'^) 100 N, 30 P, 100 Ca, and half-strength Hoagland' s solution for micronutrients only (Hoagland and Arnon, 1950) that was comprised of Mg, S, B, Cu, Mo, and Zn. The experiment was a randomized complete-block design with 5 treatments and 4 replications. Plants in Experiment 2 were grown with either the natural photoperiod extended to 16 h or with supplementary lighting for the entire 16 h photoperiod from 1000-W, highpressure sodium lamps (250 //mol -m-^ • s"^ photosynthetic photon flux) . Plants were irrigated once every two to four days, by floating flats in nutrient solution containing K at 0 or 60 mg-L"^ as KCl. Other nutrients were applied as described for Experiment 1. Peat+vermiculite (1;2, v/v) , peat+rockwool (1:1, v/v), and peat media, were used for the experiment. The factorial experiment was arranged in a split-block design. There were 3 replications within each light treatment consisting of 2 levels of K and 3 media. Plants in Experiment 3 were irrigated every other day by floating flats in nutrient solution containing K at 0,

PAGE 78

70 15, 30, 45, or 60 mg-L'^ in combination with N at 60 or 100 mg-L"^. Potassium was supplied from KCl, while N was supplied from NH4NO,. Other nutrients were applied as described for Experiment 1. The experiment was a randomized complete-block design with 10 treatments consisting of a factorial combination of 5 levels of K and 2 levels of N, replicated four times. Plant samples, 5 per treatment, were taken at approximately 14, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE) . Growth variables calculated were: root: shoot ratio (RSR = dry root mass ^ dry shoot mass), relative growth rate (RGR = [In (final total dry mass) In (initial total dry mass) ^ (final time initial time)]), net assimilation rate (NAR = [(final total dry mass initial total dry mass) ^ (final time initial time) X {(In (final leaf area) In (initial leaf area)} ^ (final leaf area initial leaf area)]), specific leaf area (SLA = leaf area ^ dry shoot mass), leaf area ratio (LAR = leaf area ^ total dry mass), leaf mass ratio (LMR = dry shoot mass ^ total dry mass), and root mass ratio (RMR = dry root mass ^ total dry mass) (Hunt, 1978; 1982; Dubik et al., 1992) .

PAGE 79

Leaf petioles were collected at 23 and 30 DAS in Experiment 2, and at the last sampling dates in Experiments 1 and 3 for sap testing. The sap was squeezed from collected petiole pieces using a hydraulic sap press onto sampling sheets according to Hochmuth (1992) . A CARDY meter (Spectrum Technologies, Inc., Plainfield, IL) was used to measure concentrations in the petiole sap. Dry shoot samples from the last sampling dates were ground to pass a 20-mesh screen and dry-ashed for K or aciddigested for total Kjeldahl N according to Wolf (1982) . For total K determination, 0.5 g subsamples were weighed into 10 mL beakers. The samples were then dry-ashed in a muffle furnance at 500 °C for 10 h. The ash was moistened with 1 N HCl and poured into 50 mL volumetric flasks, and brought to volume with 1 N HCl. The solutions were filtered through 'QS' filter papers (Fisher brand), with a particle retention of > 10 //m, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and analyzed with Model 61-E Inductively Coupled Plasma Spectrometry (Thermo Jarrell Ash Corporation, Franklin, MA) . The acid digestion procedure consisted of weighing 0.25 g subsamples into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes, which were then heated on a digestion block at 375 °C. After the

PAGE 80

72 digestion process was completed (a total of 2,5 h) , the samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through 'P8' filter papers (Fisher brand), with a particle retention of > 25 //m, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR) . Data were subjected to analysis of variance using PROC GLM and/or PROC MIXED (SAS Institute, Inc., Cary, NC) . Treatment sums of squares were partitioned into linear or quadratic polynomial contrasts in Experiments 1 and 3. Plants in the peat mix in Experiment 2 did not grow, perhaps due to poor aeration, therefore the treatment was eliminated from data analysis. Field F. xperiTnent Plants from each treatment in Greenhouse Experiment 3 were transplanted into an Arredondo fine sandy soil (loamy, siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on black polyethylene-mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville, on 29 February 1996. The soil had a water pH of 5.8, with 0 dS-m"^ for electrical conductivity, and a

PAGE 81

nutrient content (Hanlon et al . , 1994) of (in mg-kg"'') 247 P, 37 K, 695 Ca, and 43 Mg (Mehlich-1 extractant) . The experiment was a randomized complete-block design with 10 treatments consisting of a factorial combination of 5 levels of K and 2 levels of N, with each treatment replicated four times. Preplant fertilizer ( 13N-0P-10 . 8K) was applied broadcast and incorporated in the bed at 230 kg-ha"^. Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. There were 30 plants per plot planted on double offset rows with a spacing of 0.3 m between plants and between rows on the bed, equivalent to 54,000 plants/ha. Just after transplanting, 100 mL of nutrient solution (150 mg-L-^ 20N-8 . 6P-16. 7K) was applied to each transplant hole as a starter fertilizer. Water was applied daily for 45 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark (1991). starting one week after transplanting, fertilizer at a rate of 15 kg-ha"^ N and 16 kg-ha-^ K as NH4NO3 and KNO3, was injected once weekly using a venturi pump (Netafim Irrigation, Altamonte Springs, FL) , with the last application one week before harvest to give a

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74 total amount of 150 kg-ha"^ N and 180 kg-ha"^ K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al . , 1988). At lettuce head maturity, the center 20 plants in a plot were cut, individually weighed, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements. Wrapper leaves were sampled at harvest for analysis of tissue K and N according to Wolf (1982) as described for Greenhouse Experiments. Field data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc., Gary, NC) . Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts. Results and Dis cussion Greenh ouse Experiments Experiment 1 was conducted during the summer under greenhouse temperatures ranging from 22 to 36 °C (Fig. 4-1) . The average daily maximum media temperature was 32 °C, while the average daily minimum media temperature was 23 °C. During the course of the experiment, there were totals of 25 sunny days of which two were followed with rain in the afternoon, and 2 cloudy days.

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75 40 GROWING PERIOD (JUL 1 7 AUG 1 1 ) -Kair max -media max — •— air min -bmedia min Fig. 4-1. Maximum and minimum air and media temperature during transplant production for Experiment 1, Jul/Aug

PAGE 84

76 For plants sampled 15, 21, and 28 days after sowing (DAS), applied K did not influence fresh and dry shoot mass (Table 4-3) . For plants sampled 15 and 21 DAS, applied K did not influence fresh and dry root mass. However, by 28 DAS, there was a positive linear response of fresh and dry root mass to applied K. For plants sampled 15 DAS, leaf area increased linearly to applied K, but by later sampling dates, applied K did not influence leaf area. For plants grown to 21 or 28 DAS, there was a positive linear increase in petiole sap K in response to applied K. Leaf tissue K also increased linearly to applied K. Therefore, increased root growth was associated with increased tissue K. For plants sampled 15, 21, and 28 DAS, RSR values were not influenced by applied K, since shoots were unaffected while there was little increase in root growth due to added K (Table 4-4) . For plants grown to 21 DAS, RGR, and NAR values responded in quadratic fashion in response to applied K. Relative growth rates and NAR values were least with 30 mg-L-i K and greatest with 60 mg-L'^ K. By 28 DAS, RGR and NAR were not influenced by K. For plants sampled 15 DAS, SLA and LAR increased linearly in response to applied K. For plants sampled 21 and 28 DAS, SLA, and LAR were not influenced by K.

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77 Table 4-3. Root and shoot characteristics of lettuce transplants as affected by K nutrition for Experiment 1, July/August 1993. Potassium Fresh Dry Fresh Dry Leaf Leaf Leaf applied shoot shoot root root area petiole tissue mass mass mass mass sap K K (mg-L-M (mg) (mg) (mg) (mg) (cm^) (mg-L-^) (g-kg-^ 15 Days After Sowing 0 152 9.7 56 3.6 6.4 15 177 10.5 57 3.9 7.2 30 183 11.2 57 3.8 7.7 45 172 10.5 54 3.6 7.4 60 175 10.2 50 3.5 7.4 Response NS NS NS NS L* 21 Days After Sowing 0 704 40.2 167 10.4 26.4 2725 15 699 41.1 182 11.3 26.3 2800 30 721 39.4 183 10.8 27.1 2900 45 734 40.2 182 11.1 28 . 1 2950 60 726 41.2 214 13.0 26.8 3075 Response NS NS NS NS NS L** 28 Days After Sowing 0 1366 80.0 298 19.9 46.3 2300 41.9 15 1347 82.4 296 20.8 46.5 2125 42.8 30 1395 80.7 302 20.8 47. 6 2325 47.6 45 1454 88.3 324 22.4 49.6 2825 48.0 60 1474 85.6 333 22.8 50.2 2450 48.7 Response NS NS L* L** NS L** T ** Linear (L) effects significant at P = 0.05 {*), 0.01 (**), nonsignificant (NS) .

PAGE 86

78 o U) -H nj to U-l CO -H to CO -P (1) (0 to 1^ E M o 14-1 (0 -rH to 0) 4-1 0) M (0 I-:) to i-i u H 1 if E 0 10 CM 0) to (U e a 0) l4 u CO (0 c o 1 -H a: 4-1 (0 e H 4-1 CO 0) CO (t) 4J to S to M ^ E 4-) to 2^ a: C7> M E >i u H c 4-) o 0) 4-1 O •H D O O 4-1 .H O x; m rH OC CO u c H nt d) 1 im uin ^J< -H Tl (U CO 0) 0) a CO •H •H to rH XI u 4J a CP (0 0 a EH Ch 10 r~r~ in IT) in CN eg (NJ CM CM W o o o o o 2; n n in in in rrr~r-~ to o o o o o 2 00 O rH Csj '3* ^ in in in in * * O O O O O iJ rH CM CM CM ^ CM CM CM CM CM CO O O O O rH CM CM CM Cvj OO CO ooooog; 000002 CTl CD CD CX3 r~r~t~ rco O O O O CTl CXI CO CO 03 P~ CO 000002 000002 CM o ^ in (Tl in in in in '3" o o o o o CO 2 in r~in *£) o o o o o tn 0 to (1) u to to Q CTl CO o n vD IX) (£1 rIX) ^ CTl o VD (X) r~ o o o o o Q o o o o o to ^ , tl * Q o tJO CM 00 VD (Jl IX) (Tl in in in in in o o o o o o m rH in ^ in ^ CO ^CMCMCMCMCMOIco'"''^'"''-''-^^ n O rH (Tl oo n n rvj CM (*i * On 4J nt p c Q O H iM H C cn H (0 CQ P U 0) in to 2 CM CM CM CM CM a CO tr o o o o o 2 4-1 IH C o to u •H 13 (U rH to C c M cn 0 (0 -rl m o in o (U to o rH n vx> c c -H o a; c

PAGE 87

79 Applied K did not influence LMR or RMR values, regardless of sampling date (Table 4-4) . For plants sampled 15 DAS, approximately 75 I of the lettuce transplant dry matter was allocated to shoots and 25 % allocated to roots. As the plants grew older, by 28 DAS, proportionally more dry matter became allocated to shoots (80 %) compared to roots (20 %) . Results of Experiment 1 indicated that K applied to a peat+vermiculite media with 15 mg-L"^ water extractable K, increased transplant root growth, but not shoot growth. Sufficient K for shoot growth may have been available or released from the media during the growing cycle. In Experiment 2, transplant growth response to K was compared among three media types, peat, peat+rockwool, and peat+vermiculite mixes. Peat and rockwool (molten, spun basalt rock fibers) have inherently lower K levels than vermiculite (Table 4-2). Since the experiment was conducted in February when light intensities are normally low, the benefit of supplementary light for 16 h was compared with an extension of the photoperiod to 16 h. Experiment 2 was conducted during the winter, under greenhouse temperatures ranging from 8 to 37 °c (Figs 4-2 and 4-3) . The average daily maximum media temperatures were 24 and 2 6 °C under extended photoperiod and under supplementary light, respectively. Average daily minimum

PAGE 88

80 35 GROWING PERIOD (FEB 2 FEB 26) -sair max media max — •— air min -bmedia min . 4-2. Maximum and minimum air and media temperature during transplant production under extended photoperiod for Experiment 2, February 1994.

PAGE 89

81 Fig. 4-3. Maximum and minimum air and media temperature during transplant production under supplementary lighting for Experiment 2, February 1994.

PAGE 90

82 media temperature was 14 °C with both light treatments. During the course of the experiment, there were totals of 13 sunny and 16 cloudy days. Supplementary light contributed more than natural light to the light integral (PAR) received in the greenhouse by the lettuce transplants (Fig. 4-4) . For plants sampled 23 DAS, light, K, and media did not interact to influence fresh shoot mass (Table 4-5) . Light and media treatments did not influence fresh shoot mass, while more fresh shoot mass occurred in transplants grown with 60 mg-L"^ than with no K. Supplementary light for 16 h led to increased dry shoot mass compared to extending the photoperiod to 16 h, particularly in peat+vermiculite compared to peat+rockwool mix. For plants sampled 30 DAS, both K and media influenced fresh and dry shoot mass (Table 4-6) . When produced with 60 mg-L'^ K, plants grown in peat+rockwool mix had more fresh shoot mass than plants grown in the peat+vermiculite mix. There was no response in dry shoot mass to applied K in the peat+vermiculite mix. In peat+rockwool mix, applied K resulted in an increase in dry shoot mass. Plants grown with supplementary light for 16 h had greater shoot mass than those grown with the photoperiod extended to 16 h. For plants sampled 23 DAS, only the media used influenced fresh root mass; there was no effect of added K or light (Table 4-5) . Root mass of plants grown in

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83 900 GROWING PERIOD (FEB 1 FEB 27) -Bnatural supplementary . 4-4. Photosynthetically active radiation (PAR) under natural or supplementary lighting for Experiment 2, February 1994.

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84 Table 4-5. Root and shoot characteristics of lettuce transplants 23 days after sowing as affected by light, potassium, and media for Experiment 2, February 1994. Mix Light' (h) K (mg-L'M _4 16 Response 0 60 Response Fresh shoot mass (mg) Peat+vermiculite 1003 1014 NS 1005 1012 NS Peat+rockwool 1096 1057 NS 999 1154 ** Response NS NS NS Dry shoot mass (mg) Peat+vermiculite 61.4 83.6 ** 72.8 72.2 NS Peat+rockwool 59.0 69.6 * 54.7 73.9 ** Response NS * * * * NS Fresh root mass (mg) Peat+vermiculite 222 294 * 257 260 NS Peat+rockwool 195 229 NS 199 226 NS Response NS * * * * * Dry root mass (mg) Peat+vermiculite 11. 1 16.8 NS 14.1 13.8 NS Peat+rockwool 9.4 15.3 NS 11.9 12.8 NS NS NS NS NS Leaf area (cirf) 36.7 35.8 NS 36.9 35.6 NS Pea t + rn r* If won 1 36. 9 35.6 NS 31.1 41.4 ** ^ tJ^-/\^ H ^ ^ NS NS * * Leaf petiole sap K (mg-L'') Peat+vermi fiil 1 * ^ " ^ 1 V C ^ILLX UX X C 2183 1967 NS 1950 2200 * Pea t +rockwnnl 588 498 NS 101 985 ** ** * * * Root:shoot ratio 0.18 0.20 NS 0.19 0.19 NS Pea t" + KnoVwrm 1 0.16 0.23 NS 0.22 0.17 NS NS NS NS NS Specific leaf area (cm'-mg'') Peat+vermiculite 0.60 0.43 ** 0.52 0.51 NS Peat+rockwool 0.62 0.52 0.57 0.57 NS Response NS Leaf area ratio (cnt^-mg'') Peat+vermiculite 0.51 0.36 ** 0.44 0.43 NS Peat+rockwool 0.54 0.42 * 0.47 0.49 NS Response NS -It * NS ** Leaf mass ratio Peat+vermiculite 0.85 0.84 NS 0. 84 0.84 NS Peat+rockwool 0. 86 0.82 NS 0. 82 0.86 NS Response NS NS NS NS Root mass ratio Peat+vermiculite 0.15 0.16 NS 0.16 0.16 NS Peat+rockwool 0.14 0.18 NS 0. 18 0.14 NS Response NS NS NS NS 'Natural photoperiod extended by 4 h to 16 h or supplementary light f the entire 16 h. "^'"Nonsignificant (NS) or significant t-test at 5% (*), 1% {**) levels .

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85 Table 4-6. Root and shoot characteristics of lettuce transplants 30 days after sowing as affected by light, potassiiam, and media for Experiment 2, February 1994. Mix Light" (h) K (mg-L-M 16 Response 60 Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Peat+vermiculite Peat+rockwool Response Fresh shoot mass 1450 1427 NS 1575 1598 NS NS * Dry shoot mass 110.6 134.3 * 103.7 131.4 * NS NS Fresh root mass 348 450 * 289 327 NS (Tag) 1456 1329 NS (mg) 123. 9 93.1 * * (mg) 378 238 26.8 20.8 * Dry root mass (mg) 52.3 52.6 NS 36.6 ** 29.8 25.9 * 17.7 * ** Leaf area (cm^) 50.1 NS 52.0 50.7 NS 38.5 NS ** Leaf petiole sap K (mg-L'') 2467 2083 ** 1917 593 585 NS 78 * ** ** Leaf tissue K (g-kg'^) 37.8 30.1 * 30.3 11.0 9.0 NS 2.7 1422 NS 1844 ** ** 121.0 NS 142.0 ** * * 420 * 377 ** * 33.6 ** 28.9 ** 50.4 NS 64.8 ** ** 2633 ** 1100 ** ** 37.7 ** 17.3 ** ** or supplementary ^Natural photoperiod extended by 4 h to 16 h light for the entire 16 h. "Nonsignificant (NS) or significant t-test at 5% (*). 1% (**) levels. \ I ,

PAGE 94

86 peat+vermiculite mix was greater than for plants grown in peat+rockwool mix. Dry root mass of plants sampled 23 DAS was not affected by any treatment. By 30 DAS, plants grown in peat+vermiculite mix had greater fresh and dry root mass compared to those in peat+rockwool mix under both light treatments (Table 4-6) . Fresh root mass was increased by supplementary light only when peat+vermiculite mix was used. For both media types, fresh and dry root mass were greater with 60 mg-L"^ K compared to no K. The greatest dry root mass was 3 6.6 mg obtained from plants grown in peat+vermiculite mix with supplementary light. The greatest dry root mass from the peat+rockwool mix was 28.9 mg, obtained from plants grown with 60 mg-L"^ K. Regardless of sampling date, applied K did not influence leaf area when plants were grown in peat+vermiculite mix, indicating sufficient K in the media (Tables 4-5 and 4-6) . The plants had greater leaf area when 60 mg-L-^ K compared to no K was added to peat+rockwool mix. With no K, plants in peat+vermiculite mix had greater leaf area than plants in peat+rockwool mix. The opposite was true with 60 mg-L"^ K. For plants grown to 23 DAS (Table 4-5)', petiole sap K concentration was greater when plants were grown in peat+vermiculite mix instead of peat+rockwool mix. Petiole sap K increased when K was applied to either media type. By

PAGE 95

87 30 DAS (Table 4-6), plants grown with 60 mg-L"^ K had more petiole sap K than those grown with no K. Plants grown in peat+vermiculite mix had greater concentrations of petiole sap K than those grown in peat+rockwool mix. It is not clear why in peat+vermiculite mix, supplementary light resulted in lower petiole sap K, while petiole sap K values were not influenced by light treatment for plants grown in peat+rockwool mix. Plants grown in peat+vermiculite mix had greater total leaf tissue K concentration than plants grown in peat+rockwool mix due to inherently higher levels of K in vermiculite (Table 4-6) . It is unclear why supplementary light, compared with an extension of the photoperiod, resulted in lower leaf K concentration in plants grown in peat+vermiculite mix but not in peat+rockwool mix. Plants grown with 60 mg-L"^ K had greater K concentration in the leaves compared to plants grown with no K, especially those plants grown in peat+rockwool mix. For plants sampled 23 DAS, neither of the treatments influenced RSR values (Table 4-5) . For plants sampled 30 DAS, RSR was affected by both K and media (Table 4-7) . Plants grown with 60 mg-L"^ K in peat+vermiculite mix had greater RSR values than those grown with no K, because added K increased dry root mass but not dry shoot mass. Plants grown in peat+vermiculite mix also had greater RSRs than

PAGE 96

88 Table 4-7. Influence of light, potassium, and media on growth characteristics of lettuce transplants 30 days after sowing for Experiment 2, February 1994. Mix Light^ (h) K (mg 4 16 Response 0 60 Response Root : shoot ratio Peat+vermiculite 0.25 0.27 NS 0.24 0.28 * * Peat+rockwool 0.20 0.20 NS 0.19 0.20 NS Response ** * * * * * Relative growth rate (mg-mg ^ • wk'' ) Peat+vermiculite 0.64 0.54 NS 0.58 0. 60 NS Peat+rockwool 0.59 0,60 NS 0.50 0.68 ** Response NS NS NS NS Net assimilation rate (mgcm~^ • wk'^ ) Peat+vermiculite 1.48 1.68 NS 1.53 1. 63 NS Peat+rockwool 1.24 1. 66 NS 1.29 1. 61 NS Response NS NS NS NS Specific leaf area (cm" • mg'^ ) Peat+vermiculite 0.48 0.37 * 0.42 0.43 NS Peat+rockwool 0.50 0.38 ** 0.42 0.46 NS Response NS NS NS NS Leaf area ratio 'cm^ -mQ'' ) Peat+vermiculite 0.38 0.29 * 0.34 0.33 NS Peat+rockwool 0.42 0.32 ** 0.36 0.39 NS Response NS NS NS * Leaf mass ratio Peat+vermiculite 0.80 0.79 NS 0.81 0.78 * Peat+rockwool 0.83 0.84 NS 0.84 0.83 NS Response ** * ** Root mass ratio Peat+vermiculite 0.20 0.21 NS 0.19 0.22 * Peat+rockwool 0.17 0.16 NS 0.16 0,17 NS Response * * ** * * * * :Nacura± pnotoperiod extended by 4 h to 16 h or supplementary light for the entire 16 h. ccy *' "Nonsignificant (NS) or significant t-test at 5% (*) 1% (**) levels.

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89 plants grown in peat+rockwool mix. Added K did not influence RSRs for plants grown in peat+rockwool mix. For plants grown to 30 DAS (Table 4-7), RGR values were lower in peat+rockwool mix with no K than with 60 mg-L"^ K, while in the peat+vermiculite mix, RGR was not affected by K. Neither treatment influenced NAR. In Experiment 1, RGR and NAR were also not influenced by added K in a peat+vermiculite mix. When photoperiod was extended to 16 h, plants grown with no K in either mix had similar SLA values by 23 DAS, but when grown with 60 mg-L"' K, those plants grown in peat+rockwool mix had greater SLA values than plants in peat+vermiculite mix (Table 4-8). Application of 60 mg-L'^ K led to smaller SLA values compared to no K when plants were grown in peat+vermiculite compared to peat+rockwool mix. Under 16 h supplementary light, SLA values were not influenced by applied K in either media. With no K, SLA values were greater in peat+rockwool than in peat+vermiculite mix. For plants sampled 23 DAS (Table 4-5), 16 h supplementary light reduced SLA compared to extended photoperiod, particularly when plants were grown in peat+vermiculite than in peat+rockwool mix. For plants sampled 30 DAS, supplementary light for 16 h led to decreased SLA and LAR compared to simply extending the photoperiod to 16 h (Table 4-7) . A low SLA is desirable, though, because it is associated with a thicker leaf.

PAGE 98

90 Table 4-8. Influence of light, potassium, and media on SLA of lettuce transplants 23 days after sowing for Experiment 2, February 1994. Mix Light^ (h) 4 16 K (mg-L"') K (mg0 60 Response 0 60 Response Specific leaf area (cm^ -mg'' ) Peat+vermiculite 0.63 0.57 ** 0.42 0.44 NS Peat+rockwool 0.61 0.64 NS 0.53 0.50 NS Response NS ** ** ZM — 4-.1 J 1 A l_ j_. -1 ^ 1 NS ^Natural photoperiod extended by 4 h to 16 h or supplementary light for the entire 16 h. *' "'Nonsignif icant (NS) or significant t-test at 5% (*), 1% (**) levels.

PAGE 99

91 According to Masson et al . (1991a), under high photosynthetic photon flux density, the palisade layer cells generally elongate so that the leaves are thicker, resulting in a decrease in SLA. Greater leaf area ratios for plants grown in peat+rockwool than in peat+vermiculite was associated with greater leaf areas in comparison to dry shoot mass in these plants. Neither treatment affected LMR or RMR values for plants sampled 23 DAS (Table 4-5) . By 30 DAS, plants grown with 60 mg-L'^ K had smaller LMR values compared with plants grown without K. Similarly, plants grown in peat+vermiculite mix had smaller LMR values than plants grown in peat+rockwool mix. The opposite response to applied K and to media type occurred for RMR, Once again, added K increased root growth more than shoot growth. In Experiment 2, plants grown in peat+rockwool mix (3 mg'kg'^ water extractable K) responded more to applied K compared with plants grown in peat+vermiculite mix (11 mg-kg'^ water extractable K) because, unlike vermiculite, rockwool is inherently low in K. In the peat+rockwool mix, 60 mg-L"^ K compared to no K led to increases in shoot, root, and leaf growth. Potassium fertilization also led to increased K concentrations in transplant leaves. Root mass responded more to K and media treatments at 30 than at 23

PAGE 100

92 DAS, indicating that treatment effects on root growth became more apparent with time. In general, RSR, RGR, NAR, SLA, and LAR were not affected by K. Even though these transplant growth characteristics were not affected by K, transplants grown with no K in the peat+rockwool mix had inferior quality since they could not be easily removed from the transplant flat (data not provided) . Stems broke during removal in plants grown with no K, rather than breaking at the rootshoot interface as with plants without N (Chapters 5 and 6) or P (Chapter 3) . In the previous two experiments, 100 mg-L"^ N was used when growing transplants at various levels of K. Subsequent studies with N in Chapter 6, however, revealed that optimum N for lettuce transplant root growth might be in the 60 mg-L'^ range or less, supplied every second day through floatation irrigation. Therefore, in Experiment 3, N was included as a variable to compare 60 versus 100 mg-L"^ N concentration at selected levels of K. In order to further test the conclusion reached in Experiment 1 that supplemental K may not be necessary for production of high quality transplants in a peat+vermiculite mix. Experiment 3 was conducted during the winter, instead of summer, under greenhouse temperatures ranging from 14 to 38 °C (Fig. 4-5) . The average daily maximum media

PAGE 101

93 o 2. m (L D \< LU Q. lU IGROWING PERIOD (FEB 5 FEB 28) air max media max — »— air min -bmedia min Fig. 4-5. Maximiam and minimum air and media temperature during transplant production for Experiment 3, February 1996.

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94 temperature was 29 °C, while the average daily minimum media temperature was 21 °C. During the course of the trial, there were totals of 17 sunny and 9 cloudy days. For plants sampled 15, 22, or 28 DAS, there were no K by N interactions for dry shoot mass (Table 4-9) . For plants sampled 15 and 28 DAS, dry shoot mass was unaffected by K, For plants sampled 22 DAS, there was a negative linear response of dry shoot mass was to applied K. Plants grown with 100 mg-L''' N had greater shoot mass than those grown with 60 mg N-L'^ regardless of sampling date. For plants sampled 15, 22, or 28 DAS, there were no K by N interactions for dry root mass. Potassium did not influence dry root mass, regardless of sampling date. Nitrogen did not influence root mass of plants sampled 15 DAS, but by 22 and 28 DAS, plants grown with 60 mg-L"^ N had greater root mass than plants grown with 100 mg-L"^ N. There were no K by N interactions for leaf area of plants sampled 15 and 22 DAS. Potassium did not influence leaf area of plants at these sampling dates. There was greater leaf area for plants grown with 100 than with 60 mg-L-^ N. For plants sampled 28 DAS, N at 60 mg-L"^ led to a decrease in leaf area when K was applied, while at the 100 mg-L-i level, applied K did not influence leaf area. Tremblay and Senecal (1988) reported that leaf area of

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95 Table 4-9. Root and shoot characteristics of lettuce transplants as affected by K and N nutrition for Experiment 3, February 1996. Nutrient Dry Dry Leaf Leaf Leaf Root : applied shoot root area tissue tissue shoot mass mass N K ratio (mg ) (mg) ( cm ) ( g • Kg ) (gkg ') IN. 15 Days After Sowing 0 9.6 3.0 7.3 0.31 15 9.5 3.2 7.2 0.34 Q n JU 9 . 3 3 . 0 7 . 3 0.33 9 . 0 2 . 8 7 . 0 0.32 9 . 2 3 . 1 7 . 1 0. 34 Response XT N NS NS NS NS 60 8.9 3.0 6.8 0.34 100 9.7 3.0 7.5 0.31 Response NS ** K X N NS NS NS NS 22 Days After Sowing 0 45.4 11.7 31.2 0.26 15 45.5 11.7 30.8 0.26 30 45.9 12.7 30.0 0.28 45 42.6 11.9 30.5 0.29 60 42.1 12.7 31.8 0.31 Response N L** NS NS L** 60 40.0 13.4 26.3 0.34 100 48.6 10.9 35.4 0.23 Response * * ** ** K X N NS NS NS NS K 0 15 30 45 60 Response N 60 100 Response K X N 28 Days After Sowing Ni N2 93.9 25.1 51.4 67.2 23.5 36.5 0.27 91.5 24.9 47.8 71.6 23.6 38.8 0.28 95.7 25.4 51.5 73.5 23.3 45.1 0.27 91.2 23.9 45.8 74.4 22.5 49.6 0.27 88.6 24.8 45.6 72.1 22.4 53.2 0.29 NS NS L** NS L** L** NS 80.1 26.7 18.8 45.5 0.33 104.3 23.0 27,4 43.8 0.22 ** NS NS 1 . M = NS inn _™ . T -1 * NS NS NS Linear (L) . "Nonsignificant (NS) or significant at 5% (*), 1% (**) levels,

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96 lettuce transplants increased in response to applied K from 50 to 350 mg-L"^ with 350 mg-L"^ N, but not with 150 mg-L"^ N. There were no K by N interactions for leaf tissue N or leaf tissue K (Table 4-9) . There was a negative linear response of leaf tissue N to applied K. Leaf tissue K increased in a linear fashion to applied K, but this increase did not influence shoot or root growth. Plants had similar K concentrations in the leaves, regardless of applied N. Nitrogen concentrations were greater in leaves of plants grown with 100 than with 60 mg-L'^ N. For plants sampled 15, 22, or 28 DAS, there were no K by N interactions for RSR. For plants sampled 15 and 28 DAS, applied K did not influence RSR values. For plants sampled 22 DAS, RSR values increased in linear fashion in response to applied K, since there was a concomitant decrease in dry shoot mass by this sampling date. Root: shoot ratios were greater in plants grown with 60 than 100 mg-L"^ N, regardless of sampling date. There were no K by N interactions for RGR or NAR, regardles of sampling date (Table 4-10) . For plants grown to 22 and 28 DAS, applied K did not influence RGR values. However, RGR values were greater with 100 than 60 mg-L"^ N by both sampling dates. For plants grown to 22 DAS, NAR responded in quadratic fashion to applied K. Net assimilation rate was greatest with 30 mg-L"^ K and least

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97 Table 4-10. Influence of K and N nutrition on growth characteristics of lettuce transplants for Experiment 3, February 1996. Nutrient Relative Net Specific Leaf Leaf Root applied growth assimilation leaf area mass mass rate rate area ratio ratio ratio (mg • L"' ) (mg-mg"^ • wk"' ) (mgcm"^wk"' ) (cm^-mg"') (cm^-mg"') 15 Days After Sowing K 0 0.77 0.60 0.57 0.76 0.24 15 0.76 0.56 0.57 0.75 0.25 30 0.79 0.58 0.61 0.75 0.25 45 0.77 0 57 0 fin n 7 fi 60 0.78 0.55 0.61 0.74 0.26 Response N NS NS L** NS NS 60 0.77 0.75 0.25 100 0.78 0.76 0.24 Response NS k* * * K X N NS * NS NS K 22 Days After Sowing 0 1.52 2.11 0.69 0. 55 0.79 0.21 15 1.50 2.75 0.68 0.54 0.79 0.21 30 1.57 2.93 0.66 0.51 0.78 0.22 45 1.53 2.71 0.71 0.55 0 7R 60 1.50 2.60 0.75 0.58 0.76 0.24 Response N NS Q* Q* NS L** L** 60 1.50 2.87 0.66 0.49 0.75 0.25 inn 100 1 . 55 2.61 0.73 0.60 0.82 0.18 Response * ** * * It* k-k -** K X N NS NS NS NS NS NS K 28 Days After Sowing Ni Hz Ni N2 0 0.74 1.43 0.61 0.65 0.47 0.53 0.79 0.21 15 0.71 1.36 0.62 0.68 0.46 0.55 0.78 0.22 30 0.73 1.43 0.61 0.69 0.46 0.57 0.79 0.21 45 0.75 1.42 0.60 0.71 0.45 0.58 0.79 0.21 60 0.73 1.37 0.59 0.72 0.44 0.59 0.78 0.22 Response N NS NS NS L** NS L** NS NS 60 0.70 1.48 0.75 0.25 100 0.76 1.32 0.82 0.18 Response * ** ** * * K X N NS NS * ** NS NS ^Ni = 60 mg •L-'; Nj = 100 mg • L"' . Linear (L) or quadratic (Q) . • Nonsignificant (S) or significant at 5% (*), 1% (**) levels .

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98 with 60 mg-L"^ K. Potassium did not influence NAR by 28 DAS. Net assimilation rate was greater for plants grown with 60 mg-L"^ N, compared to plants receiving 100 mg-L"^ N. There were no K by N interactions for SLA of plants sampled 15 and 22 DAS (Table 4-10) . For plants sampled 15 DAS, K and N did not influence SLA. For plants sampled 22 DAS, SLA increased in quadratic fashion to applied K, and was greatest with 60 mg-L"^ K. Specific leaf area was greater when plants were grown with 100 instead of 60 mg-L'^ N. For plants sampled 28 DAS, applied K did not influence SLA with 60 mg-L"^ N, while with 100 mg-L'^ N, SLA increased in linear fashion in response to applied K. For plants sampled 15 and 28 DAS, applied K did not influence LAR at the 60 mg-L"^ N level, but with 100 mg-L'^ N, LAR increased in linear fashion to applied K. For plants sampled 22 DAS, there was no K by N interactions for LAR. Applied K did not influence LAR by this sampling date. Leaf area ratio was greater when plants were grown with 100 than with 60 mg-L"^ N. There were no K by N interactions for LMR or RMR of plants sampled 15, 22, and 28 DAS. For plants sampled 15 and 28 DAS, K did not influence either LMR or RMR because neither dry shoot mass nor dry root mass were influenced by added K. For plants sampled 22 DAS, LMR decreased in linear fashion, while RMR increased in linear fashion when K was

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applied. Plants grown with 60 mg-L'^ N had greater RMRs than those grown with 100 mg-L"^ N, regardless of sampling date. Approximately 25 % of lettuce transplant dry matter was allocated to roots when plants were grown with 60 mg-L'^ N as compared to 18 % when grown with 100 mg-L"^ N. Therefore, lower N concentration in the nutrient solution is recommended. Root and shoot growth of lettuce transplants were not increased by fertilizer K in Experiment 3. The media contained 24 mg-kg"^ water extractable K before fertilizer application. Therefore, sufficient K was probably available or released during the growing cycle for transplant growth. In general, root and shoot growth of lettuce transplants were not improved by fertilizer K applied via floatation irrigation system to a peat+vermiculite mix. In Experiment 1, leaf tissue K at the end of the experiment ranged from 42 g-kg"^ with 0 K to 49 g-kg-^ with 60 mg-L"^ K. In Experiment 3, leaf tissue analysis at the end of the experiment ranged from 37 g-kg-^ with 0 K to 53 g-kg"^ with 60 mg-L"^ K. Leaf tissue K concentrations were similar in the two experiments, perhaps leading to similar lack of response in root and shoot growth to applied K. Leaf K concentration of about 4 0 g-kg'^ appears to be adequate for production of high quality transplants, with enough roots in 28 days to fill the tray cell volume.

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100 Tremblay and Senecal (1988) reported that root growth and RSR for broccoli, celery, and lettuce were not affected by daily additions of 50 to 350 mg K-L'^ Lettuce dry shoot mass increased more sharply in response to increasing K when grown with 350 mg N-L"^ than with 150 mg N-L'^ In the present work, lettuce shoots did not respond to fertilizer K, regardless of season or fertilizer N concentration. Root: shoot ratios were not affected by K, regardless of sampling date, or season. Root: shoot ratios ranged from 0.25 to 0.27. In Experiment 3, N led to reduced RSRs probably because the plants produced more shoots and less roots with 100 mg-L-^ N than with 60 mg-L"^ N. In general, fertilizer K influenced RGR and NAR of plants grown to 21 DAS, but not of plants grown to 28 DAS, perhaps indicating that K was more important earlier in transplant shoot growth. Regardless of season, approximately 79 % dry matter was partitioned to shoots and 19 % partitioned to roots, implying that temperature differences did not influence dry matter partitioning. These values are similar to those obtained in the P experiments of Chapter 3. Fertilizer K did not influence LMR or RMR because neither dry shoot mass nor dry root mass responded to K. In Experiment 2, plants grown in peat+rockwool mix (2.5 mg-kg-^ water extractable K) responded more to applied K compared with plants grown in peat+vermiculite mix (10.9

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101 mg-kg"^ water extractable K) because, unlike veritiiculite, rockwool is inherently low in K. In the peat+rockwool mix, applied K led to increases in shoot, root, and leaf growth. In the present work, fertilizer K was not necessary when a peat+vermiculite media was used, since vermiculite will supply all the K needs for a 28-day growing period when floatation irrigation is used. For a media low in K, applying 60 mg K-L"^ resulted in improved root growth, leading to improved pulling success. During periods of low light intensity, increasing light intensity for 16 h improved root growth, resulting in high quality transplants. Field ExperimenlPlants from Greenhouse Experiment 3 were grown to maturity to evaluate the effects of pretransplant K and N on earliness, yield, and lettuce quality. Plants were harvested in May, 64 days after transplanting. There were no K by N interactions for lettuce head mass (Table 4-11) . Pretransplant K and N did not influence lettuce head mass. There were no K by N interactions for lettuce head firmness, height, diameter, stem width, or core length. Lettuce heads from all K and N treatments were equally firm. Applied K or N did not affect lettuce head height, head diameter, stem width, or core length.

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T3 m 0) xi o 4J (U o o -H u o • -P .-H C m C n (0 JJ Q) 4-) Cj> n G a ^ c O CO •H u 4-) -H -H J-) M n 4J -H C (1) s u m (t u >i M-l 4-1 0 -H fH to (fl 4-) 3 u tr 0) iw X3 M-l to W Q) x: ^ 3 1 (t CO Q) CO XI e H 102 0) 4-4 to ^ to W 0) -H ^ tji J 4-) (0 CO 4-) Qj a> M 3 O 0) 4.) 4-1 CO U Q) 4-1 CU (0 to 4-1 to -H 3 e -H M 4-) to T) CO to CO (U in 4-1 to 4-) 3 to U 3 Cn •H CO (0

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103 There were no K by N interactions for leaf tissue N or leaf tissue K. Leaf tissue N decreased in linear fashion in response to pretransplant K. Pretransplant N did not affect N concentration in plant leaves. Leaf tissue K responded in quadratic fashion to pretransplant K and was least with 60 mg-L''' K. Pretransplant N did not influence K concentration in plant leaves. Pretransplant K had no effect on posttransplant growth. Therefore, transplants grown with no K in a peat+vermiculite mix with at least 24 mg-L'^ water extractable K, produced yields equivalent to transplants grown with 15, 30, 45, or 60 mg-L"^ K, when K was applied via floatation irrigation. Summary ^South Bay' lettuce transplants were produced with different levels of K, different media types, and with extended or supplementary light in an attempt to increase transplant root and shoot growth. Plants were fertigated by floating flats in nutrient solution containing K at 0, 15, 30, 45, or 60 mg-L"^ K, and/or in factorial combination with 60 or 100 mg-L"^ N. Plants were exposed to 16 h extended natural photoperiod or to supplementary light for the entire 16 h. The media was either peat+vermiculite or peat+rockwool .

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104 Potassium applied to a peat+vermiculite media via floatation irrigation system had no significant effect on transplant growth. Plant available K in the media (11 to 24 mg-kg"^ water extractable K) may have supplied the K needs during lettuce transplant growth and development. Fertilizer K increased shoot and root growth of plants grown in peat+rockwool mix, leading to more transplant pulling success. Plants grown in peat+rockwool mix with no K could not easily be removed from the transplant flatSupplementary light for 16 h enhanced dry matter production as compared to extension of photoperiod to 16 h. Lettuce growth and yield in the field was not affected by pretransplant K. This work demonstrated that supplemental K fertilization may not be required in a peat+vermiculite mix using a floatation irrigation system, since vermiculite supplied adequate K. In a peat+rockwool mix, at least 60 mg-L"^ K is recommended to produce an ideal transplant with sufficient roots to fill a tray cell and facilitate ease of transplant removal from the transplant flat.

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CHAPTER 5 ROOT AND SHOOT GROWTH RESPONDS TO NITROGEN NUTRITION OF LETTUCE TRANSPLANTS Introduction Approximately 4,000 ha of crisphead lettuce were grown in Florida during the 1993-94 production season, mostly on the Histosols around Lake Okeechobee and Zellwood (Anon., 1995) . However, decline of the Histosols due to oxidation, and competition with other lettuce production areas such as California, have limited lettuce production on the organic soils. Cantliffe (1990) suggested that the expansion of lettuce production into the abundant sandy soils of Florida could increase lettuce production potential in Florida. Commercially acceptable yields of high quality lettuce from sandy soils require new production systems such as plastic mulch and transplants instead of the traditional directseeding used on Histosols. Florida growers have, however, been unable to produce lettuce transplants with suitable root development especially using a desirable floatation or subirrigation system. Perhaps proper fertilizer management may result in production of quality lettuce transplants. 105

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106 Fertilizers can either be applied to transplants independent of irrigation or with the irrigation water (fertigation) , When fertigation is employed, careful management of fertilization is important since large amounts of fertilizers, especially N, could be applied when irrigation demands are high, especially where floatation irrigation is employed. If overf ertilization occurs with floatation irrigation, there is no method to leach excessive salts . Tremblay and Senecal (1988) grew lettuce transplants in a growth chamber and overhead irrigated plants daily with distilled water. Fertilization treatments were initiated at emergence and were done to runoff every afternoon, while overhead irrigation was performed every morning. They found that 350 mg-L"^ N relative to 150 mg-L"' N increased leaf area and dry shoot mass of lettuce. Specific leaf area of lettuce was not affected by N. Dry root mass and root: shoot ratio (RSR) were reduced by 350 mg-L"^ N. By separating irrigation from fertilization, Tremblay and Senecal were constantly leaching salts, something that could not be achieved with the floatation irrigation system. Masson et al. (1991a) reported that 400 mg-L"^ N led to an increase in lettuce dry shoot mass of 38 % compared to applying 100 mg-L"^ N. The N treatment was applied twice daily by means of overhead fertigation for a period of 24

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days. Lettuce dry root mass and RSR decreased in response to applied N. At harvest, 400 mg-L"^ pretransplant N improved marketable head mass by 16 % and also promoted earliness compared to 100 mg-L"^ N. Guzman (1993) reported that lettuce transplants grown with 1200 mg-L'^ N applied four times over a 4-week growing period were larger than desired and bruised more during transplanting, resulting in slower recovery in the field from transplant shock. Guzman recommended 900 instead of 1200 mg-L'^ N to produce quality lettuce transplants. Kratky and Mishima (1981) reported that daily misting with 230 mg-L"^ N in combination with a preplant fertilizer was undesirable since production of excessively tender lettuce transplants, fewer saleable heads, and smaller head size at harvest occurred. They recommended daily misting with 26 mg-L'^ N when preplant fertilizer was incorporated or 78 mg-L"^ without preplant fertilizer. Experiments by Tremblay and Senecal (1988) as well as those of Masson et al . (1991a; 1991b) were conducted in Canada, while those of Guzman (1993) and Kratky and Mishima (1981) were conducted in Florida and Hawaii, respectively. Seasonal differences in photoperiod, light, and temperature, and cultural differences in frequency of irrigation and fertilization, as well as location and cultivar differences between the studies may explain why N recommendations for

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108 lettuce transplant growth, vary. In general, 350 to 400 mg-L"^ N resulted in reduced root growth (Tremblay and Senecal, 1988; Masson et al . , 1991a). Recommendations for optimum N levels for both lettuce transplant root and shoot growth are lacking. Optimizing root growth is important since transplants with well-developed root systems recover more quickly from transplant shock (Weston and Zandstra, 1986) , leading to greater earliness, improved head growth, and ultimately higher yields. The exact N nutritional needs for the production of a quality lettuce transplant, remain undefined. In the present investigation, a range of N concentrations were supplied via floatation irrigation to maximize lettuce transplant root growth in a tray cell, in an attempt to achieve a transplant that pulls from the cell, successfully establishes in the field, and leads to high yields of high quality heads. Materials and Methods Greenhouse Expe riments ^South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of 1.9 X 1.9 X 6.3 cm; 10.9 cm^ (length x width x depth;

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109 volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v) , with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg*m"^, was used for media. Four experiments were conducted during different seasons (Table 5-1) . The plants were grown Table 5-1. Sowing schedule and initial media test (Hanlon et al., 1994) for Experiments 1 to 4 . Expt Sowing date Media test' PH EC (dS-m'M NO3-N P K (mg* kg"'' Ca ) Mg 1 1 Sep 1993 4.7 0.9 1.3 12.4 14.6 14. 2 11 . 6 2 19 Nov 1993 4.6 0.1 1.0 1.3 14.3 1. 4 3. 1 3 30 Jul 1994 4.9 0.4 0.3 1.0 20.0 1. 9 4 . 3 4 10 Sep 1994 4.2 0.3 0.0 2.7 30.5 6. 6 16. 0 ^Concentrations in the saturated paste extract. with natural photoperiod extended to 16 h by 1000-W, highpressure sodium lamps (250 //mol -m"^ • s"'' photosynthetic photon flux) . A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just above the plant canopy, and media temperatures were recorded by a Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR). The flats were seeded, covered with a thin layer of vermiculite, overhead irrigated to moisten the vermiculite, and transferred to a cooler at 20 °C for germination. After 48 h, flats were returned to the greenhouse.

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Plants were irrigated every two to four days, by floating flats in nutrient solution containing N at 0, 15, 30, 45, or 60 mg-L"' as NH^NO,. The nutrient solution was absorbed by the media by capillary movement. Other nutrients were applied at equivalent rates to all plants and consisted of (in mg-L"^) 30 P, 30 K, 100 Ca, and half-strength Hoagland' s solution for micronutrients (Hoagland and Arnon, 1950) , that was comprised of Mg, S, B, Cu, Cl, Mo, and Zn. The experiments were arranged in a randomized complete-block design with 5 treatments and 4 replications. Plant samples, 5 per treatment, were taken approximately 14, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE) . Growth variables calculated were: root: shoot ratio (RSR = dry root mass hdry shoot mass), relative growth rate (RGR = [In (final total dry mass) In (initial total dry mass) ^ (final time initial time)]), net assimilation rate (NAR = [(final total dry mass initial total dry mass) ^ (final time initial time) X { (In (final leaf area) In (initial leaf area) } =(final leaf area initial leaf area)]), specific leaf area (SLA = leaf area ^ dry shoot mass), leaf area ratio (LAR = leaf area ^ total dry mass), leaf mass ratio (LMR = dry shoot mass ^ total dry mass), and root mass ratio (RMR = dry

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Ill root mass ^ total dry mass) (Hunt, 1978; 1982; Dubik et al . , 1992) . Dry shoot samples from the last sampling date were ground to pass a 20-mesh screen and acid-digested for total Kjeldahl N according to Wolf (1982) . Briefly, the digestion procedure involved weighing 0.25 g subsamples into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes that were then heated on a digestion block at 375 °C. After the digestion process was completed (a total of 2.5 h) , samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through *P8' filter papers (Fisher brand) , with a particle retention of > 25 //m, into 25 mL scintillation vials. The solution samples were then sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR) . Data were subjected to analysis of variance using the Statistical Analysis System (SAS institute. Inc., Gary, NC) . Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts. Field Experiment Plants from each treatment in Experiment 4 were transplanted into an Arredondo fine sandy soil (loamy,

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112 siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on-black polyethylene mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville, on 14 October 1994. The soil had a water pH of 6.1, with 0.1 dS-m'^ for electrical conductivity, and a nutrient content (Hanlon et al., 1994) of (in mg-kg'M 157 P, 50 K, 443 Ca, and 45 Mg (Mehlich-1 extractant) . The experiment was a randomized complete-block design with treatments replicated four times. Preplant fertilizer (13N0P-10.8K) was applied broadcast and incorporated in the bed at 230 kg-ha"''. Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. Plots consisted of 80 plants, planted on double offset rows, spaced 0.3 m between plants and between rows on the bed (equivalent to 54,000 plants per ha). Plants from the 0 N treatment were too small to transplant. Just after transplanting, 100 mL of nutrient solution (150 mg-L'^ 20N-8 . 6P-1 6 . 7K) was applied to each transplant hole as a starter fertilizer. Water was applied twice daily for 20 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark

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113 (1991) , Starting one week after transplanting, fertilizer at a rate of 15 kg-ha"^ N and 16 kg-ha"^ K, supplied from NH4NO3 and KNO3, was injected once weekly using a venturi pump (Netafim Irrigation, Altamonte Springs, FL) , with the last application one week before harvest to give a total amount of 150 kg-ha"^ N and 180 kg-ha"^ K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al., 1988). To determine optimum lettuce head maturity among the treatments, plants were harvested 53, 56, and 59 days after transplanting. At each harvest, 20 plants in a plot were cut, individually weighed, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements. Wrapper leaves were sampled a week before harvest for analysis of total Kjeldahl N as previously described for greenhouse experiments. Data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc., Gary, NC) . Treatment sums of squares were partitioned into linear and quadratic polynomial contrasts.

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Results 114 and Discussion Greenhouse Experiments Experiment 1 was conducted during the fall, under greenhouse temperatures ranging from 14 to 34 °C (Fig, 5-1) . The average daily maximum media temperature was 32 °C, while the average daily minimum media temperature was 21 °C. During the course of the trial, there were totals of 18 sunny days and 8 cloudy days . Fresh and dry shoot mass were least with 0 N and greatest with 60 mg-L''' N, regardless of sampling date (Table 5-2) . For plants sampled 15 DAS, fresh shoot mass increased in quadratic fashion in response to applied N, while for plants sampled 22 and 28 DAS, there was a positive linear response of fresh shoot mass to applied N. For plants sampled 15 and 22 DAS, there was a positive linear response of dry shoot mass to applied N, while for plants sampled 28 DAS, dry shoot mass increased in quadratic fashion in response to applied N. For plants sampled 15, 22, and 28 DAS, fresh root mass increased in quadratic fashion in response to applied N. Fresh root mass was greatest with 15 mg-L*^ N, 15 DAS. Nitrogen did not influence dry root mass by this sampling date. For plants sampled 22 and 28 DAS, dry root mass

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115 o o 40 3530UJ 25H OC D 1^ 20 cc liJ Q. 15i liJ I10-^ ai IB m T — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — I — r 29 GROWING PERIOD (SEP 5 SEP 29) air max media max — •— air min media min Fig. 5-1. Maximum and minimum air and media temperature during transplant production for Experiment 1, September 1993.

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116 Table 5-2. Root and shoot characteristics of lettuce transplants as affected by N nutrition for Experiment 1, September 1993. Nitrogen Fresh Dry Fresh Dry Leaf Leaf Root : applied shoot shoot root root area tissue shoot mass mass mass mass N ratio (mg-L-M (mg) (mg) (mg ) (mg) (cm^) (g-kg-M 15 Days After Sowing 0 40 2.5 29 1.5 1.5 0. 60 15 104 5 . 5 44 1.8 4.1 0.36 30 154 6 . 1 38 1.5 6.4 0.26 45 194 6 Q 39 1.7 8.3 0.25 60 196 6 9 36 1.8 8.4 0.34 o** T.** ±j o** NS Q** Q* 22 Dsvs After lJ IV ^ i i U 0 55 4.0 47 3 . 0 2 . 3 0.77 15 162 12.0 79 5.2 7.2 0.44 30 275 17.8 93 5.4 11.8 0.30 45 429 24.8 128 7.2 17.9 0.29 60 551 30.5 113 5.8 22.8 0.19 Response L** L** Q** Q** L** Q** 28 Days After Sowing 0 64 7.4 48 5.5 2.8 6.5 0.75 15 229 22.9 110 11.9 9.6 6.0 0.52 30 387 36.5 153 14.5 16.7 10.5 0.40 45 587 46.5 195 16.0 24.7 12.7 0.34 60 712 55.2 200 15.5 30.0 14.6 0.28 Response L** Q* Q** Q** L** L** Q** Linear (L) or quadratic (Q) effects significant at P = 0. 05 (*) 0.01 (**), or nonsignificant (NS) .

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117 increased in quadratic fashion in response to applied N. Dry root mass was least with 0 N and greatest with 45 mg-L"^ N. Added N increased shoot mass to a greater extent than root mass . Leaf area and fresh shoot mass response to applied N were similar (Table 5-2) . For plants sampled 28 DAS, there was a positive linear response of leaf tissue N to applied N. Leaf tissue N was least in plants grown with 0 or 15 mg-L''' N, and greatest in plants grown with 60 mg-L"' N, 28 DAS. In general, RSR values decreased in quadratic fashion to applied N, regardless of sampling date. With 0 N, there were greater RSR values, but the plants were extremely small for transplanting to the field. Aloni et al. (1991) reported that under low N levels, sucrose exported to the roots was rapidly hydrolyzed to support growth, presumably due to enhanced invertase activity. There was a positive linear response of RGR to applied N for plants grown to 22 DAS, but RGR values were unaffected by N for plants grown to 28 DAS (Table 5-3) . For plants grown to 22 DAS, NAR values were not influenced by applied N. However, for plants grown to 28 DAS, there was a negative linear response of NAR to applied N. Although NAR was greater in plants grown with 0 N, the total production of dry matter per plant over the same period was much greater in plants grown with added N.

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118 u o M-l to •p c o to 4-1 m •H 0 n 4J a 0 (0 (0 m a; E V4 c (0 p O <4-l to •H u ITJ (0 4J D 0) 10 (0 4J ^J E M 4-) d) <*-l O (Q O U M-l (0 •H -H (0 Q) 4-1 0) (0 « k:| (0 M •H U 0) JJ 0 (0 U u •H ro M-l jC •H U U M-l (0 (U (0 a (U -p w 10 * o V4 cn c 0 C -H o 4-) (0 c o •iH -H E •P r4 (U H • 4J m 4J M fO 0) to (0 4-) tn 2 (0 )-l 3 CTl C i-l 2 H o E ve a> •H x: Q) -P +j 4-J U 10 s 0) C 0) H 0 4-J (U H fit 3 oi H M-l -H C M +J c • (U ro E 1 -H c in U T5 00 01 >1 m Q 00 CM iH o CM in O O r-l .H tH q o to ^' -p 0) Q o CM 00 in ^ o vo ro O t-l CM CM '3' o q] CD O •sr O CT> ^ IX) 00 cn o CTl 0) CM CM in 00 ro v£) 00 r~00 CTl «) c; o 0) in o in o ro ^ >x> O O O iH o CM r~ ^ ro rH ^ o ro CO 2 00 cn CM CM
PAGE 127

119 Both SLA and LAR increased in response to applied N, regardless of sampling date (Table 5-3) . There was a positive linear response of SLA to applied N for plants sampled 15 and 22 DAS. For plants sampled 28 DAS, SLA increased in quadratic fashion to applied N. Leaf area ratios increased in linear fashion to applied N, regardless of sampling date. The reduction in SLA and LAR for plants grown with 0 N reflects the reduction in both leaf size and assimilate production (Dubik et al,, 1990). Leaf mass ratio increased in quadratic fashion, while RMR decreased in quadratic fashion to applied N, for plants sampled 15, 22, and 28 DAS. Although both shoot and root mass increased in response to applied N, the increase in shoot mass was greater than root mass, resulting in lower RMR values. Plants grown with 0 N allocated approximately 57 % of the dry matter to shoots and 43 % to roots, 28 DAS, while plants grown with 60 mg*L"^ N allocated approximately 78 % of dry matter to shoots and only 22 % went to roots, indicating that added N shifted dry matter partitioning from roots to shoots. Nitrogen was important for building a bigger plant, but shoot biomass production was favored over root biomass production. Experiment 1 indicated that at least 60 mg-L'^ N supplied via floatation irrigation, was required for

PAGE 128

120 increased transplant root and shoot growth in a peat+vermiculite mix low in NO3-N. In order to further test this conclusion, Experiment 2 was conducted during the winter instead of fall, under greenhouse temperatures ranging from 6 to 27 °C (Fig. 5-2) . The average daily maximum media temperature was 23 °C, while the average daily minimum media temperature was 11 °C. During the course of the experiment, there were totals of 19 sunny days and 8 cloudy days, with rain during two of the cloudy days . For plants sampled 15 DAS, fresh shoot mass increased in quadratic fashion in response to applied N (Table 5-4). Fresh shoot mass was least for plants grown with 0 N and greatest for plants grown with 60 mg-L"^ N. For plants sampled 21 and 28 DAS, there was a positive linear response of fresh shoot mass to applied N. Dry shoot mass increased in quadratic fashion in response to applied N for plants sampled 15, 21, and 28 DAS. Fresh root mass increased in quadratic fashion for plants sampled 15, 21, and 28 DAS. Fresh root mass of plants sampled 15 DAS increased in response to applied N from 0 to 30 mg-L"^, thereafter fresh root mass was unaffected. There was a positive linear response of dry root mass to applied N for plants sampled 15 and 21 DAS. For plants sampled 28 DAS, dry root mass increased in quadratic fashion in response to

PAGE 129

121 Fig. 5-2. Maximum and minimum air and media temperature during transplant production for Experiment 2, November /December 1993.

PAGE 130

122 Table 5-4. Root and shoot characteristics of lettuce transplants as affected by N nutrition for Experiment 2, Nov/Dec 1993. Nitrogen Fresh Dry Fresh Dry Leaf Leaf Root : applied shoot shoot root root area tissue shoot mass mass mass mass N ratio (mg-L-M (mg) (mg) (mg) (mg) (cm^) (g-kg-i) 15 Days After Sowing 0 44 3.9 37 0.7 1.8 0.17 15 106 6.5 54 1.2 4.1 0.18 30 170 9.1 62 1.8 6.3 0.20 45 233 11.3 58 2.6 8.6 0.23 60 265 11 . 4 63 2.6 9.7 0.23 O* 0** Q* L** Q* NS im/CL y ^ ^ ^ -iSowino n 8 . 4 51 4 . 4 2 . 1 0.53 15 175 16.6 92 6.9 6.3 0.42 30 323 27.7 121 8.5 11.6 0.31 45 442 32.1 142 9.5 16.0 0.30 60 547 38.2 165 11.3 18.9 0.30 Response L** Q* Q* L** Q* Q** 28 Days After Sowing 0 68 9.7 62 5.5 2.4 7.0 0.58 15 280 33.0 143 10.7 9.7 10.0 0.33 30 468 46.9 185 14 . 0 16.5 12.8 0.30 45 689 55.7 217 15.8 24.5 15.9 0.28 60 909 66.8 250 15.4 31.5 17.4 0.23 Response L** Q* Q* Q** L** L** Q* Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS) .

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123 applied N. Dry root mass was least for plants grown with 0 N, regardless of sampling date. Leaf area and fresh shoot mass response to applied N were similar. For plants sampled 28 DAS, leaf tissue N increased in linear fashion in response to applied N. Root: shoot ratios of plants sampled 15 DAS were unaffected by applied N, For plants sampled 21 and 28 DAS, RSR values decreased in quadratic fashion to applied N, indicating once again that added N caused more dry matter to be partitioned into shoots rather than to roots . Relative growth rate of plants grown to 21 DAS was unaffected by N (Table 5-5) . For plants grown to 28 DAS, RGR responded in quadratic fashion to applied N, and was least for plants grown with 0 N and greatest for plants grown with 15 mg-L"^ N. For plants grown to 21 or 28 DAS, NAR responded in quadratic fashion to applied N. In general, NAR of plants grown to 21 DAS decreased at all levels of applied N, but was greatest in plants grown with 15 mg-L'^ N, 28 DAS. For plants sampled 15 and 28 DAS, SLA and LAR increased in linear fashion in response to applied N. For plants sampled 21 DAS, SLA and LAR increased in quadratic fashion to applied N. Specific leaf area and LAR of plants at 21 DAS increased in response to applied N from 0 to 45 mg-L"^ thereafter SLA and LAR were unaffected. The reduction in SLA and LAR for plants grown with 0 N reflects the reduction in

PAGE 132

124 o 0 o V) -H CO 4-1 6 u 0 M m -H (0 to 4-1 (U nj nj U-t (0 (0 0) 0) ^ u •H IW H U U-l (U rt) a (u c o -H 4-1 nJ H •H -H 4-) to 0) to 2 03 > -H x; 4J (0 e (0 <\j M u (0 ^ (tJ c 0) Oi 4-1 e E c 0) T3 O -H H H 4-1 a •H a 2 ro in ix) (Ti (Ti iH iH .H tH rH o o o o o 2 vo in T-i 1-1 00 C£> 00 00 00 o o o o o 2 (Ti n n n m og cN rvj cvj * + O O O O O O «x) th r-~ r~ r^ t~ c~* + o o o o o o vD ^ n oj CTi n eg CN (M o o o o o o ^ yj r00 rH «5 t~ r~r00 o o o o o •ain rH CTl IX) ID o o o o o t~ (N 00 CNj n n o o o o o + eg CM o o o o o tr. c; •r1 o to HJ Q cn IX) in vc 00 o o o o o c: 0 to (j 5 in 00 eg o o 4^] eg m ^ in in * u, * o o o o o o to q O to 00 CM o in ^ 00 o o o o o in ^ n CM CNj eg o 00 CTl 00 CM in o in r~ in n CM 00 n o o O rH CM rH CM CO 2 eg eg "a" o vD in in in o o o o o O iH in o in o en ^ VD m c o a CO O rH in o in o m vo 0) CO c o a to 0) OS O rH in o in o n ^ u 4-1 (0 4-^ c l-l <0 (J H *H •H C •H CO CO 4J O (U o *M d) o o •H 4J (0 * * T3 CO O" (TJ 2 4J 14 c o m u -H (U •r| to c c M CP o (0 -H a (1) to to C c -H o 1^ c

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125 both leaf size and assimilate production (Dubik et al., 1990) . For plants sampled 15 DAS, LMR and RMR were unaffected by added N. For plants sampled 21 and 28 DAS, LMR increased in quadratic fashion, while RMR decreased in quadratic fashion in response to applied N. Plants grown with 0 N allocated approximately 64 % of the dry matter to shoots and 36 % to roots, 28 DAS, while plants grown with 60 mg-L"^ N allocated approximately 81 % of dry matter to the shoots and only 19 % went to roots. Twice as much dry matter was allocated to roots when plants were grown with 0 N than with 60 mg-L"^ N. Similar results were obtained in Experiment 1 (fall), indicating that temperature differences did not alter dry matter allocation between shoots and roots. Once again, 60 mg-L"^ N, supplied via floatation irrigation to a peat+vermiculite media low in NO3-N, led to more shoot and root growth. Experiment 3 was conducted during the summer, instead of fall or winter, under greenhouse temperatures ranging from 19 to 39 °C (Fig. 5-3) . The average daily maximum media temperature was 31 °C, while the average daily minimum media temperature was 20 °C. During the course of the experiment, there were totals of 18 sunny and 10 cloudy days, with rain during six of the cloudy days.

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126 35o 300 25oc D AT 20cc lU Q. 15lU 11050GROWING PERIOD (AUG 3 AUG 28) air max media max — •— air min -bmedia min Fig. 5-3. Maximum and minimum air and media temperature during transplant production for Experiment 3, August 1994.

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127 For plants sampled 15, 22 and 29 DAS, fresh and dry shoot mass increased in linear fashion in response to applied N (Table 5-6) . For plants sampled 15 and 29 DAS, fresh root mass increased in linear fashion to applied N. Fresh root mass of plants sampled 22 DAS increased in quadratic fashion to applied N. Fresh root mass increased in response to applied N from 0 to 45 mg•L"^ thereafter fresh root mass was unaffected. Dry root mass was least for plants grown with 0 N and greatest for plants grown with 60 mg-L"^ N, regardless of sampling date. For plants sampled 15 DAS, there was a positive linear response of dry root mass to applied N, while for plants sampled 22 and 29 DAS, dry root mass increased in quadratic fashionin response to applied N. For plants sampled 15, 22, and 29 DAS, leaf area increased in linear fashion to applied N. Leaf area and fresh shoot mass responses to applied N were similar. For plants sampled 29 DAS, leaf tissue N increased in quadratic fashion in response to applied N. Leaf N concentration was least in plants grown with 0 N, and greatest in plants grown with 60 mg-L'^ N. Therefore, leaf tissue N was related to increased root and shoot growth. Root: shoot ratios of plants sampled 15, 22, and 29 DAS decreased in quadratic fashion to applied N. With 0 N, there were greater RSR values, but the plants were extremely small for transplanting to the field. Aloni et al. (1991) reported that under low N levels.

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128 Table 5-6. Root and shoot characteristics of lettuce transplants as affected by N nutrition for Experiment 3, August 1994. Nitrogen Fresh Dry Fresh Dry Leaf Leaf Root: applied shoot shoot root root area tissue shoot mass mass mass mass N ratio (mg-L'M (mg) (mg) (mg) (mg) (cm^) (g-kg'M 15 Days After Sowing 0 40 4 . 6 41 3.0 1 A 1 . 4 0 . 66 15 108 8 . 3 50 3.5 4 . 1 0.43 30 173 11 . 8 64 4.5 7 . 2 0 . 38 45 224 13 . 9 77 4.6 Q Q n "a o u . a 60 292 16.8 62 5.1 12.1 0.31 Response L** L** L* L** L** Q** 22 Days After Sowing 0 52 7.0 49 4.7 1.8 0.67 15 184 17.1 112 9.0 6.3 0.53 30 330 27.2 132 10.7 12. 6 0.39 45 450 34.1 159 11.7 17.7 0.34 60 599 43.6 154 12. 6 24.2 0.29 Response L** L** Q* Q** L** Q** 29 Days After Sowing 0 61 9.3 56 6.0 1.9 6. 3 0.65 15 267 28.9 139 14.0 9.0 10. 6 0.49 30 488 45.4 189 19.3 17.8 12. 7 0.43 45 725 61.7 279 20.8 27. 9 14. 6 0.34 60 960 72.6 302 22.3 36.6 17. 3 0.31 Response L** L** L** Q** L** Q* Q* Linear (L) or quadratic (Q) effects significant at P = 0.05 (*) or 0.01 (**) .

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129 sucrose exported to the roots was rapidly hydrolyzed to support growth, presumably due to enhanced invertase activity. For plants grown to 22 or 29 DAS, RGR increased in quadratic fashion in response to applied N (Table 5-7) , However, the RGR values were lower by 29 than by 22 DAS. For plants grown to 22 or 29 DAS, NAR decreased in linear fashion to applied N. Net assimilation rates were least with 60 mg'L"^ N, but the total dry matter production was much greater at this N level compared to other N levels. For plants sampled 15 and 22 DAS, SLA and LAR increased in quadratic fashion to applied N, while for plants sampled 29 DAS, SLA and LAR increased in linear fashion to applied N. The reduction in SLA and LAR for plants grown with 0 N reflects the reduction in both leaf size and assimilate production (Dubik et al., 1990). Leaf mass ratio increased in quadratic fashion, while RMR decreased in quadratic fashion in response to applied N, for plants sampled 15 and 22 DAS. For plants sampled 29 DAS, LMR increased in linear fashion, while RMR decreased in linear fashion in response to applied N. Plants grown with 0 N allocated approximately 61 % of the dry matter to shoots and 39 % to roots, 29 DAS, while plants grown with 60 mg-L"^ N allocated approximately 77 % of dry matter to shoots and only 23 % went to roots, indicating once again that added N

PAGE 138

130 u o m C m rH a m c m u -p 0) u 3 P P 0) o m o •H M m -H +J o n> M (0 x: u x: » o »4 tr c o c o +J •rH U +J D C "a* a\ 2 -H iw 0 4-1 m 9) 3 U Di C D 0) iH ^ M-i n c H 4-1 C • 0) tE 1 -H in M 0) 0) a m 0 4J to •H O to 4-) O 03 (B a: E M o U-l to -H (0 to 4-1 0) ro 03 h^I E M o <4-l fO -H m 0) 4J CP OJ u u ifi •mg' u M-4 03 f>j 03 (U E a (U M u H m c 0 H 4J (0 E -H 0) 4-1 to 4-> 0) to 03 2 03 M 0) > H x; 4J 4-) (1) D> E M 03 E c tt) T3 Di 0) O -H M rH a 2 03 0^ E o o oD in n ^ n eg cNj CN * * o o o o o o o o cN in r~ >x) rt~ r~ -x + o o o o o c o in CD ID cvj ^ n CN CM Cvl o o o o o o in eg T 03 IX) ic o o o o o * iTi n o in n ro m n CM cvj * * O O O O O rH 1^ o in rvc ID r~ r~ r-K + O O O O O in o o o o ID in * • + o c CM o o o o o •K * CH CD CM o o o o c; -I 1^ 0 to ^; 4J ' to 03 Q O rH in VD VD t~ o o o o o c; •-t :^ o to 5) VD (X) VD CM ^ -f i^j CM (») 'S" in in + 0,^00000 to Q CM (Tl • * O ^^ m O iH o in o PO
PAGE 139

131 shifted dry matter partitioning from roots to shoots. Nitrogen was important for building a bigger plant, but shoot biomass production was favored over root biomass production. Once again, the highest N level used (60 mg-L'M, supplied via floatation irrigation to a peat+vermiculite media low in NO3-N, led to more shoot and root growth. Neither of the experiments conducted so far were carried to the field. Experiment 4 was conducted to evaluate the effects of N on transplant growth and subsequent yield and lettuce quality in the field. Experiment 4 was conducted during the fall, similar to Experiment 2, under greenhouse temperatures ranging from 14 to 33 °C (Fig. 5-4) . The average daily maximum media temperature was 28 °C, while the average daily minimum media temperature was 19 °C. During the course of the experiment, there were totals of 15 sunny and 13 cloudy days, with rain during seven of the cloudy days. For plants sampled 15 DAS, fresh and dry shoot mass increased in quadratic fashion to applied N (Table 5-8) . Fresh and dry shoot mass were least with 0 N and greatest with 60 mg-L'^ N. For plants sampled 22 and 29 DAS, fresh and dry shoot mass increased in linear fashion in response to applied N. For plants sampled 15 and 22 DAS, fresh root mass increased in quadratic fashion in response to applied

PAGE 140

132 Fig. 5-4. Maximum and minimum air and media temperature during transplant production for Experiment 4, September/October 1994.

PAGE 141

133 3 <«-i m -P i-4 (U 4-) dJ V -H W T3 4-1 4-1 r C iH Q) ou x; 0) M-l (tJ u m 0) D at 0) E 4-1 r-l (0 u 4-1 (U U .H Q) x> 14-1 O O 4J 4J to U >i O (0 cn to O O to E U \ Q u E •H M 0) to -H 1 to 4-1 to (U 4-1 9) O to 4-1 a U 0 to _E U [u U E to w to x; u 4-) 4J 0 to 4-1 C >i o to 0 0) t 0) 4-) O •H M •-f C 4-1 a CP to •H a EH z to CTl CX5 »H , Ci in cri CM o CTi iH '9< en CM n Cn 03 iH 00 *x> n 00 n T-l iH CM <» 1-1 (Tl O 00 * * 00 00 CM O cTi in in 1" + * ID r-n r~ i-q rH CM n m 00 n 00 CM iH CTl 00 iH CM CM 01 in o in o 00 T vo Q> to 0 (U in o in (TI in CO in r~ CM £) «3< VD 00 ^ CM t~ 00 00 ^ CM "S" r~
PAGE 142

134 N. There was a positive linear response of fresh root mass to applied N for plants sampled 29 DAS. For plants sampled 15, 22, and 29 DAS, dry root mass increased in quadratic fashion in response to applied N. Fresh and dry root mass were least in plants grown with 0 N, regardless of sampling date. Leaf area, transplant height, transplant stem diameter, leaf tissue N, and fresh and dry shoot and root mass responses to applied N were similar. For plants sampled 15, 22, and 29 DAS, RSR values decreased in linear fashion in response to applied N (Table 5-9) , With 0 N, there were greater RSR values, but the plants were extremely small. Aloni et al. (1991) reported that under low N levels, sucrose exported to the roots was rapidly hydrolyzed to support growth, presumably due to enhanced invertase activity. Applied N did not influence RGR values for plants grown to 22 DAS. For plants grown to 29 DAS, RGR increased in linear fashion in response to applied N. For plants grown to 22 DAS, NAR values decreased in quadratic fashion, while for plants grown to 29 DAS, NAR decreased in linear fashion in response to applied N. Although NAR was greater in plants grown with 0 N, the total production of dry matter per plant over the same period was much greater in plants grown with added N.

PAGE 143

135 u 0 o IH 4-* IT 0 to 4-> n 0 (0 to jj a: £ U c m H a CQ 0 c MH CQ -H la nJ CQ 4-1 u ill ril ID •p E u 0) o D +> 1 o ? rH 14H tC H E m (u P ^" M-l 10 E O (0 m u -H P is •H IM 1 Q> C2 u •H C U Mh nj eg 4-1 0) to oj e u a (u n> CO rH u (0 u c 0 x: •H p to o rH M -H 'c Cr> E K n H P V > ^* a 0 t c o 4J 4-1 \ ttJ s (U £ 2 M H 0 4J ' (U (U M iM n OS Oi O E ~ — (U u a C (U .. jj 0 0) w 4J O H O 0 p rH w 0 x: to a: to u c c • (U (T, g 1 -H c in M 0) D< 0) 1 0) a O -rl H X U^ rH 4-1 a cn (0 H a B 2 ro CM n oD n cNj ro n cvj cvj cvj + + o o o o o i-q 00 r~ csj r~ 03 ID rrr~ * + o o o o o t~ O CnJ • in in VD + o o o o o o in cTi in m m oj CNj cH tu 4J >*H to m ^ rH tX) in T CM ^ ^ in ID -Ic * o o o o o (.q n CM 00 m in 00 1^ 00 in 'J' tn q •rl 0 to ^ u 53 CQ ^' CM in m 00 CM n ^ in in o o o o o CM VD o O '3> CM C\,rOCMrHrHrHOlQ^CMCMrHrHrH CN CN ID ^ tH ID r~ t~(~ o o o o o to 2 00 CTl ^ ^ n t») CM * * O O O O O iJ) ID CQ o O rH in o in o n «s< VD rin rH ID n ill in n m * * O O O O O (J) 0) CQ o c to u H MH H c CP •H to CQ 4J U i)
PAGE 144

136 For plants sampled 15 DAS, SLA and LAR increased in quadratic fashion in response to applied N (Table 5-9) . There was a positive linear response of SLA and LAR to applied N for plants sampled 22 DAS. For plants sampled 29 DAS, SLA increased in quadratic fashion, while LAR increased in linear fashion to applied N. The reduction in SLA and LAR for plants grown with 0 N reflects the reduction in both leaf size and assimilate production (Dubik et al . , 1990). For plants sampled 15, 22 and 29 DAS, LMR increased in linear fashion, while RMR decreased in linear fashion in response to applied N. Plants grown with 0 N allocated approximately 64 % of the dry matter to shoots and 36 % to roots, 29 DAS, while plants grown with 60 mg-L"^ N allocated approximately 76 % of dry matter to shoots and only 24 % went to roots, indicating that added N shifted dry matter partitioning from roots to shoots. Nitrogen was important for building a bigger plant, but shoot biomass production was favored over root biomass production. Similar results were reported by Dufault (1985) for celery transplants. The four sowing dates were used in an attempt to assess the consistency of N treatment effect with time. Average daily maximum media temperatures were 32, 23, 31, and 28 °C, while average daily minimum temperatures were 21, 11, 20, and 19 °C for Experiments 1, 2, 3, and 4, respectively. Fresh and dry shoot mass, and fresh root mass were similar

PAGE 145

137 among the four experiments, indicating that temperature and light variations (as long as photoperiod was extended to 16 h) had minimal influence on growth. On the other hand, N nutrition had a great impact on lettuce transplant root and shoot growth. Leaf tissue N values increased in response to applied N, regardless of season. In Experiment 1, leaf tissue N ranged from approximately 6 to 15 g-kg"^, while in the other 3 experiments, it ranged from 6 to 17 g-kg'^. Although both shoot and root mass increased in response to applied N, the increase in shoot mass was greater than root mass, resulting in lower values of RSR and RMR. By the last sampling date, RSR values ranged from 0.57 to 0.75 with 0 N, and from 0.23 to 0.32 with 60 mg-L"^ N. These values were, therefore, relatively similar regardless of season. Relative growth rate, NAR, SLA, LAR, and LMR increased in response to applied N, indicating improved growth with N. It was observed that transplants could not be easily pulled from the transplant flat at all levels of applied N in these experiments. When the mean dry root mass was less than 20 mg, pulling success was observed to be even more reduced. Nitrogen at 60 mg-L"^ N was perhaps not adequate with the irrigation programs used. Therefore, additional experiments (Chapter 6) were designed to investigate the

PAGE 146

138 effect of N fertilization to 120 mg-L"' and fertigation frequency on lettuce transplant growth and development. Masson et al. (1991a) reported that 400 mg-L'^ N improved lettuce transplant shoot growth compared to 100 mg*L"\ but adversely affected dry root mass and RSRs. Tremblay and Senecal (1988) reported that 350 mg-L"^ enhanced lettuce transplant shoot growth compared to 150 mg-L''' N, but reduced dry root mass. The present experiments have demonstrated that for lettuce transplant promote root growth promotion, lower levels of N than reported by these authors seemed appropriate. Field Experiment To determine the influence of transplant conditioning on harvest maturity, plants from Experiment 4 were planted in the field. The optimum time to harvest was determined to be 56 days after transplanting (DAT) as previously described in the Materials and Methods. After the first harvest at 53 DAT (Appendix Table C-9) , plants still continued to increase in mass up to 56 DAT. Thereafter, there was no appreciable increase in head mass, and some lettuce heads started splitting because they were overmature. Lettuce heads at 59 DAT were also of poor quality due to elongated cores. Therefore, only results at 56 DAT will be discussed.

PAGE 147

139 Lettuce head mass increased in linear fashion in response to pretransplant N (Table 5-10) . The least head mass was obtained in plants grown with 15 mg-L"^ N, while the heaviest heads were from plants grown with 60 mg-L"'' N, even though all treatments received the same postplant N fertilizers and no differences in tissue N among treatments were found at harvest. Transplants grown with 60 mg-L"^ N had the greatest shoot and root mass, 29 DAS. Therefore, the bigger the plant at planting, the greater the yield. Similar results were presented in Chapters 3 and 4. In those experiments, not only did bigger plants improve yield, but also promoted early maturity. This is of particular significance in northern Florida where the growing period is shortened by either low or high temperatures. Low temperatures (fall plantings) could result in lettuce heads freezing, while high temperatures (spring plantings) could result in premature bolting. Masson et al. (1991b) reported that 4 00 mg-L"^ N improved lettuce head mass at harvest and promoted early maturity compared to 100 mg-L"^. However, N at 400 mg-L'^ adversely affected dry root mass and RSRs (Masson et al., 1991a). Pretransplant N did not influence head firmness or head diameter. Head height and core length increased in quadratic fashion to pretransplant N. Stem width increased in linear fashion to pretransplant N. The response of stem width and

PAGE 148

to B n (0 (D 0) u 4-1 -P C O C O ' •H ^ 4-) CTl u cn T3 O ^4 U 0) -P OJ c u (B 0) rH Q a CO cr> c It) Tl M 0) -U 4-) to Cn 0) c: > H M (0 C to 0 u -H -H U 4-) •H to M -H 4-1 U :3 0) C 4-1 u O U to 4J >i U 4-> Q) -H to IT) I "C to (H to tU (0 iX) ix" 00 00 r~ 00 en m cn O iD ^ ^ ^ ^ ^ 4< CNj n n C\J CM CM CM CO n vo rLO 2 CM CM CM CnJ tn r~ tTi O t^ 00
PAGE 149

141 core length to pretransplant N paralleled the response of lettuce head mass and head height to pretransplant N, respectively. At harvest, tissue N levels were equal regardless of pretransplant N applied. Hochmuth et al. (1991) reported values of 20 to 30 g-kg'' (soil type not reported) to be indicative of an adequate range for crisphead lettuce. The values of tissue N in this experiment were about 38 g-kg"\ indicating that sufficient N was supplied to the plants. The present work demonstrated that added N up to 60 mg-L"'' improved shoot and root growth, leading to a bigger transplant. A bigger plant (approximately 80 mm tall, with fresh shoot mass of 950 mg and fresh root mass of 250 mg) at transplanting led to earliness and improved head mass at harvest . Summary ^South Bay' lettuce transplants were produced with different levels of N to evaluate how much N was necessary to produce high quality transplants, and subsequent high quality crop in the field. In this study, a quality or ideal transplant was one which could be produced in the shortest period of time, having sufficient roots to fill a tray cell to facilitate ease of pulling from the transplant flat.

PAGE 150

142 Plants were fertigated by floating flats in nutrient solution containing N at 0, 15, 30, 45, or 60 mg-L"'. To avoid inconsistency in the duration of the light period, natural photoperiod was extended to 16 h. Increasing N from 0 to 60 mg-L"^ resulted in greater transplant shoot and root mass. The increase in shoot mass was much greater than for root mass, resulting in lower values of RSR and RMR due to applied N. Relative growth rate, SLA, LAR, and LMR, increased with applied N, suggesting improved transplant growth at higher N levels. Growth responses of lettuce transplant shoots and roots to applied N were consistent, regardless of season or stage of growth. Leaf tissue N always was increased by applied N. Lettuce head mass was improved at harvest by pretransplant N. The heaviest heads were obtained from plants grown with 60 mg-L"^ in the greenhouse. In the greenhouse, transplants grown with 60 mg-L'^ also had the greatest shoot and root mass, RSR of approximately 0.3, and leaf tissue N of about 17 g-kg"^ Since transplants at all levels of N could not be easily pulled from the transplant flat, further investigations are needed to relate pull force and pulling success to N nutrition of transplants. This work demonstrated that at least 60 mg-L"^ N supplied via floatation irrigation, was required for improved transplant shoot and root growth in a

PAGE 151

143 peat+vermiculite mix low in NO3-N. Transplants grown with 60 mg'L"-' N compared to 15 mg-L"-' N, were bigger at transplanting and resulted in improved head mass at harvest.

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CHAPTER 6 PROMOTION OF LETTUCE TRANSPLANT ROOT DEVELOPMENT BY PROPER MANAGEMENT OF NITROGEN AND IRRIGATION Introduction Unsatisfactory results in stand establishment of direct-seeded lettuce crops using both pelleted and raw seed, particularly during conditions of environmental stress, has led to the use of transplants as a means of establishing economically viable plant stands (Cliffe, 1989). Guzman et al. (1989) found that superior plant stand was the major factor resulting in increased marketable yields from transplanted crisphead and romaine lettuce. They concluded that perhaps growers in south Florida, with harsh and unreliable weather, could minimize economic losses and become more reliable suppliers of lettuce if a portion of the lettuce crop was transplanted. According to Klassen (1986), other reasons growers were transplanting rather than direct-seeding included better plant-to-plant uniformity especially for a once-over harvested crop such as lettuce, early season weed control, more precise spacing of plants, and elimination of the need to thin densely seeded rows. 144

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145 Containerized vegetable transplants grown in greenhouses can either be overhead irrigated, or subirrigated. Using subirrigation (floatation system) , Leskovar and Cantliffe (1993) improved uniformity and quality of pepper transplants, compared to using overhead irrigation. When drought stress and root pruning methods were used to harden and prevent stem elongation in freshmarket tomato transplants grown with a floatation system, an increase in lateral root elongation and a decrease in shoot: root ratio were reported (Leskovar et al . , 1994). A reduction in shoot: root ratio and an improvement in wateruse efficiency of pepper transplants were also reported by Leskovar and Heineman (1994) when plants were produced via the floatation system of irrigation. However, growers have not been able to produce the highest quality lettuce transplants on a seasonal basis using the floatation system. A well developed root system is essential so that transplants can be easily pulled from the transplant flat, or pushed out utilizing a mechanical transplanter. If shoots are too long, the plants will tend to fall over, resulting in easily damaged plants and scorched leaves especially when transplanted onto plasticmulched beds. If shoots are too short, they cannot be easily handled and can be trapped under plastic mulch. When using the floatation system of irrigation, careful management of

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146 fertilization is important since large amounts of fertilizers, especially N, can greatly increase lettuce transplant shoot growth at the expense of root growth (Tremblay and Senecal, 1987; 1988; Masson et al., 1991a). This study was conducted (a) to determine the optimum fertigation frequency and optimum N concentration for maximizing lettuce transplant root growth when N was supplied via the floatation irrigation system, and (b) to determine if N applied at different times during transplant growth, was a factor in promoting root growth. Materials and Methods Greenhouse Expe riments ^South Bay' lettuce transplants were grown in a glass greenhouse at the University of Florida, Gainesville, FL. Speedling styrofoam planter flats, model F392A [392 cells of 1.9 X 1.9 X 6.3 cm; 10.9 cm^ (length x width x depth; volume)], were used for growing plants. A peat+vermiculite+styrofoam bead mix (1:2:1, v/v/v) , with AquaGro wetting agent (Aquatrols, Cherry Hill, NJ) at 0.2 kg'm"^ was used for media. Five experiments were conducted during different seasons (Table 6-1) . The plants were grown with natural photoperiod extended to 16 h by 1000-W, highpressure sodium lamps (250 /imol -m'^ • s"^ photosynthetic photon

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147 Table 6-1. Sowing schedule and initial media test (Hanlon et al . , 1994) for Experiments 1 to 5. Expt Sowing date Media test" PH EC (dS-m-M NO3 -N P K (mgkg"MCa Mg 1 18 Jun 1994 5.1 0.1 0 1 . 0 16.6 1.3 4.0 2 23 Feb 1995 5.0 0.1 0 0.6 12.6 1.1 4.5 3 27 Jun 1995 4.8 0.1 0 0.5 13.1 1.2 4.3 4 18 Sep 1995 4.4 0.3 0 0.3 20.9 1.6 3.9 5 02 Apr 1996 4.5 0.2 0 0.4 23.0 0.8 9.9 ^Concentrations in the saturated paste extract. flux) . A record of cloud cover was kept as an indication of the evaporative demand of the atmosphere. Greenhouse air temperature just above the plant canopy, and media temperatures were recorded by a Series 3020T Datalogger (Electronic Controls Design, Inc., Mulino, OR). The flats were seeded, then covered with a thin layer of vermiculite, overhead irrigated enough to moisten the vermiculite, and transferred to a cooler at 20 °C for germination. After 48 h, flats were returned to the greenhouse. In Experiment 1, the effect of irrigation frequency on lettuce transplant growth was evaluated with four treatments, consisting of 0, 20, 40, and 60 % moisture deficit from field capacity (FC) . Flats were weighed twice daily, and irrigation was performed when the flats had lost a predetermined amount of moisture. Irrigation was by means of floating flats directly in water or nutrient solution. All the treatments were fertigated seven times during the

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148 duration of the trial. Fertilizers (in mg-L"') consisted of 100 N, 30 P, 30 K, 100 Ca, and half -strength Hoagland' s solution for micronutrients only (Hoagland and Arnon, 1950), that was comprised of Mg, S, B, Cu, CI, Mo, and Zn. The experiment was a randomized complete-block design with 4 treatments and 4 replications. Plants in Experiments 2, 3, and 4 were grown with N at 0, 30, 60, 90, or 120 mg-L'^. Other nutrients were applied at equivalent rates to all plants and consisted of (in mg-L'M 30 P, 30 K, 30 Ca, and half-strength Hoagland' s solution for micronutrients only (Hoagland and Arnon, 1950) . Fertigation frequency treatments were sub-irrigated daily, or sub-irrigated every second, third, or fourth day. The experiments were arranged in a randomized complete-block design with 20 treatments consisting of a factorial combination of 5 levels of N and 4 levels of fertigation frequency in 4 replications. Plants in Experiment 5 were grown with N at 0 or 60 mg*L'^ applied at four different times using sub-irrigation. The first 60 mg-L'^ N was applied every other day for the first 14 days, then no further N was applied. The second N treatment was applied every other day only during the last 14 days of a 28-day growing period. The third N treatment was applied every fourth day, while the fourth N treatment was applied every other day for a 28-day growing period.

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149 Other nutrients were supplied every other day as described for Experiment 2. The experiment was a randomized completeblock design with 5 treatments and 4 replications. Plant samples, 5 per treatment, were taken at 13, 21, and 28 days after sowing (DAS) for growth measurements. Measurements included shoot and root fresh and dry mass, and leaf area (measured by a LI-3100 leaf area meter; LI-COR, Lincoln, NE) . Growth variables calculated were: root: shoot ratio (RSR = dry root mass dry shoot mass), relative growth rate (RGR = [In (final total dry mass) In (initial total dry mass) ^ (final time initial time)]), net assimilation rate (NAR = [(final total dry mass initial total dry mass) ^ (final time initial time) x { (in (final leaf area) In (initial leaf area) } (final leaf area initial leaf area) ] ) , specific leaf area (SLA = leaf area dry shoot mass), leaf area ratio (LAR = leaf area ^ total dry mass), leaf mass ratio (LMR = dry shoot mass ^ total dry mass), and root mass ratio (RMR = dry root mass ^ total dry mass) (Hunt, 1978; 1982; Dubik et al . , 1992). At the last sampling date in Experiment 5, fresh roots were scanned with a Hewlett Packard desktop scanner and analyzed with MacRHIZO software (Regent Instruments Inc., Quebec, Canada) at 300 dpi for length, area, and diameter. Additionally, pull force, the force required to pull a lettuce transplant out of a flat using Model DPP Dial Push-

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150 Pull Gauge (John Chatillon and Sons, Kew Gardens, NY) attached to a binder clip, was measured. Pulling success was calculated as the percentage of 5 plants per treatment that could be pulled from the transplant flat without any breakage . Leaf petioles were collected 28 DAS in Experiment 5, for sap testing. The sap was squeezed from collected petiole pieces using a hydraulic sap press onto sampling sheets according to Hochmuth (1992) . A CARDY meter (Spectrum Technologies, Inc., Plainfield, IL) was used to measure NO3-N concentrations in the petiole sap. Dry shoot samples from the last sampling dates were ground to pass a 20-mesh screen and dry-ashed for P and K in Experiment 1 or acid-digested for total Kjeldahl N in all experiments according to Wolf (1982) . For total P and K determination, 0.5 g subsamples were weighed into 10 mL beakers. The samples were then dry-ashed in a muffle furnance at 500 °C for 10 h. The ash was moistened with 1 N HCl and poured into 50 mL volumetric flasks, and brought to voliame with 1 N HCl. The solutions were filtered through 'Q8' filter paper (Fisher brand), with a particle retention of > 10 //m, into 25 mL scintillation vials. The solution samples were sent to the Analytical Research Laboratory, University of Florida, and analyzed with Model 61-E

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151 Inductively Coupled Plasma Spectrometry (Thermo Jarrell Ash Corporation, Franklin, MA) . For total Kjeldahl N, 0.25 g subsamples were weighed into 50 mL digestion tubes. Sulfuric acid and 30 % hydrogen peroxide were added to the tubes which were then heated on a digestion block at 375 °C. After the digestion process was completed (a total of 2.5 h) , the samples were allowed to cool, and deionized water was used to bring the volume to 25 mL. The solutions were filtered through *P8' filter papers (Fisher brand) , with a particle retention of > 25 //m into 25 mL scintillation vials. The solution samples were then sent to the Analytical Research Laboratory, University of Florida, and N was determined on a 300 Series Rapid Flow Analyzer (ALPKEM Corporation, Wilsonville, OR) . Data were subjected to analysis of variance using the Statistical Analysis System (SAS Institute, Inc., Cary, NC) . Treatment sums of squares were partitioned into linear or quadratic polynomial contrasts for Experiment 1. Data in Experiments 2, 3, and 4 were subjected to regression analysis. Bonferroni multiple comparison procedure (Neter et al., 1990) was used for multiple pairwise comparisons of treatment means.

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152 Field Experiments Plants from each treatment in Experiments 2 and 4 were transplanted into an Arredondo fine sandy soil (loamy, siliceous, hyperthermic Grosarenic Paleudults) in beds covered with white-on-black polyethylene-mulch (0.038 mm thick) at the University of Florida Horticultural Unit, Gainesville (Table 6-2) . The experiments were arranged in a Table 6-2. Transplanting schedule and initial soil test (Hanlon et al., 1994) for Experiments 1 and 2. Experiment Transplanting Soil test" date PH EC P K Ca Mg (dS-m(mgkg-M 1 23 Mar 1995 6.3 0.1 208 46 452 45 2 2_TI J 17 Oct 1995 5.9 0.1 185 30 733 54 "pH and EC determined on 2 : 1 water to soil ratio procedure, while elements are from a Mehlich-1 extractant. randomized complete-block design with 20 treatments consisting of a factorial combination of 5 levels of N and 4 levels of fertigation frequency in 4 replications. Preplant fertilizer (13N-0P-10 . 8K) was applied broadcast and incorporated in the bed at 230 kg-ha"^ Raised beds spaced 1.2 m center to center, were fumigated with methyl bromide and then covered with the polyethylene mulch. There were 30 plants per plot planted on double offset rows with a spacing of 0.3 m between plants and between rows on the bed (equivalent to 54,000 plants/ha).

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153 Just after transplanting, 100 itiL of nutrient solution (150 mg-L"^ 20N-8 . 6P-1 6 , 7K) was applied to each transplant hole as a starter fertilizer. Water was applied twice daily for 20 min each cycle, using drip irrigation lines placed on the center of the bed with emitters spaced 0.3 m apart. Tensiometers (Irrometer Company, Inc., Riverside, CA) were used to monitor soil moisture adequacy in the beds. The root zone area was maintained at approximately -10 kPa according to Hochmuth and Clark (1991) . Starting one week after transplanting, fertilizer at a rate of 15 kg-ha"^ N and 16 kg-ha"^ K, supplied from NH4NO3 and KNO3, was injected weekly using a venturi pinap (Netafim Irrigation, Altamonte Springs, FL) , with the last application one week before harvest to give a total amount of 150 kg-ha"^ N and 180 kg-ha'^ K. Cultural management practices were similar to those used commercially in Florida (Hochmuth et al., 1988), At head maturity, the center 20 plants in a plot were cut, weighed individually, and then 10 heads were assessed for firmness, cut longitudinally for height, diameter, stem width, and core length measurements. Wrapper leaves were sampled at harvest for analysis of tissue N according to Wolf (1982) as described for Greenhouse Experiments. Field data were subjected to regression analysis using the Statistical Analysis System (SAS Institute, Inc., Cary, NC) .

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154 Results and Discussion Greenhouse Expe riments Experiment 1 was conducted to assess the effect of water deficit on lettuce growth. Greenhouse temperatures ranged from 19 to 39 °C (Fig. 6-1). The average daily maximum media temperature was 32 °C, while the average daily minimum media temperature was 20 °C. During the course of the trial, there were 19 sunny and 9 cloudy days with rain during three of the cloudy days. For plants sampled 15, 22, and 29 days after sowing (DAS) , fresh and dry shoot mass responded in quadratic fashion to irrigation frequency (Table 6-3) . Fresh and dry shoot mass were least at the 0 % level of moisture deficit from field capacity (FC = 368 %, m/m) and greatest at the 40 % level for plants grown to 15 DAS. However, dry shoot mass was least at the 60 % level of moisture deficit by this sampling date. For plants sampled 22 DAS, fresh and dry shoot mass were least at the 0 % level of moisture deficit from FC, and greatest at the 40 % level, with no further increases at higher moisture deficits. For plants sampled 29 DAS, fresh and dry shoot mass were increased by increasing moisture deficit from 0 to 60 % below FC.

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155 LU 25H d D ^ 20 cc III Q. 15H m I10 5m-m0~h I I I I — I — I — I — I — I — \ — I — I — I — I — I — I — I — I — I — I — f — I — I — I — r 21 301 16 GROWING PERIOD (JUN 21 JUL 1 6) air max media max — •— air min media min Fig. 6-1. Maximum and minimum air and media temperature during transplant production for Experiment 1, Jun/Jul 1994.

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156 c o •H 4-> (0 o> -H M M >i X) T3 0> 4J U 0) IH IH ID n ID to •P c ID H Q. n c ID P V U 3 P P 0) 0) 3 M m nj m 0) -iH -p 3 m CO CP 0) 3 ip m n) m -p 2 2; ip rfl >i o W Di M o to e Que — x: O en to 4-> 10 0) 0 (0 CP CO rH M 0 to B u Cu p B •H >1 4-1 rH m 3 H ^^ 4-1 \ o in (1) (D >i o (0 4-1 C P x: to U 3 Q n e to ^o M to j: rH j:: 4J u to 0 to 4-> 0) 0 to CP 4-) c p x; to E o m e o e -H m er te c to 0 x: E -H o to u H 4-1 c to 0) 4-1 u 0 CP 3 to o 0 u -H cr >, 0 ip sx P 0) to P p •D >i •H »p u n 1 en 0) VD 3 p 4J u tr 3 •H b (1) 0) 4J U rH p (0 -H e oP JQ H IP o to O 0) p Eh D «p I~ CNJ ^ * . . . . -X m r~ "a* ro n «3< ^ '3« rH ro in 0 CO CX3 en tn o o q rH eg CM CVJ •g o to rH rH Csl CM lOVDOCTl* in^oo^ • • • • + rOCnO (j^rHtTrHO csj n •>3' lo rVD n -tt . . . . + rr tn O CO 0) •P to in cvj m °° CM rH in in 00 O rH rH o r•^r * • • • * rH CM Qj rH rH 4J "P to Q rH rH rH rH in n VD * . . . . * ^ 00 cn in O CM n n n o to p (U 4J >P o en o 00 rH O CM CM eg CM CM CM 00 OI cn 00 to Q cn CM in in n cn o CM CM 00 m * * in z ID 4-1 c to u * •H IP •rH c cn •H to k to 11 4-1 OI U 0) IP


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For plants sampled 15 and 22 DAS, fresh root mass responded in quadratic fashion to irrigation frequency. For plants sampled 29 DAS, fresh root mass increased linearly in response to moisture deficit from FC. For plants sampled 15 DAS, dry root mass responded in quadratic fashion to irrigation frequency, but irrigation frequency did not affect dry root mass for plants sampled 22 and 29 DAS. Leaf area responded in quadratic fashion to irrigation frequency, regardless of sampling date (Table 6-3) . For plants sampled 15 DAS, leaf area was least at the 0 % level of moisture deficit from FC, and greatest at the 40 % moisture deficit. For plants sampled 22 and 29 DAS, leaf area was increased by increasing the moisture deficit from 0 to 60 % below FC. For plants sampled 29 DAS, leaf tissue N increased in quadratic fashion, while leaf tissue P and K increased linearly in response to increasing the moisture deficit from 0 to 60 % below FC. Lower levels of tissue N, P, and K in transplants which were irrigated more frequently compared with those which were irrigated less frequently, indicated that some nutrient leaching might have occurred with frequent irrigations. Therefore, further investigations were needed to determine optimum fertigation frequency rather than irrigation frequency, due to the potential leaching problem.

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158 For plants sampled 15 DAS, RSR was not influenced by increasing moisture deficit from FC (Table 6-4) . For plants sampled 22 and 29 DAS, RSR decreased quadratically as moisture deficit increased. For plants sampled 22 DAS, RSR was greatest at the 0 % level of moisture deficit. For plants sampled 29 DAS, RSR was decreased by increasing moisture deficit from 0 to 40 %, and thereafter there was no further decrease in RSR. For plants grown to 22 DAS, RGR increased linearly in response to irrigation frequency, while for plants grown to 29 DAS, RGR increased in quadratic fashion in response to irrigation frequency. Relative growth rate values were lower by 29 DAS compared to 22 DAS. Irrigation frequency did not influence NAR for plants grown to 22 DAS, but NAR decreased in quadratic fashion in response to irrigation frequency for plants grown to 29 DAS. For plants sampled 15, 22, and 29 DAS, both SLA and LAR increased linearly in response to irrigation frequency. Specific leaf area and LAR were least at the 0 % level of moisture deficit from FC, and greatest at the 60 % level of moisture deficit. For plants sampled 15 DAS, irrigation frequency did not influence LMR or RMR. For plants sampled 22 and 29 DAS, LMR increased in quadratic fashion, while RMR decreased in quadratic fashion in response to irrigation frequency. Leaf mass ratio was least at the 0 I level of moisture deficit

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159 0) u 3 P P V U-t o o V) -H cn p to m o m m p (U ro (0 (0 -H ra 0) 4J ID M ro ^^ (d M u •H iw •H U flJ a Cn 6 ro •H x; 4-) to OS P 6 to CP •H 4-1 4-1 O P c 4-) O -H U 0) O o 4-) •H E 0 to •H cC to P

i C E •rH U (U p H +J c 3 0 to (U .H o cn 3 (days IM p •H CT c to a P (U ant P -H fr 1 pl 0) to p 4-> u c 3 •H (U to P U H p to -H E 4J •H IM o to O (U p 2 T3 O O i 10 Q CM 00 CM O tH rH rH 0 to m cn csj n o 4^ CM ^ n r-K CO C • • . • * 2 CM rH rH rH O to to Q Oi CM + ^ CM VD O ^ ^ ^ ^ o o o o CO 2 VD CM rH CM ^ n n cn * • . . . * o o o o o CM I I CM n CM I I CM n o o O CM ^ 01 q O a; o o o O CM T tX) 0) 01 q O a: 00 eg cn «3 in 00 * to * p O o o o o u «p ip 0) CM rCM CM o o o o u •r4 4J •0 P T3 CO (0 2 3 tr rH CM 1 4J CM p c o to u -H


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160 from FC, and greatest at the 20 % level of moisture deficit by 22 DAS, with no further increases in LMR at higher levels. The opposite response occurred for RMR. For plants sampled 29 DAS, LMR was increased by increasing moisture deficit from 0 to 40 % below FC, with no further increases in LMR at higher levels. The opposite response occurred for RMR. It was observed that transplants kept close to field capacity (FC) and those which were irrigated at 60 % moisture deficit from FC were inferior because they could not be easily pulled from the transplant flat compared with those irrigated at 20 or 40 % moisture deficit from FC. Roots of transplants which were irrigated less frequently were observed as being finer and they penetrated the sides of cells, apparently causing root systems not to pull out completely from the transplant flat. In experiment 1, optimum irrigation frequency for lettuce transplants was investigated independent of the amount of fertilizer applied. Leaf tissue analysis indicated that frequent irrigations may have resulted in leaching of fertilizer nutrients. The next three experiments, therefore, were conducted to determine optimum fertigation frequency, by investigating simultaneously both the nutrition and the water requirements of the transplants. Since the lettuce transplants could not be easily pulled from the transplant

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161 flat and total potential growth due to N additions was not achieved after 28 days by growing plants with 60 mg-L'^ N (Chapter 5), N was increased to 120 mg-L"^ in experiments which follow. Experiment 2 was conducted during the spring in order to determine the effects of N fertigation frequency on lettuce growth. Greenhouse temperatures ranged from 13 to 46 °C (Fig. 6-2). The average daily maximum media temperature was 32 °C, while the average daily minimum media temperature was 20 °C. During the course of the trial, there were totals of 18 sunny and 8 cloudy days, with rain during four of the cloudy days. Since dry root mass was maximized with 60 mg-L''' in Experiment 2, this N level was used for comparison whenever there were interactions between fertilizer N and fertigation frequency. For plants sampled 13 DAS (Fig. 6-3), 21 DAS (Fig. 6-4) and 28 DAS (Fig. 6-5), dry shoot mass increased in quadratic fashion to applied N when fertigation frequency was daily, every second or every third day. Dry shoot mass increased linearly in response to applied N when fertigation frequency was every fourth day. For plants sampled 13 DAS, fertigating every third day was as adequate as daily fertigation for increased shoot growth. However, for plants sampled 21 and

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162 50 T — I — I — 1 — I — I — I — I — I — I — r "1 — I — 1 — I — I — I — I — I — I — r 28 1 24 GROWING PERIOD (FEB 27 MAR 24) air max media max — •— air min media min Fig. 6-2. Maximum and minimum air and media temperature during transplant production for Experiment 2, Feb/Mar 1995.

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0 30 60 90 120 APPLIED NITROGEN (mg/L) -Hfertig freq 1 ; Q* ^ fertig freq 2; Q* -bfertig freq 3; Q* -•fertig freq 4; L** N at 60 mg-L-' Fertigation frequency 4 3 2 1 Treatment means 5,6 8.6 8.6 10.5 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-3. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 13 days after sowing for Experiment 2, Feb/Mar 1995.

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164 30 60 90 APPLIED NITROGEN (mg/g 120 fertig freq 1 ; Q* fertig freq 2; Q* -bfeitig freq 3; Q* -*fertig freq 4; L** N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 23.0 35.7 48.9 61.6 All means are significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-4. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 21 days after sowing for Experiment 2, Feb/Mar 1995.

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165 200 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -+fertig freq 4; L** N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 43.1 72.0 104.8 135.6 All means are significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-5. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 28 days after sowing for Experiment 2, Feb/Mar 1995.

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166 28 DAS, dry shoot mass was increased by each level of fertigation frequency. For plants sampled 13 DAS, dry root mass responded in quadratic fashion to applied N at all fertigation frequencies, except when fertigating every fourth day wherein N did not affect dry root mass (Fig. 6-6) . The greatest increases in dry root mass to applied N occurred between 0 and 30 mg-L'-', when transplants were fertigated every day, every second day or every third day. For plants sampled 21 DAS (Fig. 6-7) and 28 DAS (Fig. 6-8), dry root mass increased in quadratic fashion in response to applied N, regardless of fertigation frequency. For plants sampled 21 DAS, dry root mass was least when fertigation frequency was every fourth day. For plants sampled 28 DAS, the optimum N to maximize dry root mass was 60 mq-L'\ regardless of fertigation frequency. Fertigating every other day was as effective as daily fertigation for increasing root growth. For plants sampled 13 DAS (Fig. 6-9), leaf area increased in quadratic fashion to applied N, regardless of fertigation frequency. For plants sampled 22 DAS (Fig. 610) , leaf area increased in quadratic fashion to applied N only when the fertigation frequency was daily to every third day, but increased linearly when the fertigation frequency was every fourth day. For plants sampled 28 DAS (Fig. 6-11),

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167 ^ feitig freq 1 ; Q* fertig freq 2; Q* -efertig freq 3; Q* -»fertig freq 4; NS N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 2 . 6 3_M 3_;_6 3 . 6 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-6. Lettuce transplant dry root mass response to N nutrition and fertigation frequency 13 days after sowing for Experiment 2, Feb/Mar 1995.

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168 30 60 90 APPLIED NITROGEN (mg/L) 120 fertig freq 1 ; Q* fertig freq 2; Q* -afertig freq 3; Q* -•fertig freq 4; Q* N at 60 mg-L'^ Fertigation frequency 4 3 2 1 Treatment means 11.0 16.3 18.7 21.9 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-7. Lettuce transplant dry root mass response to N nutrition and fertigation frequency 21 days after sowing for Experiment 2, Feb /Mar 1995.

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169 30 60 90 APPLIED NITROGEN (mg/g 120 fertig freq 1 ; feitig freq 2; Q* -bfertig freq 3; Q* — ifertig freq 4; N at 60 mg-L'^ Fertigation frequency 4 3 2 1 Treatment means 19.8 27.2 37.9 39.2 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-8. Lettuce transplant dry root mass response to N nutrition and fertigation frequency 28 days after sowing for Experiment 2, Feb/Mar 1995.

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170 E o LU < LL < LU 30 60 90 APPLIED NITROGEN (mg/g 120 feitig freq 1 ; Q* feitig freq 2; Q* -bfertig freq 3; feitig freq 4; N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 4.5 5.8 7.4 9.6 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-9. Lettuce transplant leaf area response to N nutrition and fertigation frequency 13 days after sowing for Experiment 2, Feb/Mar 1995.

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171 50 APPLIED NITROGEN (mg/g fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* — »fertig freq 4; L N at 60 mg-L-' Fertigation frequency 4 3 2 1 Treatment means 11.5 15.7 22.5 32.1 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-10. Lettuce transplant leaf area response to N nutrition and fertigation frequency 21 days after sowing for Experiment 2, Feb/Mar 1995.

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172 120 APPUED NITROGEN (mg/L) -Hfeitig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; L** fertig freq 4; L** N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 21.9 31.5 47.9 65.9 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-11. Lettuce transplant leaf area response to N nutrition and fertigation frequency 28 days after sowing for Experiment 2, Feb/Mar 1995.

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173 leaf area increased in quadratic fashion to applied N when the fertigation frequency was daily or every second day, but increased linearly when the fertigation frequency was every third or fourth day. Leaf area was greatest when fertigation was daily and least when fertigation was every third or fourth day, regardless of sampling date. For plants sampled 28 DAS, the force required to pull transplants from the transplant flat increased in quadratic fashion to applied N (Fig. 6-12) . The greatest force was required to pull transplants produced with daily fertigation of 90 mg-L'^ N. There were no N by fertigation frequency interactions for pulling success (Table 6-5) . Pulling success increased in quadratic fashion to applied N. Pulling success was improved dramatically from 16 to 95 % when N was increased from 0 to 60 mg-L'^. Low pull force was associated with low pulling success due to plants breaking when pulled. With added N, the stems and roots were strong enough to prevent breakage. For plants sampled 28 DAS, leaf tissue N increased linearly in response to applied N (Fig. 6-13) . Leaf tissue N was increased from about 5 to 40 g-kg"^ by applied N, regardless of fertigation frequency. Fertigation frequency did not influence N concentration in transplant leaves. There were no N by fertigation frequency interactions for RSR and RGR (Table 6-5) . For plants sampled 13, 21, and

PAGE 182

174 0.035 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* fertig freq 2; Q* -efertig freq 3; Q* -fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 0.014 0.017 0.021 0.029 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-12. Lettuce transplant pull force response to N nutrition and fertigation frequency 28 days after sowing for Experiment 2, Feb/Mar 1995.

PAGE 183

175 Table 6-5. Root and shoot characteristics of lettuce transplants as affected by N nutrition and fertigation frequency for Experiment 2, February/March 1995. Treatment Z Pulling Root : Relative success (%) shoot ratio growth rate (mg-mg'^ • wk"M 13 Days After Sowing N (mg-L-') 0 0. 88 30 0.52 60 0.41 90 0.34 120 0.33 Response Q** F (days) 1 0.48 2 0.51 3 0.46 4 0.54 Response NS N X F NS 21 Days After Sowing Nitrogen (mg-L-M 0 1.03 0.54 30 0.50 1.40 60 0.43 1.59 90 0.38 1.69 120 0.31 1.62 Response Q** Q** F (days) 1 0.45 1.51 2 0.53 1.50 0.55 1.30 4 0.59 0.16 Response L* L** N X F NS NS 28 Days After Sowing Nitrogen (mgL"M n u Id 1.01 0.51 30 86 0.47 0.68 60 95 0.38 0.69 90 94 0.29 0.74 120 85 0.21 0.79 Response Q** Q** L** F (days) 1 74 0.41 0.72 2 83 0.46 0.72 3 79 0.47 0.65 4 65 0.55 0.65 Response Q** L** L* N X F NS NS NS ^N = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS) .

PAGE 184

176 30 60 90 APPLIED NITROGEN (mg/g 120 fertig freq 1 ; L** -fertig freq 2; L ** fertig freq 3; L fertig freq A; L N at 60 mg-L'^ Fertigation frequency 3 4 2 1 Treatment means 1.80 2.11 2.31 2.32 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-13. Lettuce transplant leaf tissue N response to N nutrition and fertigation frequency 28 days after sowing for Experiment 2, Feb/Mar 1995.

PAGE 185

177 28 DAS, RSR values decreased in quadratic fashion when N was applied. For plants sampled 13 DAS, fertigation frequency did not influence RSR values, but for plants sampled 21 and 28 DAS, RSR values increased linearly when the period between each fertigation was increased. For plants grown to 21 DAS, RGR increased in quadratic fashion to applied N, while for plants grown to 28 DAS, RGR increased linearly in response to applied N (Table 6-5) . Larger values of RGR by 21 DAS compared to 28 DAS, indicated that younger transplants had greater efficiency for growth than older ones. However, RGR values decreased when the period between each fertigation was increased. There were no N by fertigation frequency interactions for NAR (Table 6-6) . For plants grown to 21 or 28 DAS, NAR decreased in quadratic fashion to applied N. Net assimilation rate responded in quadratic fashion to fertigation frequency for plants grown to 21 DAS, but for plants grown to 28 DAS, fertigation frequency did not influence NAR. For plants sampled 13 DAS, SLA (Fig. 6-14) and LAR (Fig. 6-15) increased in quadratic fashion in response to applied N, regardless of fertigation frequency. The greatest increases in SLA or LAR to applied N were between 0 and 30 mg-L'^. Fertigating every day was better than fertigating every third day in increasing SLA or LAR. For plants sampled 21 and 28 DAS, there were no N by fertigation frequency

PAGE 186

178 Table 6-6. Influence of N nutrition and fertigation frequency on growth characteristics of lettuce transplants for Experiment 2, Feb/Mar 1995. Treatment" Net Specific Leaf Leaf Root assimilation leaf area mass mass ratio ratio rati o 13 Dciys After Sowing w \ mg • Li ) n 0 . 54 0 .46 30 0.66 0.34 fif) 0.71 0.29 Qn ^ u 0 . 75 0 .25 Ton 0 . 24 Response o** y F (days) 1 U . / U u u o 3 0 .70 0 . 30 4 •1 0 . 66 0 . 34 J?espo/jse Mo isio 21 Days After Sowing Mo rjo w ( mg jj / 0 3 60 0 27 0 . 14 0 . 50 30 4,03 0.39 0.26 0.33 60 3.81 0.48 0 . 34 0.30 90 3.43 0.57 0.41 0.27 120 3.09 0.61 0.46 0.24 ResTDons^ o** n** n** n** F (davs) 1 2 3.97 0 46 0 . 32 0 . 32 3 3.79 0.43 0.29 0.34 4 3.07 0.49 0.32 0.36 n** n** L* * N X F no no Days Alter bowing no rJo n ( mg Jj / 0 n in n u • o u u . ou 30 2 73 f) "^6 n 7i u . ^ *1 V . o o V/ . 60 2.02 0 47 0 . 35 0.73 90 1 81 OS"? n 7ft 120 1.73 0.55 0 4 S n R'^ n 17 Response Q** Q** Q** Q** Q** F (days) 1 2.41 0.43 0.33 0.74 0.26 2 2.76 0.41 0.31 0.71 0.29 3 2.53 0.40 0.30 0.70 0.30 4 2.47 0.45 0.31 0. 67 0.33 NS Q** Q** L** L** N X F NS NS NS NS NS = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0. 05 (*), 0.01 (**), or nonsignificant (NS) .

PAGE 187

179 1.2 UJ 0.2Q. 0-1 , 1 ^ 0 30 60 90 1 20 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* fertig freq 2; Q* -bfeitig freq 3; Q* — •— fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 3 4 2 1 Treatment means 0.68 0.82 0.87 0.92 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-14. Lettuce transplant specific leaf area response to N nutrition and fertigation frequency 13 days after sowing for Experiment 2, Feb/Mar 1995.

PAGE 188

180 0.85 0.7c c — , w 0.6E o. 0.50 1< 0.4< 0.3UJ < u. 0.2| < UJ 0.10-30 60 90 APPUED NITROGEN (mg/L) 120 fertig freq 1 ; Q* -•^ fertig freq 2; Q* -bfertig freq 3; fertig freq 4; N at 60 mg-L'^ Fertigation frequency 3 4 2 1 Treatment means 0.49 0.56 0.61 0.68 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-15. Lettuce transplant leaf area ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 2, Feb/Mar 1995.

PAGE 189

181 interactions for SLA and LAR (Table 6-6) . Both SLA and LAR increased in quadratic fashion in response to applied N. Fertigating every day or every fourth day increased SLA more than fertigating every second or third day. However, LAR was only increased by daily fertigation. For plants sampled 13 and 2 8 DAS, there were no N by fertigation frequency interactions for LMR. Leaf mass ratio increased in quadratic fashion in response to applied N. Fertigation frequency did not influence LMR for plants sampled 13 DAS, but frequent fertigations increased LMR for plants sampled 28 DAS. For plants sampled 21 DAS (Fig. 616) , LMR increased in quadratic fashion to applied N when the fertigation frequency was daily or every second or third day. When the fertigation frequency was .e.very fourth day, LMR increased linearly in response to applied N. The greatest increases in LMR to applied N occurred between 0 and 30 mg-L"^, when transplants were fertigated every day, or every second and third day. Fertigation frequency did not influence LMR for plants sampled 21 DAS. For plants sampled 13, 21, and 28 DAS, there were no N by fertigation frequency interactions for RMR (Table 6-6) . Root mass ratios decreased in quadratic fashion when N was applied, regardless of sampling date. For plants sampled 13 DAS, RMR was not affected by fertigation frequency, but for

PAGE 190

182 1 0.20 30 60 90 1 20 APPLIED NITROGEN (mg/L) ^ fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -•fertig freq 4; L** N at 60 mg-L'^ Fertigation frequency 3 4 2 1 Treatment means 0. 67 0 . 69 0.72 0.74 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-16. Lettuce transplant leaf mass ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 2, Feb/Mar 1995.

PAGE 191

183 plants sampled 21 and 28 DAS, RMR was increased by less frequent f ertigations . Results of Experiment 2 indicated that, overall, high quality transplants could be produced with 60 mg-L"^ N, supplied every second day via floatation irrigation, especially when evaluating transplant quality based on dry root mass 28 DAS. Quality transplants had dry root mass of about 38 mg and dry shoot mass of about 100 mg, 28 DAS. In order to further test this conclusion, Experiment 3 was conducted during the summer, instead of spring, under greenhouse temperatures ranging from 2 6 to 48 °C (Fig. 617) . The average daily maximum media temperature was 38 °C, while the average daily minimum media temperature was 28 °C. During the course of the trial, there were totals of 22 sunny and 5 cloudy days, with rain during four of the cloudy days . Since dry root mass was maximized with 30 mg-L'^ in Experiment 3, this N level was used for comparison whenever there were interactions between fertilizer N and fertigation frequency. For plants sampled 13 DAS (Fig. 6-18) and 21 DAS (Fig. 6-19) , dry shoot mass increased in quadratic fashion to applied N when the fertigation frequency was daily, every second day, or every third day. When the fertigation frequency was every fourth day, dry shoot mass increased

PAGE 192

184 GROWING PERIOD (JUN 30 JUL 26) air max media max -*air min media min Fig. 6-17. Maximum and minimum air and media temperature during transplant production for Experiment 3, July 1995.

PAGE 193

185 30 60 90 APPLIED NITROGEN (mg/L) 120 fertig freq 1 ; Q* -*fertig freq 2; Q* -efertig freq 3; fertig freq 4; L N at 30 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 6.2 8,6 10.7 14.5 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-18. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 13 days after sowing for Experiment 3, July 1995.

PAGE 194

186 120 APPUED NITROGEN (mg/L) -Kfertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -tfertig freq 4; L** N at 30 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 15.4 24.2 33.3 43.0 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-19. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 195

187 linearly in response to N. Daily fertigation, compared to other frequencies, increased dry shoot mass for plants sampled 13 DAS. For plants sampled 21 DAS, fertigating every other day was as effective as fertigating daily in order to increase shoot growth. For plants sampled 28 DAS, dry shoot mass increased in quadratic fashion in response to applied N, regardless of fertigation frequency (Fig. 6-20) . Fertigating every third day was as effective as daily fertigation in order to increase shoot growth. For plants sampled 13 and 28 DAS, there were no N by fertigation frequency interactions for dry root mass (Table 6-7). Dry root mass increased in quadratic fashion in response to applied N by both sampling dates. For plants sampled 28 DAS, increasing N application from 0 to 30 mg-L"^ dramatically increased dry root mass from about 5 to 28 mg, thereafter dry root mass decreased. Fertigation frequency did not influence dry root mass for plants sampled 13 DAS, but for plants sampled 28 DAS, frequent fertigations led to increased root growth. For plants sampled 21 DAS (Fig. 621), dry root mass increased in quadratic fashion to applied N when fertigation frequency was every day to every third day. When fertigation frequency was every fourth day, dry root mass increased linearly in response to N. The greatest increases in dry root mass to applied N occurred between 0

PAGE 196

188 250 APPUED NITROGEN (mg/L) sfertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -ifertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 37.9 51.4 65.9 76.2 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-20. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 28 days after sowing for Experiment 3, July 1995.

PAGE 197

189 Table 6-7. Root and shoot characteristics of lettuce transplants as affected by N nutrition and fertigation frequency for Experiment 3, July 1995. Treatment^ Dry Pull Pulling Root ; root force success shoot growth mass ratio rate (mg) (N) (%) (mg mg'' • wk"' ) 13 Days After Sowing N (mg-L"') 0 2.1 30 4 . 8 60 4 . 9 90 4 . 2 120 4 . 1 Response Q** F (days) 1 4.1 2 4 . 5 3 3.9 4 3.5 Response NS N X F NS 21 Days After Sowing ** N (mg-L-M 0 0.41 30 1. 04 60 1.17 90 1.26 120 1.26 Q** F (days) 1 1 1 . 09 o 1 . 11 1 . 00 A 4 0.80 RBsponsB L** N X r 28 Days After Sowing NS n 'k , 0 U . UUO J 1 . 01 OR A r\ Cioo , H U . U^Z / O n c.n U . OU DU ZD . 4 U . UZD DO 0 . 30 90 25.7 0.029 69 0.18 120 19.2 0.027 43 0.13 Response Q** Q** Q** Q** F (days) 1 25.1 0.025 45 0.41 2 23.2 0.024 50 0.43 3 17.6 0.019 55 0.49 4 14.2 0.016 49 0.53 Response L** L** NS NS N X F NS NS NS NS ''N = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 {**), or nonsignificant (NS) .

PAGE 198

190 120 APPUED NITROGEN (mg/L) feitig freq 1 ; Q* feitig freq 2; Q* -efertig freq 3; Q* -ifertig freq 4; NS N at 30 mg-L'^ Fertigation frequency 4 3 2 1 Treatment means 8.0 12.8 16.0 19.8 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-21. Lettuce transplant dry root mass response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 199

191 and 30 mg-L'^ when transplants were fertigated every day, every second day, or every third day. For plants sampled 13 DAS (Fig. 6-22), 21 DAS (Fig. 623), and 28 DAS (Fig. 6-24), leaf area increased in quadratic fashion in response to applied N, regardless of fertigation frequency. The exception was plants sampled 21 DAS wherein leaf area increased linearly in response to N when fertigation was every fourth day. For plants sampled 13 DAS, leaf area was increased by daily fertigation, while for plants sampled 21 and 28 DAS, fertigating every other day was as adequate as daily fertigation in order to achieve greater leaf area. For plants grown to 28 DAS, there were no N by fertigation frequency interactions for pull force and pulling success (Table 6-7) , Once again, the amount of force required to pull transplants from the transplant flat as well as pulling success increased in quadratic fashion in response to applied N. Approximately 73 % of transplants could be successfully pulled from the transplant flat when 30 mg-L'^ N was applied, compared to only 3 % with 0 N and 43 % with 120 mg-L"^ N. Low pull force was associated with low pulling success due to root systems breaking when pulled, especially with 0 N.

PAGE 200

192 30 60 90 APPLIED NITROGEN (mg/g 120 feitig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -•fertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 4 3 Treatment means 3.6 5.6 2 6.4 1 9.1 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-22. Lettuce transplant leaf area response to N nutrition and fertigation frequency 13 days after sowing for Experiment 3, July 1995.

PAGE 201

193 APPLIED NITROGEN (mg/g fertig freq 1 ; Q* -•^ fertig freq 2; Q* -bfertig freq 3; Q* -ifertig freq 4; L** N at 30 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 7.7 11.9 16,1 21.9 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-23. Lettuce transplant leaf area response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 202

194 140 APPUED NITROGEN (mg/g feitig freq 1 ; Q* feitig freq 2; Q* -bfertig freq 3; Q* — •— fertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 17.0 23.3 27.6 35.1 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-24. Lettuce transplant leaf area response to N nutrition and fertigation frequency 28 days after sowing for Experiment 3, July 1995.

PAGE 203

195 For plants sampled 28 DAS (Fig. 6-25), leaf tissue N increased linearly in response to applied N, regardless of fertigation frequency. Fertigation frequency did not influence N concentrations in transplant leaves. For plants sampled 13 DAS (Fig. 6-26), RSR values decreased in quadratic fashion to applied N when the fertigation frequency was every day to every three days. When the fertigation frequency was every fourth day, RSR values decreased linearly in response to N. Fertigating every fourth day increased RSRs compared to daily fertigation. For plants sampled 21 DAS (Fig. 6-27), RSR values decreased in quadratic fashion in response to applied N, regardless of fertigation frequency. For plants sampled 28 DAS, there were no N by fertigation frequency interactions for RSRs (Table 6-7) . Fertigation frequency did not influence RSR values, but RSR values decreased in quadratic fashion in response to applied N. For plants grown to 21 DAS, there were no N by fertigation frequency interactions for RGR. Relative growth rate increased in quadratic fashion to applied N, but decreased linearly in response to a decrease in fertigation frequency. For plants grown to 28 DAS (Fig. 6-28), RGR increased linearly in response to applied N when fertigation frequency was daily, but when fertigation frequency was every two to every three days, RGR increased in quadratic

PAGE 204

196 30 60 90 APPLIED NITROGEN (mg/g 120 fertig freq 1 ; Q* fertig freq 2; L** -bfertig freq 3; L** -*fertig freq 4; L** N at 30 mg-L-^ Fertigation frequency 3 4 2 1 Treatment means 1.26 1.40 1.41 1.60 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-25. Lettuce transplant leaf tissue N response to N nutrition and fertigation frequency 28 days after sowing for Experiment 3, July 1995.

PAGE 205

197 1 -I 1 1 1 — — I 0 30 60 90 1 20 APPUED NITROGEN (mg/L) ^ feitig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -*fertig freq 4; L** N at 30 mg-r' Fertigation frequency 13 2 4 Treatment means 0.38 0.52 0.54 0.59 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-26. Lettuce transplant root: shoot ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 3, July 1995.

PAGE 206

198 1.2 0-1 1 , , 0 30 60 90 1 20 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* feitig freq 2; Q* feitig freq 3; Q* -ifertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 12 3 4 Treatment means 0.47 0 .49 0. 54 0 . 56 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-27. Lettuce transplant root: shoot ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 207

199 fertig freq 1 ; L** feitg freq 2; Q* feitig freq 3; Q* -*fertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 13 2 4 Treatment means 0.57 0.70 0.74 0.88 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-28. Lettuce transplant relative growth rate response to N nutrition and fertigation frequency 28 days after sowing for Experiment 3, July 1995.

PAGE 208

200 fashion to N. The greatest increases in RGR in response to applied N occurred between 0 and 30 mg-L"\ when transplants were fertigated every second, third or fourth day. Fertigating every fourth day increased RGR values more than daily fertigation. For plants grown to 21 and 28 DAS, there were no N by fertigation frequency interactions for NAR (Table 6-8) . Net assimilation rate decreased in quadratic fashion in response to applied N. For plants grown to 21 DAS, NAR responded in quadratic fashion to fertigation frequency, but fertigation frequency did not influence NAR values for plants grown to 28 DAS. Although NAR was greatest with 0 N regardless of sampling date, the total production of dry matter over the same period was greater with any level of N. For plants sampled 13, 21, and 28 DAS, there were no N by fertigation frequency interactions for SLA. Specific leaf area increased in quadratic fashion in response to applied N, regardless of sampling date. Fertigation frequency did not influence SLA. For plants sampled 13 and 28 DAS, there were no N by fertigation frequency interactions for LAR. For plants sampled 13 DAS, LAR increased in quadratic fashion in response to applied N, but decreased as the interval between each fertigation was delayed by one day. For plants sampled 28 DAS, LAR increased in quadratic fashion in response to applied N. Fertigation frequency, however, did not influence

PAGE 209

201 Table 6-8. Influence of N nutrition and fertigation frequency on growth characteristics of lettuce transplants for Experiment 3, July 1995. Treatment^ Net Specific Leaf Leaf Root assimilation leaf area mass mass rate area ratio ratio ratio (mg cm"^' •wk"') (cm^-mg"M (cm^-mg'M 13 Days After Sowing N (mg • L ) 0.25 0.14 30 0.62 0.41 fin 0.74 0.54 y u 0.70 0.56 12U 0.70 0.57 Response Q** Q** F (days) 1 0.62 0.49 2 0.59 0.43 0.62 0.45 0.58 0.40 i?esponse NS L* N X F 21 NS ; Days After Sowing NS N ( rag • L ) 0 3.27 0.23 30 2.87 0.50 60 2.40 0. 62 90 2.29 0.66 120 2.20 0.68 Q* Q** F (davs) 1 2.54 0.55 2 2.86 0.54 3 2.61 0.53 4 2.45 0.48 Q** NS N X F NS NS ** 28 Days After Sowing M f mn . T "1 1 ri vxug ij / 0 2.92 0.25 0.12 30 2.33 0.45 0.30 60 1.50 0.57 0.44 0 77 0 ')'>. \J » ^ o 90 1.35 0.55 0.47 0 . 85 0 . 15 120 1.20 0.58 0.51 0.89 0.11 Response Q** Q** Q** Q** Q** F (days) 1 1.85 0.47 0.37 0.75 0.25 2 1.72 0.48 0.37 0.73 0.27 3 1.85 0.47 0.35 0.70 0.30 4 2.46 0.45 0.32 0.68 0.32 i?esponse NS NS NS NS NS N X F NS NS NS NS NS = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0. .05 (*), 0.01 (**), or nonsignificant (NS) .

PAGE 210

202 LAR values. For plants sampled 21 DAS (Fig. 6-29), LAR increased in quadratic fashion in response to applied N at all levels of fertigation frequency. Fertigation frequency did not influence LAR values. The reduction in SLA and LAR values for plants -grown with 0 N reflects the reduction in both leaf size and assimilate production (Dubik et al . , 1990) . For plants sampled 13 DAS (Fig. 6-30), LMR increased in quadratic fashion in response to applied N under daily fertigation, but when the fertigation frequency was every two to every four days, LMR increased linearly in response to N. Fertigation frequency did not influence LMR values. For plants sampled 21 DAS (Fig. 6-31), LMR increased in quadratic fashion in response to applied N at all levels of fertigation frequency. Leaf mass ratios were not influenced by fertigation frequency. For plants sampled 13 DAS (Fig. 632), RMR decreased in quadratic fashion in response to applied N under daily fertigation, but when the fertigation frequency was every two to every four days, RMR decreased linearly in response to N. Fertigation frequency did not influence RMR values. For plants sampled 21 DAS (Fig. 6-33), RMR decreased in quadratic fashion in response to applied N at all levels of fertigation frequency. Root mass ratios were not influenced by fertigation frequency. I

PAGE 211

203 0.7 0-1 1 1 , 0 30 60 90 1 20 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* feitig freq 2; Q* -bfeitig freq 3; Q* -*fertig freq 4; Q* N at 30 mg-L-^ Fertigation frequency 12 3 4 Treatment means 0.47 0.49 0 . 54 0 . 56 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-29. Lettuce transplant leaf area ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 212

204 30 60 90 APPUED NITROGEN (mg/g 120 feitig freq 1 ; Q* -•^ feitig freq 2; L** -bfertig freq 3; L** -fertig freq 4; L** N at 30 mg-L'^ Fertigation frequency 12 3 4 Treatment means 0.64 0.65 0.66 0.73 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-30. Lettuce transplant leaf mass ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 3, July 1995.

PAGE 213

205 o p < (0 Hi < LL < 30 60 90 APPUED NITROGEN (mg/L) 120 feitig freq 1 ; Q* fertig freq 2; Q* -afertig freq 3; Q* -ifertig freq 4; N at 30 mg-L-^ Fertigation frequency Treatment means 1 0. 65 2 0. 65 3 0.67 4 0. 68 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test Fig. 6-31. Lettuce transplant leaf mass ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 214

206 0.5 0-11 1 1 0 30 60 90 1 20 APPUED NITROGEN (mg/L) feitig freq 1 ; Q* feitig freq 2; L** -bfertig freq 3; L** fertig freq 4; L** N at 30 mg-L-' Fertigation frequency 12 3 4 Treeatment means 0.27 0.34 0.35 0.36 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-32. Lettuce transplant root mass ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 3, July 1995.

PAGE 215

207 0.6 0-1 ^ 1 , 0 30 60 90 120 APPLIED NITROGEN (mg/L) -Hfertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -*fertig freq 4; Q* N at 30 mg-L'^ Fertigation frequency 12 3 4 Treatment means 0.32 0.33 0.35 0.35 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-33. lettuce transplant root mass ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 3, July 1995.

PAGE 216

208 For plants sampled 28 DAS, there were no N by fertigation frequency interactions for LMR and RMR (Table 68) . Leaf mass ratios increased in quadratic fashion, while RMR decreased in quadratic fashion in response to applied N. Root mass ratios were reduced from 0.5 to 0.1 with applications of 0 to 120 mg-L'^ N. Fertigation frequency did not influence LMR or RMR values. The results of Experiment 3 indicated that, overall, high quality transplants could be produced with 30 mg-L'^ N, supplied daily via floatation irrigation, especially when evaluating transplant quality based on dry root mass 28 DAS. Quality transplants had dry root mass of about 28 mg and dry shoot mass of about 75 mg, 28 DAS. Results of Experiment 3 (summer) are different from those obtained in Experiment 2 (spring) , where high quality transplants were obtained with 60 mg-L'^ applied every other day. Since different growing seasons provided different results in Experiment 3 compared to Experiment 2, further investigations were deemed necessary in order to determine the seasonal effect of N fertilization practices. Experiment 4 was conducted during the fall, instead of spring and summer, under greenhouse temperatures ranging from 19 to 4 3 °C (Fig. 6-34) . The average daily maximum media temperature was 33 °C, while the average daily minimum media temperature was 26 °C. During the course of the experiment, there were

PAGE 217

209 o O, LU QC D I< UJ Q. m IGROWING PERIOD (SEP 22 OCT 1 6) air max media max — — air min -smedia min Fig. 6-34. Maximum and minimum air and media temperature during transplant production for Experiment 4, Sep/Oct 1995.

PAGE 218

210 11 sunny and 15 cloudy days, with rain during three of the cloudy days. Since dry root mass was maximized with 60 mg-L"^ in Experiment 4, this N level was used for comparison whenever there were interactions between fertilizer N and fertigation frequency. For plants sampled 13 DAS (Fig. 6-35) and 21 DAS (Fig. 6-36) , dry shoot mass increased in quadratic fashion in response to applied N at all levels of fertigation frequency. For plants sampled 13 DAS, daily fertigation improved dry shoot mass over fertigating every third or fourth day. For plants sampled 21 DAS, dry shoot mass increased when fertigation was more frequent. For plants sampled 28 DAS (Fig. 6-37), dry shoot mass increased linearly in response to N when the fertigation frequency was daily or every second day. When the fertigation frequency was every third or fourth day, dry shoot mass increased in quadratic fashion to applied N. Daily fertigation increased dry shoot mass more than the other frequencies. For plants sampled 13, 21, and 28 DAS, there were no N by fertigation frequency interactions for dry root mass (Table 6-9) . Dry root mass increased in quadratic fashion in response to applied N, regardless of sampling date. The greatest increase in dry root mass occurred between 0 and 30 mg-L'^N. Fertigation frequency did not influence dry root

PAGE 219

211 1614B E 124O 2i 00 30 60 90 APPUED NITROGEN (mg/g 120 feitig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -»fertig freq 4; Q* N at 60 mg-L-' Fertigation frequency 4 3 2 1 Treatment means 8.7 9.6 11.1 12.0 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-35. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 13 days after sowing for Experiment 4, Sep/Oct 1995.

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212 -«-fertigfreq1; Q* fertig freq 2; Q* -bfertig freq 3; Q* -»fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 28.2 36.3 42.6 47.0 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-36. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995.

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213 175 APPUED NITROGEN (mg/L) -sfeitig freq 1 ; L** fertig freq 2; L** fertig freq 3; Q* fertig freq 4; Q* N at 60 mg-L-' Fertigation frequency 4 3 2 1 Treatment means 55.9 68.4 80.0 97.1 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-37. Lettuce transplant dry shoot mass response to N nutrition and fertigation frequency 28 days after sowing for Experiment 4, Sep/Oct 1995.

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214 Table 6-9. Root and shoot characteristics of lettuce transplants as affected by N nutrition and fertigation frequency for Experiment 4, September/October 1995. Treatment^ Drv Pull Pulling Root : Relative ^ tju ^ shoot n r out" Vi rate (nig) ^ TTirr . TTirr"^ * ulf \ ij Days Aitej: bowing w \ lug JLi I 0 1.4 •J \j fin Q n o . Z Response r ( aays ) 1 X ^ . o 2 3 . 0 3 3 . 1 4 3 . 1 n o n o 21 Days A£t.eT Sowing * 0 2.2 0.81 30 10 . 4 0 52 60 13 2 yj . 90 u . z o 120 u • ^ ^ F Mavc:^ 1 U . o / 2 10 . 9 U . 1 o 3 10.3 4 9 . 9 no 1 1 " N X F no uays Axzej. oowmg Na N (mg'L'M 0 3.7 0.004 0 0.86 0 .47 30 19.2 0.013 88 0.48 0.65 60 23.3 0.017 96 0.32 0.64 90 21.8 0.018 98 0.22 0.70 120 18.9 0.018 79 0.16 0.70 Response Q** Q** Q** Q** Q** F (days) 1 18.2 0.013 73 0.37 0. 68 2 18.9 0.016 68 0.39 0.64 3 16.6 0.013 76 0.42 0.61 4 15.7 0.013 69 0.44 0.62 /Response NS Q* NS L* L* N X F NS NS NS NS NS = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS) .

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215 mass, regardless of sampling date. In Experiments 2 and 3, frequent fertigations increased dry root mass. For plants sampled 13 DAS (Fig. 6-38), leaf area increased in quadratic fashion in response to applied N at all levels of fertigation frequency. Daily fertigation increased leaf area more than the other frequencies. For plants sampled 21 DAS (Fig. 6-39), leaf area increased in quadratic fashion in response to applied N when fertigation frequency was daily or every fourth day. When fertigation frequency was every two or three days, leaf area increased linearly to applied N. Daily fertigation improved leaf area more than the other frequencies. For plants sampled 28 DAS (Fig. 6-40), leaf area increased in quadratic fashion to applied N only when daily fertigation was applied, but increased linearly when fertigation frequency was every second to every fourth day. Leaf area increased at each level of fertigation frequency, indicating that both N and fertigation frequency were responsible for increased leaf growth. For plants sampled 28 DAS, there were no N by fertigation frequency interactions for pull force and pulling success (Table 6-9) . Pull force increased in quadratic fashion in response to applied N and fertigation frequency. The greatest force was required to pull out transplants fertigated every second day than all other

PAGE 224

216 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -•fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 8.6 9.1 10.0 11.8 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-38. Lettuce transplant leaf area response to N nutrition and fertigation frequency 13 days after sowing for Experiment 4, July 1995.

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217 60 APPLIED NITROGEN (mg/L) fertig freq 1 ; Q* feitig freq 2; L** -bfertig freq 3; L** fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 4 3 2 1 Treatment means 19.5 21.8 28.3 33.6 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-39. Lettuce transplant leaf area response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 226

218 120 APPUED NITROGEN (mg/L) feitig freq 1 ; Q* fertig freq 2; L** -bfertig freq 3; L** -*fertig freq 4; L** N at 60 mg-L'^ Fertigation frequency 4 3 2 1 Treatment means 32.8 39.4 4 6.2 59.1 All means are significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-40. Lettuce transplant leaf area response to N nutrition and fertigation frequency 28 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 227

219 frequencies. Pulling success increased in quadratic fashion to applied N, but was not influenced by fertigation frequency. Applied N dramatically improved pulling success from 0 % with 0 mg-L"^ to 8 8 % with 30 mg-L"^ and 98 % with 90 mg-L"^ For plants sampled 28 DAS (Fig. 6-41), leaf tissue N increased linearly in response to applied N. Fertigation frequency did not influence N concentrations in transplant leaves when 60 mg-L"^ N was used. For plants sampled 13 DAS (Fig. 6-42), RSR increased in quadratic fashion in response to applied N, regardless of fertigation frequency. Fertigating every fourth day increased RSR values compared to daily fertigation. For plants sampled 21 and 28 DAS, there were no N by fertigation frequency interactions for RSR (Table 6-9). Root: shoot ratios decreased in quadratic fashion in response to applied N. Root: shoot ratios also increased with less frequent f ertigations . For plants grown to 21 DAS (Fig 6-43), RGR increased in quadratic fashion in response to applied N at all levels of fertigation frequency. Daily fertigation led to greater RGR values than fertigating every fourth day. For plants grown to 28 DAS, there were no N by fertigation frequency interactions for RGR (Table 6-9) . Relative growth rate

PAGE 228

220 O it z m D (0 (0 F IL < liJ _J 30 60 90 APPLIED NITROGEN (mg/L) 120 fertig freq 1 ; L ** . fertig freq 2; L fertig freq 3; L' fertig freq A; L N at 60 mg-L'^ Fertigation frequency 2 13 Treatment means 1.55 1.59 1.59 4 1.67 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test Fig. 6-41. Lettuce transplant leaf tissue N response to N nutrition and fertigation frequency 28 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 229

221 0.230 60 90 APPUED NITROGEN (mg/L) 120 fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -^fertig freq 4; Q* N at 60 mg-L-' Fertigation frequency 1 2 Treatment means 0.28 0.33 3 0.38 4 0.42 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-42. Lettuce transplant root: shoot ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 4, Sep/Oct 1995.

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222 0.20-1 1 1 , 0 30 60 90 1 20 APPUED NITROGEN (mg/L) -Hfeitig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* fertig freq 4; Q* N at 60 mg-L'^ Fertigation frequency 4 3 2 1 Treatment means 1.17 1.31 1.35 1.38 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig, 6-43. Lettuce transplant relative growth rate response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 231

223 increased in quadratic fashion in response to applied N, but decreased linearly when the interval between each fertigation increased. For plants grown to 21 DAS (Fig 6-44), NAR decreased in quadratic fashion in response to applied N under daily fertigation, but decreased linearly when fertigation was every second day. However, NAR was unaffected by N when fertigation was every third or fourth day. Fertigation frequency did not affect NAR values. For plants grown to 28 DAS, there were no N by fertigation frequency interactions for NAR (Table 6-10) . Net assimilation rate decreased in quadratic fashion in response to applied N, but was unaffected by fertigation frequency. For plants sampled 13 DAS, there were no N by fertigation frequency interactions for SLA (Table 6-10) . Specific leaf area responded in quadratic fashion to applied N and to fertigation frequency. Fertigating every day or every fourth day led to greater SLA values compared to fertigating every second or third day. For plants sampled 21 DAS (Fig. 6-45) and 28 DAS (Fig. 6-46), SLA increased in quadratic fashion in response to applied N at all levels of fertigation frequency. In general, fertigation frequency did not affect SLA values. For plants sampled 13 and 28 DAS, there were no N by fertigation frequency interactions for LAR (Table 6-10) .

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224 3 0.50-1 1 1 1 0 30 60 90 1 20 APPLIED NITROGEN (mg/L) -sfertig freq 1 ; Q* fertig freq 2; L** -bfertig freq 3; NS -•fertig freq 4; NS N at 60 mg-L-^ Fertigation frequency 4 12 3 Treatment means 2 . 07 2 .18 2 . 40 2.46 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-44. Lettuce transplant net assimilation rate response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995.

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225 Table 6-10. Influence of N nutrition and fertigation frequency on growth characteristics of lettuce transplants for Experiment 4, Sep/Oct 1995. Treatment'' KT ^ C v~\ T ^ T T -f L Rnni" dSSj.ITLJ.XaL.XUii X^dX a rea ma s s iUCt O i9 rate area ratio ratio ratio (mg ' cin''^ ' wk M (cm''*mg"M ( cm^ • itig ' ) 13 Days After Sowing N (mg-L"M 0 A OA 30 0.82 0.55 f A u . y D A m y (J u . y / A T U . / 3 120 0 . y 0 A no 0 . / 0 Response Q** Q** F (days) 1 0. 84 0 . 65 2 0 . 78 0 . 58 3 0.80 0 . 58 A H 0.84 A C A 0.59 Response Q** Q** N X F NS 21 Days After Sowing NS * * * * N (mg-L'') 0 0.55 0.45 30 0. 66 0.34 oU 0 .74 0 .26 Qn y u 0 .78 0 . 22 0 . 82 0 . 18 RGSponsG Q** Q** r \ uoyS y 1 X 0 .75 0.25 2 A TO 0 . 72 0.28 3 A OA 0 . / 0 0 . 30 4 A £ O A O O 0 . 32 L* * L** N X F ** ** 2S Days After Sowing it ir NS NS IN \ mg jj ^ V i . U4 0. 15 0. 53 0.46 30 J. . 0 . 32 0 . 68 0. 32 60 1 TO 0.45 0 .76 0.24 J. . oU 0 . 51 0.82 0. 18 120 1.22 0 . 57 0 . 86 0.14 Response Q** Q** Q** Q** F (days) 1 1.76 0.41 0.76 0.24 2 1.74 0.39 0.74 0.26 3 1.73 0.40 0.73 0.27 4 1.86 0,40 0.71 0.29 J?espo/3se NS NS L** L** N X F NS ** NS NS NS = nitrogen; F = fertigation frequency. Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS) .

PAGE 234

226 0.8 It 0.2O lU 0 30 60 90 1 20 APPLIED NITROGEN (mg/L) ^ fertig freq 1 ; Q* feitig freq 2; Q* -bfertig freq 3; Q* -ifertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 12 3 4 Treatment means 0.61 0.67 0.70 0.71 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-45. Lettuce transplant specific leaf area response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995,

PAGE 235

227 0.8 t 0.2O lU 0 30 60 90 1 20 APPUED NITROGEN (mg/L) ^ fertig freq 1 ; Q* feitig freq 2; Q* -afertig freq 3; Q* -•fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 3 2 4 1 Treatment means 0.58 0.58 0.59 Q. 61 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-46. Lettuce transplant specific leaf area response to N nutrition and fertigation frequency 28 days after sowing for Experiment A, Sep/Oct 1995.

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228 Leaf area ratios increased in quadratic fashion in response to applied N. For plants sampled 13 DAS, daily fertigation increased LAR, while for plants sampled 28 DAS, fertigation frequency did not influence L7^ values. For plants sampled 21 DAS (Fig. 6-47), LAR increased in quadratic fashion in response to applied N at all levels of fertigation frequency. Daily fertigation resulted in greater LAR values than fertigating every third or fourth day. For plants sampled 13 DAS, LMR (Fig. 6-48) increased in quadratic fashion, while RMR (Fig. 6-49) decreased in quadratic fashion in response to applied N. Daily fertigation led to greater LMR values than fertigating every third or fourth day. However, fertigating every third or fourth day led to greater RMR values than daily fertigation. For plants sampled 21 and 28 DAS, there were no N by fertigation frequency interactions for LMR and RMR (Table 610) . Leaf mass ratios increased in quadratic fashion, while RMRs decreased in quadratic fashion in response to applied N. Leaf mass ratios decreased when the interval between each fertigation was increased, while RMRs increased when the interval between each fertigation increased. Results of Experiment 4 in the fall indicated that, overall, high quality transplants could be produced with 60 mg-L-i N, supplied daily to every fourth day via floatation irrigation, especially when evaluating transplant quality

PAGE 237

229 0 30 60 90 120 APPLIED NITROGEN (mg/L) fertig freq 1 ; Q* fertig freq 2; Q* -bfertig freq 3; Q* -•fertig freq 4; Q* N at 60 mg-L'^ Fertigation frequency 3 4 2 1 Treatment means 0.45 0,4 9 0.50 0.55 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-47. Lettuce transplant leaf area ratio response to N nutrition and fertigation frequency 21 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 238

230 30 60 90 APPLIED NITROGEN (mg/L) 120 fertig freq 1 ; Q* feitig freq 2; Q* -efertig freq 3; Q* fertig freq 4; Q* N at 60 mg-L-^ Fertigation frequency 4 3 2 Treatment means 0.70 0.72 0.75 1 0.78 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-48. Lettuce transplant leaf mass ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 239

231 0.5 0.10-1 1 — , , 0 30 60 90 1 20 APPUED NITROGEN (mg/L) fertig freq 1 ; Q* -4fertig freq 2; Q* -efertig freq 3; Q* -ifertig freq 4; Q* N at 60 mg-L'^ Fertigation frequency 12 3 4 Treatment means 0.22 0.25 0.28 0.30 Means connected by a common line are not significantly different at 5 % level. Mean separation of fertigation frequency by Bonferroni test. Fig. 6-49. Lettuce transplant root mass ratio response to N nutrition and fertigation frequency 13 days after sowing for Experiment 4, Sep/Oct 1995.

PAGE 240

232 based on dry root mass 28 DAS. Quality transplants had dry root mass of about 23 mg and dry shoot mass of about 75 mg, 28 DAS. Regardless of season of the year, there were similar trends in the response of dry shoot mass to applied N and to fertigation frequency. However, the greatest dry shoot mass was obtained in Experiment 3, which was conducted during the summer, potentially due to the greater light intensities and temperatures . The concentration of N necessary for obtaining high quality transplants, especially in terms of root growth, was seasonally related. High quality transplants were obtained with 60 mg-L"^ N in Experiment 2 (Feb/Mar) and Experiment 4 (Sep/Oct) supplied every other day, but with 30 mg-L"^ N supplied daily in Experiment 3 (July) . The corresponding levels of tissue N at these N concentrations were approximately 20, 15, and 16 g-kg-^ for Experiments 2, 3, and 4, respectively. The seasonal response of dry root mass to applied N was probably related to proportionally greater shoot growth compared to root growth during July due to higher temperatures and probably higher sunlight intensities for a longer duration than the other two growing periods. Shoots grew at the expense of root growth. Roots are, therefore, weaker sink during periods of excellent shoot growth. Root

PAGE 241

growth was improved at lower minimum temperatures (20 °C) than at higher minimum temperatures (26 or 28 °C) . However, pulling success was adequate in both Experiments 2 and 4, compared to Experiment 3, indicating that very high temperatures were detrimental to pulling success. When considering the optimum N level of 60 mg-L'^ for fall and spring, the optimum RSR ranged between 0.24 and 0.27. The optimum RSR during summer was 0.33 with 30 mg-L"^ N. Regardless of season, RGR increased in response to applied N, while NAR decreased in response to N. However RGR and NAR values were greater at 21 than at 28 DAS, implying that younger transplants had higher efficiency of growth than older ones. Specific leaf area and LAR were less affected by seasonal differences, and they were both improved by applied N as well as frequent f ertigations . In Experiment 2 (spring) and Experiment 4 (fall), 60 mg-L-^ N applied every second day through floatation irrigation generally was shown to produce high quality transplants. Nitrogen at 60 mg-L'^ applied every other day was next used to determine if N applied at different times during the growth cycle for transplant production was a factor in promoting root growth. Growth at this N level was compared to growth of transplants receiving no N. Experiment 5 was conducted during the spring, similar to Experiment 2, under greenhouse temperatures ranging from

PAGE 242

234 16 to 37 °C (Fig. 6-50) . The average daily maximum media temperature was 32 °C, while the average daily minimum media temperature was 22 °C. During the course of the trial, there were 19 sunny and 8 cloudy days, with rain during three of the cloudy days. For plants sampled 22 DAS, fresh shoot mass was least at 0 N, and greatest at 60 mg-L"^ N applied every two days over a 28-day growing period (Table 6-11) . Dry shoot mass was also least at 0 N during this sampling date, but greatest at 60 mg-L'^ N applied at the first 14 days or the entire 28-day growing period. For plants sampled 28 DAS, fresh and dry shoot mass were least at 0 N, and greatest at 60 mg-L'^ N applied every two days for the entire growing period. Plants grown with 60 mg-L"^ N during the first 14 days, allocated more dry matter to shoots than plants grown with 60 mg*L"^ N applied every four days for a 28-day growing period. For plants sampled 22 DAS, transplants which received N at the second half of the growing period, apparently partitioned N for shoot growth rather than root growth. Roots of these transplants were not any larger than those grown with 0 N. For plants sampled 22 and 28 DAS, fresh or dry root mass was greatest with 60 mg-L'^ N applied every two days over the entire growing period. By the last sampling date, root length, area, and diameter were least

PAGE 243

235 40 50~n I — I — I — I — I — I — I — I — I — I — I — I — i— I — I — I — I — I — I — I — I — I — I — I — r 5 30 GROWING PERIOD (APR 5 APR 30) air max media max air min media min Fig. 6-50. Maximum and minimum air and media temperature during transplant production for Experiment 5, April 1995.

PAGE 244

236 •M 0 c tir > i3 0 01 P O fe af w Id IQ p C IB iH a W e Id M 4J o; u 3 4J 4J 01 H M-l 0 • U Ol •H Ol J-1 iH H i-l -W (U M 4J U m M 10 ID Xi u 4-1 c H U V 4-1 (1) E 10 •H 4J O O a -D 4J (0 O 0) o w 4-1 4-1 & O C 0 dJ OC rH 4-1 10 >1 O to M 0 (0 QMS o 10 >i O 10 M X (0 Q 10 g JZ 4J 10 o v o in in in ic E c o C 4.) (1) (0 C71 Di U C O -H H M rH e 4J Q, H -H a H C (0 n CTi o OD rO ^ OO ID O M rH lO CNJ "S" C— cc o (NJ o m vD O rH tn in "3" rH CM rH ro c c r1 0 0 C/5 10 fNJ E iH In u 0) CJ 4J 4J >*H Mh to to >1 >, ? 0 0 u Q Q CM 00 CM CN in 00 o cN i~ r~ n o ^ •^i> rH Cv) rH CM « rH 00 ro IT) in 00 ^ in O rH (N CM CM ro 00 CM 00 CM O ro o in rH lO 1X1 CM ^ o in 00 U3 rin in VD rH rjrH ID O Q 1(N «r in W E-" H E-i H rj in o in o o o o o o rH rH rH rH lO rH CM rH 00 ro o (NJ rH (NJ CM o o O O O O o Q "an rH rsj ID rH cri ro ID in vo in 00 o CM in [ rH ID Cvj (Tl CM CM CM CM in VD CM CTl rH ro rH CM ro CM ro ro o O o o o o o 00 r~ CTl in (T> ro rH o CM CM ro 00 CTl O CM CM T in o , — 1 , — 1 CM ro CM ro in VD l£) CTl V in CM in CM 00 ro 00 00 CM CTl rin ro rn ro IN ro in rH cn r00 00 in CTl oo CM m O 01 rro rH rH o rH rsj o o r~ m cn CM o 00 00 CTl rH Q -I , CO H M >, '-I tt) IB 1^ > T3 o 4-1 >, M > cn c •H 0) ja c o •rt 4-1 IB cn •H M M 2 XI ttJ 4-1 J (B • cn cn -H E M M O -H VD 2 II 3 O l*H >. M OJ > 0) D 0) V -H J rH » cn 0 e o " 4J H a (B Is 4J CP c cn .H S o M M H cn p T3 lU 00 N (M II (B ItH o > HI TI 4-1 O f -H cn M -H m M r( C 2 •rH 3 7 O J M • cn cn E >i IB O

PAGE 245

237 with 0 N, and greatest with 60 mg-L"'' N applied every two days for the entire growing period. For plants sampled 22 and 28 DAS, both leaf area and transplant height were least with 0 N and greatest with 60 mg*L"^ N applied over the 28-day growing period (Table 611) . By the termination of the experiment, transplants grown with 0 N or with 60 mg*L"^ N applied at the second half of a 28-day growing period, could not be easily pulled from the transplant flat. Transplants broke after only minimal pull force was applied compared to the treatments which pulled successfully. Petiole sap NO3-N was not sampled for transplants grown with 0 N because they were too small to obtain a necessary quantity of sap for testing (Table 6-12) . The greatest concentration of NO3-N was in leaves of transplants grown with 60 mg-L"^ N applied every four days. However, the highest concentration of tissue N was obtained in leaves of transplants which received N only in the second half of a 28-day growing period, probably because N was now available. For plants sampled 22 DAS, RSR was greatest with 0 N, and least with 60 mg-L"^ N applied every second day only in the first half of a 28-day growing period. For plants sampled 28 DAS, RSR continued to be greatest with 0 N, but least with 60 mg-L"' N applied every two days for a 28-day growing period.

PAGE 246

238 in -H (TJ IB o r-{ 4-1 0) (0 m O m (u -p (u M n) (0 M •H 6 U >H « • II) (0 0) '^g c o •H (0 H Hi U j-> n 4-) • 0) U) IT) D> 2 m M e > H J3 m S o M tp 1-1 10 u o 4J O -H O O 4J O (0 o: 0] M CD 00 O l£> CNJ T CNj n r\j (Nj o CTl in Oi CM 'a' (N) r\J CNj (Nj o o o o o o o (N) O 00 '3' tSJ iT) r~ r~ o o o o o o o iH lD n rH ^ CM in rr1^ ro o o o o o o rH n CTl VD CM iH CM 00 CM ro o o o o o o o O Cn T-l CM 00 CM tH t-H "SCM 00 O 3 <4-l 10 +J 2 01 (1) (0 M (U (0 •^^ ? c o >t-l -H o c -u OJ (0 Di en u C O -H M M e -u a H c m o o o o o o tH O 00 CD 00 CM 00 I/O 00 "a" o o o o o o o ai UO VD iH UO CM i-H CM iT) PO O o o o o o o o o o o o o c o Q 00 rvj 00 CM •g ,5 iH rO CD .H o ^ OO O lO 00 rH •M Q 00 00 CM rCM oo CO 'T IT) VO 1-1 00 cn "3" 00 >i) CTl CM 00 00 O o o ^ o o o r00 vo .-( ^ lo ^ in t/3 H Eh H Eh Eh J I M 00 >i CM TJ ns 4J (0 m >, ^ nj H T3 , " o ^ 1 M w J >, > to •0 o 0) 2 4J 4J (0 & >, H M M (1) w > •H (1) >, 10 T3 o s >, M 0) > (U c •H 0) XI c o •H 4J 10 Cn H M M •H J-) D XI (0 >, (0 TI 2 V D 0) 0 4J vt-l ID cn >i •H M E M i) M > o •H II 2 ed •w IN Eh a Di a in e m o 2 O " >i . ^. -D O H M (U a w (0 T3 01 2 >, O -o I D 00 N csj II (0 „ ^ ^•D » 0) T3 +J 0 * H Oi M OJ 14 ?2 •rH S 7 O M . 01 0> E >, 10 o TI VD

PAGE 247

For plants grown to 2 8 DAS, RGR was least with 0 N, and greatest with 60 mg-L"'' N applied every two days in the second half of a 28-day growing period. Net assimilation rate was least with 60 itig-L"^ N applied every second day over the entire growing period, and greatest when N was applied in the second half of a 28-day growing period. Therefore, transplants which received N during the second half of a 28-day growing period had a high efficiency for growth due to the now available N. For plants sampled 22 and 28 DAS, SLA and LAR were greatest with 60 mg'L'^ N applied every second day during the second half of a 28-day growing period. For plants sampled 22 DAS, LMR was greatest with 60 mg-L"^ N applied every second day during the first week of a 28-day growing period, and least at 0 N. For plants sampled 22 DAS, transplants grown with 0 N had the greatest RMR values, followed by plants produced with 60 mg-L"^ N applied during the second half of a 28-day growing period. By 28 DAS, RMR continued to be greatest in plants grown with 0 N. Experiment 5 demonstrated that frequent applications (every second day) of 60 mg-L"^ N throughout the period of transplant growth led to more root growth than applying N at the first half or last half of a 28-day growing period. Also, less frequent applications (every fourth day) of N reduced root and shoot growth compared to applying N every

PAGE 248

240 other day. However, all transplants could be pulled from all the treatment flats except for 0 N for 28 days or 60 mg-L"^ N applied in the second half of a 28-day transplant growing period. Pull force ranged from 0.021 to 0.028 N for quality transplants. It appeared that N supplied earlier was needed for continued root growth during the 28-day growing period. Widders (1989) reported that for tomato transplants grown under a moderately low mineral nutrient regime, increasing N during the last 10 days before transplanting led to the best quality transplants. Field Experimpnt.q Plants from Greenhouse Experiment 2 (spring) and Experiment 4 (fall) were grown to maturity to evaluate the effects of pretransplant N and fertigation frequency on earliness, yield and lettuce head quality. Harvesting was done at head maturity in the spring crop of Experiment 1 on 11 May or 16 May (Table 6-13) . Plants grown with pretransplant N at 0 or 30 mg-L'^ were harvested 54 days after transplanting (DAT) . Others were harvested 49 DAT. At harvest, there were no N by fertigation frequency interactions for head mass or head quality characteristics. Head mass was increased by pretransplant N to 60 mg-L'^ but was unaffected by pretransplant fertigation frequency.

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241 o , ^ CP 1-^ -p ^ — 0) CP s-i c O Q) u ^ 0) -P 0) -M -H W 13 u 0) -p 0) ro to 0) -H C -p dj tt) X x; c S-l -P T3 W ro u) Q) ro rVD r CM CO ^ q< »3< <3< <3< 2 CM CNj rvj CNj CNj K + >H rrU3 o rCTi lh IT) tn + ri-t 5' ^ 00 O 00 00 w ^ LO T ^ ^ 2 in in i-H 00 CM C ro ixi o CTi >^ (» rrr^ 1^ O IX) • • • • OT in I* T 2 2 CM (N CM CM CO CO IX) cTi 2 2 IX) IX) 1X> IX) CO CO cn CTi CT^ CD 2 2 CM CM CNJ CM CO CO CM ro tH ^ 2 2 ro ro n n r00 00 ro ro n ro CO CO 2 2 00 CX3 00 cr^ ^ ^ ^ CO CO 2 2 CO CO iH (Tl >T (M 2 2 ro in U5 IX) rr~ CP o E o o o CNJ ^ o ro *X) cn rH to m C >i 0 ro ^ .H CM 00 o 0) ft; VJ 0) (D » — 1 4—' ' Q fU o u iH II in X w in 0 o +J ro >1 II 4-1 u c c iH ro 01 u D U -H CP 0 H-l (L) -H Q) C ip CP ro H C u w o w H p) ro 0 ro CP c •H o CO -P 2 Q) V) ip 0) c E u •H C T3 Q) ro CP (U x; u " -I ro u • ro • O 2

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242 Lettuce head height, head diameter, and stem diameter were increased by pretransplant N. Plants grown with 0 or 30 mg-L"^ pretransplant N were late maturing and consequently had elongated cores at harvest. Therefore, larger plants at transplanting due to pretransplant N can be important for earliness. At harvest, tissue N levels were equal regardless of pretransplant N applied, and ranged from 24 to 25 g-kg"'. Hochmuth et al. (1991) reported values of 20 to 30 g-kg'^ (soil type not reported) to be indicative of an adequate range for crisphead lettuce. All treatments in the fall crop of Experiment 2 were harvested in December, 64 DAT (Table 6-14) . There were no N by fertigation frequency interactions for head mass or head quality characteristics. Head mass was increased by pretransplant N to 90 mg•L-^ but was unaffected by pretransplant fertigation frequency. Firmness, head height, head diameter, and stem width were all increased by pretransplant N. Core length was enlarged, indicating an effect of pretransplant N on earliness. Neither of these quality parameters were influenced by pretransplant fertigation frequency. Leaf tissue N was not influenced by pretransplant fertigation frequency. Leaf tissue N was greatest in plants with no pretransplant N. This was probably a dilution effect since heads were smaller at this level than all the other

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243 C o •rH 4-1 O U (M 0) CO •D 4-1 x; n x: C E -H M 4-1 U4 M •a n 0) to = E CP 4-1 u c O (U U -1 M 0) 4-1 0) E E 0) m 4-) -H CO 73 0) 4-1 0) •D E "a* CM (N iH ^ T ^ ro n m ro n + (Ti rin n O CM n ^ in in iH P00 o CM (M CM CM n K (31 CN) CX5 O O CD O H ix> CO in ^ ID IX) IXI IX) CM T CVJ in ^ T c^ CO CO 2 Z CO CO in in tH 2 2 <^ ^ ^ ^ CO CO rrr^ 2 2 CM CM CM CM 00 >X) o r~ rH r-l CM ^ CO CO 2 2 r~ lO 00 in CO CO 2 2 ^ ^ IX) in • • • • CO CO T '3' >T 2 2 CTi 00 in m en cn o iH in in ^X) CXI CO CO 2 2 D) E — o o o o n IX) cTi (U „ CO CO C >, O rt! CO — (D a; u, iH CM ^ CO c O 6u CO X q: 2 4-1 u to a £ o u CO o o >, u c Q) D cr cu u CM c o 4-1 ro Di H 4-1 u (U (0 u to c o CO CO (U c E k4 -H C T3 x; u

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244 treatments. Hochmuth et al. (1991) reported values of 20 to 30 g-kg"^ (soil type not reported) to be indicative of an adequate range for crisphead lettuce. The values of tissue N in this experiment were about 34 g-kg'\ indicating that sufficient N was supplied to the plants. The lettuce production season is from September to May in Florida. Consequently, if plants are left later than mid May they will bolt and in north Florida if they are unprotected and left later than mid December, they will likely freeze. Plants which were grown with 0 or 30 mg-L"^ pretransplant N were small at transplanting and matured later than those produced with at least 60 mg-L'^ N during spring planting. Plants were, therefore, harvested later and consequently had elongated cores, which is an indicator of poor lettuce quality. During the fall planting, all plants were harvested at the same time even though those which were produced with no pretransplant N were less mature. If the plants were left longer in the field, they would have frozen. A similar result was described in Chapters 3 and 5 where plants produced with no pretransplant P or pretransplant N matured later because they were small at transplanting. Earliness is extremely important to the lettuce producer and transplant nutrition can affect earliness .

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245 Summary In order to determine N concentration and fertigation frequency required for production of high quality transplants and subsequent high yield, 'South Bay' lettuce transplants were fertigated every day or every second, third, or fourth day with N at 0, 30, 60, 90, or 120 mg-L'^. A quality transplant is defined as one that can fill a 10.9 cm^ tray cell with roots, to facilitate easy removal of transplants from the transplant flat, and for rapid field establishment. Nitrogen concentration and fertigation frequency that resulted in quality transplants were subsequently used to determine if N applied at different times during transplant growth, was a factor in promoting root growth. To avoid inconsistency in the duration of the light period, natural photoperiod was extended to 16 h in all experiments. Regardless of fertigation frequency, applied N increased dry shoot mass, leaf area, pull force, pulling success, leaf tissue N, RGR, SLA, LAR, and LMR, but reduced RSR, NAR, and RMR. In general, the effect of N on transplant growth was enhanced by frequent f ertigations . Nitrogen at 30 mg-L"^ during the summer or 60 mg-L"^ during fall and spring, maximized root growth, provided that fertigation frequency was daily (summer) or every other day (spring) .

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246 In general, pulling success was reduced during summer compared to spring or fall crops. Low pull force was associated with low pulling success due to the root systems not pulling out completely from the transplant flat. Quality transplants had dry shoot mass of no more than 136 mg, dry root mass of at least 23 mg, RSRs ranging from 0.30 to 0.48, with leaf tissue N ranging from 16 to 23 g-kg'^. Pretransplant N of 60 mg-L'^ led to increased head mass at harvest and reduced time to maturity. Earliness is extremely important to the lettuce grower and transplant nutrition can affect earliness. In investigating the significance of when N was applied during transplant growth, 60 mg-L"^ N applied every second or every fourth day throughout the period of transplant production or at the first half of a 28-day growing period, led to more root growth compared to N applied at the second half. Therefore, N is more important earlier in growth than later on, for production of quality transplants. This work demonstrated that at least 60 mg-L"^ N applied every other day via floatation irrigation to a peat+vermiculite media was required for production of high quality transplants during fall and spring, which led to more lettuce head mass at harvest and reduced time to maturity. During summer, 30 mg-L"^ N applied daily was adequate for production of quality transplants.

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CHAPTER 7 SUMMARY Lettuce transplants grown with floatation irrigation system often show limited root growth, resulting in root systems not pulling out completely from the transplant flat, and poor establishment in the field. In the present investigation, 'South Bay' lettuce transplants, grown in a peat+vermiculite media in the greenhouse, were fertilized with varying concentrations of N, P, and K via floatation irrigation at selected fertigation frequencies, to determine optimum nutrient and water management requirements for production of high quality lettuce transplants, with sufficient roots to fill a 10.9 cm^ tray cell, and that ultimately establish in the field rapidly. To avoid inconsistency in the duration of the light period, natural photoperiod was extended to 16 h in all experiments. To determine the optimum P concentration necessary for production of high quality transplants, plants were propagated by floating flats in nutrient solution containing either 0, 15, 30, 45, or 60 mg-L'^ P in summer and fall experiments, and either 0, 15, 30, 60, or 90 mg-L"^ P in 247

PAGE 256

248 factorial combination with 60 or 100 mg-L"' N in a winter experiment. When the concentration of P in the media (saturated paste extract) was more than 12 mg-kg'^ (summer experiment), P at 0, 15, 30, 45, or 60 mg-L"^ sub-irrigated every two to four days, did not influence fresh or dry root mass. However, when the concentration of P in the media was about 0.5 mg-kg'^ (fall experiment), fresh and dry root mass increased with each level of P f ertigatigated every two to four days. When the fertigation frequency was every two days (winter), fresh and dry root mass increased in response to 15 mg-L"^ P, with no further increases in root mass at higher P concentrations up to 90 mg-L'^ even though the media P concentration was only 0.4 mg-kg'^. The major transplant growth responses to applied P occurred between 0 and 15 mg-L'^ P, regardless of fertigation frequency and media P concentration. Added P increased fresh and dry shoot mass, root length and area, leaf area, pulling success, leaf tissue P, relative growth rate (RGR) , specific leaf area (SLA), leaf area ratio (LAR) , leaf mass ratio (LMR) , but reduced root: shoot ratio (RSR) , net assimilation rate (NAR) , and root mass ratio (RMR) . Only about 30 % of plants grown with 0 P could be pulled from the transplant flat, compared to approximately 90 % pulling success with any level of applied P. Quality transplants had dry shoot mass of not more than 115 mg, dry root mass of at

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249 least 21 mg. Rootrshoot ratio of 0.25 and leaf tissue P of 4 g-kg"^ can be considered optimum for production of high quality transplants. All pretransplant P concentrations had similar effects of increasing head mass at harvest time, and reducing time to maturity regardless of season. At transplanting, plants grown with pretransplant P were larger than those grown with no P. Therefore, larger plants at transplanting led to earlier harvests, and larger head size at harvest. This work demonstrated that at least 15 mg-L"^ P, supplied every two days via floatation irrigation, was required for production of high quality lettuce transplants in a peat+vermiculite media that contained less than 0.5 mg-kg"^ P (saturated paste extract). Floatation fertigation with K at 0, 15, 30, 45, or 60 mg-L"^ K applied every two to four days, increased fresh and dry root mass when the concentration of K in the media (saturated paste extract) was less than 15 mg•kg"^ but with higher media K (24 mg-kg"M/ root mass was unaffected. Fresh and dry shoot mass, leaf area, RSR, RGR, LMR, and RMR were unaffected by applied K, regardless of the initial K concentration in the media. Plant available K in the media (11 to 24 mg-kg"^ K in the saturated paste extract) may have supplied the K needs during lettuce transplant growth and development. In an experiment comparing 60 with 100 mg-L"^ N

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250 at various levels of K, applied K did not influence SLA at 60 mg-L'' N, while at 100 mg-L"' N, SLA increased at each level of applied K. Lettuce growth and yield in the field was not affected by pretransplant K fertilization. Since transplants had minimal response to K in peat+vermiculite media, peat+rockwool media (2.5 mg*kg"^ water extractable K) was used to investigate lettuce transplant growth in a mix inherently low in K. The benefits in promoting improved lettuce growth by supplementary light for 16 h or extended photoperiod to 16 h were also evaluated. Potassium at 60 mg-L"^ increased shoot and root mass, leaf area, petiole sap K, leaf tissue K, and RGR, but did not influence RSR, SLA, LAR, LMR, or RMR. It was observed that transplants grown with 0 K in peat+rockwool mix could not be easily removed from the transplant flat. Stems broke during removal, rather than breaking at the root-shoot interface similar to transplants that received no N or P fertilizer. Under periods of low light intensity, supplemental lighting (250 Mmol-m-^-s"^ photosynthetic photon flux) led to improved transplant root growth. Potassium fertilizer programs revealed that supplemental K may not be required in a peat+vermiculite mix using a floatation irrigation system, since vermiculite supplied adequate K to the growing seedlings. In a

PAGE 259

251 peat+rockwool mix, at least 60 mg-L'^ K is recommended to produce a transplant with sufficient roots and a strong stem to facilitate ease of transplant removal from the transplant flat. Nitrogen was the nutrient with the greatest impact on lettuce transplant growth. Nitrogen at 0, 15, 30, 45, or 60 mg-L"^ sub-irrigated every two to four days, increased fresh and dry shoot and root mass, leaf area, transplant height, stem diameter, RGR, SLA, LAR, and LMR, but reduced RSR, NAR, and RMR. Transplants grown with 60 mg-L'^ N were about 80 mm tall, had dry shoot mass ranging from 55 to 73 mg, dry root mass ranging from 15 to 22 mg, and RSR ranging from 0.23 to 0.32, and leaf tissue N ranging from 15 to 17 g-kg-^ It was observed that transplants could not be easily pulled from the transplant flat at all levels of applied N in these experiments. When the mean dry root mass was less than 20 mg, pulling success was observed to be even more reduced. Nitrogen at 60 mg-L'^ was perhaps not adequate with the irrigation programs used. Therefore, additional experiments were designed to investigate the effect of N fertilization to 120 mg-L'^ and fertigation frequency on lettuce transplant growth and development. In the field, lettuce head mass was improved at harvest by pretransplant N. The heaviest heads were obtained from plants grown with 60 mg-L"^ pretransplant N. In the

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252 greenhouse, transplants grown with 60 mg-L"^ N also had the greatest shoot and root mass. Nitrogen fertilizer programs revealed that at least 60 mg-L'^ N supplied every two to four days via floatation irrigation, was required for improved transplant shoot and root growth in a peat+vermiculite mix low in NO3-N. Transplants grown with 60 compared to 15 mg-L"^ N were larger at transplanting, resulting in improved head mass at harvest. To determine the optimum N concentration and fertigation frequency, transplants were fertigated every day or every second, third, or fourth day with N at 0, 30, 60, 90, or 120 mg-L"^. Nitrogen concentration and fertigation frequency that resulted in quality transplants were subsequently used to determine if N applied at different times during transplant growth, was a factor in promoting root growth. In order to determine the seasonal effect of N fertilization practices on lettuce transplant growth, the N by fertigation frequency experiments were conducted in spring, summer and fall. Regardless of fertigation frequency, N from 30 to 120 mg-L"^ increased dry shoot and root mass, leaf area, pulling success, leaf tissue N, RGR, SLA, LAR, and LMR, but reduced RSR, NAR, and RMR. The concentration of N necessary for obtaining high quality

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253 transplants, especially in terms of root growth, was seasonally related. High quality transplants were obtained with daily fertigation of 30 mg-L'^ N in summer, and with 60 mg-L"^ N in the fall or spring, supplied every other day via floatation irrigation. Therefore, N concentration and fertigation frequency must be considered together. Pulling success was improved from less than 16 % with 0 N to about 88 % with the initial N application of 30 mg-L'^ in the spring and fall experiments. In general, pulling success was reduced during summer compared to spring or fall crops, indicating that very high temperatures (average daily maximum media temperature of 38 °C) were detrimental to pulling success. Quality transplants had dry shoot mass of not more than 136 mg, dry root mass of at least 23 mg, RSRs ranging from 0.30 to 0.48, with leaf tissue N ranging from 16 to 23 g-kg-i. Pretransplant N, but not fertigation frequency, improved head mass at harvest and reduced time to maturity. This is of particular significance in northern Florida where the growing period for lettuce is short. Earliness is extremely important to the grower and transplant nutrition can affect earliness. In investigating the significance of when N was applied during transplant growth, 60 mg-L"^ N applied every second or every fourth day throughout the period of transplant

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254 production or at the first half of a 28-day transplant production period, improved root growth compared to N applied at the second half. Transplants had 100 % pulling success compared to 5 % pulling success when transplants were grown with 0 N or with 60 mg-L"'' N applied every second day during the second half of a 28-day growing period. Therefore, N is more important earlier in growth for production of quality transplants. Quality transplants were 54 mm (N applied every fourth day) to 87 mm tall (N applied every second day) , had dry shoot mass ranging from 79 to 133 mg, dry root mass ranging from 33 to 4 6 mg, RSRs ranging from 0.34 to 0.41, with leaf tissue N ranging from 5 g-kg"^ (N applied during the first half of a 28-day growing period) to 17 g-kg-^ (N applied for the entire 28-day period). Transplant height, therefore, could be controlled by increasing the period between each N application, and fertigating every second day with the other nutrients. In conclusion, a quality transplant can be produced with no supplemental K if media K (saturated paste extract) is at least 15 g-kg'^ Phosphorus at 15 to 30 mg-L"^ P, applied every other day, is adequate for production of a quality transplant all-year-round if media P (saturated paste extract) is less than 12 g-kg-K Nitrogen at 30 mg-L'^ applied daily during the summer or 60 mg-L"^ N applied every other day during the fall or spring can be considered

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255 adequate for production of a quality transplant. Pretransplant N at 60 mg-L'^ in the spring or fall led to more lettuce head mass and reduced time to maturity. A quality transplant should be about 80 mm tall, fill-up a 10.9 cm' tray cell with roots in 28 days to facilitate ease of removal from the transplant flat, have dry root mass of no less than 25 mg, and dry shoot mass of about 100 mg to achieve a RSR of approximately 0.25. Adequate tissue levels for N, P, and K are about 17, 4, and 40 g-kg~\ respectively. Identifying and understanding the differential growth responses of roots and shoots of lettuce transplants to N, P, and K fertilization, has provided new guidelines for the production of quality transplants. Quality lettuce transplants can be produced with lower fertilizer inputs than growers are currently using, which could lead to lower transplant production costs, and reduce the risk of polluting the environment. The fertilizer and irrigation programs for lettuce could potentially be applied to other vegetable transplants.

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APPENDIX A PHOSPHORUS EXPERIMENTS

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257 m m m p c (0 . . JJ o H P 0 •H a 0 0 o n 0 x; (D C DC (0 M (0 p ce ) tu 4-1 3 10 • +j (T3 V fH M 4-J O rH 1-1 0 ics Mh fO CM e u +^ (0 (U 0} d) 1 ^J] (0 o (U * — ' 1 \ fT) L) — cn E M rH a) -p to m >, 0 to IT) o (C U .H n Q u E o cr w — o \ O (U c £ c nj 0) x: n to X) 0) o to c 0 to E m rH p J-1 0 c 0 tt) e •H JJ CP 0 to 0 0) >, o to t-i a u x; 10 x: Q to o 0) u f— j u c u m o x; 4-1 nj c to 0 to > o (U o to Di -H x; (TJ by to E O -H M m -P •H 3 •D m C >i << J3 c 0 •H 4-1 U rtj 3 O n c\j rO rH + rH CM ro o ro 1-^ CD + IT) rH in in cri «3< O rH ro (T^ ^ "3" CD , m Q o tn (U 4J 00 o o oj rH ^r q IS 0 to (IJ •p (Ti ro eg rH in CD CTl CD rH rH JJOCvJrH JJcMtDro •^rocDin "^tDor-^ IOy3mini5°^f^ rH "-l rH eg rH CM + r-~ o o r-{ * * rH O rH CM rH CM c tu e 4-> m tu u Eh U O u u 4J c e 4-1 nj u c o •H 4-1 nj u rH O a u Q) U ro 00 00 VD CM rH ro ^ ro CM C o -H 4-1 ro u •rl in r-< O a M 4J C (U 4-1 UJ 0) >H Eh 4-) nJ 4J c: to u H >H H C 01 •H to to (U 3 H >

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258 u 3 P •P 0 n u H *i m •H u V u « u 10 u O M 0 w •rH (0 w o (U (0 e M o 10 •H 0 in -M O 0 « (0 i-H ec e 0 E u + (0 H rH n cn 00 (0 0) ^ in rH m 'J' 00 (U M (0 ri-H J (0 M b CNJ rH o u O C O W -H 0) -P u CNJ 00 OO 00 kD i-H CM 00 rrH 00 IT) 'J' 00 rr rH O rH IT) ID rH u e bi •r( >»H •H g 2 U >4H ID U 0 0 (U (0 0) to to a OJ M CO rH 10 o >H rH 0) *H c to o s >, •H ID ID 4-) Q Q (0 E rH u •-i H •-H e H i) E 4J in 4J (1) in (0 ri 2 (0 M b E u m rH o M OJ M H CC ti] tn c; o + csj in to CM CNj ro 00 IX) o CM u IB Q CM . iT) CM O CJl rH IT) rH + 00 rH 00 rm rH r•^f n CM c o 4-> -H C 4J ^ n CM c o C -P 0) m u M o UJ P P -H m c o P -H C P lU 10 E u P -H P 10 rH O 0) a p P lU P E-" a u p n) .p B lit u •H >P H C o> H n m lU 3 H 10 >

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259 >i Ji V t) 4J U t) t-l « M c ro rH a M C ID U 4J 0) u 3 4J U V x; u IT) (Tl jj O) 0 iH o i .H M (0 0 M-l c 0 ro •H 1 4J < •H 4J .H P XI c 10 Eh 3 U >t-i m 0) 4-1 0) E O ro O -H x: •P Di O C O (1) a: ^ 4-1 >i o M o Q M x: to 4-) n 01 o M Mora 4J o o u) E Q w E o x: P 10 o 0) o M x; n 5* Bl E •o O m t) u M 3 O w * O (X) n + * r03 00 C\J CTi o "ain 00 c-H CNJ VO o ro n m lo CTl CTl ^ (NJ 00 CSJ VD IT) 00 00 in 00 en rrin r-i ro c o to 0> E to O rH Da tn r~in Cn C •2 o to »H (U U >M r~ 00 ro to ro (NJ t>, ^ 10 03 Q o o in o CM ro 00 rro I" in i-H CM ro q •2 o to 0) u to T tJ C\) q ,-1 00 ro ro CTi in tH CTl + in cn CM CM rH CTl CM ro ro + ro u> 00 VD O in t-i o ro CM ro 00 in r-i ro O T-i CM ro CM C O 4-1 -H + o in .-I 00 o VD rrCM 00 .-I VD csi rvi in 00 Ol a\ o 00 CTi in CM ro ro ro H fH OS w E 4J (0 (U ni ^ u •H 0) a u M a; M H a u E 4J (0 0) a IH U ot; 4J (0 4-1 n 10 u H MH rH C CI> H (0 (0 0) 3 H 10 >

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260 u D J-i 4-> 0) V) U H P in •H M Q) JJ U ni m V •H 4-1 *rM 4J 3 C m CTl CIl 0 i-i (U M o (U c X! 01 0 D 4J .H U O C \ •H M 1) £ ^ P 4J M a 0 (U M-l w (U u CM c m •H c M m e > H U it-i i 0 rH (0 w 4J c (C --I 1 a < 10 c ID M in Tl in C m * + fB B in + + + + 0 n m o 00 o 03 in 4J •H O CTl in I— 1 o oo 00 (0 0 4-1 1— 1 ID CTl Ol in CTl KD i-i 0) 0 (0 cn J M E o g (0 0) 4J OJ M (0 'V J m M o u e H <4-l •H U >t-l ITJ 01 10 0) + 00 in o in in T in N 10 (U M (0 c 3 0 cr •H w 4J ro c •H (0 •H (U g S •H d) in 2 (0 (U > H x; 4-> (U 10 4-) , (0 M O 4J 2 o M 4-1 o 4J O -H O O 4J „ o x: m 2 (U 3 M-l 10 (0 W 0) -H 7 I-:] 4-) o c o •H 4J U 10 3 O W o in CTl rrin 00 .-I in in in o n •* + 'a* o iH O + + + * VD O CM O CM CO CM in ro o rin O VO TJ< CTl CM o M 4J Q c; o to + M o I-I oi 0) CM CM 00 u in o to to Q CM 0) 00 u ^ to >l Q 03 CN O CM 00 •^r CM rvj in in r~ CM CM en O iH ro CO T ro vo OD CD ^ * oo o r•V in iH r00 CM T CO CM ro CM rH iH iH C C C 0 0 0 •H 4J -H 4J H c 4J c 4J c 4-> 0) (0 (D m 01 (0 g U g u g U 4J •H M 4-1 iH M 4-) -H u (0 0 n) 0 IT) iH o i) a I-I 01 Q. M 0) a M 0) lU >-l M M E-i a w oc u E-i U c m u H i«-l H c H in

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261 04 >1 J3 tt) D (fl to 01 -H J ^ a. ^' 2; (8 0) n U U ttl N 4J ^ m (U n tt) 4J E H 0 m b M (0 0 -H V 3 a T3 U tr U IQ U n 4J (0 IB ro 0 x) 0 CTl x; W i-H 4J 4-1 Cn ? V >i 0 c C M 0 01 u ra « oc. .H D 4J M 0 ja O (U 4J (0 M ^ >i 0 w e 0 n M 0 m >t-i Q g b , 0 10 > x U jC <0 u Q CO e o m O M 0 n >M •H w c >i 0 >t-l rH -H m 4J 6 5t o tt) +J O (0 M -H 3 W O ro W > + + * + -)! + + •* .-I [~IT) CM 00 rH I" i-H O CN 03 >£l n m CTi lD CM O o in rH 00 CM 00 in rcn in rH o o n in CTl U3 CM ID 00 Oi Ol 00 m ^ in O n rH in CN) •s o iH tt) •tH to Q to c •g o to 0) «<; Cl tn * c: + + H 00 VD cri in in s m o po o CD O in rH eg n to M t) u * + + H. Kj; W3 OO CTl 00 rH m 0 00 CN Ht He t— vo cn 00 vc f\J M o CD n oo m + + * + * + + + + + + rH CN rH rH m in 00 T VD CN \D CN 00 rin rH rn in VD rH cn T rin t~rrH rH CD rH rH rin rH CN CN CN o ro CN rH rH + + + * HI + + + + + * + >r •"3" n ro m O CD 00 00 rin rH rH ro rOO m rH o o cn CD in CM rrCN rH r~o rH CN o CN rH "T CD CN CM O CM rH rH n rrH ro r~^ rH ro rCM CN rsi C a C 0 o 0 H •H •H 4J U +J (0 10 m 0 U u 2 •H M 2 •H U 2 rH U rH 0 rH 0 rH 0 X a X a >H X a IH ttJ M 0) M 0) M Ol 2 Ol cc 2 Ol OS U 2 04 U 4J •0 JJ c CO O H CM H -H to CO 0) D H CO >

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II) 1 1 +J V i-H O w (J •"H 4-1 (0 *H M HI 4J U m M x: u s 0 u Di C 0 c 0 rH 4J •H M 4J C 2 •0 c of (U 0 c (U 3 •H Ol <4-l Ol C 1-1 •H >. (U M x; (0 M XI 0 (U IM bu 0) u CO c to •p •H c M v (0 e > •H M OJ 0 a 10 tc •H in M >i 0 .H H-l (0 10 c IB VD .-1 1 a < (0 c 10 •-i u XI (0 Eh 262 T3 w to 10 >W to -rH U «3 i) i) W to , to M H x; to 2 o HI M (1) C 4J •P O O O o x: a: (0 tw 4-1 o c o to -H D 4-) o to U -H 3 M O to CO > o — •H T 4J O to iH 1-1 — + + + o * •sjo i-t CNj r\j rlD CD O T-H o ^ »-i in in in in + O 03 CTl lO O CM r-l CD CD OD CD T --I IX) CTt CTt CO in CNJ in cji .H ro PO o 1-1 T ro CNJ CD iH in T CNl rH csj cr\ » Hi + + + + CD CD in 1-1 CM 1-1 m CM in in CD n in in in m CO •H E u * + + IH + + + + -H O •«J' o VD rH CD CTl CTl in u to in in VD VD CNJ O ro to (D m in 1-1 m rin iH 1-1 a ID u b ID in CO to b> iH to i-H q 0) ri M to 0 0 P to to cr c CO 0 •H IN ID lU c J-> u -u to to 'e ID rH u s TUI to + CO + + + •H tu e >1 VD in 00 in jj 10 p IB IB CD VD ro VD 0) to to Q Q VD o Ol 2 to M OT) CN m + + CM VD O O CO in 00 in rin CM CM b> rH c; •s o to 0) >i rin CD n to CM rCM rQ 00 VD CO rH 00 CM * * + + o t-t CD cn VD CTl rH n ro rH t~ CO CM [~ 00 rCM * + * -k + » + + VD rH CM o CO O 00 in rH rH 00 VD rCvj CM O CTl in CO rH CM CM CM "arH 00 CsJ cn CTl CM 1-1 rH "a" CO rrH 00 r^ rH 00 rCM CM CM C c c O 0 0 •H •H H P *J p to to to u u u 2 •H M 2 •H M 2 •H P 0 rH 0 rH 0 X a u X a M X a p OJ p U p Ol 2 Ol a: u Ol 2 On a a Ol 2 oc

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263 C cr. 0 .H -H 4-> U QJ D U x; CO S-l 4J r~ 00 00 c to rH o r~nj x: u c CM n u 0 i -p Cn C -H C * O iH E -H in 00 CM ro P 4-1 1 ^ in ^ (U p H ro O "3> CM ^ U CP tu p rH CM rH c ro Ti -H ro U D to cT\ ^ r~ro (0 ro 00 "P c 0) ro in CO t^ 0 ro K E — ro CM rH rH n CO to -^^ to CO ro >l C ( p r-i ^ ro CM ro T3 •D rH c ro a; (1) x: c o M-l C 4-1 -H 1 u 0 0 C 4J < 3 H (u ro 4J 0) 4-) E u 0) 4-) u ro 4J -H P rH (U l-l -H ro rH o XI -H 3 P Q) U ra D ro P Eh QS t] in C ro u H ip H c H 10 to 0)

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u 3 4-) 4-> (1) iH C 0 c 0 •H ri U 3 TJ 0 )-l a c (0 H Ol c i-H 10 M >1 4-1 to X Cn C CM •H M TI 3 01 T3 •P n C (U 0 > H M JJ (0 •H J3 M +J 3 Csl C 4-) 2 c T3 E C •H 10 M (U Ou a x: >M u 0 M to O 4J <«-l U a> V] <*-! U >4-l •H i> 4J n -rH M 4J v 4J U U 0 « <*-l M (0 (U x: 0 u c n) >i •H 4J M •H 10 .H > (0 3 tr 0 T3 V) (0 H (U w >1 TJ (0 C 10 10 w CD 10 1 B V (0 (U (0 264 (1) 3 M to to to 0) -H 4-1 Ol 3 >t-l 10 ro n •rH 4J 2 o XI 4J 0) D^ M C o o 0) TJ •rH TJ E 10 10 (1) •H :-; X TJ x TJ Cn to •H (U (U C E -H 4J 10 M (0 to w 0) 10 E T3 o 10 ID U U 3 O r-( ^ in in ro in T O OD T ^ .-H O rH OD CTl 1£> 00 CN TT n (D o (Ti r00 [— o CM rH t— r3< in OO o rH iT) rn CTi U3 in U3 cT» in o ^ 00 I-H m O CO VO CM VO CM T m CM in ro o ro r~in o in rH vD ro O rH o 03 rrH [~ rH cn •» 00 CM in r~ ro 00 in ro CM T o CTl in Ol I~ T Ol rH rH rH rH ^ rH ro r~ CM c 0 •rH 4J 10 u 2 •rH M rH 0 X a u u 2 CLl a >*H H c H to 10 01

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APPENDIX B POTASSIUM EXPERIMENTS

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266 m 0) (C to (1) -H h:i -P ic: Q) 10 0) rO D CP W c m 0 o (C -P (U 0) CJ1 (TJ a to 4-> to >i o to M o m u E x; to 4J to tU O to U O ns tx4 (-1 E 4-1 o to >i o to cn E ^1 x: Q to x: 4J to o OJ o E o to ^ to cn tu to E ^ U-l o to 0) U (0 M -H O tn + -K ro n cNj CD cn ^ o CM + + * in n in og n CM n o eg c\j + in o eg tn c\j in eg iH in c\j CTl cn CD in rH CM 00 VD bi O r-eg eg c; .-1 c; eg q CM 00 eg "I wi wi rH rH 0 0 0 to to Is Q) 0) (U •u ^ n CD in in rin cn iH in in cn vo to to n rH eg in rH rH to rH >, >, >, fO tj Q Q Q lO 00 O o [~ro CT^ in rH eg rH tH r~ CO rH cn rH rH * in n ro rH o 00 o ,-1 n tj^ eg rH in U3 in rH rH n CM CD r~ 00 o rH >3< n CM * (~ o ^ O 00 CM cn ID CM in cn rH CM in in o in o eg CD CM rH rH o 1-1 in n CM ro CM ^ n eg C 0 •H P 0) to E -p m (u a i4 iH tu M Eh OS U T n eg c o -P -H C +J tU (0 E u -p ro ro eg C O C 4J (u ro M 0 tu a M M

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267 10 in to g m O 4-1 -H O 4J o m u u m 0) !-> (U M (0 J 18 M u H U •H U m (U (0 (U a (U M CO rH n 01 M 10 3 c 0* 0 S OT ti C 10 'g « rH u •rl s e rl (U g 4J OT +J 0) m Z (0 (0 M b (U > 4-1 JJ s o 4-> O — 4J O -H O O -U O O £ fO rH a; w M ^ MH •o 0 c o m -H 01 4J U 10 M -H 3 M O 10 W > ro r\j CTi OD CNJ OD n Tj" 00 rH r~ o ^ o r~ U3 n + cn cn o rCN in lO o vo in iT) CD rH O CM VD CN VD ro o o CN rH CTt lD CO ovj oo cn n ^ n T o 00 cri n o CD 00 CO rH cn 3< CN rH CN rH Cn tn c: c c •s •3 0 0 0 to to to M »H 0) (U Q) •u 4J -u Mh cn 10 CN [~ in CN >i cn •«r CO 0 "0 rH o • CN 1/1 «J> O CN r~ 00 rH rH CN ro CN C o H -rl C -W M O (1) m o H g m rH o 0) a M M 0) M H a: u O C7> CO i£) in ro in VD CO rH cN 00 CN d n ^ CO CN c o P -H C P t> 10 g u P -H M to rH O n) a M U 0) u f-i w

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268 Table B-3. Sources of variation in the analysis of variance for the effects of light, potassium, and media on root and shoot characteristics of lettuce transplants for Experiment 2, February 1994. Treatments^ Fresh Dry Fresh Dry Leaf Leaf Leaf shoot shoot root root area petiole tissue mass mass mass mass sap K K (mg) (mg) (mg) (mg) (cm^) (mg-L-M (g-kg-^) 23 Days After Sowing Li Ki Ml 1036 62.4 226 11.2 39.1 2000 Li Kj M2 1037 52.5 192 8.5 32.2 110 Li K2 Ml 970 60.5 219 11.0 34 . 3 2367 Li K2 M2 1155 65. 5 199 10.4 41 . 6 1067 L2 Ki Ml 974 83.3 288 17.1 34 . 6 1900 L2 Ki M2 961 57.0 205 15.4 30 . 0 92 L2 K2 Ml 1054 83. 9 301 16.5 37.0 2033 I12 K2 M2 1153 82.2 252 15.1 41.2 903 Source Light (L) NS ** NS NS NS NS Potas (K) * ** NS NS ** ** Media (M) NS ** ** NS NS ** L X K NS NS NS NS NS NS L X M NS * NS NS NS NS K X M NS ** NS NS ** * * L X K X M NS NS NS NS NS NS 30 Days After Sowing Li Ki Ml 1504 113. 1 325 24.6 54.3 2033 33.7 Li Ki M2 1366 83.8 218 16.2 40.6 85 3.0 Li K2 Ml 1398 108.1 370 29.1 50.3 2900 41.9 Li K2 M2 1784 123.6 359 25.3 64.7 1100 19.0 L2 Ki Ml 1408 134.7 430 35.1 59.7 1800 26.8 L2 Ki M2 1291 102.4 259 19.2 36.3 71 2.4 L2 K2 Ml 1446 133. 9 470 38.0 50.6 2367 33.5 I12 K2 M2 1904 160.4 395 32.5 65.0 1100 15.5 Source Light (L) NS * NS * NS * NS Potas (K) ** ** ** ** ** ** Media (M) ** NS ** ** NS ** ** L X K NS NS NS NS NS NS NS L X M NS NS * * NS * K X M ** ** ** NS * L X K X M NS NS NS NS NS NS NS ^Li = photoperiod extension to 16 h; L2 = supplementary light for 16 h; Kj = 0 mg-L"* K; K2 = 60 mg-L'^ K; Mi = peat+vermiculite mix; Mj = peat+rockwool mix. *' "Nonsignificant (NS) or significant at 5 % (*), 1 % (**) levels.

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269 Table B-4. Sources of variation in the analysis of variance for the effects of light, potassium, and media on growth characteristics of lettuce transplants for Experiment 2, February 1994. Treatments^ Root : Vie* 1 ^5 T* i VP Xica X T £k a -F j-iear Root shoot y XT OWUfl assimilation leaf area mass mass ratio I TTifT mn"^ • wk ~M ( mrr • r^tn"^ • \ I r'm^ • mn~^ \ I \ my ill WA. ) \ y^m iiiy I jra ti 0 \ cm lug ratio ratio 23 Days After Sowing Li Ki Ml U . D J 0 . 53 0 . 85 0.15 Li Ki Mj u • Id 0 . 61 0 . 53 0 . 86 0. 14 Li K2 Ml KJ , ±0 A CI 0 . 57 0.48 0 . 85 0.15 Li K2 M2 0.16 n U . D*i U • 3 0 U . 0 / 0.13 L2 Ki Ml n /1 0 A 0 C 0.83 0 . 17 L2 Ki M2 U . 0.79 0 . 21 L2 K2 Ml U . 4 4 A 0 T 0.84 0 . 16 L2 K2 M2 0.18 U . 0 U A yl 0 0.85 0 . 15 Source Light (L) NS NS Potas (K) NS M C No NS NS NS Media (M) NS A-A^ ^ NS NS L X K NS NS NS NS NS L X M XT e ANS NS NS K X M NS XT C NS NS NS L X K X M NS NS NS NS Li Ki Ml 30 Days After Sowing 0.22 0 . 63 X.J? u . 4 0 U . J 9 0 . 82 0 . 18 Li Ki M2 0. 19 0 .50 X«UO U*fiO 0.41 0 . 84 0. 16 Li K2 Ml 0.27 0 . 65 X.J/ U.flO U.J/ 0.79 0.21 Li K, M2 0.20 0 . 43 0 . 83 0. 17 L2 Ki Ml 0.26 0.53 1.66 0.37 0.29 0.79 0.21 L2 Ki M2 0.19 0.51 1.50 0.36 0.31 0.84 0. 16 L2 K2 Ml 0.28 0.54 1.69 0.38 0.30 0.78 0.22 L2 K2 M2 0.20 0.68 1.83 0.41 0.34 0. 83 0.17 Source Light (L) NS NS NS * * NS NS Potas (K) * NS NS NS * Media (M) ** NS NS NS * ** ** L X K NS NS NS NS NS NS NS L X M NS NS NS NS NS NS NS K X M NS * NS NS NS NS NS L X K X M NS NS NS NS NS NS ^Li mg' L-' K; K2 = 60 mg-L-' K; Mi ' "Nonsignificant (NS) or ;o 16 h; Lj = supplementary = peat+vermiculite mix; Mj significant at 5 % (*), 1 light for 16 h; Kj = 0 = peat+rockwool mix. % (**) levels.

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270 (U u m D XT c (0 0) m -H -P (U •-^ ;^ • a) to CM ^ ^ m o 4-> o o s: m T3 O c o to -H 0) 4-1 u m M -H o to m e 10 o m Q) n w M r (0 He + rcn cTi ro cTi cn o vo in in n eg OD n CM CM *-* OD 00 X) cTi in cv) CM CD rCM CM + n in m o cn CM CM rcri o o CTl rH <-l 00 CM t< S 2 M (0 o X X X X Cl, M 0) ^4 CM r00 iH CM CM iH CM iH "am n rH to '--IrHCriVCODOO^iniHrH roinrHCMinincri^ [|piX)rooo^c\iooiHiX)iX) * + + * + * r-roncMCMcx3Vocriin OrHOooooocnr-co rH in [~o CM CM VD rH cn rH rH * * It * •»: o CM 00 rH VD (Tl IX) rH 00 rH rrH 00 CTl CM CM rH n CM O CO CM n CM 00 rH CM m CO * * •x * + (Tl rH rH CTl r00 eg ^ rH o in o t~ CM 00 CT^ CTl CTl VD n in rH -K * o rH 00 m o in in in r00 CM n rrH n CTl rH rH rH o n rH n * + rH in 00 rH rH CO o in ro VD rcn in o cn CM rH o^ CM VD o CM n rH rH ro in rH rH rH rH rH rH rH rH rH CM ^ 2 2 i^: u to o X X X X Cli M

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271 in in C (0 IT) e in 0 M-l u •H o 10 0 JJ t-l 0) 0 (0 M M •l-l (0 (0 0) u 4-1 H 0 >« 0) 10 a (u c o H 4J (0 .H H 0) (U <0 (0 (0 M ID > 4J (0 S 0) ^ O JJ (U l-l 10 a 01 M JJ o o -H , O O 4J o jz la 4-1 •d o c o W -H 4) 4J U (0 3 O to i-ivcoooiooo'^r.-itri rovD inoiror-CTioo ID i-H rH i-H r-l i-H i-l oor-rvjTt-icMooCTicsj OOr-mr-ILn'3'i-Hr— ICNJ 1-H Ol + O CD ro CN 1-1 + * ^ CM Ol CTi rO CD CN (D rH c\j rc\j IX) ^ ^ 'a" ro IX) CD CN CM IX) "a" .H ro o CO i-{ * * * * + + * cMfon^i-HoncritniD oovcocM^rnocDixi in 00 in vD in CM o o r~.H cn cri m CM iH O VD CM o in CO «H u to g <0 Q o tn rH CM c: •s o to « to CD cn . . ^ ^ m m (T\ "0 CD rQ in CM o CTl CM O IX) 00 CT) r~ CM 00 rH iH o rCM IX) ix> rH cn 1— I OD O O VO rH CM CM rro IX) (Ti cn rH m rH n CTl 00 o r~ o ro CM 00 CMLnr-voooiniX)rHo r^OrHCTlrHinrHCrirH rH^ ro^ncMco^ inrHrocMoooono-^r cncMrHrocMrH^inm "J* CM rH rH CM rHrHi-HrHrH'^CM rHrH s X X M 10 O X X X X M T-i r-l r~-l r-i rr X X X X X X in 0) u t^uix^^>^i^>^a:u dp (0 rH M O M o x: — vo H rH o T3 O •H ' M ' lU a o • o jn o< II •

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272 m 0) o a o x; u u M 0 0 3 C o 2 n T3 •H C 10 ro >, i-H ro c >1 < X3 73 rV 1 4-1 00 U Q) (U >*-l -H <4-l XI ro *-> o ; +J O -W 1 O O -t-J o o x; ro 3 •4-1 CO ro to CP 3 10 (0 •H 4-" CP M-l ro ro 0) 4-1 >i O u o 10 ^ ro 1 4-> O >i O p x: ro 7 Q to E o 73 o c o to -H (U 4J u ro U -H 3 o CO * It CTl U5 O CNJ IT) o in r~ iH iH tH o X) CD ^ 00 •tt 4t n ro 00 ^ CO 00 o n 00 cri CM in in CM ro O to 4J + KD CM + 'a' CM CM O tn "3 0 to 00 ro ro * + * * CM ^ CM vo ^ 00 VD CTl rH ^ &>CM -H o to to >1 ro Q in (Ti in CTl + in o cn ro 00 CTl CM CM to Q in CM ro ro 00 rH CM ro cn o rro '9' in IX) o eg * n «5 in rH T •< < to ro Q 00 * O O in in * vo o ro ro rH 00 00 T ^ CM a* cn CO in CT^ ro ^ r~CO in + ro CTl ^3" ro ro ro ro ro CTl CTl 00 CTl '3< O rH ro CD CM CM CM CM in rH CM CO ro ro rH ro in rH * * + K 41 + + + K + * 00 rH o cn O o CM CO rH VD 00 rH CO rH cn in rin O CM in cn CO CM CM in <£) in
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274 cr * 0) a: en LD o o 4-1 CTl (0 tj CNJ o C iH ro W 00 rH in nj 0) -H rH rH (J 4-1 d fO n 2 c (0 CO 1 •P "0 Cr 9) (U tJi A-> 3 • o 00 in C CO to Cr o rH o o 00 -H (1) (C to n "a" n M > 0) •H iH rH 3 M 4-) 2 o — C O . — . •H to x; -P U 4-) •H -H (U tJi r~ rH CM M 4-1 M C fNJ ro 00 in 4-1 to 0 (U o CD 00 in D -H u iH C U N 0) to Z -P 0) U T) (0 ID + C M D E 4-1 (T> VD n 00 rO (0 cr (U T) LT) o CM CM x: CO 4-1 •H 1 rCTl in ^ u c CO o rH •h >, O 4-1 d) -H 2 (U m .H 4-) 4-) nj (U IT) CD ro in U P T) E O CTl C (C •H rvj o M (0 0) (1) rH n CM 0 X X CM M-t U) ^ [ 10 Q) (C CTi U £ C c E •H irt 1 IT) o in n rH (0 T3 M 4-1 o o Oi ro rH •H td H 10 tH rH CM rH U 0) [u U rH > (U tJ to iH VD o cn •p 0 (0 to — . rH rH rH 0 D OJ nJ OvJ rH IT) IT) 00 4-' E iH rH rH to 4-) •H 0) W .H >i -H C rH ro TO (J T3 CM c < c 0 c • -H iw 0 CTl 4-> o c -H 1 u o 4-1 OQ 3 to (0 n 0) 4-1 U (U o u m 2 -H U .H u 1-1 H iH O Xi a D X a u (0 o u Eh CO > 2 oc: u 4-1 m 4J c (0 u H IM H c

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APPENDIX C NITROGEN EXPERIMENTS

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279 o to u H P m •H U 0) 4J U (0 to O » O M & C o 0 -H PO P Ol •H (Tl U iH +J p C (!) c o -H z (U p P M c 0 v 0

" 0 m ro i2 ro P ^ Q) > H s: P p ro dJ .H O p P ro Qc; p O C O to -H (U pi u ro P -H 3 o CTl 00 T rH rH rH ro + rH 'a' ^ U3 n o + m 00 o CM + + r~ n rH O rH r~ n eg T -K ix> tn "3" ro ro (M ro •K + T 00 00 00 00 ro * CN IT) ro * ro o IX) tn * rH « r~ CTl q IT) CTl ro IT) T q O en ro •-1 (Tl IX) OJ IX) •ri CD r~ rH -a* IS ro 0 0 0 CO to to p p p 0) 0) Q) p -p P •p •p "P •<< to to to >1 >, ro ro ro tt + Q Q + Q * * to eg rH rH VD t~ 00 CTl rH 00 eg W3 00 "H '3< ro ro IX) CM CNJ o cn CM ro o cn LT) in CM rH ro rH rH rH ro CM ro CM n CM rH H rH c C c 0 o o p -H P •H P H c P c P c P -H P ro rH O ro H O ro rH O (U a P 0) a P P p 0) P p

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280 n 10 m +j c m H a n c Rt U 4J (U U 3 P P V O m u -H n •H M (U U (0 m x: u p o o s: 10 4-) « D < n c (0 ro 4-1 O C 0) e H M (U 0) u u o u u o c as •H u to > u o c o 0 -H u to 4-) -H D m c >1 H 2 to C >i < • -D in (u 1 4-1 u u 0) 0) <4-l nj Eh (0 Q) M fO D tr c ro 0) ••4-10-^ 4-1 O -H O O +J o o x; ro 0) m to Cn (U -H 4J ro ro 0) ro 4-1 >i o u o Q i-i to 4J (U O u o hi U to ^ to 2^ ro E E — to 4-1 o >i O to u jz m Q to e o x: 4-1 to o to ^ ro ro vo * o o IX) 0 + to cn 'h 00 ro ro 03 cri CM ^ CD CM in CO CO 00 00 cn CM rtX3 IX) T iH rH 00 00 ^ 00 CM 4-) c V E 4-1 ro u E-i c o -H 4J ro u •H a ID a: u IXI 00 ro in ro CM in rH cn 00 cn o in vxi IX) ro 00 + .H IX) (» IX) 00 00 + + CM O q o * to o tu rI* ro uo '--> 00 ,^ o * * CM c\j in in in IX) O CM CM t~rin CM cn ro 00 (~IX) ^ ro o in 00 ro c\j tn ro 00 O tTl 00 IX) ^ r~cn IX) in o in 'J' 00 CM 4-) C

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281 o u -H to T3 CO m c ro ro £ u 0) M-l 4-1 p ro o o (D O r" o r! 1 i — ' > •> ft W (_1 Hi ro (1) Q) M *— i fU n w o 4J -rH (J i4 .p-l Xi m D -rH c U U-H CO 0) ro u CO >H m • ro 0 'S* CP CO c U rH o c c rH 0) 4-1 ro 3 m 4) ro H 3 i-H •4-1 cr rH C 3 •H «; H p CO dj ^ (1) CO 2 ro M c O (1) IM E -H (U M U Q) C Cli (0 X 0) -H U > u m V4 4J P > o ro 3 <«-i H O CO c -rH ro CO .H >i a tH to ro c TD C ffl < u p U-l vo dj O C 1 U 0 U 3 CO -H 0) 4-) (U 4-1 u ro H (U U -H X) H 3 H ffl o ro Eh w > to o •H O P ro u p ro u o p ro o 0} p e u E 4-> ro u IT) m m O cri cn * in eg o CD o tH T o -K rr~ CO VD cn n IT) in + in in in n ro n i-H CX5 * r~ T in in n CTl '5' * reg * + in * CTl iH + VD n o o m 00 in r!^ n CD n rH r•rH [~ o in in 0 0 0 CO to CO tH (U (U (D 4J 4J 4J CO CO CO >i >1 nj ro ro Q Q Q in a\ og 00 in T eg 00 00 VD iH eg eg eg ^ n eg in o n rH vo cn eg r~ rH rn eg C o H P (1) ro c o H P
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282 (U 3 M-l m nj in i) •^^ J -P 2 M 0) J-) E e >i o M O Q M in JJ d) O u o cn in E in 10 E <=> in in 10 c e o >i O W M x: m Q VI in E E o 0) E in ro o E -H 1J3 t-H >JD rH n O rCM cv) IT) + + cn r-l in rIT) in O rH i-H (M CM CM CvJ m CM CM •» •! + •* CT^ 00 cn i-l Cji ro rG IT) (Tt rc ^ q IS) crH d ^ CM 'H 00 o 00 S O li O S rrH 0 .-1 0 n 0 rto (0 to 0) + H ro O CM VD «i; IT) in rH rCM O o ri-H r-l rH in CM in to CM m Q Q Q «o »
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283 u 3 m 0 H V 4J O m M 10 JS V £ s o o> c o c o 4-) H M 4J 3 C 2 cn M-l CTl 0 iH M u dJ c M o c \ -H M (U •i ^ i) M a 0 <*-! to 0) 0 c (0 •H c M (U (0 E > •H M (U 0 a n u H 10 l-l >, 0 H >(-l 10 (0 4J c m 00 I U XI 10 a m C fO M •P CO V w c 10 (0 B SO M-l 4J •H nj 0 4J Q) 0 (0 M M 0 10 •H rO *-l (0 (U rO 0) ^ a 0) c o •H 4J 10 iT) 0% 1-1 TJ* TJ* TJ* CTl iH + o 03 in CM m m o O n (s) CNj eg ro M r-rrvj CTl n m o o 00 r~liJ IT) 4J Z 10 •H e •rS (U 10 4-) (0 M o (1) > H 4-1 4J (0 i) 0 4-1 M 10 Di M 4J O — O -H ? O 4J O O £ (0 iH " 10 l-l — 6 4-1 o c o W -H 0) 4J U 10 3 o * + + + + 1X1 00 o a\ CTl cn 00 00 oo o CO CTl l-l CD cn n rr I-l CM CM in l-l Cn CTl tjl CTl Cn oo c; s •g 0 0 0 to to to ^^ M *H + » to CTl CO VO <0 T o CM iH o >, CTl "0
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284 Table C-9. Effects of N nutrition during transplant production on lettuce head mass and head quality characteristics, harvested 6 and 12 December 1994. Nitrogen Head Firm Head Head Stem Core applied mass rating^ height diameter width length (mg-L-') (g) (1-5) (mm) (mm) (mm) (mm) 53 Days After Transplanting 15 556 4.5 104 115 21 32 30 577 4.7 109 119 22 34 45 579 4.8 107 116 22 35 60 577 4.8 106 115 21 37 NS NS NS NS NS NS 59 Days After Transplanting 15 641 4.8 116 126 22 41 30 606 4.9 120 121 22 45 45 613 4 . 8 121 123 22 48 60 664 4.7 122 129 23 49 NS NS NS Q* NS L** ^Lettuce head firmness on a scale of 1 = loose, 5 = compact. Linear (L) or quadratic (Q) effects significant at P = 0.05 (*), 0.01 (**), or nonsignificant (NS) .

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u 3 •D 0 D U 0) a 3 •P 01 in ro 01 Cs] c • 1) -H rn CM (U ^ f— f rH Q, (Tl CO rH "g J_) o 4-' "i C U D n) \j w o ro -H QJ T ^ _j M Q "O cr» C T) r; 0 (U 00 -H -P (11 "D •P CO -P -H o CM H 0) CO 3 »-H ro M > -P c x: M -~ (D ro +j E E CO 3 (1) CD IW u cr O -H CO ro ro +J 0) -H o rH 01 01 c X n rH -P -H ro u u £) P u rH — M >i 0 -P 4-1 -rH H C ^ — ^ (U (0 E -H Q U 3 P 4J o c cr rH ro tu p •H TJ ^ (0 m 0) > JZ "O 01 IT) •4-1 TI ro 01 0 C 0) ro cr> o ro E E 00 01 •H 01 01 01 >i ro <4-l ro C*l c -D o < ro dj • x; o tp rH tt) o c p • 1 u o C U 3 01 -i-l 01 E u ro +j • H 0) P -H ro r 3 P d) ro 0 ro u Eh w > Eh ( o O CD CM CTl CM rin ro in o C O U 01 P -p ro -p c ro u H H c cn

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APPENDIX D NITROGEN AND IRRIGATION EXPERIMENTS

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287 4J u a (U M-l 3 in ^* M-l 4-1 to OJ CO rH rH CNJ -H • in rH W 4-> rH OS Vi — " 4J c m (U Ht a 3 V) M-l CO C>J iH c (0 CO r-) CN 00 CM m 0) •H o yo M 4-1 rH JJ u ' 4-1 u (U 0) 3 + + M-l CO in in 00 10 CO rO M-l 01 •H in 1— 1 0 1-3 4-1 z o r10 u •H tn tn 10 IN £ q c + C * to 1 M 0 (0 >,-l t— 1 CM ss o in 00 o o iH Q E 0 >T "0 (0 CNJ ro CO o 3 dQ Q Q rH to CO > CN + CJl Si •» CN + CN Tj C CO 4-1 CO n ro vo CNI rH C A) 0 CO vo c^ CTl 00 CNJ vo to (TJ o to cx> in 00 ro ro ro CD M E c: c rH iH •P 0 0 M p * * * C 4J * + + » M 0 CO ro CNJ in 00 o 0 £ >, 0 CO M^ n 0 to C\J ro CM ro 00 CNJ vo vo iTi i o M-l .H M m ro at ro 00 cn ro ro CT^ m M-l •d c 0 C c c •H M-l 0 0 0 iH 4J 0 c 4J -H 4J ri 4J H m 0 C 4J C 4J C 4-> Q Di (0 •H 1) 10 10 0) 4J £ U £ u £ U (U M u 10 4-1 •H u 4J -H M u •H M iH M M •H (0 0 (0 rH 0 10 rH 0 XI H 3 (U a u 0) a u ID a M (0 0 <0 u 0) M M (U u M (U M H OT > E(t u H a u H 4J (0 4J C 10 u •H MH H C D> •H CO CO 0) 3 rH (0 >

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288 •H T CTv tM 1 V c P d) 3 •-< OJ M-l c C 3 •H t) x: i-H 4J M c O Q) M-l B H S) U (U c a <0 •H u M 10 > o M-l M-l O m 4J 10 C •H m to >i a H to 10 c (0 u •p 0) 1 0 Q 3 4J 01 4J ID rH to o o H 4J (0 M 4-1 (0 10 01 H cif M-l 10 i u 0) (0 0) IT) a 0) M o iH N to w (0 01 M « 3 cr c w OT c J-> (0 (0 'e 0) <-l u TUI -H V e 4J to 0) to 2 (0 M o P o o o x: to M-l •D M-l o c o (0 -H (U 4J u m JJ o (0 iH 3 O CO li) cri -H oa CM ro >-H m o in in iT) in cv) in (X) oo yD r(Nj in » in 1-1 •^ rCM CM CM CM + en 00 00 (D o 00 1~ Io 00 o o •^r o 00 CM CM CM Cn s o to 0) to 10 C o to 0) 4J >, <0 Q CN cn ex> cri iH o m CM T-t 4J c a; E 4J 10 0) O 0C3 00 rcn CTi r^ "T tn q o to to , m 00 00 00 cn + VD in VC 00 VD en o CM CM 00 00 CTl c (U B JJ 10 OJ M E-i ro U •H M O a M 01 M a: u * 00 in 00 in
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289 0] m 0) u (0 D cr d (0 2 4J 0 o -H 0 0 4J o n r' CC 10 0) 1 3 <4-t to IB 10 0) •rH 4J CO c to rH 0) .H u rH u 0 D cu to + + og rH n H 0) (C P (0 >i o to u o m QUE -P O (0 >i o to M x; ro Q to E MH o c o to -H tt) +J u m vh -h o CO o CO 0) . k •>: •>: * + * rH in n 00 in 00 00 m c ••-i o to 0) •u 0 ^ lo 1^ ix> CO CT) ,3. ^ ^ CO *^ ^ rH to ro cvj CM , in CD eg rH o fQ rH tH rH rH rH CO >1, eg * * -K •K -K + o n »j< 00 cT^ in m 00 cn ^ 15 <^ CM in in 00 CM * + * * * tc • * •K * * •x + x • rH c^ 00 eg 0 00 in eg rCT> 00 eg in eg eg in eg VO CO 0 rH rH in in t^ rH in eg rrH * * * Ht Ht * * + + + + • • Ht + 00 0 in Q) rH + + >1 dP u c rH cu U tr 0 u UH * c 0 dP H H-) in ro en -P •H ro JJ U H-> Q) c KH ro u II -H IM b -H c Cn C •H 0) (0 D> 0 to M (U H-l D •H rH C ro > II N ?

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290 o m -H in 4J ic nj e M O m -H (0 in jj o d) (0 nj J g M — O m 0) •!-> (u M m J (0 M ' u <*-! V H t U U-l 10 £ 0) (0 OJ • c o H (U to (0 7 4^ 0 c o OJ -H 01 4J U (0 M -H D M 0 (0 to > -tt + * + * * + * + + + + o CTl 00 00 CM 'am o cn IX> 1X> o o cn o n 00 CTl i-H in rrH r~ rH CO IT) (N 03 in rH in 1X1 i-H 1-1 in rH 1—1 rH CM + + * * + + + + + * o cn 00 n 00 CM 'a* 'a' in o CTl 3" CTl ri-< o in oo in CD r~en T 00 'J' CTv CM n in in •ST •c in i-H VD i-H CM CNJ ro c; "ri & & 0 * 0 » 10 + + (0 + + + o rH CM rH oo rH rH in rH rH rH »H m rH t) CM d) -u 2, •B »-< 00 CM + + + » * + in o (*) o o CM o ro 00 CO rH in 00 rH 00 in o o rH CM CM ro rCM o CM * •» ro rvj ro rrH in 00 I~ t~ rH ro in ^ o •V r00 CM po vo m CM 00 CM ro rrH m 00 rH IX) t~ CM cn «T rH CM rH rro rH CM rH ro CM ro r~ rH in a C C o 0 0 H •H •H P 4J 4J 10 (0 ID U U U •H M -H M •H M rH 0 rH 0 rH o a M X a IH X a u OJ M lU 0) u 2 b4 2 a 2 U4 2 DC u 2 a, 2 a; u rH (U > 1 dp o c rH V 3 M tr 0 t) u IM * c o dP •H in n cn +J •H nj •p M P (V C MH 10 u II H >«H b •H c D> C -H 0 n M (U •P D -H rH c ro > II 2

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291 0) u (0 CP CO c (0 0) s -P o 0 -H 1 o 0 +J o o x; (0 qS w (U y 3 IM m (0 CO (1) ij +j 2 O " — ry in c m H (U u i-H u CU CO Q) 2 •H U rrH U r D o O Ou M-l T-i e (0 u (0 (U 0) u m 1 O 4J 10 > O CO Mora o u B O to >i O CO Q CO E CP O CO 0) U M D O w c o •H m •H >^ (0 > • * * * + •)< CTi CM rg ID in r00 ri-( CTl + o .H l£) * + + + + * + , Q •K * + * -K + n CM o cri rH n o ro rH "S" rH rH T rH c; o ^ 0) ro tn 0 o <0 in o ^ rH CM CM in in ro r~o >, ro CM in (XJ ro CM CM Q 1^ rH ro ro in r/v * * * [Ji in ro rH o in 00 ro oi ^5 CM in "Xl 00 rH -J CTl CM in „ CNJ T 2? • * * * K + • K K * + VD 00 CM CTl rrH PO in O ro rH 00 00 00 IX) CM rH ro * * * •K * * + * •K * o 00 00 ro 00 00 ro in in rH r1X> 00 ro CM in 00 rH ro ro CNJ ro ro CM 00 r-~ rH in rH in C c 0 0 H -H 4-1 4-> (0 nJ O u bn H U Du •H iH rH O rH 0 X a X a U 0) (U Z 2 PS 2 2 a: K + CNJ CTl rH 00 'J' (T> in ro ro CM K 00 CM r~ CM CM rrH 00 * 00 r00 CM ro ro DO rH CM ro CN ro r~rH in C O H 4-) ttJ U •H rH a rH * + dp U c rH 0) U cr 0 a> H 4-1 Q) c I4H ro u II •H Cu -H C CP C -H II N

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292 in c m a> c 0 •H 4J 3 U U * 3 C 4J z C (U E 0 •H M (U (U u a c X (U u 3 .-1 1-1 >*-l 0 c >l-l n c V (0 u a 0 m C 10 t) M u 4J c (0 (U •H 0 3 4J > 4J 9) •4-1 •-i 0 t*A m O •H 0) m >l 0 iH H ID •P 10 ^ •rH u t> U 1 O Q <0 M H H U •*-! 0) 10 a CM VO O CNl 1" n CO * + + o T en CM 00 00 o CM "S00 o CTl CM + in CTl CTl o c o in -H (U -u o ro M -H 3 M O ID OT > b< in q oo ID H OO iH ^ ^ .H O VD to U 0) -u >i 10 Q + » CM d i-H m tn CD VD C CD •H rCM S o O in ^ 0) u •X CM q CTl O CM to t) ro cri m Q * * + "-I 00 r~00 rCM 03 -i in o 00 CM o ro CM ro CM ro CM ro ro Tj< 00 O ^ CM iH r~ £) 00 00 oo in 00 CM ro r00 CM 00 r~ 1" 00 CM 00 in iH in i-l in a c c o 0 0 •H -H •r-l 4J •U 4J 10 10 10 U u u •H M Du •H M bu M •-i 0 0 0 X a M X a, M X a M lU M lU M 2 Z b: u 2 2 cc U 2 2 a: u w (1) > V * * dp U c 1-1 3 tr O 0) M 14-1 •» C3 O dp •H +J in 10 4J •H 10 +J M +J (U C •4-1 m u II H •4-1 bu •H C Cn C H lU m 01 0 in M V 4-1 3 •H .H c 10 > II 2

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293 m in (Tl m a c 0) 2 •• -U o +J O -H O O +J 0X1(0 0) (0 w P 2 O (Tl in C w .H U .H U CLl CO 0) H U H M D O 04 U-l <4-( m fO 0) * + * iH CTl ^ <3< 00 eg en o IX) ^ r^ tH rID iH CTl OD rn (Ni rH ro CTl CTl CTl O CM + + + IT) cvi in OD (J^ CO iH -P OT >i O to M o ro c Q M E ^ 4-) o >i o Q to o n (U u u o rH rin o CM CM O VD in ^ eg IX> CM o 10 K + -X * + * * * o rin T "3" o i-i n !5 W5 CM Q Cn •2 o to (U u «H • £5 * + + «a< CX5 cn IX) ro in tH + CO IX) to >,n o oj r~in Q ^ O to 0) u to + tX) 00 r~ in cn c»i in CM in >l CO 00 VX) (M CO CTl CO [~ rH 00 reg CM * * * X X * + X X 00 CTl 00 n tH n eg O IX) 00 cn tH cn reg tH tH r~ 00 CM cn in in r~iH eg in CM iH eg n eg 00 tH o iH o 00 in tH in e^J cn "a* tH (O CTl * * K * • + * * X * * • •X * * X * n tH iH VO CT^ in 00 n in >X) vo (o rm CM 00 tH IX) in eg iH n eg o in co o eg IX) rCTl CTl tH cn eg cn in n r00 CTl \D 00 tH r~ in C*) (O eg '9' eg (O eg CO rtH in iH in tH in c C C o o 0 •H •H -H +J JJ 4J ro (0 ro u u u Dy •H U bu •H M b •H U -H o .H 0 .H O X a X a u X a M u Q) u 0) 2 2 2 2 a: a 2 2 u oP u c tH ID a n cr o 0} M X c 0 dP •H 4-1 in ro CJ) 4J -H ro J-J +j 0) c IM (0 u II •H •H c CD c •H (U 10 o to M (U 4-1 •H .H c ro > II

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294 c r\ iM OJ CO 4J D C IJ 2 c 0) M-l e 0 H M ID 01 U a C V u 3 M >«-4 0 C M-l •H (0 0) x: c 4J (0 M a 0 M >t-l c 4J t) r-t 0 (0 o H W (0 >i O .-t •H (B 4J 10 H M (U 00 JJ 1 U Q m M V ID .-1 J3 u to TI in 4J O 0) o to IB to O 0> O g to O 4J Q) u n ^ J to M ' u H H U M-l (U 10 a t) to « ,-1 on wk ti to 'g U •w E •H g 4-) to ^-> (U 10 to 2 to M o > 10 S (1) >4-l O o -u . M (0 10 -H cvj Oi ro cri o r~ 00 CTi ID n i-H CD 00 00 tNJ 1—1 'J" VD o in 00 I/) ro ri-H CD >£l CNJ ^ rr~^ CTi l£l n VD CM + » + •» •» rCM iH VD IT) O CD r~ IT) tTl rrH iH CM CM CM >-l + * + + + + + + IT) CN) "a* i-H 00 OO O CTl IT) CTi CM 00 rH i-H iT) + n n o rH o r>-H CM 1-H IT) + VD CM CM CM in •» + * * » » in CM CO rH CM VO CTl C 00 CM OO rH CM rH rH 3 rH O rH to >H lU 4J >M «^ 0 Q q o o to u s. rH O in m m 00 > •X rH C 00 OO + * CTl CM O r~ TT r~ 00 in * + CM 00 r~OO 00 o o en (Ti CM T CM CM •U Q VO 00 in CM CM CM 00 00 VD 00 CD CTl CM 00 CT> rH in 00 CM 00 r~ rH in * * + + 00 OO 00 ^ in cn cn 00 in O rH CM 'J' 00 CM 00 rrH in * CM 00 rT CM O 00 rH rH rH O CM rH T 00 CM 00 r~ rH m C C C 0 o 0 •H -rl •H P JJ JJ IT) (0 (0 O U U H M [14 H M H M rH 0 rH 0 rH o X a M X a M X a IH 2 2 a; M (U M (U M a U 2 2 a 2 fcu 2 a U 10 rH (U > rH * + >1 u c rH OJ 3 M cr 0 u M + c o dP •H 4J in 10 Di •P •H m 4J M 4J 0) c MH 10 u II H UH Eu •H c Di C •H lU 10 «y 0 to u ID JJ 3 -H rH c (0 > II i4 2

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295 10 i o n M O (0 e o u e 3. (0 go 4J o m >i O (0 M £ ID e Q n 6 — x; 4J in o flJ O M . ID . w g O •d o c o m -H 0) 4J U ID 3 O + q« rrm IT) + + o rCO ro c\j ^ 03 o + * , ID Q CM 00 00 >H O CM * CM CVJ CM O 1-1 ro 00 rO » , *H 00 ro in i^i; 1-1 00 (N ID * Q + ro 05 . cri ro ro 00 o o * TT CM ^ CM 00 in + CM o CM + in (71 ro T CM >-i 00 "S" ro CM C o 4J -H C 4J i) ID E 4J ID ID a u W tt( M H a: ba iH iH [~ T-i o m vo CM + * + ro 1-1 iH vo iH ro o ^ 00 CTi CO CM ^ CM cn ^ CO CM c o -H 4J ID u •H 4J c ID e 4-1 ID 01 U Eh ^ O 0) u OP in 4J ID 4J C ID U H <4-l H c W

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o to u •H 4J W T-i M 01 4J U to M (8 u 4-> S o M C o c o •H P fO V •H H a a 10 of cn C •H E •H 4J Of 0) vo u CTi c (Tl (U i-l 3 fH M-l •H C H §: (U x: 4J m M 0 c IM 0) B 01 •H u M c 0) la a H u u > o 0 10 to 4J -H c (0 m >l.-l a (0 to c m •p o rH 1 u Q 3 4J -H 0 10 H to OJ (V M to (0 u fi E u * •H + * + u 10 CM (1) to m T CM i-H a M o CD at CO ID t-H c o •H JJ to •H E •H 10 <0 M ^ 0) e 4-1 _ 4J P O o o o — H P O O J3 (0 (X n M — 3 >*-i to to to 2 I to a OJ ro O D> to 2 E O o 0> 00 CD ^ CD OD n O 00 CM CM in r~ CM 296 C o 0) -u to >. m CN CN 1= O to rIT) rCM CTl o CM vc m CM M 0) 4J <0 Q . CN » CTl i-H + CM in q' CM E-" a: u Eh a: u (U •p m u 4J H c — 10 m dp

i > ^ c It, o

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LIST OF REFERENCES Aloni, B., T. Pashkar, L. Karni, and J. Daie. 1991. Nitrogen supply influences carbohydrate partitioning of pepper seedlings and transplant development. J. Amer. Soc. Hort. Sci. 116:995-999. Anon. 1986. Beating bacterial leaf spot in Bushnell. Amer. Veg. Grower 34:28+. Anon. 1995. Florida agricultural statistics: Vegetable summary 1993-1994. Florida Agricultural Statistics Service, Orlando. Basoccu, L. and S. Nicola. 1990. Light conditions, timing fertilization and water availability influence on nursery development of lettuce seedlings and their effect on field productivity. Acta Hortic. 287:399-404. Boivin, C, M.-J. Trudel, and A. Gosselin. 1986. Influence du niveau d'irradiance d' appoint (HPS) en pepiniere sur la croissance d'une cluture de tomate de serre. Can. J. Plant Sci. 66:961-970. Cantliffe, D.J. 1990. Performance of crisphead lettuce cultivars on plastic-mulched, drip-irrigated sandy soils in Florida. Belle Glade EREC Research Report EV1990-7:48-56. Chipman, E.W. 1961. The effect of seeding and plant topping on the production of early and total yields of ripe tomatoes. Proc. Amer. Soc. Hort. Sci. 77:483-486. Cliffe, D.O. 1989. Production and scheduling of lettuce transplants for commercial crop production. Acta Hortic. 247:49-51. Costigan, P. A. and G.P. Mead. 1987. The requirements of cabbage and lettuce seedlings for potassium in the presence and absence of sodium. J. Plant Nutr. 10:385401. 297

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298 Craker, L.E. and Seibert, M. 1983. Light and the development of Grand Rapids lettuce. Can. J. Plant Sci. 63:277-281. Decoteau, D.R. and H.H. Friend. 1991. Growth and subsequent yield of tomatoes following end-of-day light treatment of transplants. HortScience 25:1528-1530. Dennis, D.J. and W.M. Dullforce. 1975. The response of the heated glasshouse lettuce crop to in situ supplements of low illuminance flourescent light. Acta hortic. 51:185-201. Dubik, S.P., D.T. Krizek, and D.P. Stimart. 1990. Influence of root zone restriction on mineral element concentration, water potential, chlorophyll concentration, and partitioning of assimilate in spreading euonymus (E. kiautschovica Loes. 'Sieboldiana' ) . J. Plant Nutr. 13:677-699. Dubik, S.P., D.T. Krizek, D.P. Stimart, and M.S. Mcintosh. 1992. Growth analysis of spreading euonymus subjected to root restriction. J. Plant Nutr. 15:469-486. Dufault, R.J. 1985. Relationship among nitrogen, phosphorus, and potassium fertility regimes on celery transplant growth. HortScience 20:1104-1106. Dufault, R.J. and L. Waters, Jr. 1985. Container size influences broccoli and cauliflower transplant growth but not yield. HortScience 20:682-684. Dufault, R.J. and R.R. Melton. 1990. Cyclic cold stresses before transplanting influence tomato seedling growth, but not fruit earliness, fresh market yield, or quality. J. Amer. Soc. Hort. Sci. 115:559-563. Dufault, R.J. and J.R. Schultheis. 1994. Bell pepper seedling growth and yield following pretransplant nutritional conditioning. HortScience 29:999-1001. Dullforce, W.M. 1971. The growth of winter glasshouse lettuce with artificial light. Acta Hortic. 22:199-210. Garton, R.W. and I.E. Widders. 1990. Nitrogen and phosphorus preconditioning of small-plug seedlings influence processing tomato productivity. HortScience 25:655-657.

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299 Guzman, V.L. 1990. Effect of high temperatures during the seedling stage on yield and quality of crisphead lettuce. Belle Glade EREC Research Report EV-19907:101-108. Guzman, V.L. 1993. Effect of rootball volume and three soluble fertilizer formulas applied at the seedling stage on yields and quality of transplanted crisphead lettuce. Belle Glade EREC Research Report EV-1993-2 : 1117. Guzman, V.L., C.A. Sanchez, and R.T. Nagata. 1989. A comparison of transplanted and direct-seeded lettuce at various levels of soil fertility. Soil Crop Sci. Soc. Fla Proc. 48:26-28. Hall, M.R. 1989. Cell size of seedling containers influences early vine growth and yield of transplanted watermelon. HortScience 24:771-773. Hanlon, E.A., J.G. Gonzalez, and J.M. Bartos. 1994. IFAS extension soil testing laboratory chemical procedure and training manual. Fla Coop. Ext. Serv., IFAS, Univ. Fla, Circ. 812. Hoagland, D.R. and D.I. Arnon. 1950. The water-culture method for growing plants without soil. Circ. 347. California Agricultural Experiment Station, California. Hochmuth, G.J. 1992. Tips on plant sap testing. Amer. Veg. Grower 40:23-25. Hochmuth, G.J. and G.A. Clark. 1991. Fertilizer application and management for micro (or drip) irrigated vegetables in Florida. Fla Coop. Ext. Special Series Report SSVEC-45. Hochmuth, G.J., D.N. Maynard, and M. Sherman. 1988. Tomato production guide for Florida. Vol. 98C. Fla Coop. Ext. Serv., IFAS, Univ. Fla, Circ. 98C. Hochmuth, G.J., D.N. Maynard, C.S. Vavrina, and E.A. Hanlon. 1991. Plant tissue analysis and interpretation for vegetable crops in Florida. Fla Coop. Ext. Special Series Report SS-VEC-42.

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300 Hunt, R. 1978. Plant growth analysis. Edward Arnold, London. Hunt, R. 1982. Plant growth curves. The functional approach to plant growth analysis. Edward Arnold, London. Jaworski, C.A. and R.E. Webb. 1966. Yield and growth uniformity of tomato transplants in relation to nutrition levels. Proc. Amer. Soc. Hort. Sci. 79:216221 . Jaworski, C.A., R.E. Webb, and D.J. Morgan. 1967. Effects of storage and nutrition on tomato transplant quality, survival and fruit yield. Hort. Res. 7:90-96. Karchi, Z., D.J. Cantliffe, and A. Dagan. 1992. Growth of containerized lettuce transplants supplemented with varying concentrations of nitrogen and phosphorus. Acta Hortic. 319:365-370. Kemble, J.M., J.M. Davis, R.G. Gardner, and D.C. Sanders. 1994. Root cell volume affects growth of compactgrowth-habit tomato transplants. HortScience 29:261262. Klassen, P. 1986. Economics dictate using transplants. Amer. Veg. Grower 34:9-14. Knavel, D.E. 1965. Influence of container, container size, and spacing on growth of transplants and yields in tomato. Proc. Amer. Soc. Hort. Sci. 86:582-586. Kratky, B.A. and H.Y. Mishima. 1981. Lettuce seedling and yield response to preplant and foliar fertilization during transplant production. J. Amer. Soc. Hort. Sci. 106:3-7. Krizek, D.T. and D.P. Ormond. 1980. Growth response of 'Grand Rapids' lettuce and 'First Lady' marigold to increased far-red and infrared radiation under controlled environments. J. Amer. Soc. Hort. Sci. 105: 936-939. Leskovar, D.I. and D.J. Cantliffe. 1991. Tomato transplant morphology affected by handling and storage. HortScience 26:1377-1379.

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301 Leskovar, D.I. and D.J. Cantliffe. 1993. Comparison of plant establishment method, transplant, or direct-seeding on growth and yield of bell pepper. J. Amer. Soc. Hort. Sci. 118:17-22. Leskovar, D.I. and R.R. Heineman. 1994. Greenhouse irrigation systems affect growth of 'TAM-Mild Jalapeno1' pepper seedlings. HortScience 29:1470-1474. Leskovar, D.I., D.J. Cantliffe, and P.J. Stoffella. 1991. Growth and yield of tomato plants in response to age of transplants. J. Amer. Soc. Hort. Sci. 116:416-420. Leskovar, D.I., D.J. Cantliffe, and P.J. Stoffella. 1994. Transplant production systems influence growth and yield of fresh-market tomatoes. J. Amer. Soc. Hort. Sci. 119:662-668. Liptay, A. and D. Edwards. 1994. Tomato seedling growth in response to variation in root container shape. HortScience. 29:633-635. Lorenz, O.A. and M.T. Vittum. 1980. Phosphorus nutrition of vegetable crops and sugar beets. The role of phosphorus in agriculture. ASA-CSSA-SSSA, Madison, Wis. p. 737-762 (cited by Widders, 1989) . Masson, J., N. Tremblay, and A. Gosselin. 1991a. Nitrogen fertilization and HPS supplementary lighting influence vegetable transplant production. I. Transplant growth. J. Amer. Soc. Hort. Sci. 116:594-598. Masson, J., N. Tremblay, and A. Gosselin. 1991b. Effects of nitrogen fertilization and HPS supplementary lighting on vegetable transplant production. II. Yield. J. Amer. Soc. Hort. Sci. 116:599-602. Maynard, E.T., C.S. Vavrina, and W.D. Scott. 1996. Containerized muskmelon transplants: Cell volume effects on pretransplant development and subsequent yield. HortScience 31:58-61. Melton, R.R. and R.J. Dufault. 1991. Nitrogen, phosphorus, and potassium fertility regimes affect tomato transplant growth. HortScience 26:141-142. Neter, J., W. Wasserman, and M.H. Kutner. 1990. Applied linear statistical models: regression, analysis of variance, and experimental designs. 3'^'^ ed. Richard D. Irwin, Inc., Boston.

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302 Nicklow, C.W. and P. A. Minges. 1962. Plant growing factors influencing the field performance of the Fireball tomato variety. Proc. Amer. Soc. Hort. Sci. 81:443-450. Nicola, S. and D.J. Cantliffe. 1996. Increasing cell size and reducing medium compression enhance lettuce transplant quality and field production. HortScience 31 : 184-189. Poniedzialek, M., T. Wojtaszek, E. Kunicki, and R. Suchodolska. 1988. Effect of temperature, supplementary lighting, and pricking-out on the length of the growing period and quality of lettuce transplants for greenhouse production. Bull. Pol. Acad. Sci., Biol. Sci. 36:53-60. Sadler, R. and D.J. Cantliffe. 1990. Lettuce bolting problems as related to greenhouse grown transplants. Belle Glade EREC Research Report EV-1990-7 : 55-59 . Soffe, R.W., J.R. Lenton, and G.F.J. Milford. 1977. Effects of photoperiod on some vegetable species. Ann. Appl. Biol. 85:411-415. Tesi, R. And R. Tallarico. 1984. L ' indurimento delle piantine di pomodoro in vivaio e loro resistenza al freddo. Colture Prolette 11:49-54 (cited by Masson et al. , 1991a) . Thomas, B.M. 1993. Overview of the Speedling, Inc., transplant industry operation. HortTech, 3:406-408. Thomas, S.H. 1990. A look at supplementary light fixtures. Greenhouse Manager August : 114-115 (cited by Decoteau and Friend, 1991) . Tibbits, T.W., D.C. Morgan, I, J. Warrington. 1983. Growth of lettuce, spinach, mustard, and wheat plants under four combinations of high-pressure sodium, metal halide, and tungsten halogen lamps at equal PPFD. J. Amer. Soc. Hort. Sci. 108:622-630. Tremblay, N., S. Yelle, and A. Gosselin. 1987. Effects of CO2 enrichment, nitrogen and phosphorus fertilization on growth and yield of celery transplants. HortScience 22: 875-876.

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303 Tremblay, N. and M. Senecal. 1988. Nitrogen and potassium in nutrient solution influence seedling growth of four vegetable species. HortScience 23:1018-1020. Weston, L.A. 1988. Effect of flat cell size, transplant age, and production site on growth and yield of pepper transplants. HortScience 23:709-711. Weston, L.A. and B.H. Zandstra. 1986. Effect of root container size and location of production on growth and yield of tomato transplants. J. Amer. Soc. Hort. Sci. 111:498-501. Weston, L.A. and B.H. Zandstra. 1989. Transplant age and N and P nutrition effects on growth and yield of tomatoes. HortScience 24:88-90, Widders, I.E. 1989. Pretransplant treatments of N and P influence growth and elemental accumulation in tomato seedlings. J. Amer. Soc. Hort. Sci. 114:416-420. Wolf, B. 1982. A comprehensive system of leaf analysis and its use for diagnosing crop nutrient status. Comm. Soil Sci. Plant Anal. 13:1035-1059. Wurr, D.C.E. and J.R. Fellows. 1982. The influence of plant raising conditions on the head weight of crisp lettuce at maturity. J. Agric. Sci., Camb. 99:417-423. Wurr, D.C.E. , J.R. Fellows, and P. Hadley. 1986. The influence of supplementary lighting and mechanicallyinduced stress during plant raising, on transplant and maturity characteristic of crisp lettuce. J. Hort. Sci. 61:325-330.

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BIOGRAPHICAL SKETCH Puffy Soundy was born on 15 May 1962, in Louis Trichardt, South Africa, to Keystone and Flora Soundy, and is third of five sons. He completed high school in 1979 at Good Hope College, Cape Province, South Africa. He then attended the University of Fort Hare, Alice, South Africa, where he obtained a B.Sc. (Agric.) degree in 1986, majoring in crop science and horticultural science. Through a grant from the Council for Scientific and Industrial Research, Pretoria, South Africa, he pursued an M.Sc. (Agric.) degree at the University of Natal, Pietermaritzburg, South Africa, majoring in horticultural science, which he completed in 1990. Having completed his master's degree, he went back to the University of Fort Hare as a Junior Lecturer in the Department of Agronomy. One year later, he was promoted to Lecturer, a position he held until he got a Fulbright Scholarship to pursue a Ph.D degree at the University of Florida. After completing his degree requirements, he is going back to the University of Fort Hare, where he is currently on study leave. He is presently a member of the 304

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305 Southern African Society for Horticultural Sciences and the American Society for Horticultural Science.

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy, Daniel J. Proressor oi Science ntliffe. Chair Horticultural I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Georg^ J. Hochmuth, Cochair Professor of Horticultural Science I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Russell T. Nagata Associate Professor of Horticultural Science I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Do^topj of Phi4os9pJ;iy , ^eter J. Stoff^ Professor of Horticultural Science I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Edward A. Hanlon, Jr. Professor of Soil and Water Science

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This dissertation was submitted to the Graduate Faculty of the College of Agriculture and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy, December, 1996 Dean, College of Agriculture Dean, Graduate School