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Growth of young 'Hamlin' orange trees as influenced by microsprinkler irrigation, fertilization, and nursery tree type

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Growth of young 'Hamlin' orange trees as influenced by microsprinkler irrigation, fertilization, and nursery tree type
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Marler, Thomas E., 1959-
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English
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xi, 173 leaves : ill. ; 28 cm.

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Citrus trees ( jstor )
Fertilizers ( jstor )
Irrigation ( jstor )
Irrigation systems ( jstor )
Plant roots ( jstor )
Plants ( jstor )
Soil science ( jstor )
Soil water ( jstor )
Tree growth ( jstor )
Tree trunks ( jstor )
Dissertations, Academic -- Horticultural Science -- UF
Horticultural Science thesis Ph.D
Oranges -- Growth ( lcsh )
Oranges -- Roots ( lcsh )
City of Gainesville ( local )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1988.
Bibliography:
Includes bibliographical references.
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Typescript.
General Note:
Vita.
Statement of Responsibility:
Thomas E. Marler.

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University of Florida
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University of Florida
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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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AA00004816_00001 ( sobekcm )

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GROWTH OF YOUNG 'HAMLIN' ORANGE TREES AS INFLUENCED
BY MICROSPRINKLER IRRIGATION, FERTILIZATION, AND
NURSERY TREE TYPE








By

THOMAS E. MARLER


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

1988




GROWTH OF YOUNG 'HAMLIN' ORANGE TREES AS INFLUENCED
BY MICROSPRINKLER IRRIGATION, FERTILIZATION, AND
NURSERY TREE TYPE
By
THOMAS E. MARLER
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
1988


ACKNOWLEDGEMENTS
Sincere appreciation is extended to Dr. F. S. Davies, chairman
of the supervisory committee, for his supervision of this project
and guidance in preparation of this manuscript. Appreciation is
also extended to Dr. J. J. Ferguson, Dr. L. K. Jackson, Dr. R. C. J.
Koo, and Dr. A. G. Smajstria for serving on the supervisory
committee and offering helpful suggestions in the planning and
conducting of this research.
Special thanks are extended to Dr. P. C. Andersen for provision
of equipment and to the UF faculty members whose timely counsel made
the continuation of this project possible.
- ii -


TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
LIST OF TABLES v
LIST OF FIGURES viii
ABSTRACT x
CHAPTERS
I INTRODUCTION 1
II REVIEW OF THE LITERATURE 4
Vegetative Growth and Development of Citrus .... 4
Water Relations 7
Irrigation of Mature Citrus 15
Fertilization 18
Young Citrus Tree Care 22
IIIMICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. I. CANOPY GROWTH AND
DEVELOPMENT 29
Introduction 29
Materials and Methods 31
Results and Discussion 42
IVMICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. II. ROOT GROWTH AND
DISTRIBUTION 69
Introduction 69
Materials and Methods 70
Results and Discussion 73
VSOIL MOISTURE STRESS AND FOLIAR GAS EXCHANGE
OF YOUNG, FIELD-GROWN 'HAMLIN* ORANGE TREES 92
Introduction 92
Materials and Methods 93
Results and Discussion 95
- iii -


VIGROWTH OF YOUNG 'HAMLIN' ORANGE TREES USING
STANDARD AND CONTROLLED-RELEASE FERTILIZERS 114
Introduction 114
Materials and Methods 116
Results and Discussion 119
VII GROWTH OF BARE-ROOTED AND CONTAINER-GROWN
'HAMLIN' ORANGE TREES IN THE FIELD 125
Introduction 125
Materials and Methods 126
Results and Discussion 129
VIII CONCLUSIONS 141
APPENDICES
A MEAN MONTHLY PAN EVAPORATION AND MAXIMUM AND
MINIMUM AIR TEMPERATURES FOR 1985, 1986, AND 1987 . 145
B WHOLE PLANT FRESH AND DRY WEIGHTS AND SHOOT:
RATIO OF YOUNG 'HAMLIN' ORANGE TREES AS RELATED
TO IRRIGATION BASED ON SOIL WATER DEPLETION 146
C REGRESSION EQUATIONS AND COEFFICIENTS OF
DETERMINATION ACCOMPANYING FIGS. 3-7, 3-8,
AND 3-9 147
D MEAN AND MAXIMUM LEAF TO AMBIENT AIR TEMPERATURE
DIFFERENCE OF 'HAMLIN' ORANGE TREES IN JUNE 1987
AS INFLUENCED BY SOIL WATER DEPLETION 148
LITERATURE CITED 149
BIOGRAPHICAL SKETCH 173
- iv -


LIST OF TABLES
Tables Page
3-1. Length of irrigation and amount of water applied at
each irrigation related to soil water depletion for
young 'Hamlin' orange trees 43
3-2. Number of irrigations and cumulative water applied
for young 'Hamlin' orange trees under scheduling
treatments based on soil water depletion 46
3-3. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1985 49
3-4. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1986 51
3-5. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Ham!in'
orange trees as related to irrigation based on soil
water depletion, 1985 53
3-6. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1986 54
3-7. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1987 56
3-8. Canopy volume, dry weight, shoot length, leaf area
and trunk cross-sectional area of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion 58
4-1. Total root dry weight, dry weight of new roots, and
root volume of young 'Hamlin' orange trees as related
to irrigation based on soil water depletion 74
- v -


4-2. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1985 77
4-3. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1986 78
4-4. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1987 79
4-5. Partitioning of variance into main and interaction
effects for 1987 circular trench profile root growth
variables 82
4-6. Total root concentration at four depths and two
distances of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1987 83
4-7. Concentration of fibrous roots at four depths and
two distances from trunk of young 'Hamlin' orange
trees as influenced by irrigation based on soil
water depletion, 1987 85
4-8. Concentration of non-fibrous roots at four depths
and two distances from trunk of young 'Hamlin'
orange trees as influenced by irrigation based on
soil water depletion, 1987 86
4-9. Percentage distribution of root concentration with
depth of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1987 .... 87
4-10. Root length density at four depths and two distances
from trunk of young 'Hamlin' orange trees as related
to irrigation based on soil water depletion, 1987 ... 89
5-1. Mean and maximum CO assimilation, transpiration, and
stomatal conductance of young 'Hamlin' orange trees on
18 Oct. 1986 as influenced by soil water depletion . 99
5-2. Mean and maximum CO^ assimilation, transpiration,
stomatal conductance, and mean internal CO2
concentration of young 'Hamlin' orange trees in June
1987 as influenced by soil water depletion 105
6-1. Effects of controlled-release fertilizers on growth
of young 'Hamlin' orange trees in the field 120
- vi


6-2. Effects of standard fertilizer rate on growth of
young 'Hamlin' orange trees in the field 122
6-3. Influence of standard fertilizer rate on leaf analysis
of young 'Hamlin' orange trees in the field 123
7-1. Effect of nursery tree type on growth of 'Hamlin'
orange trees after 8 months in the field 131
7-2. Effect of removing medium prior to planting on growth
of containerized 'Hamlin' orange trees, 1986 135
7-3. Growth of bare-rooted and container-grown 'Hamlin'
orange trees as influenced by planting procedure,
1987 137
- vii -


LIST OF FIGURES
Page
Figure
3-1. The relationship between soil water content and water
potential of Kanapaha sand at the Horticultural Unit . 32
3-2. The relationship between neutron probe ratio and
volumetric water content of Kanapaha sand at the
Horticultural Unit 35
3-3. Relationship of leaf area and fresh weight from eight
representative 'Hamlin' orange trees after 8 months
in the field 39
3-4. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1985 45
3-5. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1986 47
3-6. Distribution of rainfall and microsprinkler Irrigation
at the Horticultural Unit, April-Dee. 1987 48
3-7. Cumulative percentage of trees in three irrigation
treatments growing over the 1985 season 60
3-8. Cumulative percentage of trees in three irrigation
treatments growing over the 1986 season 62
3-9. Cumulative percentage of trees in three irrigation
treatments growing over the 1987 season 63
3-10. Trunk cross sectional area and canopy volume of young
'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern, 1986 66
3-11. Trunk cross sectional area and canopy volume of young
'Hamlin' orange trees as influenced by microsprinkler
Irrigation spray pattern, 1987 67
5-1. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 18 Oct. 1986 96
5-2. Diurnal cycle of CO2 assimilation, transpiration, and
stomatal conductance of young 'Hamlin' orange trees on
18 Oct. 1986 as influenced by soil water depletion ... 97
- viii -


5-3. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 12 June 1987 100
5-4. Diurnal cycle of CO 2 assimilation, transpiration,
stomatal conductance, and water use efficiency of
young 'Hamlin' orange trees on 12 June 1987 as
influenced by soil water depletion 101
5-5. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 13 June 1987 102
5-6. Diurnal cycle of assimilation, transpiration,
stomatal conductance, and water use efficiency of
young 'Hamlin' orange trees on 13 June 1987 as
influenced by soil water depletion 104
5-7. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 15 June 1987 106
5-8. Diurnal cycle of xylem potential, CO2 assimilation,
transpiration, stomatal conductance, and water use
efficiency of young 'Hamlin' orange trees on 15
June 1987 as influenced by soil water depletion .... 107
7-1. Effect of 'Hamlin' orange nursery tree type on
increase in trunk cross sectional area and canopy
volume from May 1985 to Dec. 1986 ........... 130
7-2. Effect of 'Hamlin' orange nursery tree type on
increase in trunk cross sectional area and canopy
volume from May 1986 to Dec. 1987 133
ix -


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
GROWTH OF YOUNG 'HAMLIN' ORANGE TREES AS INFLUENCED
BY MICROSPRINKLER IRRIGATION, FERTILIZATION, AND
NURSERY TREE TYPE
By
Thomas E. Marler
December, 1988
Chairman: Frederick S. Davies
Major Department: Horticultural Science (Fruit Crops)
Young tree care is a costly and important part of any citrus
production program. Irrigation, fertilization, and nursery stock
characteristics have a major influence on the success of a young
tree care program. Therefore field experiments were conducted to
study the effects of various microsprinkler irrigation and
fertilization rates and nursery tree types on growth and development
of young 'Hamlin' orange [Citrus sinensis (L.) Osb.] trees.
Three experiments were conducted to determine the effects of
scheduling microsprinkler irrigations at 20 (high frequency), 45
(moderate frequency), and 65% (low frequency) available soil water
depletion on one season's canopy and root growth and leaf gas
exchange. Canopy and root growth were similar for high and moderate
treatments in 2 out of 3 years, but were reduced by the low
treatment, even though the moderate received about 50% less water
than the high treatment. Reduced shoot number and delayed
- x -


initiation of late growth flushes sometimes occurred with moderate
and low treatments, and may have been related to decreases in CO^
assimilation in spring and early summer. Root concentrations wejre *
greatest between 10 and 30 cm depths, and >90% of the root dry
weight was within 80 cm of the trunk.
Two studies with various fertilizer rates and two studies with
controlled-release fertilizer sources were conducted to determine
the influence of fertilizer rate and source on growth. Growth was
comparable over one season using 0.07-0.09, 0.14-0.18 (recommended),
and 0.22-0.27 kg N/tree/season, implying that adequate rates may
sometimes be lower than recommended. In addition, application of
standard fertilizers (4-5X/season) compared with controlled-release
fertilizers (2X) at the same seasonal rates resulted in similar
growth over 2 seasons, suggesting that controlled-release sources
may sometimes be used as alternatives to standard sources.
Commercial bare-rooted, field-grown nursery trees and
containerized, greenhouse-grown trees were used in three experiments
to compare establishment and initial growth in the field. Bare-
rooted trees remained larger than containerized trees through 2
seasons and initial growth of container-grown trees was highly
variable. Removal of medium prior to planting container-grown trees
improved tree growth in one of two experiments.
- xi


CHAPTER I
INTRODUCTION
Citrus plantings in Florida decreased by more than 75,000
hectares between 1984 and 1986 (Division Plant Industry, 1986), more
than any other 2-year period since the beginning of the Citrus Tree
Census Survey in 1966. This decrease was largely attributable to
the series of freezes since 1982; however, blight, tristeza, and
urbanization have also contributed to the reduction in land area
planted to citrus. Growers estimated in 1983-1984 that 6.1% of
their young trees died each year (Jackson et al., 1986). Estimates
of the number of trees being planted annually in the mid-1980s range
from greater than 3 million (Division Plant Industry, 1986) to 6-10
million (Jackson et al., 1986).
Muraro (personal communication, Citrus Research and Education
Center, Lake Alfred) estimates that more than $20 is required to
maintain a tree in a solid-set planting for the first 4 years in the
flatwood areas, while $24 to $40 per tree is required for resets,
depending on number of resets per hectare. Consequently, young tree
care costs are immense on an industry-wide basis. More efficient
management practices for young citrus trees are important in the
success of any profitable citrus production venture due to the
increased need for cost containment in the Florida citrus industry.
1


2
Vital to the improvement of field management practices is the
understanding of how these practices influence the growth and
physiology of young citrus trees.
Irrigation practices for mature Florida citrus trees are based
on many years of field research, however, information on water
requirements of young citrus trees, irrigation scheduling, and tree
response has not been reported with micro irrigation under field
conditions in Florida. Objectives of the first part of this
research were to investigate the effects of microsprinkler
irrigation scheduling based on soil water depletion on canopy and
root growth of young citrus trees. In addition, the influence of
soil water deficit on gas exchange processes of young citrus trees
was studied. Separate experiments were designed to investigate the
effect of microsprinkler irrigation spray pattern on growth of trees
during the second season in the field.
Fertilization practices for mature Florida citrus are well-
defined and based on years of production records, but fewer studies
have been made on fertilization of young citrus trees. Previous
studies suggest that under many conditions fertilizer rates for
young trees are excessive. Use of controlled-release fertilizers
may reduce nitrogen losses and fertilizer costs by decreasing the
number of yearly fertilizer applications, resulting in reduced labor
and equipment costs. Objectives of the second portion of this
research were to compare growth of young citrus trees using standard
fertilizer applied at recommended seasonal rates and controlled-
release fertilizer applied fewer times per season than the standard
source. A second set of experiments was designed to determine if


3
recommended seasonal fertilization rates could be reduced without a
reduction in young citrus tree growth.
Florida nurserymen have been producing bare-rooted citrus trees
in field nurseries for many years, however, recently many citrus
trees have been produced in containers in greenhouses. While
opinions vary concerning post-plant growth and survival rates of
container-grown and field-grown trees, there have been no
replicated, controlled comparisons of these two nursery tree types
under Florida conditions. The purpose of the third part of this
research was to compare establishment and post-plant canopy and root
growth of l)containerized, greenhouse-grown and 2)bare-rooted,
field-grown citrus trees. Further experiments were designed to
determine the effect of the removal of potting medium prior to
planting on root and canopy growth of container-grown citrus trees.
Results from these experiments may be useful in improving
management programs for young citrus trees by reducing water and
fertilizer costs and may lead to further research on optimizing
growth and development of young citrus trees in Florida.


CHAPTER II
REVIEW OF THE LITERATURE
Citrus tree growth and productivity is influenced by many
environmental conditions and cultural factors. Adequate soil
moisture and nutrient levels are two important factors necessary for
optimum tree growth. Moreover, initial growth and development of
newly-planted trees may be influenced by the nursery conditions
under which they were produced. Understanding the influence of
nursery and field management decisions on growth and physiological
processes of young citrus trees is important to maximize early
productivity.
Vegetative Growth and Development of Citrus
Citrus shoot growth is cyclic with two to five flushes per
season, depending on climate (Cooper et al., 1969; Mendel, 1969).
Environmental conditions may alter both number and timing of growth
flushes. Following cessation of shoot elongation the terminal bud
normally aborts and abscises (Schroeder, 1951). Duration of flush
growth is largely dependent upon climate, as cooler spring
temperatures result in longer periods of growth (Cooper et al.,
1963). Both the spring (Cooper et al., 1963) and summer
(Krishnamurthi et al., 1960) flushes have been reported to occur
over the longest time interval.
4


5
Shoot growth is mainly dependent on stored substrates in citrus
(Sinclair, 1984). Available reserves influence shoot length and
number of leaves per shoot (Van Noort, 1969). The spring flush
normally has more shoots than other flushes, but average shoot
length is shorter (Mendel, 1969; Krishnamurthi et al., 1960). Later
flushes have fewer buds breaking and longer average shoot length,
thicker shoots, and larger leaves (Reuther, 1973; Mendel, 1969). A
large proportion of spring shoots do not continue growing during
subsequent flushes (Sauer, 1951). Summer (Mendel, 1969) and spring
(Crider, 1927; Krishnamurthi et al., 1960; Syvertsen et al., 1981)
flushes have each been attributed with having more total growth than
the other growth flushes.
In the spring root growth usually occurs after shoot growth
(Crider, 1927; Cooper et al., 1963; Bevington, 1983; Hatton, 1949;
Krishnamurthi et al., 1960; Reed & McDougal, 1937), except in one
case where shoot growth occurred first (Waynick & Walker, 1930).
This occurs possibly because soil temperatures are lower than air
temperatures in early spring (Krishnamurthi et al., 1960).
Shoot and root growth flushes tend to alternate. During
periods of very active shoot growth the roots are inactive or nearly
so (Crider, 1927; Bevington, 1983; Bevington & Castle, 1985;
Krishnamurthi et al., 1960; Reed & McDougal, 1937). Mendel (1969)
suggests that increases and decreases in the amount of growth
inhibiting substances may govern the cycles. Trunk growth may be
cyclical, coinciding generally with root growth (Krishnamurthi et
al., 1960), or continuous throughout the growing season (Cooper et
al., 1963).


6
Alternating periods of root and shoot growth and extent of
growth are greatly influenced by the environment. Shoot growth is
reduced below 12.5-13.0 C and above 33-39 C, with optimum growth
occurring between 23-33 C (Mendel, 1969; Reuther, 1973). Root
growth is maximum at soil temperatures of 25-26 C (Reuther, 1973),
and limited below 13 or above 36 C (Castle, 1978). Increases in
soil temperature increase translocation of assimilates to roots
(Vinokur, 1957).
Daylength influences shoot growth of most citrus. While some
citrus is non-responsive to daylength (Warner et al., 1979), many
rootstocks and scions produce more and longer shoots in longer
daylengths (Lenz, 1969; Reuther, 1973; Warner et al., 1979).
The root and shoot relationship is measured many times as a
ratio of fresh or dry weight of shoot and root systems. Mature
citrus trees in California had shoot:root ratios of ca. 3.5
(Cameron, 1939; Cameron & Compton, 1945) and in Florida of 2.2
(Castle, 1978). This ratio varies within a species with age and
environmental or cultural conditions. Some factors that influence
shoot:root ratio of a number of crops are soil moisture levels
(Harris, 1914; Hilgeman & Sharp, 1970; Hsiao & Acevedo, 1974;
Kramer, 1983; Kriedeman & Barrs, 1981; Levy et al., 1983; Menzel et
al., 1986; Rodgers, 1939; Tinus & Owston, 1984), mineral nutrition,
especially nitrogen (Brouwer, 1962; Harris, 1914; Russel, 1977;
Turner, 1922), light intensity (Russell, 1977), and root restriction
(Hameed et al., 1987).


7
Water Relations
Growth Responses to Water Deficits
Most aspects of plant metabolism are affected by tissue water
relations, but turgor-dependent processes, such as cellular
expansion, are especially sensitive. Water deficits reduce cell
expansion and division, organ growth, or whole plant growth (Hagan
et al., 1959; Hsiao, 1973). Studies with forest trees indicate that
water deficits during bud formation can have as much influence on
shoot length as stress during shoot expansion (Kozlowski, 1983;
Doley, 1981). In addition to reduced production of new leaf area
during periods of water deficits, abscission of older existing
leaves may increase the loss in leaf area (Marsh, 1973; Kriedemann &
Barrs, 1981).
Soil water deficits may result in cessation of growth of a
flush or prolong the period of quiescence between flushes in citrus
(Cooper et al., 1969; Mendel, 1969). Ford (1964) stated that the
alternations between root and shoot growth may become less
pronounced during periods of drought.
Growth reduction in response to water stress may be immediately
followed by an increased growth rate upon relief of stress. This
has been shown with corn (Acevedo et al., 1971; Hsiao et al., 1970),
soybean (Bunce, 1977), tomato (Gates, 1955), pine (Miller, 1965),
barley and rye (Williams & Shapter, 1955) under short term stress.
Similarly, early season moisture stress suppressed dogwood shoot
growth initially, but stimulated late season growth following
regular irrigation (Williams et al., 1987).


8
Root growth is likewise affected by moisture stress, although
to a lesser degree than shoot growth. Reduced meristematic activity
and root elongation as well as reduced water and nutrient uptake due
to suberization occur in response to water deficits in most crops
(Slatyer, 1969). Working with wheat, Cole and Alston (1974) state
that root volume or weight are generally more affected by water
deficits than root length. The rate of suberization may exceed that
of elongation during periods of water deficits, substantially
reducing the non-suberized root area (Kaufmann, 1968; Kramer, 1950;
Turner & Kramer, 1980). Periods of water deficits have reduced
citrus root conductivity (Levy et al., 1983; Ramos & Kaufmann, 1979)
and root elongation (Bevington, 1983; Bevington & Castle, 1985;
Marsh, 1973). Water deficits may result in development of long
roots with restricted branching in citrus (Castle, 1978).
Resumption of root growth following rain or irrigation typically
occurs with most crops (Kozlowski, 1983), however, pine roots
reportedly matured toward the tip during water deficits and became
inactive (Kaufmann, 1968; Lesham, 1965).
Total root growth and distribution of citrus trees are often
altered by irrigation practices. Infrequent irrigation increased
root density or the proportion of total roots deep in the soil
profile when compared to frequent irrigation (Cahoon et al., 1961;
Cahoon et al., 1964; Hilgeman & Sharp, 1970; Marsh, 1973; Ruggiero &
Andiloro, 1985). Partial root zone irrigation in an arid
environment results in an increase in root growth in the wetted soil
volume (Bielorai, 1977, 1982; Bielorai et al., 1981; Cohen et al.,
1987; Rodney et al., 1977). Conversely, high root concentrations


9
within the wetted areas do not occur in regions of abundant rainfall
(Koo, 1980).
Gas Exchange Responses to Water Deficits
Water influences plant growth and development through its
effect on biochemical and physiological processes, including gas
exchange. It is clearly established that storaatal conductance
generally declines under conditions of water deficit. This stomatal
response has been considered to be a feedback response to leaf water
status, involving a reduction of leaf water potential, turgor
potential, or relative water content to a critical point, at which
time the stomata begin to close (Begg & Turner, 1976; Hsiao, 1973;
Raschke, 1975; Turner, 1979). However, correlations of bulk leaf
water status and stomatal conductance are typically weak, and the
relationship is not consistent, but is dependent on developmental
conditions. Numerous recent reports with a variety of crops show
that decreases in stomatal conductance resulting from soil moisture
depletion are not closely correlated with changes in bulk leaf water
status (Bates & Hall, 1981; Bennett et al., 1987; Black et al.,
1985; Blackman & Davies, 1985b; Cock et al., 1985; Gollan et al.,
1985; Lorenzo-Minguez et al., 1985; Osonubi, 1985; Turner et al.,
1985). Furthermore, stomata respond directly to epidermal water
relations and are only indirectly related to bulk leaf water
relations (Edwards & Meidner, 1978). The pressure probe has
recently been used to directly measure turgor of individual cells
(Shackel & Brinckmann, 1985; Frensch & Schulze, 1987) and it is now
evident that large cell turgor gradients may exist between mesophyll


10
and epidermis, illustrating the problems associated with relating
stomatal conductance with bulk leaf water status.
Stronger correlations of stomatal conductance to soil moisture
status than to leaf water status (Bates & Hall, 1981, 1982; Blackman
& Davies 1985b; Gollan et al., 1985; Gollan et al., 1986; Jones et
al., 1983; Osonubi, 1985; Turner et al., 1985) suggest that soil
water deficits may influence stomata in a manner independent of
shoot water relations. The most clear evidence of this comes from
stomatal closure in response to soil water deficits, in spite of
maintenance of leaf turgor by partial root zone irrigation (Blackman
& Davies, 1985 a,b) or by pressurizing the root system (Gollan et
al., 1986). In these cases, stomata responded directly to a signal
from roots under soil water deficits. An interruption in cytokinin
supply has been proposed as this message (Blackman & Davies 1985
a,b; Davies et al., 1986). It has been known for two decades (Itai
& Vaadia, 1965) that soil water deficits affect concentrations of
growth substances in roots and shoots. The well-known involvement
of abscisic acid (ABA) in stomatal action (Davies et al., 1981) and
the interaction of this ABA effect with other phytohormones
(Blackman & Davies, 1985 a,b; Cox et al., 1985; Snaith & Mansfield,
1982) lend support to the suggestions that plant growth substances
play a role in the stomatal response to soil water deficits.
However, stomatal closure of some plants is not related to leaf ABA
levels (Davies & Lakso, 1978; Raschke et al., 1976).
The discovery that stomata respond directly to humidity (Lange
et al., 1971) in a feed-forward manner is well-documented in many
plants. Feedforward responses are important in enabling a plant to


11
restrict excessive water loss before developing a severe water
deficit. Sharkey (1984) has shown that high transpiration may
result in reduced photosynthetic capacity in a manner similar to
soil water deficits, and suggests that stomatal responses to
humidity may guard against this reduction in photosynthetic
capacity. Stomatal responses to humidity can greatly confuse
interpretation of stomatal responses to soil water deficits. This
stomatal sensitivity to humidity or vapor pressure deficit (VPD) is
well-documented in citrus (Camacho-B., 1977; Camacho-B. et al.,
1974; Fereres et al., 1979; Hall et al., 1975; Kaufmann, 1977;
Kaufmann & Levy, 1976; Khairi & Hall, 1976b; Kriedemann & Barrs,
1981; Levy, 1980; Syvertsen, 1982) and is important in maintaining
favorable leaf water relations under arid environments. Well-
watered citrus trees in environments of drastically different
evaporative demand have been shown to have nearly equal
transpiration and equal or increased leaf water potential in arid
environments due to VPD-induced partial stomatal closure (Kaufmann,
1977; Levy, 1980; Levy & Syvertsen, 1981). Hilgeman (1966) reported
nearly equal transpiration but greater leaf water deficits of citrus
in Arizona compared to Florida. Annual evapotranspiration values
for mature citrus in different environments are quite similar,
ranging from 1.07 to 1.21 m in Florida (Koo, 1963; Rogers et al.,
1983; Rogers & Bartholic, 1975; Reitz et al., 1978; Gerber et al.,
1973), 0.91 to 1.14 m in Texas (Weigand & Swanson, 1982a), and 0.80
to 1.47 m in Arizona (Hilgeman & Sharp, 1970; Hilgeman et al., 1969;
Hoffman et al., 1982).


12
The literature concerning soil water deficit effects on
photosynthesis is extensive and has been reviewed by many [e.g.
Boyer (1976), Hsiao (1973), Lawlor (1979), Slatyer (1967)]. Early
research suggested that stomatal conductance and CC^ assimilation
are closely correlated for some species, and led to conclusions that
CO2 assimilation was reduced by soil water deficits through stomatal
closure. However, water stress also impairs non-stomatal factors
which may be causal, yet closely correlated with stomatal
conductance. The correlation could result from a concommitant yet
independent effect of water deficits on stomatal and non-stomatal
processes, or from a direct reduction in mesophyll photosynthetic
capacity, followed by stomatal closure in response to reduced CO^
assimilation (Farquhar & Sharkey, 1982; Redshaw & Meidner, 1972;
Wong et al., 1979).
Non-stomatal limitations on photosynthesis are well-documented.
Residual conductance to CC^ is reduced by water stress in many
plants (Brown & Simmons, 1979; Bunce, 1977; Collatz, 1977; Mederski
et al., 1975; OToole et al., 1976; Pearcy, 1983; Pellegrino et al.,
1987; Radin & Ackerson, 1981). Stress-induced reduction of
photochemical activity has been shown by using isolated chloroplasts
and artificial electron acceptors or with measurements on
fluorescence (Boyer, 1976; Boyer & Bowen, 1970; Genty et al., 1987;
Keck & Boyer, 1974; Nir & Poljakoff-Mayber, 1967; Sharkey & Badger,
1982; Von Caemmerer & Farquhar, 1981). Reduced carboxylation
capacity in response to stress has also been reported due to
decreases in activity of ribulose bisphosphate carboxylase-
oxygenase, carbonic anhydrase, phosphoenol pyruvate carboxylase,


13
fructose-1,6-bisphosphatase, or sedoheptulose-1,7-bisphosphatase
(Berkowitz & Gibbs, 1982; Boag & Portis, 1984; Huffaker et al.,
1970; Jones, 1973; O'Toole et al., 1976). In situations where
intercellular CO2 concentration is unchanged or higher in stressed
than in control plants, reduced stomatal conductance of stressed
plants is of little importance in photosynthetic reduction (Briggs
et al., 1986; Farquhar et al., 1980; Radin & Ackerson, 1981; Wong et
al., 1979).
Inhibition of leaf growth by water deficits may contribute to a
whole plant reductions in photosynthesis. Water deficits may reduce
the rate of leaf initiation (Husain & Aspinall, 1970) as well as
foliar expansion (Hagan et al., 1959; Hsiao, 1973). Additional loss
of leaf area through abscission of existing leaves may also result
from water deficits. Stress-induced foliar abscission in citrus
normally occurs at a secondary abscission zone between the leaf
blade and petiole (Schneider, 1968). These effects may be quite
damaging, since recovery does not occur until new growth replaces
the lost photosynthetic surface (Hsiao, 1973).
Effects of water deficits on photosynthesis are certainly not
fully understood. The fact that such diverse mechanisms of action
are discussed in the literature is not surprising and all mechanisms
could be applicable under certain situations.
Water deficit effects on gas exchange of citrus has received
fairly limited attention. Hilgeman (1977) stated stomatal closure
of mature citrus in Arizona occurs at progressively earlier times in
the day as soil water is depleted over several weeks. Irrigated and
nonirrigated mature sweet orange trees in Italy began days with


14
similar storaatal conductances, but conductance of nonirrigated trees
declined steadily throughout the day, while that of irrigated trees
increased to peak around midday before declining (Ruggiero &
Andiloro, 1985). Similarly, leaf conductance of severely-stressed
mature sweet orange trees increased slightly until mid-morning
before declining steadily throughout the day, while conductance of
irrigated trees increased rapidly until mid-morning and did not
decline any until mid-afternoon (Cohen & Cohen, 1983). Hilgeman et
al. (1969) reported increased transpiration of mature sweet orange
trees in Arizona at a soil matric potential of -29 kPa compared to
-79 kPa, especially during the middle part of the day. Koo (1953)
reported citrus tree transpiration in Florida was unchanged at soil
moisture levels between field capacity and depletion of two-thirds
of the readily available water. A reduction in transpiration or
storaatal conductance by soil water deficits has been reported under
many conditions (Bielorai & Mendel, 1969; Brakke et al., 1986; Cohen
& Cohen, 1983; Hilgeman, 1977; Kaufmann & Levy, 1976; Koo, 1953;
Kriedeman, 1971; Ruggiero & Andiloro, 1985; Thompson et al., 1968;
Zekri, 1984). Only controlled conditions utilizing containerized
plants have been used to illustrate a reduction of citrus
photosynthesis by soil water deficits (Bielorai & Mendel, 1969;
Brakke et al., 1986; Kriedemann, 1968, 1971; Ono & Hirose, 1984;
Thompson et al., 1968). Bielorai & Mendel (1969) reported that
sweet lime and rough lemon photosynthesis was reduced more than
transpiration with soil water deficits, while Kriedemann (1971)
reported a greater reduction of transpiration with sweet orange.


15
Irrigation of Mature Citrus
Irrigation in Florida
Although Florida's climate is characterized by an average
annual rainfall of 120-150 cm, the rainfall is distributed in an
irregular pattern. Consequently, periods of minimal and infrequent
rainfall result in unfavorable soil moisture conditions for optimum
citrus growth and production. Periods of drought combined with the
low water-holding characteristics of most Florida soils create
favorable conditions for the use of irrigation to obtain maximum
production.
Many early studies presented differing results concerning the
need to irrigate mature citrus trees (DeBusk, 1928, 1933; Hoard,
1908; Koo & Sites, 1955; Savage, 1951, 1952; Sites et al., 1951;
Staebner, 1919; Stanley, 1913, 1914, 1916; Stevens, 1909; Williams,
1909; Young, 1948; Ziegler, 1955b). Many of these reports were
based on observations and correlations between rainfall or soil
moisture content and tree response, while some were actual
irrigation experiments. Responses to irrigation and opinions on its
cost effectiveness varied widely. Ziegler (1955b) stated that
irrigation was needed in Florida only in years of light spring
rainfall, and was not essential every year, and suggested the use of
temporary wilt or "anticipated tree wilt combined with records of
rainfall for practical irrigation timing (Ziegler, 1955a).
Results of a 3-year study were published in 1963 indicating
that irrigation based on soil moisture depletion increased yields
for a variety of scions on rough lemon rootstock (Koo, 1963).
Recommendations were to irrigate from January to June based on


16
one-third soil moisture depletion, and from July to December based
on two-thirds depletion. Since that time irrigation has ranked high
among requirements for maximum production of Florida citrus, even in
years of high rainfall. Thus, irrigation has been used in Florida
to increase yields of many cultivars on different rootstocks (Koo,
1963, 1969, 1975, 1979, 1985; Koo & Hurner, 1969; Koo & McCornack,
1965; Koo & Sites, 1955; Koo et al., 1974; Reese & Koo, 1976).
Likewise, irrigation increases vegetative growth of mature citrus
trees under Florida conditions (Koo, 1963, 1969, 1979, 1985; Koo &
Hurner, 1969; Sites et al., 1951; Zekri, 1984). Increases in citrus
yield due to irrigation may result from an increase in canopy volume
(Koo, 1969; Koo & Hurner, 1969; Levy et al., 1978). Recommendations
similar to Koo's (1963) concerning more frequent irrigation during
the spring than other parts of the year are reported for other
citrus growing regions (Hilgeman, 1956; Hilgeman & Sharp, 1970;
Mantell, 1977; Weigand & Swanson, 1982a).
Micro Irrigation of Mature Citrus
Micro irrigation systems, which direct water to only a portion
of the soil volume, have received widespread use in citrus culture
around the world. Yields of navel oranges were comparable using
trickle, basin, and dragline sprinklers in South Africa, with ca.
50% savings of applied water for the trickle systems over the other
systems (Bester et al., 1974). Similarly, water savings of 30% were
possible while using trickle Instead of flood irrigation on
grapefruit in Arizona (Roth et al., 1981). Irrigating oranges with
trickle or basin systems in Arizona reduced water application by
greater than 90% when compared with border or high volume sprinkler


17
methods (Roth et al., 1974). Increased tree growth and reduced weed
control problems also resulted from the use of trickle and basin
irrigation. Drip irrigation increased yields and fruit size of
sweet oranges by 30% over flood irrigation in Spain (Legaz et al.,
1981). Similar comparisons of trickle systems and conventional
systems with mature citrus are numerous (Alijbury et al., 1974; Cole
& Till, 1974; Leyden, 1975b; Rathwell & Leyden, 1976; Yagev &
Choresh, 1974) and include those cited in the next discussion.
Irrigation studies frequently use several types of systems or
emitters to apply water to variable percentages of soil surface.
Bielorai (1977, 1978, 1982) reported similar grapefruit yields and
tree growth resulted in Israel from wetting 30-40 as compared to 70%
of the soil surface. Similarly, irrigation of 35, 70, and 90% of
the soil surface for oranges resulted in increased or comparable
yields with the smaller wetted areas (Bielorai et al., 1981). These
results involved data from only the first 3 years following a
reduction in wetted soil volume. Following this period, however,
yields of the 90% wetting treatment were greater than for the other
treatments (Cohen et al., 1985). Another comparison of orange
performance In Israel showed a reduction in growth rate and yield
with partial compared to full wetting (Moreshet et al., 1983).
Other responses included reduced water uptake and tree hydraulic
conductance due to reduced wetted soil volume (Cohen et al., 1985,
1987). Koo and Tucker (1974) recommended in Florida that systems be
designed to wet 60% of the citrus root volume. However, irrigating
high-density oranges at 15-25%, 30-40%, and 70-78% of the under-tree
area showed that yield Increased with increasing coverage (Koo,


18
1978). Grapefruit irrigated at 14, 64, and 100% coverage of under
tree area also responded with increased yield and growth with
increasing coverage (Koo, 1985). Irrigation at 19, 88, and 100% of
under-tree area gave similar results (Zekri, 1984). Five to 10%,
and 28 to 51% coverage of the under-tree area increased yield of
oranges 41-44 and 65%, respectively, over nonirrigated controls in
Florida (Smajstrla & Koo, 1984, 1985).
Fertilization
Uptake and Function of Minerals
Plants require adequate supplies of essential elements for
optimum growth and development. These elements affect plant growth
through their involvement in physiological and biochemical
processes. The amount required by plants, form that is absorbed,
and function vary considerably among the essential elements.
Agricultural soils are more commonly deficient in nitrogen (N)
than any other element. Nitrogen is an essential component of amino
acids, chlorophyll, and many other organic materials, and is
involved in many enzymic processes. Various forms of N are absorbed
by roots, but nitrate and ammonium are the major forms.
Preferential absorption of ammonium (Wallace, 1954; Wallace &
Mueller, 1957) or nitrate (Hilgeman, 1941) has been suggested in
citrus. Several studies have provided estimates of the partitioning
of N throughout the citrus plant. Leaves contained 31% of the N in
a mature orange tree In Florida; with fruit, small roots, branches,
large root, and the trunk containing 27, 21, 13, 5, and 3%,
respectively (Smith, 1963). Cameron and Compton (1945) presented
data from California suggesting that leaves contain 40-50%; twigs


19
and shoots, 10%; trunk, and branches, 20-30%; and roots, 15-20% of
the total N. In contrast, leaves contained greater than 60% of the
total N in 3.5 year old orange trees (Cameron & Applemen, 1933).
Averaged over the entire season, N content was 1.86% of the total
tree dry weight.
Absorption and function of the remaining essential elements are
covered in detail by Clarkson and Hanson (1980) and Mengel and
Kirkby (1982) and are briefly outlined here. Carbon, hydrogen,
oxygen, and sulfur have metabolic functions similar to N, in that
they are structural components of essential plant compounds and are
involved in enzymic processes. Sulfur is absorbed from the soil as
sulfate, or from the atmosphere as sulfur dioxide. Carbon,
hydrogen, and oxygen are absorbed in gaseous forms from the
atmosphere. Phosphorus, boron, and silicon are involved in
esterification with alcohol groups in plants, and are absorbed as
phosphates, borate or boric acid, and silicate, respectively.
Potassium, sodium, magnesium, calcium, manganese, iron, chlorine,
copper, zinc, and molybdenum are absorbed by plants in the form of
ions or chelates. Their metabolic functions are numerous, and range
from controlling osmotic and electro-potentials and membrane
permeability to enzyme activation.
Fertilization of Mature Citrus in Florida
Plant growth and yield increase with added fertilizer up to a
critical level, after which no significant increase occurs. As this
level of maximum production is approached, the increase in response
for a specific rate increase becomes less and less. Citrus tree
responses to fertilization do not increase indefinitely with


20
increased fertilization rates, nor do trees accumulate excessive
amounts when minerals are present in large quantities (Koo et al.,
1984; Sites et al., 1953; Sites et al., 1961; Smith & Rasmussen,
1961; Stewart et al., 1961; Wallace et al., 1952). Optimum
fertilization rates depend not only on response to increased
fertilizer application, but on fertilizer and fruit prices as well.
Current recommendations for fertilizing mature citrus groves in
Florida (Koo et al., 1984) are based on many years of controlled
experiments. Mature citrus trees require from 100 to 340 kg
N/ha/year to maintain optimum production, however, rates above 225
kg/ha are rarely justifiable in terms of yield increase for most
citrus cultivars.
Many field experiments have been conducted to determine the
optimum number and timing of fertilizer applications and the effect
of elemental sources on mature citrus trees. Results generally
agree that frequency or timing are of far less importance than
fertilizer rate (Calvert & Reitz, 1964; Reitz, 1956; Reuther &
Smith, 1954; Sites et al., 1953, 1961; Smith, 1970). One to two
applications per year are adequate when recommended rates are used.
Nutrient source is also of little consequence in yield responses of
mature citrus trees (Camp, 1943; Leonard et al., 1961; Reitz, 1956;
Reitz & Koo, 1959; Sites, 1949; Sites et al., 1953; Smith, 1970;
Stewart et al., 1961).
Nitrogen Losses
Applied N may be lost from the soil in many ways. Ammonium
salts and urea may be lost by volatilization of ammonia, especially
on calcareous soils (Volk, 1959; Wahhab et al., 1957).


21
Nitrification, the bacterial oxidation of ammonia to nitrate, can
lead to increased losses of N from the root zone by leaching of
nitrate. Reduction of nitrate to volatile forms of N is carried out
by many species of bacteria in the soil. This process
(denitrification) is promoted by high soil moisture and temperatures
(Mengel & Kirkby, 1982).
About a third of the applied N was lost through leaching in a
newly-developed Florida citrus grove (Calvert, 1975; Calvert &
Phung, 1971). An average of 50 kg N/ha may be lost annually through
leaching in Florida groves (Barnette, 1936). Annual leaching losses
from some southern California citrus orchards have been estimated at
about 67 kg N/ha (Bingham et al., 1971), which is equivalent to
about 45% of the applied N. Wallace et al. (1952) estimated a 15%
recovery of applied N by citrus trees in southern California.
Similarly, annual losses of up to 67 kg/ha have been attributed to
volatilization (Chapman, 1951; Chapman et al., 1949).
Controlled-Release Fertilizer Sources
Controlled-release fertilizer sources that are N carriers can
be used to substantially reduce losses of applied N (Maynard &
Lorenz, 1979; Oertli, 1980). Some benefits from using
controlled-release fertilizers are reduced losses of applied N
through leaching, denitrification, or volatilization; the
possibility of less frequent applications; and increased efficiency
in use of materials (Allen & Mays, 1974; Oertli, 1980; Terman &
Allen, 1970). Controlled-release fertilizer sources have proven
beneficial with many horticultural crops (reviewed by Maynard &
Lorenz, 1979). Sulfur-coated urea and isobutylidene diurea have


22
increased mature orange tree yields over ammonium nitrate in Florida
(Koo, 1986).
Young Citrus Tree Care
Objectives of a young citrus tree care program center on
obtaining the greatest amount of growth in the shortest amount of
time. There is little concern for marketable yield or fruit quality
as with mature trees.
Irrigation
Information on irrigation of young citrus is limited. In
Arizona, basin and trickle irrigation systems provided greater
growth and lower water use of young orange trees through 5 years in
the field, compared to border-flood and sprinkler systems (Rodney et
al., 1977; Roth et al. 1974). The trickle system was operated
daily, basin and sprinkler were operated weekly, and flood
irrigations were at intervals of at least 2 weeks. Trees in the
trickle treatment received 11%, in the basin treatment, 13%, and in
the sprinkler treatment, 38% as much water as those in the flood
treatment. Leyden (1975a) compared strip watering, ring watering,
and drip irrigation on newly-planted grapefruit trees in Texas.
Tree growth was similar over a 2-year period, with drip requiring
about 18% and ring watering 25% of the water utilized with strip
watering. Strip and ring watering were scheduled at 30% soil water
depletion, and drip irrigation was scheduled on the basis of pan
evaporation. De Barreda et al. (1984) used drip and basin
irrigation in Spain to compare the application of varying amounts of
water at the same frequency. Applications were based on
coefficients of pan evaporation, and results suggested that


23
coefficients of 0.10-0.15, 0.20, and 0.30 for the first 3 years
maintained adequate growth. They made no comparison of basin and
drip irrigation methods. Split-root containers have been utilized
to simulate the effects of partial root wetting of young citrus
trees in Florida (Brakke et al., 1986). Preliminary results of gas
exchange measurements suggested that growth may be only slightly
reduced by irrigating 50-75% of the root volume compared to 100%.
Field lysimeters equipped with rain shelters have been used to
study irrigation requirements of young orange trees in Florida
(Aribi, 1985; Smajstrla et al., 1985). Irrigations were scheduled
at matric potentials of -10, -20, and -40 kPa, which corresponded to
available soil water depletions of 30, 45, and 55%. Water was
applied to return the upper 60 cm of soil to field capacity, thus
23, 34, and 42 liters were applied per irrigation per tree in the
-10, -20, and -40 kPa treatments, respectively. Maintaining
weed-free conditions around trees resulted in a 50% reduction in
water use compared to trees with a bahiagrass cover. Tree growth
was greatest when irrigations were scheduled at -20 kPa and the
ground was maintained weed-free.
General recommendations have been given for irrigation of
newly-planted citrus in Florida. Jackson and Ferguson (1984)
recommend that newly-planted trees irrigated by the basin method
should be watered 2 to 3 times per week for 8 weeks, and once per
week thereafter. Ziegler and Wolfe (1975) suggest applying 30-38
liters per tree each 2 weeks in the spring, with no irrigation
needed in the fall. Jackson and Lawrence (1984) recommend a
"generous supply" of water applied every 7-10 days.


24
Fertilization
Optimum nutrient levels for maximum canopy growth should be
maintained by fertilization of young citrus trees (Koo & Reese,
1971). Young tree fertilization practices in Florida have changed
over the years. Collison (1919) made a 10-year comparison of
fertilizer rates. The standard treatment used 0.136 kg N/tree/year
in year-1, and was gradually increased to 0.408 kg N/tree/year by
year-10. Some plots received one-half, twice, and four times this
amount. After several years, trees in the standard and one-half
standard plots had larger trunk diameters than two-fold and
four-fold treatments. Although not properly replicated, these data
suggest as little as 0.068 kg N/year are needed for fertilization of
young citrus trees. Bryan (1940) recommended applying 0.07 kg
N/year per meter of canopy spread for young citrus trees in Florida.
Several field experiments conducted in the 1950s suggested that
annual rates of 0.073 kg N/tree for the first 2 years were
sufficient to obtain optimum tree growth on the Ridge (Rasmussen &
Smith, 1961, 1962). These rates and the authors' recommended
application frequency of three times during the first year and two
times thereafter differed from the average grower practice. Up to
seven applications per year and rates of 0.13 and 0.25 kg N/tree for
the first 2 years, respectively, were not uncommon at the time.
Calvert (1969) reported that trees responded with greater
growth to rates of 0.22 to 0.32 kg N/tree than 0.11 kg/tree when
growing in the flatwoods in marginal soils. Furthermore, four
applications per year were superior to three. These data illustrate


25
the importance of considering soil type when evaluating
fertilization needs.
There is a wide range of adequate fertilization methods
available for growers, and considerable judgement is required to
choose the most efficient. Current recommendations (Koo et al.,
1984) call for 0.18-0.22 kg N/tree in year-1 and 0.29-0.36 kg N/tree
in year-2. Other elements should be applied in proportion with N in
the following ratio: N-l, P20,--1, K20-1, Mg-1/5, Mn-1/20, Cu-1/40,
B-l/300.
Controlled-release N sources have been used for fertilization
of young citrus trees. Controlled-release isobutylidene diurea
increased growth of young container-grown citrus compared to soluble
N sources (Khalaf, 1980; Khalaf & Koo, 1983). In the same study,
isobutylidene diurea and sulfur coated urea reduced N leaching
losses compared to soluble sources. Fucik (1974) similarly
demonstrated an increase in growth of young, container-grown citrus
trees with controlled-release compared to soluble fertilizer.
Jackson & Davies (1984) reported similar growth rates of young,
field-grown 'Orlando' tngelo trees occurred with sulfur coated urea
and a soluble fertilizer source, but application frequency was
reduced by 50% with sulfur coated urea.
Container- and Field-grown Nursery Trees
Cultural practices and growing conditions of any nursery affect
growth and development of plant material in the nursery, but also
after transplanting to the field. Production systems for field and
greenhouse citrus nurseries are well-developed, however, little
effort has gone into understanding the effect of nursery tree


26
characteristics on growth after field planting. Webber (1932), with
'Washington' navel orange, and Gardner and Horanic (1959) with
'Parson Brown and 'Valencia' orange reported no relationship
between initial size of nursery trees and mature tree size. Effects
on precosity were not reported. Grimm (1956, 1957) stated the most
important factor affecting initial growth of bare-rooted nursery
trees was the protection of roots while they were out of the ground.
The advantages of containerized, greenhouse nursery systems are
varied and include greater control over the production system, lower
land requirement, and shorter production cycle (Castle et al., 1979;
Moore, 1966; Platt & Opitz, 1973; Richards et al., 1967). Nursery
trees produced under these conditions are much different from the
traditional field-grown nursery trees. Shoot:root ratio of nursery
trees is substantially increased in this system over the field
system (W.S. Castle, Citrus Research and Education Center, Lake
Alfred, personal communication). Very high nitrogen and water
applications, reduced light intensity, and root restriction by
containers may all contribute to this, as all of these factors have
been shown to increase shoot:root ratio. Vigorous root growth is
altered when the available soil volume is permeated, at which time
the growth pattern may be shifted to fibrous roots (Castle, 1978).
This results in a substantial increase in the proportion of fibrous
to non-fibrous roots of container-grown compared to field-grown
citrus nursery trees (W.S. Castle, Citrus Research and Education
Center, Lake Alfred, personal communication).
Initial expansion of the root system is critical for successful
establishment of any containerized transplant (Castle, 1987). The


27
potential for rapid initial root growth of containerized forest
seedlings has been studied in detail and is directly linked to
survival and tree growth after field planting. Container
characteristics such as size and shape have altered post-planting
shoot and root growth of container-grown transplants (Elam et al.,
1981; Hiatt & Tinus, 1974; Hite, 1974; Tinus & Owston, 1984; Van
Eerden & Arnott, 1974). Container medium has also affected tree
growth following field planting in a number of species (Elam et al.,
1981; Helium, 1981) including citrus (Warneke et al., 1975). Elam
et al. (1981) reported considerable variation between oak species
with respect to the effects of container and media characteristics
on growth. Less root growth of pine seedlings occurred as the
length of time plants were maintained in containers was lengthened
(Helium, 1981). Reduced irrigation frequency or nitrogen
fertilization prior to removal from the nursery has been used to
increase root and shoot growth of container-grown trees (Rook, 1973;
Timmis, 1974; Tinus & Owston, 1984).
Drying of container medium after planting in the field may
induce plant water stress in some cases. A substantial increase in
drainage out of the medium occurred following removal from a
container and placement in contact with field soil (Costello & Paul,
1975; Nelms & Spomer, 1983; Warneke et al., 1975). Low survival
rates have been attributed to severe water stress due to these
conditions.
Post-plant growth comparison of container- and field-grown
citrus nursery trees has been conducted in Texas. Leyden and Timmer
(1978) observed growth of grapefruit on sour orange trees for 2.5


28
years in the field and concluded that container-grown trees would be
less productive and smaller than field-grown trees during the early
years of bearing. Maxwell and Rouse (1980, 1984) reported that
container-grown grapefruit on sour orange trees remained smaller
than field-grown trees through 10 years after planting, but yield
did not differ. Container-grown trees were not produced under
greenhouse conditions as is the case in Florida, and field-grown
trees were transplanted as ball and burlapped stock in both studies.


CHAPTER III
MICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG 'HAMLIN'
ORANGE TREES. I. CANOPY GROWTH AND DEVELOPMENT
Introduction
Comparable or increased growth or yield and decreased water use
by mature citrus has been observed in several countries when micro
irrigation systems were compared with flood or high volume sprinkler
systems (Alijbury et al., 1974; Bester et al., 1974; Bielorai, 1982;
Bielorai et al., 1981; Legaz et al., 1981; Roth et al., 1981). Some
have found that increasing the area of coverage by irrigation
systems has increased growth and yield of mature citrus (Moreshet et
al., 1983; Koo, 1978, 1985; Zekri, 1984). Smajstrla and Koo (1984,
1985) reported an increase in yield over non-irrigated trees in
Florida when irrigating between 5 and 50% of the under-tree canopy
area of mature citrus. Koo and Tucker (1974) recommended that 60%
of a citrus tree's root volume be covered by an irrigation system.
A number of studies have been conducted comparing growth and
development of young citrus trees using various irrigation methods.
Border-flood, sprinkler, basin, and trickle irrigation methods were
compared on young 'Campbell Valencia' orange trees in Arizona
(Rodney et al. 1977; Roth et al. 1974). Tree growth and yield were
greater and water use lower in the first 5 years with the basin and
29


30
trickle systems compared with the other systems. Growth of young
navel orange trees has been evaluated using drip and basin
irrigation methods in Spain (De Barreda et al., 1984). Treatments
consisted of applying different volumes of water at the same
frequency based on coefficients of pan evaporation. Coefficients
were increased with each year of age, suggesting that coefficients
of 0.10-0.15, 0.20, and 0.30 for the first 3 years maintained
adequate growth. Leyden (1975a) compared irrigation systems on
newly-planted 'Star Ruby' grapefruit trees in Texas. Strip and ring
watering were scheduled based on 30% soil water depletion, and drip
irrigation was scheduled on the basis of pan evaporation. Drip
irrigation used 18 and ring watering used 25% of the cumulative
irrigation water required for strip watering. Tree growth was
similar between the three systems over a 2-year period.
Irrigation of young citrus trees in Florida is also necessary
to obtain optimum growth. Irrigation studies on newly-planted
'Valencia' orange trees were conducted using a series of lysimeters
under rain shelters (Aribi, 1985; Smajstrla et al., 1985).
Comparing irrigation scheduling at matric potentials of -10, -20,
and -40 kPa, tree growth was greatest when irrigations were
scheduled at -20 kPa and the ground was maintained weed-free.
Split-root containers were utilized to simulate the effects of
partial root-volume wetting of young 'Hamlin' orange trees (Brakke
et al., 1986). Preliminary results of gas exchange measurements
suggested that growth may be only slightly reduced by irrigating
50-75% of the root volume compared to 100%. Allen et al. (1985)


31
stated that frequent irrigation of containerized citrus rootstock
seedlings was required to sustain high photosynthesis.
An estimated 6-10 million young citrus trees are being planted
in Florida annually (Jackson et al., 1986), however, water
requirements, irrigation scheduling, and growth responses to
microsprinkler irrigation have not been studied under field
conditions. The purpose of this field study using 'Hamlin' orange
trees was to determine 1) the optimum level of soil water depletion
at which irrigations should be scheduled to maximize growth, 2) the
amount of irrigation time needed to replenish soil water to field
capacity, and 3) the effect of irrigation pattern on growth of
second season trees.
Materials and Methods
Site and Soil Characteristics
Three field experiments were conducted at the Horticultural
Research Unit near Gainesville, FL, in 1985, 1986, and 1987. Soil
type was Kanapaha sand (Carlisle et al., 1988) (loamy, siliceous,
hyperthermic, Grossarenic, Paleaquults) underlain by a hardpan.
Particle size distribution was 93.4% sand, 3.9% silt, and 2.7% clay.
The soil has a field capacity of ca. 11.3%; permanent wilting point,
3
ca. 2%; mean bulk density, 1.56 g/cm ; pH, 6.4; and percent organic
matter, 0.65. A desorption soil water characteristic curve was
determined using a vacuum desiccator employing undisturbed soil
cores in weighable pressure cells fitted with fritted glass plates
(Fig. 3-1). Saturated hydraulic conductivity was 9.3 cm/hr as
determined by using undisturbed cores in a constant head
permeameter.


Log, soil water potential (kPa)
32
0.1 0.2 0.3
Volumetric water content (m3*m3)
Fig. 3-1. The relationship between soil water content
and water potential of Kanapaha sand at the Horticultural
Unit.


33
Beds (16.75 m width x 0.60-0.75 m height x 85 m length) were
constructed in March, 1985 to facilitate drainage and simulate
flatwoods growing conditions. Ground water table was monitored
through observation wells located in the tree rows and averaged 1.1
m deep between June and Sept, (rainy season) with a minimum of 0.45
m, and greater than 1.6 m during other months. A ground cover of
bahiagrass was developed between rows and was mowed as needed, while
tree rows (about 2.1 m width) were maintained weed-free with
herbicides.
Irrigation Treatments
The irrigation system was designed so that each treatment could
be monitored and controlled individually using a flow meter and
pressure gauge controlled by a gate valve. Water was conveyed from
a manifold through 2.54 cm PVC main lines and 1.91 cm black
polyethylene laterals down tree rows. Three tubes, one per
treatment, were positioned 50-55 cm west of each tree row which ran
the full length of the bed, allowing unrestricted randomization at
each experimental site. Irrigation water containing 287 mg/liter
total dissolved solids was applied via 90, 38 liter/hr Maxijet"
microsprinkler emitters positioned 1 m northwest of each tree. This
positioning also provides optimum cold protection for young citrus
trees (Rieger et al., 1986). The area of ground covered was about 5
2
m for these emitters. Irrigation was controlled by hand regulation
of gate valves to maintain the system pressure at about 140 kPa.
Irrigation scheduling was based on soil water content as
monitored by the neutron scattering method (Hillel, 1982) using a
Troxler Model 1255 neutron probe. A calibration curve for the


34
neutron probe was developed by the gravimetric method (Fig. 3-2)
(Hillel, 1982). Access tubes constructed of 48 mm i.d. aluminum
irrigation tubing were driven into the soil 1 m from the emitters
and 35 cm from each of four randomly chosen trees per treatment in
1985 and 1986 and from each of three trees per treatment in 1987.
Soil moisture measurements were made daily or as needed during the
rainy season at a depth of 30 cm. The soil volume around the tree
was irrigated to field capacity when a prespecified level of
available soil water depletion (SWD) was reached. Irrigations were
initiated when any of the four neutron probe readings reached the
level of SWD in 1985 and 1986, and based on the average of the three
neutron probe readings in 1987. Three levels of irrigation were
designated as high (20% SWD), moderate (45% SWD), and low (65% SWD).
The amount of irrigation water needed for each treatment to
replenish soil moisture to field capacity was determined during the
first few irrigations in 1985. Soil water content at 30 cm depth
was monitored using the neutron scattering method at 15-min
intervals following an irrigation. By initially varying the length
of irrigation time, the approximate length of time needed for each
treatment to return soil moisture from the specified SWD level to
field capacity was determined.
Plant Material and Experimental Design
Commercially obtained, bare-rooted 'Hamlin' orange [Citrus
sinensis (L.) Osb.] on sour orange (£. aurantium L.) trees were
planted on double row beds at 7.6 m between and 4 m within rows.
Initial trunk diameter averaged 1.2, 1.4, and 1.5 cm in 1985, 1986,
and 1987, respectively. A randomized complete block design


35
Volumetric water content (m3*m-3)
Fig. 3-2. The relationship between neutron probe ratio and
volumetric water content in Kanapaha sand at the
Horticultural Unit. R = counts per min. in soil,
R = standard counts per min.
o r


36
consisting of four blocks was utilized in 1985 and 1986, employing
six single-tree replications per treatment per block, resulting in a
total of 24 replications per treatment. A completely randomized
design with 13 replications was utilized in 1987 because the block
effect was not significant in 1985 and 1986. Trees were planted on
10-12 May, 3-4 May, and 2 April for the 1985, 1986, and 1987
experimental periods, respectively. All trees were irrigated every
2 days during an establishment period of 10-14 days in each
experiment. Commercial fertilization rates recommended for Florida
(Koo et al., 1984) were followed using an 8N 2.6P 6.6K 2Mg -
0.2Mn 0.12Cu 0.2Zn 1.78Fe dry formulation. Four applications
were made at regular intervals in 1985, while five were made in 1986
and 1987.
Plant Measurements
Seasonal measurements. Shoot growth was measured throughout
the growing season during 1985 and 1986. As shoots began growth,
three per tree were randomly chosen and tagged. Shoot length was
measured with a ruler at about 7-day intervals thereafter. The
length and width of the third leaf from the shoot base was measured
at the same time. This leaf position was used because the bottom
two leaves were many times of unrepresentative size or shape. Leaf
expansion was determined by using the product (x) of length and
2
width in an equation for leaf area (area = 2.33 + 0.63x, r = 0.94).
This equation was derived from a sample of 100 leaves of various
sizes by linear regression of leaf area, determined with a LI-COR
Model LI-3000 leaf area meter, on leaf length x width (data not
shown). Final shoot length and leaf area were compared, as were


37
expansion rates. Rates were calculated on a daily basis by
subtracting the initial length or area when shoots were tagged from
the final length or area, and dividing by the number of days
required to reach the final size. Although shoot and leaf expansion
were not measured in 1987, dates of initial shoot growth were
recorded for each tree in each of the three flushes. Intervals of
about 7 days were again used. The percentage of the tree population
growing for each irrigation treatment and at each date was
calculated for all 3 years.
Canopy and trunk measurements. Canopy height and width in two
directions were measured initially and in December of each year (15
May and 10 Dec. 1985, 6 May and 7 Dec. 1986, 4 April and 18 Dec.
1987). Width measurements were averaged and canopy volume was
calculated as (4/3)(3.14)(1/2H)(1/2W)^, where H=height and W=width
(Westwood, 1978). This formula most closely approximates the canopy
shape of a young tree which is taller than it is wide. A mark was
painted with latex paint about 5 cm above the bud union where trunk
diameter was measured in two directions with a hand-held caliper.
Measurements were made on the same days as canopy measurements. The
two measurements were averaged and trunk cross sectional area
calculated. In addition total shoot length for each tree was
measured with a ruler within 1 week of planting dates.
Final measurements. Root excavation of 20, 21, and five plants
per treatment in 1985, 1986, and 1987, respectively, was conducted
by hand in Dec. of each year. Initially a circular trench was dug
about 40 cm deep at a distance of 120 cm from the trunks. The few
roots extending beyond this distance were individually recovered by


38
excavation. The entire shallow root system was recovered almost
completely intact by undercutting it to a depth of 40 cm until the
sand loosened exposing the roots. The depth was increased near the
trunk to ensure recovery of the taproots. This operation was not
difficult due to the loose, sandy nature of the soil and shallowness
of the root systems. Trees were taken indoors where roots and
canopies were separated. Root growth and distribution are discussed
in Chapter IV.
Leaves were removed from canopies for fresh weight
determination. Leaf area was calculated from the fresh weight
utilizing a linear relationship obtained from sampling eight trees.
Leaf area was measured on these trees using a LI-COR Model LI-3000
leaf area meter. The relationship shown in Fig. 3-3 was derived
from linear regression of leaf area on fresh weight. Total shoot
length was measured with a ruler following leaf removal.
A random sample of four, six, and five trees in 1985, 1986, and
1987, respectively, were separated into the three growth flushes
that had occurred during the experimental period. Shoots within
each flush were counted and measured with a ruler. Mean and total
shoot length and mean shoot number were calculated for each growth
flush.
All canopy parts were dried in an oven at 80 C to determine
canopy dry weight.
Analysis of Data
Shoot length, leaf area, and expansion rates measured
throughout the growing season were analyzed separately for each of
the three flushes in 1985 and 1986. Shoots in each flush were


Leaf area (m
39
Fig. 3-3. Relationship of leaf area and fresh weight from
eight representative 'Hamlin' orange trees after 8 months
in the field.


40
blocked into groups of shoots that had begun growth within periods
of time not exceeding 2 weeks in length. Data were analyzed by
analysis of variance as a treatment x time period factorial
experiment.
Data concerning the percentage of each treatment population
growing throughout the season were analyzed using linear, quadratic,
and cubic regression models. Models with the highest level of
significance and best fit for each treatment population were chosen.
Resulting equations were tested by analysis of covariance for
homogeneity within each growth flush.
Shoot number, mean length, and total length measured by
dividing excavated trees into different flushes were analyzed by a
split-plot analysis with treatments as main plots and flushes as
subplots.
Final plant measurements from different years were analyzed
separately. Data on dry weight and leaf area were subjected to
analysis of variance and data on canopy volume, trunk cross
sectional area, and shoot length to analysis of covariance to
standardize differences in initial plant measurements. Where
irrigation levels were significantly different, Williams' (Williams,
1971) test was used to compare means. This test is useful in cases
where curve-fitting is not desired or is difficult due to a small
number of dose levels. It was employed due to the small number of
treatments to compare irrigation levels to the most frequent
scheduling treatment to determine the level at which a significant
response occurred


41
Microsprinkler Irrigation Spray Patterns
Two experiments were conducted on the same site to test the
effect of microsprinkler irrigation spray pattern on 'Hamlin' orange
tree growth the second season in the field. Site characteristics
were as described for microsprinkler irrigation scheduling
experiments, except trees were set 3.4 m apart in the rows. Two
microsprinkler spray patterns were utilized, 90 and 180.
Approximately 89 liters were applied per tree at each irrigation,
delivered through 89 liter/hr emitters and distributed over ca. 5.9
2
and 9.3 m of ground area for the 90 and 180 patterns,
respectively.
Treatments begun on 10 May 1986 in the first experiment on
trees planted in May 1985. The factorial experiment performed from
May 1985 May 1986 was used to study two types of nursery trees
receiving four types of fertilizer (Chapters VI & VII). Each 2x4
treatment combination had been assigned to two trees within each of
four randomized complete blocks in 1985. Subsequently, each spray
pattern was randomly assigned to one of the two trees of each tree
type fertilizer type combination in May 1986. Treatments were
begun on 2 May 1987 in the second experiment on trees planted in May
1986 (Chapters VI & VII). Each 2x4 treatment combination had been
assigned to two trees within each of two randomized complete blocks
in 1986. Each spray pattern was again randomly assigned to one of
the two trees of each tree type fertilizer type combination in May
1987.
Trees were grown during the first year under 90 microsprinkler
irrigation on 20Z SWD scheduling. During the two experimental


42
periods irrigations were scheduled when 20% SWD was reached in
accompanying irrigation scheduling experiments.
Trunk cross sectional area and canopy volume were measured as
described previously. Measurements were made on 9 May and 7 Dec.
1986 for the first experiment and 30 April and 18 Dec. 1987 for the
second. Measurements were analyzed separately for the two
experimental periods. Split-plot analysis was utilized with
fertilizer type x tree type as main plots and irrigation patterns as
subplots. Analysis of covariance was used to standardize
differences in plant measurements from the beginning of the
experimental periods.
Environmental Variables
Rainfall was recorded daily with a Science Associates Model 503
rain gauge located about 200 m east of the experimental site.
Relative humidity and temperature were recorded continuously using a
hygrothermograph (WEATHERtronics Model 5021) located at the
experimental site.
Results and Discussion
Irrigation Amount and Frequency
Thirty-eight, 50, and 76 liters/tree were needed for the high,
moderate, and low irrigation treatments, respectively, to return the
soil to field capacity (Table 3-1). These durations resulted from
measurements of soil moisture content by the neutron probe at a
depth of 30 cm, and did not take into account the level of depletion
in the surface soil. Irrigation durations of 1-2 hr were used with
the 38 liter/hr, 90 emitters used in this study. An industry
survey has shown that many times growers do not take advantage of


43
Table 3-1. Duration of irrigation and amount of water
applied at each irrigation as related to soil water
depletion for young 'Hamlin' orange trees.
Soil water
Irrigation
Amount
applied
depletion
duration
liters/
(%)Z
(hr)
tree
mmy
20 (HighX)
1.0
38
7.5
45 (Mod.)
1.3
50
10.0
65 (Low)
2.0
76
15.1
2
Based on neutron probe measurements at 30 cm ^epth.
yBased on area wetted by emitters of about 5 m .
X *
High, moderate, and low refers to irrigation frequency.


44
the water savings that are possible with micro irrigation systems
due to excessive operating time (Hutcheson & Bellizio, 1974).
The number of irrigations needed to maintain soil moisture at
the specified levels differed greatly (Table 3-2). An average of
31, 11, and 2 irrigations per season were required for the high,
moderate, and low schedules, respectively. During dry periods,
irrigations were 2-3 days apart in the high treatment and 4-6 days
apart in the moderate schedule. The number of irrigations in the
moderate schedule varied from year-to-year more than the other
scheduling treatments. The 16 irrigations needed in 1986 were
nearly twice the number needed in the other years. Different
factors were probably responsible for the reduced number of
irrigations needed in 1985 and 1987 compared with 1986. In 1985,
more frequent rainfall (Fig. 3-4) did not allow SWD to reach 45%
(moderate) as often as occurred in 1986 (Fig. 3-5). Rainfall was
not responsible for the reduced number of irrigations in 1987 since
frequency and cumulative amount of rainfall was lower (Fig. 3-6)
than in 1986. Tree growth and water use was less during the 1987
season regardless of irrigation treatment, thus the plants required
fewer irrigations. Over the 3-year period, trees in the moderate
and low treatments received respectively 49 and 13% as much
irrigation water as those in the high treatment (Table 3-2).
Individual Shoot and Leaf Growth Seasonal Measurements
During the first flush of 1985 trees under the high and
moderate treatments made significantly greater average shoot growth
(P<.0165) and leaf growth (P<.0315) than trees under the low
treatment (Table 3-3). Shoot and leaf size in the second and third


Rainfall (cm) Low(cm) Mod. (cm) High (cm)
45
Fig. 3-4. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1985. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.


46
Table 3-2. Number of irrigations and cumulative water applied
for young 'Hamlin' orange trees under scheduling treatments
based on soil water depletion.
Soil Cumulative
water Irrigations/ Cumulative water applied irrigation
depletion
(%)Z
year
(no.)
liters/
tree
/N
o
B
and rain
(cm)
1985
20 (HighX)
31
1173.5
23.3
141.2
45 (Mod.)
8
402.8
8.0
125.9
65 (Low)
1
75.7
1.5
119.4
1986
20 (High)
35
1324.9
26.4
145.8
45 (Mod.)
16
805.5
16.0
135.5
65 (Low)
4
302.8
6.0
125.5
1987
20 (High)
27
1022.0
20.3
112.1
45 (Mod.)
10
503.5
10.0
101.8
65 (Low)
1
75.7
1.5
93.3
2
Based on neutron probe measurements at 30 cm ^Based on area wetted by emitters of about 5 m .
xHigh, moderate, and low refers to irrigation frequency.


Rainfall (cm) Low (cm) Mod. (cm) High (cm)
47
Time (months)
Fig. 3-5. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1986. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.


Rainfall (cm) Low (cm) Mod. (cm) High (cm)
48
Fig. 3-6. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, April-Dee. 1987. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.


49
Table 3-3. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1985.
Soil water
depletion
(%)
Shoot
Leaf
n
length
(cm)
rate
(cm/day)
n
area
(cm )
rate
2
(cm /day)
Flush
one (5/30 -
6/25)Z
20 (High)
70
9.7
0.5
67
22.6
0.8
45 (Mod.)
69
10.8
0.6
67
23.3
0.9
65 (Low)
64
8.6
0.4
61
19.6
0.7
SEy
0.7
0.1
1.4
0.1
Flush
two (7/12 -
8/19)
20 (High)
49
21.5
1.2
49
38.7
1.4
45 (Mod.)
40
22.3
1.3
40
40.9
1.6
65 (Low)
34
17.4
1.0
33
28.1
1.0
SE
ns
ns
ns
ns
Flush three (9/11
- 11/25)
20 (High)
36
28.8
1.1
37
44.1
1.7
45 (Mod.)
28
31.5
1.2
28
43.8
1.6
65 (Low)
10
25.8
1.2
10
40.7
1.5
SE
ns
ns
ns
ns
2
Range in dates of growth initiation.
ySE = standard error of mean.


50
flush were not dependent on irrigation treatments, although the low
treatment consistently had the lowest means.
Irrigation treatment effects on growth of the first flush in
1986 were in contrast to 1985, in that trees in the moderate and low
treatments had greater individual shoot length (P<.0077) and leaf
area (P<.0005) than trees in the high treatment (Table 3-4). A
different pattern occurred in flush two, when shoot length of trees
in the high and moderate treatments was larger (P<.0117) than those
in the low treatment. Leaf size in flush two and both shoot and
leaf size in flush three were independent of irrigation treatments.
Shoot length, leaf area, and rates of growth generally increased
from flush one to flush three in both 1985 and 1986.
Shoot and leaf expansion rates averaged less in flush one than
the other flushes. Similarly, growth rates of the spring flush of
mature orange trees in Florida, estimated as gain in biomass, were
less than the summer flush (Syvertsen et al., 1981). Shoot growth
rates ranged from 0.4-1.0 cm/day in flush one and 0.7-1.4 cm/day in
the other flushes, which was considerably more than previously
reported for mature trees of various citrus species in subtropical
India (Krishnamurthi et al., 1960).
Shoot Number and Length Within Flushes Final Measurements
Shoot number, average shoot length, and total shoot length were
not significantly affected by irrigation treatments in 1985.
Cumulative shoot length during the season (three flushes summed) was
650, 760, and 537 cm/tree in the high, moderate, and low treatments,
respectively. Lack of significant differences may have been be due
to the small number of replications (four) or to the loss in


51
Table 3-4. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1986.
Soil water
depletion
(%)
Shoot
Leaf
n
length
(cm)
rate
(cm/day)
n
area
(cm )
rate
2
(cm /day)
Flush one (5/25 -
6/17)Z
20 (High)
65
13.7
0.8
65
15.9
0.6
45 (Mod.)
45
17.5
1.0
44
22.0
0.9
65 (Low)
49
17.8
1.0
49
21.4
0.7
SE7
1.5
0.1
1.9
0.1
Flush
two (7/7 -
9/13)
20 (High)
61
20.3
1.1
60
31.3
1.1
45 (Mod.)
39
21.6
1.1
44
34.9
1.3
65 (Low)
40
17.4
0.7
40
30.4
1.1
SE
1.5
0.1
ns
ns
Flush three (8/30
- 12/3)
20 (High)
58
30.6
1.4
53
46.6
2.1
45 (Mod.)
49
31.7
1.4
46
46.3
1.8
65 (Low)
45
28.2
1.3
42
37.3
1.5
SE
ns
ns
ns
ns
ZRange in dates of growth initiation.
ySE = standard error of mean.


52
precision in the whole plot analysis of treatments in the split-plot
design. Across all treatments, however, individual shoot length
significantly increased (P<.0001) and shoot number per flush
decreased (P<.0108) successively from flush one to flush three
(Table 3-5). Flushes two and three added significantly (P<.0124)
more total shoot length than flush one during the season (Table
3-5).
In contrast, irrigation treatments significantly affected shoot
number per flush (P<.0079) and total shoot length per flush
(P<.0164) in 1986, although there was no effect on mean shoot length
(Table 3-6). A mean of 19 shoots per flush occurred on trees in the
high treatment, with 13 and 9 shoots per flush occurring on trees in
moderate and low treatments, respectively (SE=3). Total length per
flush was 335 cm for trees in the 20% SWD treatment, and 258 and 134
cm for trees in the 45 and 65% SWD treatments (SE=64). Reduction in
total length per flush by 65% SWD was more pronounced in flush three
(Table 3-6). Cumulative shoot length for the entire season was 1004
cm in high, and 774 and 402 cm in moderate and low treatments,
respectively. As in 1985, mean shoot length, pooled over the
treatments, significantly (P<.0001) increased from flush one to
flush three, while flush three had significantly (P<.0012) fewer
shoots than flushes one and two (Table 3-6). These trends balanced
total length of individual flushes such that the flushes were not
different.
Irrigation treatments did not affect mean shoot length or
number in 1987, but significantly (P<.0204) altered total shoot
length per flush, producing a total length of 408, 235, and 212 cm


53
Table 3-5. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1985.
Soil water
Shoot
Total
depletion
Shoot
length
length
(%)
z
no.
(cm)Z
(cm)Z
Flush one
(5/30 6/25)y
20 (High)
17
9.5
163.0
45 (Mod.)
15
7.9
118.7
65 (Low)
18
7.9
146.4
Mean
17
8.4
142.7
Flush two
(7/12 8/19)
20 (High)
13
20.5
261.3
45 (Mod.)
10
28.2
289.1
65 (Low)
8
25.7
211.7
Mean
10
24.8
256.4
Flush three
(9/11 11/25)
20 (High)
7
34.8
225.9
45 (Mod.)
11
32.2
352.4
65 (Low)
4
42.9
179.1
Mean
7
36.6
262.1
SEX
2
3.4
36.9
2
Means of 4 trees/treatment.
y
'Range in dates of initiation.
SE for comparison of means among growth flushes,
among irrigation levels within growth flushes is
Comparison
inappropriate
since irrigation x flush interaction is not significant.


54
Table 3-6. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1986.
Soil water
Shoot
Total
depletion
Shoot
length
length
(%)
z
no.
(cm)2
(cm)2
Flush one
(5/25 6/17)y
20 (High)
22
13.1
284.3
45 (Mod.)
16
13.0
203.7
65 (Low)
12
10.8
126.3
Mean
16
12.3
201.9
Flush two
(7/7 9/13)
20 (High)
23
15.9
366.4
45 (Mod.)
15
18.2
283.5
65 (Low)
12
20.1
234.1
Mean
17
18.1
301.1
Flush three
(8/30 12/3)
20 (High)
12
29.5
353.6
45 (Mod.)
9
33.4
286.7
65 (Low)
2
22.9
41.7
Mean
8
29.5
219.9
SEX
3
1.8
ns
2
Means of 6 trees/treatment.
yRange in dates of initiation.
XSE for comparison of means among growth flushes. Comparison
among irrigation levels within growth flushes is inappropriate
since irrigation x flush interaction is not significant.


55
of growth per flush (SE=59) in the high, moderate, and low
treatments, respectively (Table 3-7). As in 1986, the treatment
effect was most pronounced in flush three. Mean shoot length
significantly increased (P<.0001) and shoot number decreased
(P<.0001) successively from flush one to flush three, although total
length was not different among the three flushes. Cumulative shoot
length for the season averaged 1225 cm for the high, and 706 and 635
cm for the moderate and low treatments.
Severe defoliation shortly after transplanting trees from the
nursery to the field in 1987, possibly due to greasy spot, produced
a large number of short shoots during flush one. An average of 68
shoots were initiated per tree, compared to 17 and 16 in 1985 and
1986. Although these shoots in 1987 were much shorter than in the
other years, total shoot length in flush one was greater due to the
large number of shoots.
There was no uniform effect of irrigation treatment on average
shoot length or leaf size. This seems in contrast to the
well-established effect of decreased organ expansion due to water
deficits (Hsiao, 1973). However, Hanson and Hitz (1982) state that
under conditions of extreme diurnal variation in leaf water status,
expansion may be inhibited during the day, but long-term growth may
not be affected in some plants. There was little contribution of
soil water deficit to the pronounced midday decreases in xylem
potential during the early summer in this study, but a considerable
decrease in CO^ assimilation occurred with increased soil water
deficit (Chapter V). In contrast to the lack of effect on average
shoot length, treatments more consistently affected the number of


56
Table 3-7. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1987.
Soil water
depletion
(%)
Shoot
z
no.
Shoot
length
(cm)Z
Total
length
(cm)Z
Flush one (4/12 5/6)^
20 (High)
77
4.9
376.4
45 (Mod.)
63
4.0
254.8
65 (Low)
64
4.2
264.5
Mean
68
4.4
297.4
Flush two (6/18 8/2)
20 (High)
22
16.2
362.4
45 (Mod.)
21
12.3
259.1
65 (Low)
20
11.0
216.8
Mean
21
13.1
277.4
Flush three (8/9 11/2)
20 (High)
15
32.4
486.6
45 (Mod.)
6
32.1
192.4
65 (Low)
6
27.4
153.5
Mean
9
30.6
271.7
SEX
6
1.5
ns
2
Means of 5 trees/treatment.
^Range in dates of initiation.
XSE for comparison of means among growth flushes. Comparison
among irrigation levels within growth flushes is inappropriate
since irrigation x flush interaction is not significant.


57
shoots that initiated growth during the later flushes of the season.
Perhaps the level of carbon reserves at the time of flush initiation
affects the number of shoots that grow.
Average shoot length consistently increased with the three
successive flushes during all years of the irrigation scheduling
experiments, and shoot number generally decreased from flush one to
flush three. This is consistent with previous studies with citrus
growth and development in both subtropical and arid regions
(Krishnamurthi et al., 1960; Mendel, 1969).
Final Measurements
Final canopy size was not different for trees in the high and
moderate treatments in 1985 or 1986. The low treatment, however,
significantly decreased canopy volume (P<.0003), trunk cross
sectional area (P<.0001), dry weight (P<.0043), total shoot length
(P<.0040), and leaf area (P<.0029) when compared to the high
treatment in 1985 (Table 3-8). Similarly, in 1986 trees in the low
treatment had significantly less canopy volume (P<.0023), trunk
cross-sectional area (P<.0017), dry weight (P<.0086), and leaf area
(P<.0217) than in the high treatment (Table 3-8).
Irrigation treatment effects on canopy growth followed a
different pattern in 1987. Both the moderate and low treatments
reduced tree growth measured as trunk cross sectional area
(P<.0029), canopy dry weight (P<.0050), shoot length (P<.0257), and
leaf area (P<.0112) when compared to the high treatment (Table 3-8).
In addition, final canopy size for all treatments was generally
smaller in 1987 when compared to the other years. This difference
in growth response may be due to the nursery trees used in 1987. As


58
Table 3-8. Canopy volume, dry weight, shoot length, leaf area
and trunk cross-sectional area of young Hamlin' orange trees
as related to irrigation based on soil water depletion.
Trunk
Soil
cross
Canopy
water
Canopy
sectional
dry
Shoot
Leaf
depletion
volume
area
wt.
length
area
(%)
, 3.
(m )
(cm )
(g)
(cm)
(m2)
1985
20 (High)
0.57
5.1
424.6
950.8
1.3
45 (Mod.)
0.52
4.9
425.8
950.9
1.2
65 (Low)
**
0.33
**
4.2
**
336.6
**
752.4
**
1.0
1986
20 (High)
0.51
8.0
379.2
950.4
1.3
45 (Mod.)
0.54
8.0
383.3
942.8
1.3
65 (Low)
**
0.31
**
6.8
**
300.0
894.3
**
0.9
1987
20 (High)
0.56
4.7
393.2
1428.6
1.4
45 (Mod.)
0.37
*
3.8
**
259.0
937.4*
*
0.7
65 (Low)
0.36
*
3.3
**
229.6
872.2*
*
0.5
* **
Response
is significant when compared with
the 20% soil
water
depletion
treatment by
the Williams
method,
5% and 1%,
respectively.


59
discussed earlier, abscission of most leaves occurred shortly after
transplanting to the field, inducing a flush with more than three
times the number of shoots than occurred in 1985 or 1986. Newly-
emerged shoot growth in citrus is mainly dependent upon stored
reserves (Sinclair, 1984; Van Noort, 1969), and this heavy flush may
have severely depleted the level of reserves in these trees. Webb
(1981) stated that following a heavy defoliation of fir trees,
recovery was related to the level of carbohydrate reserves in the
trees, and trees with a low level of reserves died. Survival was
not a problem in this study, but a severe depletion of reserves
early in the spring may have caused reduced canopy growth in general
in 1987, especially in the moderate and low treatments. Weak trees
typically respond to more frequent irrigation with increased growth.
Seasonal Distribution of Shoot Growth
More than two-thirds of the trees in 1985 initiated their first
growth flush the last week in May, and all had begun growth by 25
June (Fig. 3-7). Initiation of the second flush occurred from 12
July to 19 Aug.. Irrigation treatments did not affect the dates of
initial growth in either of the first two flushes. Initiation of
the third flush occurred over a longer time period than for flushes
one and two. The entire population of trees receiving the high
irrigation treatment had begun growth of flush three in October.
The percentage of trees initiating this flush in the moderate and
low treatments was shifted to later in the season, and approximately
40% of the trees in the low treatment did not initiate a third
flush. The shapes of curves in flush three differed significantly
among treatments


60
J J A S O N
Time (months)
Fig. 3-7. Cumulative percentage of trees in three irrigation
treatments growing over the 1985 season. ( MMN ) 20% soil
water depletion (SWD), ( ) 45% SWD, ( # ) 65% SWD.
ns,** indicates nonsignificance or significant at the 1% level,
respectively, according to analysis of covariance test of
homogeneity of the three equations.


61
Similarly, initiation of the first growth flush in 1986 was not
affected by irrigation treatments, and occurred between 25 May and
17 June (Fig. 3-8). The two subsequent flushes were initiated over
a longer time interval, ranging from 7 July to 13 Sept, in flush two
and 30 Aug. and Dec. in flush three. The shapes of the curves in
both flushes two and three differed significantly among treatments.
Again, the percentage of trees initiating a flush in the moderate
and low treatments shifted growth to later in the season compared to
the high treatment. Approximately 25 and 35% of the trees in the
moderate and low irrigation treatments had not initiated growth of
the third flush by the end of the season.
Initiation of flush one in 1987 occurred earlier than in the
other years due to an earlier planting date (Fig. 3-9). Trees
across all treatments began growth between 12 April and 6 May.
Flushes two and three were again spread over a longer time interval
than flush one. Treatment effects were similar to those in 1986 in
that the shapes of the curves for flushes two and three differed
significantly. All trees initiated a third flush by the end of the
season, in contrast to 1985 and 1986, possibly resulting from the
earlier planting date.
Microsprinkler irrigation of these young trees at 45 and 65%
SWD clearly prolonged the period between initiation of flushes two
and three in some cases when compared to 20% SWD. Similarly, Cooper
et al. (1969) stated that a prolonging of the period of quiescence
between growth flushes of citrus commonly occurs in response to
drought. This delay in shoot growth may result from decreased
levels of available reserves, since a considerable reduction in CO2


62
Tim* (months)
Fig. 3-8. Cumulative percentage of trees in three irrigation
treatments growing over the 1986 season. () 20% soil
water depletion (SWD), ( ) 45% SWD, ( ) 65% SWD.
ns,** indicates nonsignificance or significant at the 1% level,
respectively, according to analysis of covariance test of
homogeneity of the three equations.


63
so
100
Time (days)
1S0
L
200
I
M
Time (months)
Fig. 3-9. Cumulative percentage of trees in three irrigation
treatments growing over the 1987 season. ( MNH ) 202 soil
water depletion (SWD), ( ) 452 SWD, ( eeeee ) 652 SWD.
ns,*,** Indicates nonsignificance or significant at the 5
and 12 level, respectively, according to analysis of
covariance test of homogeneity of the three equations.


64
assimilation occurred in response to increased soil water deficit
(Chapter V). Perhaps a critical level of available reserves must be
met before subsequent shoot growth begins. The period between
flushes of citrus has been shortened with longer daylengths
(Piringer et al., 1961). Both increased irrigation frequency and
daylength would fit well into a hypothesis of available reserves
controlling the length of time between flushes, as both would
increase CO^ assimilation over time. Shoot length and number of
leaves per shoot are dependent upon available reserves before growth
begins (Van Noort, 1969), and the date of growth may be dependent as
well.
Other factors may be involved with this delay in growth. The
amount of growth inhibitors reportedly decreases during the period
between flushes (Mendel, 1969). It would be of value to understand
the interactions of growth delays caused by less frequent irrigation
with the levels of growth promoters and inhibitors. Knowledge of
the interactions of root growth periodicity would also add
understanding.
Time of shoot growth initiation was uniform from plant to plant
in the spring flush all 3 years and across all irrigation
treatments, but became more widespread in the subsequent flushes.
This pattern has been previously documented with mature citrus trees
in Australia (Sauer, 1951).
Microsprinkler Spray Patterns
2
Distributing irrigation water over 5.9 and 9.2 m by using 90
and 180 spray patterns during the second growing season did not
result in different trunk cross sectional area or canopy volume


65
(Figs. 3-10, -11). Furthermore, measurements of root distribution
suggested that 90 emitters placed water over the majority of the
citrus tree's root system after one season of growth (Chapter IV).
When beginning with 90 spray patterns, there appears to be no
advantage in changing to a larger pattern after the first season in
the field. By directing more water on tree trunks, 90 patterns are
more efficient than larger patterns for freeze protection purposes
(Rieger et al., 1986). Maintaining 90 patterns for irrigation
purposes more than one season allows their use throughout another
winter for freeze protection.
In summary, growth of young 'Hamlin' orange trees was similar
in 2 out of 3 years with the high (20% SWD) and moderate (45% SWD)
irrigation treatments, but was reduced by the low treatment (65%
SWD). The seasonal amount of water applied to the moderate
treatment averaged about 50% of the amount applied to the high
treatment. There was a pronounced delay in summer and fall growth
flushes and In some cases a reduction in the number of shoots per
tree in the low irrigation treatment.
The optimum level at which irrigations should be scheduled
cannot be precisely determined from these studies, but is most
likely between 20 and 45% SWD. These values are in general
agreement with other reports. Smajstria et al. (1985) obtained
optimum growth of young 'Valencia' orange trees while scheduling
irrigations at 45% SWD in a field lysimeter study. Leyden (1975a)
presented no data, but suggested scheduling basin and strip watering
methods of irrigation on 30% SWD, based on field observations.
Therefore, it appears that on a per tree basis, microsprinkler


TCA (cm
66
Fig. 3-10. Trunk cross sectional area (TCA) and canopy volume
of young 'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern (90 and 180), 1986. There were no
significant differences between patterns, 5% level.


TCA (cm
67
Fig. 3-11. Trunk cross sectional area (TCA) and canopy volume
of young 'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern (90 and 180), 1987. There were no
significant differences between patterns, 5% level.


68
irrigation at 45% SWD is as effective as at 20% SWD, resulting in a
considerable reduction in water use. However, on a population basis
the final growth flush of some trees will be reduced at the moderate
irrigation level probably because of inherent variability in SWD
among trees within the treatment. Variability in soil moisture
content is a particular problem in flatwoods areas of Florida where
soil type is extremely variable. Consequently, growers must be
certain to monitor soil moisture content carefully in a
representative portion of their grove to ensure that the entire
irrigated area receives adequate water.


CHAPTER IV
MICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. II. ROOT
GROWTH & DISTRIBUTION
Introduction
Perennial tree root systems respond to a variety of
environmental and management factors, including irrigation.
Irrigation method and scheduling directly affect soil water content
and indirectly affect edaphic factors like physical impedence,
fertility, and aeration, thus altering root growth and distribution.
Increases in soil water content generally increase root growth
provided that oxygen or salinity levels are not limiting.
Irrigation has been observed to increase root growth or density of
many crops (Goode et al., 1978; Goode & Hyrycz, 1970; Ponder &
Kenworthy, 1976; Richards & Cockroft, 1975), including citrus
(Bielorai, 1982; Bielorai et al., 1981; Bielorai et al., 1984;
Hilgeman & Sharp, 1970; Rodney et al., 1977). In addition to growth
responses, irrigation also affects root distribution. Frequent
irrigation commonly increases the proportion of roots in shallow
zones (Beukes, 1984; Cahoon et al.; 1961, Goode et al., 1978; Goode
& Hyrcyz, 1964; Hilgeman et al., 1969; Hilgeman & Sharp, 1970; Layne
et al., 1986; Levin et al., 1980).
69


70
Increasing demands on water resources require that irrigation
be used more efficiently. Micro irrigation systems are becoming
widely used in Florida, however, there is little information
available on irrigation scheduling for young citrus trees under
Floridas subtropical climate and sandy soil conditions. Therefore,
our objectives were to study the influence of three microsprinkler
irrigation treatments on root growth and distribution of young
citrus trees.
Materials and Methods
Three field experiments were conducted at the Horticultural
Research Unit near Gainesville, FL, with site and environmental
variables as described In Chapter III. Microsprinkler irrigation
was scheduled based on available soil water depletion (SWD) In the
root zone. Treatments were defined as high (20% SWD), moderate (45%
SWD), and low (65% SWD).
Root Growth and Lateral Distribution, Excavated Root Systems
Twenty, 21, and five root systems per treatment were excavated
by hand in 1985, 1986, and 1987, respectively. Excavation procedure
was described in Chapter III. Included in this group were samples
of 10 (1985, 1986) and five (1987) randomly selected plants per
treatment in which the root system was dyed prior to planting.
Dyeing was accomplished by dipping roots for 15 sec in a 1% solution
of safranin-0 as described by Kaufmann (1968). Root dry weights
were determined following oven-drying for 3 days at 80 C. Roots
which had developed during the experimental period were easily
distinguished from initial dyed roots, and were oven-dried and
weighed separately. Random samples of 10 (1985, 1986) and five


71
(1987) plants per treatment were utilized to determine the increase
in root volume. Water displaced by submersing root systems into
water-filled containers was collected to determine root volume at
planting time and after excavation.
Samples of 10 (1985) and five (1986, 1987) randomly selected,
excavated root systems per treatment were used to determine lateral
root distribution. After excavation these root systems were
separated into three concentric zones (0-40, 40-80, and >80 cm) from
the trunk. Distribution of total and fibrous (<_ 1.5 mm) roots was
determined on a dry weight and percentage basis.
Circular Trench Profiles
A modification of the trench profile method (Atkinson, 1980)
for root distribution studies was used to determine irrigation
effects on vertical root distribution at two distances from tree
trunks. The method was modified by Huguet (1973) by using a
curvilinear instead of a straight trench, based on the assumption
that roots radiate concentrically from trunks. Based on the same
assumption, circular trenches were dug by hand around each of three
randomly selected trees per treatment in Dec. 1987. The initial
trench was excavated 80 cm from the trunk to a depth of 50 cm. The
profile was smoothed with a spade, then ca. 2 cm of soil was removed
by hand with the aid of a brush to expose roots. It was not
difficult to remove a 2 cm layer of soil from the circular profile
walls due to the loose, sandy nature of the soil (Kanapaha sand).
Roots were counted at each of four depth increments (0-10, 10-20,
20-30, and >30 cm) and categorized on the basis of total root
number, and the number of fibrous (<^ 1.5 mm) and non-fibrous (> 1.5


72
mm) roots. Vertical distribution was determined on a concentration
2
(roots/m ) and percentage basis.
Roots were counted to allow calculation of root length density
immediately after recording root numbers. Lengths of exposed roots
were estimated by visually counting the number of 2-cm sections of
exposed root length as described previously (Bohm, 1976) in each
depth increment. Roots that were perpendicular to the profile wall
in situ extended ca. 2 cm, and were assigned a value of one, while
roots longer than 2 cm were given values greater than one. For
example, roots which were 3, 4, and 5 cm long were given values of
1.5, 2.0, and 2.5, respectively. Summation of these values
estimated the number of 2-cm increments of exposed roots. Root
-3
length density (mm dm ) was determined for each depth by
calculating total root length (multiplying the sum of all values by
2 cm) and volume of the 2 cm layer of soil on each profile wall. A
second circular profile wall was subsequently exposed on the same
tree at 40 cm from the trunk and the process repeated.
Analysis of Data
Data from excavated root systems were analyzed separately for
the 3 years. Data on root dry weights were subjected to analysis of
variance and data on root volumes to analysis of covariance to
standardize initial root volumes. Where measurements were
significantly different among irrigation treatments, Williams'
(Williams, 1971) test was used to compare means.
Lateral distribution of total and fibrous (<_ 1.5 mm) root dry
weights were analyzed separately by a split-plot analysis with
irrigation treatments as main plots and lateral zones as subplots.


73
Root percentage distribution among zones was analyzed in the same
manner following arc sine transformation.
Measurements from profiles in 1987 of root density and root
concentration of fibrous, non-fibrous, and total roots were analyzed
separately. Percentage distribution data were transformed using an
arc sine transformation prior to analysis. All data sets were
analyzed by a split-split plot analysis with irrigation treatments
as main plots, distance from the trunk as sub-plots, and depth
increment as sub-sub-plots.
Results and Discussion
Root Weight and Volume Measurements
Irrigation treatments had variable effects on root dry weight
and volume from year to year (Table 4-1). Dry weights were not
affected in 1985, but root volume was decreased significantly at the
low compared to the high irrigation treatment (P<.0083). Irrigation
levels significantly affected root growth in 1986 and 1987. Trees
receiving the low irrigation treatment in 1986 had less total root
dry weight (P<.0052), new root dry weight (P<.0249), and root volume
(P<.0116) than those in the high treatment. Both moderate and low
treatments in 1987 decreased total root dry weight (P<.0170), new
root dry weight (P<.0455), and root volume (P<.0345) compared to the
high treatment.
3
Root volume ranged from 850 to 1190 cm in 1985 and 1986, and
3
from 550 to 860 cm in 1987 (Table 4-1). Root dry weight ranged
from 260 to 350 g in 1985 and 1986, and from 160 to 260 g in 1987.
These values were in close agreement with those of Bevington (1983)
and Bevington and Castle (1982) for 13-month-old 'Valencia' orange


74
Table 4-1. Total root dry weight, dry weight of new roots,
and root volume of young 'Hamlin' orange/sour orange
trees as related to irrigation based on soil water
depletion.
Soil water
Total root
New root
Root
depletion
dry wt
dry wt
volume
(%)
(g)
(g)
(cm3)
1985
(n=20)
20 (High)
348.7
156.3
1189.2
45 (Mod.)
333.9
158.8
1058.4
65 (Low)
297.3
139.4
**
920.5
1986
(n=21)
20 (High)
342.1
191.0
1180.9
45 (Mod.)
337.8
185.4
1396.5
65 (Low)
262.1*
128.2*
**
854.3
1987
(n=5)
20 (High)
257.8
126.2
858.2
45 (Mod.)
169.4*
84.8*
559.7*
65 (Low)
159.6*
81.2*
554.0*
Response is significant when compared with the 20%
SWD treatment by the Williams' method; 5% and 1%
levels, respectively.


75
on 'Carrizo' citrange or rough lemon trees grown in root observation
chambers. Tree age in this study was about 7 months in 1985 and
1986, and 8 months from planting in 1987. The comparable root sizes
in the field and root chamber studies despite the much shorter
growing period in the field study may be due to rootstock
differences or to the allowance of unrestricted root development
under the field conditions.
Shoot:root ratio was calculated using dry weight measurements
reported in Chapter III (Table 3-8). The SWD ranges used in this
study did not significantly influence shootrroot ratio, which ranged
from 1.16 to 1.54, with a mean of 1.30. These values are lower than
those reported for 1 1/2-year-old mandarin seedlings in India, where
healthy trees had a shoot:root ratio of 1.92 (Aiyappa & Srivastava,
1965). Comparatively, excavated mature citrus trees in California
had a ratio of about 3.5 (Cameron, 1939; Cameron & Compton, 1945),
and in Florida a ratio of about 2.2 (Castle, 1978).
Root growth was less in 1987 compared to the other years
despite a longer growing season. Furthermore, trees under moderate
irrigation grew similarly to those in the high treatment in 1985 and
1986, but had less root growth than those in the high treatment in
1987. These discrepancies may be due to the nursery trees used in
1987. Excessive leaf abscission occurred shortly after
transplanting trees to the field in 1987, inducing a growth flush
with a large number of shoots per tree (Chapter III, Table 3-7).
Such a growth flush early in the spring may have depleted available
reserves thus decreasing subsequent root growth.


76
In all 3 years root growth was decreased at the lowest soil
moisture content. Similarly, decreased root growth in response to
decreased soil moisture has been frequently reported for citrus
trees of various ages (Bevington & Castle, 1985; Bielorai, 1982;
Bielorai et al., 1981; Hilgeman & Sharp, 1970; Rodney et al., 1977).
Lateral Root Distribution
Lateral distribution of roots on a dry weight basis was
affected by irrigation treatments. The irrigation treatment x
lateral zone interaction for total root weight was significant in
1985 (PC.0238), 1986 (PC.0405), and 1987 (PC.0141). A greater
quantity of roots within 40 cm of the trunk occurred in response to
the high treatment when compared to moderate and low treatments
(Tables 4-2, -3, -4). Root weight in the 40-80 and >80 cm zones was
not affected by irrigation, but in all 3 years the high treatment
produced more roots than other treatments in the 0-40 cm zone. In
contrast, irrigation treatments did not affect lateral distribution
of fibrous (Cl.5 mm) root dry weight or distribution of both fibrous
and total roots on a percentage basis.
Across all irrigation treatments and years root dry weights and
percentages decreased significantly (PC.0001) with increased
distance from trunk (Tables 4-2, -3, -4). One exception occurred in
1986, when fibrous root weight was not different among the lateral
zones (Table 4-3). The percentage of fibrous roots within 40 cm of
the trunk ranged from 40 to 60%, between 40 and 80 cm from 26 to
39%, and >80 cm from 13 to 29%. A much larger percentage of total
root weight was located close to the tree, with 68 to 84%, 11 to


Table 4-2. Lateral dry weight and percentage distribution of fibrous (jG.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1985.
Distance
Irrigation treatment
from
20% SWD
45% SWD
65% SWD
trunk
High
Mod
Low
Mean
(cm)
(g) (%)
(g) (%)
(g) (%)
(g) (%)
SEZ
0-40
47.9
61.3
46.6
Fibrous
53.0 44.8
56.5
46.4
56.7
3.6
40-80
20.1
25.7
26.0
29.5
22.3
28.1
22.8
27.9
3.6
>80
10.2
13.0
15.4
17.5
12.2
15.4
12.6
15.4
3.6
Total
SEy
78.2
88.0
79.3
__ _
81.8
5.9
7.0
0-40
292.0
83.9
257.3
Total
80.0
roots
256.3
82.0
269.0
82.0
10.4
40-80
40.0
11.5
44.9
13.9
40.6
13.0
41.9
12.8
10.4
>80
16.0
4.6
19.7
6.1
15.3
5.0
17.1
5.2
10.4
Total
SEy
348.0
10.0
_
321.9
10.0
312.2
10.0
328.0
7.6
2
SE for comparison of irrigation treatment means within the same zone.
ySE for comparison of zone means within columns. Where no SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.


Table 4-3. Lateral dry weight and percentage distribution of fibrous (_<1.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1986.
Distance
Irrigation treatment
from
20% SWD
45% SWD
65% SWD
trunk
High
Mod
Low
Mean
(cm)
(g) (%)
(g) (%)
(g) (%)
(g) (%)
SEZ
0-40
40.2
40.3
32.4
Fibrous
40.3 27.0
40.1
33.2
40.2
7.9
40-80
31.2
30.7
26.6
33.1
27.4
38.8
28.4
34.2
7.9
>80
30.2
29.0
21.4
26.6
16.0
21.1
22.5
25.6
7.9
Total
SEy
101.6
80.4
__
70.4
84.1
5.6
5.9
0-40
239.8
68.3
189.2
Total
70.7
roots
163.0
69.2
197.3
68.6
20.0
40-80
72.6
20.1
53.8
19.1
51.0
21.6
59.1
20.6
20.0
>80
44.0
11.6
27.4
10.2
21.6
9.2
31.0
10.8
20.0
Total
SE7
356.4
13.1
__
270.4
13.1
235.6
13.1
__
287.4
4.7
2
SE for comparison of irrigation treatment means within the same zone.
ySE for comparison of zone means within columns. Where no SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.


Table 4-4. Lateral dry weight and percentage distribution of fibrous (<1.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1987.
Distance
Irrigation treatment
from
trunk
(cm)
20% SWD
High
45% SWD
Mod
65% SWD
Low
Mean
SEZ
(g)
(%)
(g)
(%)
(g)
(%)
(g)
(%)
Fibrous
0-40
32.0
46.6
25.2
49.0
23.0
50.7
26.7
48.5
4.8
40-80
26.0
37.7
19.4
38.1
14.6
32.1
20.0
36.4
4.8
>80
10.4
15.7
6.8
12.9
7.8
17.2
8.3
15.1
4.8
Total
68.4
51.4
45.4
55.0
SEy






7.8
13.9
Total
roots
0-40
197.0
76.4
132.6
78.3
124.8
78.0
151.5
77.5
15.3
40-80
44.0
17.1
28.2
16.6
26.0
16.4
32.7
16.7
15.3
>80
16.8
6.5
8.6
5.1
8.8
5.6
11.4
5.8
15.3
Total
257.8
169.4
156.4
195.6
SEy
13.5
13.5
13.5
9.1
ZSE for
comparison of
irrigation
treatment
means
within the
same zone.
ySE for
comparison of
zone means
within columns.
Where no
SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.


80
21%, and 4 to 11% of root dry weight in the three concentric lateral
zones, respectively.
The decrease in root growth with increasing distance from the
trunk has been reported for citrus trees of all ages (Aiyappa &
Srivastava, 1965; Bielorai, 1977, 1982; Bielorai et al., 1981;
Cahoon et al., 1964; Minessy et al., 1971; Yagev & Choresh, 1974).
Healthy 1 1/2-year-old mandarin trees in India had 60.0, 14.7, 13.2,
and 12.1% of their root dry weight in 0-30, 30-60, 60-90, and >90 cm
concentric zones, respectively (Aiyappa & Srivastava, 1965). These
values are similar to those from the young 'Hamlin' orange trees in
this study.
Maximum distance of lateral root spread pooled over the 3
seasons averaged 137 cm for the high treatment, compared with 127
for moderate and 121 cm for the low treatments, respectively (data
not shown). In comparison, 2-year-old 'Valencia' orange trees in
Australia had a lateral root spread of about 150 cm (Till & Cox,
1965), and 1 1/2-year-old mandarin trees in India had a spread of
160 cm (Aiyappa & Srivastava, 1965). Savage et al. (1945) observed
that 6-year-old trees budded on common rootstocks and spaced 30 x 90
cm in a Florida nursery had lateral root spreads up to 150 cm.
There is considerable discussion about the optimum irrigation
pattern to use for young citrus trees in Florida. Roots covered an
2
area of about 5.2 m with 64% of this area covered by the 90
emitters in this study. Furthermore, 96% of the roots on a dry
weight basis were within the wetted zones due to the high
concentration of roots within 80 cm of the trunk (Tables 4-2, -3,
-4). These data imply that 90 microsprinkler emitters cover the


81
majority of the young tree's roots after one season of growth. Tree
growth comparisons utilizing 90 and 180 emitters during the second
season of growth also suggest 90 emitters are adequate for up to 2
years in the field (Figs. 3-10, -11).
Weight of fibrous roots accounted for 27% of the total. This
value is similar to those found on root systems of rough lemon and
'Carrizo' citrange in the nursery (Bevington & Castle, 1982), but
considerably less than found on healthy 1 1/2-year-old mandarin
seedlings (37.5%) growing in India (Aiyappa & Srivastava, 1965). In
comparison, feeder roots on 8- to 9-year-old orange trees on sweet
orange rootstock accounted for less than 15% of the root system dry
weight (Cameron & Compton, 1945).
Circular Trench Profiles
Analysis of variance results showing effects of irrigation
treatments, distance from tree trunks, and depth increments on root
-3 -2
density (mm dm ) and concentration (roots m ) of total, fibrous,
and non-fibrous roots are summarized in Table 4-5. A larger
proportional increase in total root concentration occurred at 40
than at 80 cm from the trunk in response to the high treatment
(Table 4-6). This corroborates results of irrigation treatment
effects on lateral root dry weight distribution (Tables 4-2, -3,
-4). The high irrigation treatment produced a proportionally larger
increase in root number in the 10-20 cm layer than in the other
layers (Table 4-6). The greatest number of roots grew in this
layer, regardless of irrigation treatment and there was little root
growth in the 0-10 cm depth regardless of treatment, and trees under
the high treatment had the highest root concentration at all depths


82
Table 4-5. Partitioning of variance into main and interaction
effects for 1987 circular trench profile root growth variables of
'Hamlin' orange trees as described in Tables 4-6, 4-7, and 4-8.
F tests on variance (MS) ratios
Total
Fibrous
Non-fibrous
Total
root
root
root
root
Source
df
cone.
cone.
cone.
density
Irrigation (I)
2
*
5.39
6.05*
1.ions
*
5.73
Distance (Di)
1
***
55.40
***
54.88
***
44.03
***
55.60
I x Di
2
5.90*
6.40*
1.55nS
6.10*
Depth (De)
3
***
60.12
***
60.08
***
38.06
***
58.58
I x De
6
**
5.76
**
6.11
1.7lnS
5.32
Di x De
3
***
21.61
***
21.35
***
15.79
***
19.91
I x Di x De
6
2.27ns
2.43ns
0.92nS
2.2ins
* * Nonsignificant or significant F values at the 5Z,
1%, and .1% levels, respectively.


83
Table 4-6. Total root concentration at four depths and two distances
from trunk of young 'Hamlin' orange/sour orange trees as influenced
by irrigation based on soil water depletion (SWD) as determined
using circular trench profiles, 1987.
Irrigation treatment
Depth
(cm)
20% SWD
High
45% SWD
Mod.
65% SWD
Low
Mean
Root cone, (roots m
2) at 40 cmZ
0-10
19.9
37.2
11.9
23.0
10-20
965.5
655.2
299.7
640.1
20-30
447.0
244.0
202.9
298.0
>30
291.8
156.5
70.3
172.9
Mean
431.1
273.2
146.2
283.5y
Root cone, (roots m
at 80 cmZ
0-10
13.9
6.6
0
6.8
10-20
215.5
222.8
45.1
161.1
20-30
159.1
172.4
87.5
139.7
>30
65.7
24.6
43.1
44.5
Mean
113.6
106.6
43.9
88.0y
zn=3 for irrigation x distance x depth means.
yMean for distance from trunk.


84
below 10 cm. This increase in root concentration in response to
maintaining higher soil moisture levels corroborates the dry weight
measurements made on excavated root systems (Table 4-1).
The concentration of fibrous roots ( similar to total root concentration in regard to irrigation
treatments, distance from trunk, and depth because fibrous roots
comprised the majority of the total roots (Table 4-7). Differences
in non-fibrous (>1.5 mm) root concentration were not associated with
irrigation treatments. There were four times as many large roots at
40 cm from tree trunks than 80 cm, and almost half were located
between 10 and 20 cm in depth (Table 4-8). Trees receiving the low
irrigation treatment had 10 times as many fibrous as large roots;
whereas, trees in the high and moderate treatments developed
approximately 15 times the number of fibrous as large roots.
The percentages presented in Table 4-9 were pooled over the two
lateral distances since they did not interact with percentage
distribution with depth. Greater than 50% of the root concentration
was between 0-20 cm with high and moderate irrigation, but was
deeper than 20 cm with low irrigation. Because of the small number
of replications and the variability, this difference was not enough
to be significant. Across all treatments, percentages of roots were
significantly (P<.0001) different with depth (Table 4-9). Roughly
half of the roots were 10-20 cm deep, with 35% between 20-30 cm, and
less than 15% below 30 cm. More than 85% of the roots were located
between 0-30 cm in depth.
The concentration of roots in the top 30 cm after 1 season of
growth suggests that when soil moisture monitoring is used for


85
Table 4-7. Concentration of fibrous (<1.5 mm) roots at four depths
and two distances from trunk of young 'Hamlin orange/sour orange
trees as influenced by irrigation based on soil water depletion
(SWD) as determined using circular trench profiles, 1987.
Depth
(cm)
Irrigation i
treatment
Mean
20% SWD
High
45% SWD
Mod.
65% SWD
Low
Root
cone, (roots
-2 z
m ) at 40 cm
0-10
19.9
33.2
11.9
21.7
10-20
903.2
614.1
263.9
593.7
20-30
415.1
228.1
181.7
275.0
>30
279.9
149.9
65.0
164.9
Mean
404.5
256.3
130.6
263.8:
Root
cone, (roots
2 z
m ) at 80 cm
0-10
13.3
6.0
0
6.4
10-20
199.6
211.5
43.1
151.4
20-30
150.5
157.8
84.9
131.1
>30
63.0
23.9
41.8
42.9
Mean
106.6
99.8
42.5
83.0:
2
n=*3 for irrigation x distance x depth means
^Mean for distance from trunk.


86
Table 4-8. Concentration of non-fibrous (>1.5 mm) roots at four
depths and two distances from trunk of young 'Hamlin' orange/sour
orange trees as influenced by irrigation based on soil water
depletion (SWD) as determined using circular trench profiles, 1987.
Irrigation treatment
Depth
20% SWD
45% SWD
65% SWD
(cm)
High
Mod.
Low
Mean
Root
_2
cone, (roots m )
at 40 cmZ
0-10
0
4.0
0
1.3
10-20
62.3
41.1
35.8
46.4
20-30
31.8
15.9
21.2
23.0
>30
11.9
6.7
5.3
8.0
Mean
26.5
16.9
15.6
19.7y
Root
-2
cone, (roots m )
at 80 era2
0-10
0.7
0.7
0
0.5
10-20
15.9
11.3
2.0
9.7
20-30
8.6
14.6
2.7
8.6
>30
2.7
0.7
1.3
1.6
Mean
7.0
6.8
1.5
5.1
zn=3 for irrigation x distance x depth means
yMean for distance from trunk.


87
Table 4-9. Percentage distribution of root concentration with depth
of young 'Hamlin' orange/sour orange trees as influenced by
irrigation based on soil water depletion (SWD). Percentages are
means of measurements from 40 and 80 cm distances from trunk, 1987.
Irrigation treatment
Depth
(cm)
20% SWD
High
45% SWD
Mod.
65% SWD
Low
Mean
<1.5 mm
0-10
1.4
2.6
1.6
1.8
10-20
47.9
57.2
38.0
47.7
20-30
34.1
30.6
41.4
35.4
>30
16.6
9.6
19.0
15.1
>1.5 mm
0-10
0.6
3.1
0
1.2
10-20
50.8
48.4
45.2
48.2
20-30
36.3
38.9
38.5
37.9
>30
12.3
9.6
16.3
12.7
Total
0-10
1.3
2.7
1.4
1.8
10-20
48.1
56.5
38.4
47.7
20-30
34.2
31.3
41.6
35.7
>30
16.4
9.5
18.6
14.8


88
irrigation scheduling, measurements should be concentrated in this
zone. Furthermore, irrigation times should be limited to replenish
soil moisture only in these shallow zones. Irrigations of short
duration are adequate when using systems which direct applications
to a small areas on sandy soils. Soil moisture was monitored at a
depth of 30 cm in this study, and at 20% SWD the 90, 38 liter/hr
emitters required approximately 1 hr to replenish the root zone to
field capacity.
-3
Root length density (mm dm ) at two distances from tree trunks
and four soil depths as influenced by irrigation treatment followed
patterns similar to root concentrations. The greatest difference
between treatments occurred between 10-20 cm, with differences of
less magnitude deeper in the profile (Table 4-10). Densities ranged
-3
from 0.5 mm dm at 80 cm from the trunk on trees under the low
-3
irrigation treatment to 9.8 mm dm at 40 cm from the trunk on trees
under the high irrigation treatment.
Mature citrus responded to frequent irrigation with an
increased proportion of roots in shallow root zones and a decrease
in root growth in deep root zones using flood (Cahoon et al., 1961;
Cahoon et al., 1964; Hilgeman & Sharp, 1970) and drip (Ruggiero &
Andiloro, 1984) irrigation systems. Results from this study
indicated that irrigating more often in the high treatment had no
effect on root growth in the 0-10 cm depth, and increased the
proportion of root growth in the 10-20 cm depth compared to deeper
zones. However, in contrast to the previous studies, there was no
decrease in root concentration or length density in the deeper zones
with the frequent irrigation. These results are similar to those


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FILES


GROWTH OF YOUNG 'HAMLIN' ORANGE TREES AS INFLUENCED
BY MICROSPRINKLER IRRIGATION, FERTILIZATION, AND
NURSERY TREE TYPE
By
THOMAS E. MARLER
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
1988

ACKNOWLEDGEMENTS
Sincere appreciation is extended to Dr. F. S. Davies, chairman
of the supervisory committee, for his supervision of this project
and guidance in preparation of this manuscript. Appreciation is
also extended to Dr. J. J. Ferguson, Dr. L. K. Jackson, Dr. R. C. J.
Koo, and Dr. A. G. Smajstria for serving on the supervisory
committee and offering helpful suggestions in the planning and
conducting of this research.
Special thanks are extended to Dr. P. C. Andersen for provision
of equipment and to the UF faculty members whose timely counsel made
the continuation of this project possible.
- ii -

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
LIST OF TABLES v
LIST OF FIGURES viii
ABSTRACT x
CHAPTERS
I INTRODUCTION 1
II REVIEW OF THE LITERATURE 4
Vegetative Growth and Development of Citrus .... 4
Water Relations 7
Irrigation of Mature Citrus 15
Fertilization 18
Young Citrus Tree Care 22
IIIMICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. I. CANOPY GROWTH AND
DEVELOPMENT 29
Introduction 29
Materials and Methods 31
Results and Discussion 42
IVMICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. II. ROOT GROWTH AND
DISTRIBUTION 69
Introduction 69
Materials and Methods 70
Results and Discussion 73
VSOIL MOISTURE STRESS AND FOLIAR GAS EXCHANGE
OF YOUNG, FIELD-GROWN 'HAMLIN* ORANGE TREES 92
Introduction 92
Materials and Methods 93
Results and Discussion 95
- iii -

VIGROWTH OF YOUNG 'HAMLIN' ORANGE TREES USING
STANDARD AND CONTROLLED-RELEASE FERTILIZERS 114
Introduction 114
Materials and Methods 116
Results and Discussion 119
VII GROWTH OF BARE-ROOTED AND CONTAINER-GROWN
'HAMLIN' ORANGE TREES IN THE FIELD 125
Introduction 125
Materials and Methods 126
Results and Discussion 129
VIII CONCLUSIONS 141
APPENDICES
A MEAN MONTHLY PAN EVAPORATION AND MAXIMUM AND
MINIMUM AIR TEMPERATURES FOR 1985, 1986, AND 1987 . . 145
B WHOLE PLANT FRESH AND DRY WEIGHTS AND SHOOT:
RATIO OF YOUNG 'HAMLIN' ORANGE TREES AS RELATED
TO IRRIGATION BASED ON SOIL WATER DEPLETION 146
C REGRESSION EQUATIONS AND COEFFICIENTS OF
DETERMINATION ACCOMPANYING FIGS. 3-7, 3-8,
AND 3-9 147
D MEAN AND MAXIMUM LEAF TO AMBIENT AIR TEMPERATURE
DIFFERENCE OF 'HAMLIN' ORANGE TREES IN JUNE 1987
AS INFLUENCED BY SOIL WATER DEPLETION 148
LITERATURE CITED 149
BIOGRAPHICAL SKETCH 173
- iv -

LIST OF TABLES
Tables Page
3-1. Length of irrigation and amount of water applied at
each irrigation related to soil water depletion for
young 'Hamlin' orange trees 43
3-2. Number of irrigations and cumulative water applied
for young 'Hamlin' orange trees under scheduling
treatments based on soil water depletion 46
3-3. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1985 49
3-4. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1986 51
3-5. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Ham!in'
orange trees as related to irrigation based on soil
water depletion, 1985 53
3-6. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1986 54
3-7. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1987 56
3-8. Canopy volume, dry weight, shoot length, leaf area
and trunk cross-sectional area of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion 58
4-1. Total root dry weight, dry weight of new roots, and
root volume of young 'Hamlin' orange trees as related
to irrigation based on soil water depletion 74
- v -

4-2. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1985 77
4-3. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1986 78
4-4. Lateral dry weight and percentage distribution of
fibrous and total root systems of young 'Hamlin'
orange trees as related to irrigation based on soil
water depletion, 1987 79
4-5. Partitioning of variance into main and interaction
effects for 1987 circular trench profile root growth
variables ...... 82
4-6. Total root concentration at four depths and two
distances of young 'Hamlin' orange trees as
influenced by irrigation based on soil water
depletion, 1987 83
4-7. Concentration of fibrous roots at four depths and
two distances from trunk of young 'Hamlin' orange
trees as influenced by irrigation based on soil
water depletion, 1987 85
4-8. Concentration of non-fibrous roots at four depths
and two distances from trunk of young 'Hamlin'
orange trees as influenced by irrigation based on
soil water depletion, 1987 86
4-9. Percentage distribution of root concentration with
depth of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1987 .... 87
4-10. Root length density at four depths and two distances
from trunk of young 'Hamlin' orange trees as related
to irrigation based on soil water depletion, 1987 ... 89
5-1. Mean and maximum CO assimilation, transpiration, and
stomatal conductance of young 'Hamlin' orange trees on
18 Oct. 1986 as influenced by soil water depletion . . 99
5-2. Mean and maximum CO^ assimilation, transpiration,
stomatal conductance, and mean internal CO2
concentration of young 'Hamlin' orange trees in June
1987 as influenced by soil water depletion 105
6-1. Effects of controlled-release fertilizers on growth
of young 'Hamlin' orange trees in the field 120
- vi

6-2. Effects of standard fertilizer rate on growth of
young 'Hamlin' orange trees in the field 122
6-3. Influence of standard fertilizer rate on leaf analysis
of young 'Hamlin' orange trees in the field 123
7-1. Effect of nursery tree type on growth of 'Hamlin'
orange trees after 8 months in the field 131
7-2. Effect of removing medium prior to planting on growth
of containerized 'Hamlin' orange trees, 1986 135
7-3. Growth of bare-rooted and container-grown 'Hamlin'
orange trees as influenced by planting procedure,
1987 137
- vii -

LIST OF FIGURES
Page
Figure
3-1. The relationship between soil water content and water
potential of Kanapaha sand at the Horticultural Unit . . 32
3-2. The relationship between neutron probe ratio and
volumetric water content of Kanapaha sand at the
Horticultural Unit 35
3-3. Relationship of leaf area and fresh weight from eight
representative 'Hamlin' orange trees after 8 months
in the field 39
3-4. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1985 45
3-5. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1986 47
3-6. Distribution of rainfall and microsprinkler Irrigation
at the Horticultural Unit, April-Dee. 1987 48
3-7. Cumulative percentage of trees in three irrigation
treatments growing over the 1985 season 60
3-8. Cumulative percentage of trees in three irrigation
treatments growing over the 1986 season 62
3-9. Cumulative percentage of trees in three irrigation
treatments growing over the 1987 season 63
3-10. Trunk cross sectional area and canopy volume of young
'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern, 1986 66
3-11. Trunk cross sectional area and canopy volume of young
'Hamlin' orange trees as influenced by microsprinkler
Irrigation spray pattern, 1987 67
5-1. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 18 Oct. 1986 96
5-2. Diurnal cycle of CO2 assimilation, transpiration, and
stomatal conductance of young 'Hamlin' orange trees on
18 Oct. 1986 as influenced by soil water depletion ... 97
- viii -

5-3. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 12 June 1987 100
5-4. Diurnal cycle of CO 2 assimilation, transpiration,
stomatal conductance, and water use efficiency of
young 'Hamlin' orange trees on 12 June 1987 as
influenced by soil water depletion 101
5-5. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 13 June 1987 102
5-6. Diurnal cycle of assimilation, transpiration,
stomatal conductance, and water use efficiency of
young 'Hamlin' orange trees on 13 June 1987 as
influenced by soil water depletion 104
5-7. Diurnal cycle of photosynthetic photon flux, air
temperature, relative humidity, and vapor pressure
deficit on 15 June 1987 106
5-8. Diurnal cycle of xylem potential, CO2 assimilation,
transpiration, stomatal conductance, and water use
efficiency of young 'Hamlin' orange trees on 15
June 1987 as influenced by soil water depletion .... 107
7-1. Effect of 'Hamlin' orange nursery tree type on
increase in trunk cross sectional area and canopy
volume from May 1985 to Dec. 1986 130
7-2. Effect of 'Hamlin' orange nursery tree type on
increase in trunk cross sectional area and canopy
volume from May 1986 to Dec. 1987 133
ix -

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
GROWTH OF YOUNG 'HAMLIN' ORANGE TREES AS INFLUENCED
BY MICROSPRINKLER IRRIGATION, FERTILIZATION, AND
NURSERY TREE TYPE
By
Thomas E. Marler
December, 1988
Chairman: Frederick S. Davies
Major Department: Horticultural Science (Fruit Crops)
Young tree care is a costly and important part of any citrus
production program. Irrigation, fertilization, and nursery stock
characteristics have a major influence on the success of a young
tree care program. Therefore field experiments were conducted to
study the effects of various microsprinkler irrigation and
fertilization rates and nursery tree types on growth and development
of young 'Hamlin' orange [Citrus sinensis (L.) Osb.] trees.
Three experiments were conducted to determine the effects of
scheduling microsprinkler irrigations at 20 (high frequency), 45
(moderate frequency), and 65% (low frequency) available soil water
depletion on one season's canopy and root growth and leaf gas
exchange. Canopy and root growth were similar for high and moderate
treatments in 2 out of 3 years, but were reduced by the low
treatment, even though the moderate received about 50% less water
than the high treatment. Reduced shoot number and delayed
- x -

initiation of late growth flushes sometimes occurred with moderate
and low treatments, and may have been related to decreases in CO^
assimilation in spring and early summer. Root concentrations wejre *
greatest between 10 and 30 cm depths, and >90% of the root dry
weight was within 80 cm of the trunk.
Two studies with various fertilizer rates and two studies with
controlled-release fertilizer sources were conducted to determine
the influence of fertilizer rate and source on growth. Growth was
comparable over one season using 0.07-0.09, 0.14-0.18 (recommended),
and 0.22-0.27 kg N/tree/season, implying that adequate rates may
sometimes be lower than recommended. In addition, application of
standard fertilizers (4-5X/season) compared with controlled-release
fertilizers (2X) at the same seasonal rates resulted in similar
growth over 2 seasons, suggesting that controlled-release sources
may sometimes be used as alternatives to standard sources.
Commercial bare-rooted, field-grown nursery trees and
containerized, greenhouse-grown trees were used in three experiments
to compare establishment and initial growth in the field. Bare-
rooted trees remained larger than containerized trees through 2
seasons and initial growth of container-grown trees was highly
variable. Removal of medium prior to planting container-grown trees
improved tree growth in one of two experiments.
- xi

CHAPTER I
INTRODUCTION
Citrus plantings in Florida decreased by more than 75,000
hectares between 1984 and 1986 (Division Plant Industry, 1986), more
than any other 2-year period since the beginning of the Citrus Tree
Census Survey in 1966. This decrease was largely attributable to
the series of freezes since 1982; however, blight, tristeza, and
urbanization have also contributed to the reduction in land area
planted to citrus. Growers estimated in 1983-1984 that 6.1% of
their young trees died each year (Jackson et al., 1986). Estimates
of the number of trees being planted annually in the mid-1980s range
from greater than 3 million (Division Plant Industry, 1986) to 6-10
million (Jackson et al., 1986).
Muraro (personal communication, Citrus Research and Education
Center, Lake Alfred) estimates that more than $20 is required to
maintain a tree in a solid-set planting for the first 4 years in the
flatwood areas, while $24 to $40 per tree is required for resets,
depending on number of resets per hectare. Consequently, young tree
care costs are immense on an industry-wide basis. More efficient
management practices for young citrus trees are important in the
success of any profitable citrus production venture due to the
increased need for cost containment in the Florida citrus industry.
1

2
Vital to the improvement of field management practices is the
understanding of how these practices influence the growth and
physiology of young citrus trees.
Irrigation practices for mature Florida citrus trees are based
on many years of field research, however, information on water
requirements of young citrus trees, irrigation scheduling, and tree
response has not been reported with micro irrigation under field
conditions in Florida. Objectives of the first part of this
research were to investigate the effects of microsprinkler
irrigation scheduling based on soil water depletion on canopy and
root growth of young citrus trees. In addition, the influence of
soil water deficit on gas exchange processes of young citrus trees
was studied. Separate experiments were designed to investigate the
effect of microsprinkler irrigation spray pattern on growth of trees
during the second season in the field.
Fertilization practices for mature Florida citrus are well-
defined and based on years of production records, but fewer studies
have been made on fertilization of young citrus trees. Previous
studies suggest that under many conditions fertilizer rates for
young trees are excessive. Use of controlled-release fertilizers
may reduce nitrogen losses and fertilizer costs by decreasing the
number of yearly fertilizer applications, resulting in reduced labor
and equipment costs. Objectives of the second portion of this
research were to compare growth of young citrus trees using standard
fertilizer applied at recommended seasonal rates and controlled-
release fertilizer applied fewer times per season than the standard
source. A second set of experiments was designed to determine if

3
recommended seasonal fertilization rates could be reduced without a
reduction in young citrus tree growth.
Florida nurserymen have been producing bare-rooted citrus trees
in field nurseries for many years, however, recently many citrus
trees have been produced in containers in greenhouses. While
opinions vary concerning post-plant growth and survival rates of
container-grown and field-grown trees, there have been no
replicated, controlled comparisons of these two nursery tree types
under Florida conditions. The purpose of the third part of this
research was to compare establishment and post-plant canopy and root
growth of 1)containerized, greenhouse-grown and 2)bare-rooted,
field-grown citrus trees. Further experiments were designed to
determine the effect of the removal of potting medium prior to
planting on root and canopy growth of container-grown citrus trees.
Results from these experiments may be useful in improving
management programs for young citrus trees by reducing water and
fertilizer costs and may lead to further research on optimizing
growth and development of young citrus trees in Florida.

CHAPTER II
REVIEW OF THE LITERATURE
Citrus tree growth and productivity is influenced by many
environmental conditions and cultural factors. Adequate soil
moisture and nutrient levels are two important factors necessary for
optimum tree growth. Moreover, initial growth and development of
newly-planted trees may be influenced by the nursery conditions
under which they were produced. Understanding the influence of
nursery and field management decisions on growth and physiological
processes of young citrus trees is important to maximize early
productivity.
Vegetative Growth and Development of Citrus
Citrus shoot growth is cyclic with two to five flushes per
season, depending on climate (Cooper et al., 1969; Mendel, 1969).
Environmental conditions may alter both number and timing of growth
flushes. Following cessation of shoot elongation the terminal bud
normally aborts and abscises (Schroeder, 1951). Duration of flush
growth is largely dependent upon climate, as cooler spring
temperatures result in longer periods of growth (Cooper et al.,
1963). Both the spring (Cooper et al., 1963) and summer
(Krishnamurthi et al., 1960) flushes have been reported to occur
over the longest time interval.
4

5
Shoot growth is mainly dependent on stored substrates in citrus
(Sinclair, 1984). Available reserves influence shoot length and
number of leaves per shoot (Van Noort, 1969). The spring flush
normally has more shoots than other flushes, but average shoot
length is shorter (Mendel, 1969; Krishnamurthi et al., 1960). Later
flushes have fewer buds breaking and longer average shoot length,
thicker shoots, and larger leaves (Reuther, 1973; Mendel, 1969). A
large proportion of spring shoots do not continue growing during
subsequent flushes (Sauer, 1951). Summer (Mendel, 1969) and spring
(Crider, 1927; Krishnamurthi et al., 1960; Syvertsen et al., 1981)
flushes have each been attributed with having more total growth than
the other growth flushes.
In the spring root growth usually occurs after shoot growth
(Crider, 1927; Cooper et al., 1963; Bevington, 1983; Hatton, 1949;
Krishnamurthi et al., 1960; Reed & McDougal, 1937), except in one
case where shoot growth occurred first (Waynick & Walker, 1930).
This occurs possibly because soil temperatures are lower than air
temperatures in early spring (Krishnamurthi et al., 1960).
Shoot and root growth flushes tend to alternate. During
periods of very active shoot growth the roots are inactive or nearly
so (Crider, 1927; Bevington, 1983; Bevington & Castle, 1985;
Krishnamurthi et al., 1960; Reed & McDougal, 1937). Mendel (1969)
suggests that increases and decreases in the amount of growth
inhibiting substances may govern the cycles. Trunk growth may be
cyclical, coinciding generally with root growth (Krishnamurthi et
al., 1960), or continuous throughout the growing season (Cooper et
al., 1963).

6
Alternating periods of root and shoot growth and extent of
growth are greatly influenced by the environment. Shoot growth is
reduced below 12.5-13.0° C and above 33-39° C, with optimum growth
occurring between 23-33° C (Mendel, 1969; Reuther, 1973). Root
growth is maximum at soil temperatures of 25-26° C (Reuther, 1973),
and limited below 13 or above 36° C (Castle, 1978). Increases in
soil temperature increase translocation of assimilates to roots
(Vinokur, 1957).
Daylength influences shoot growth of most citrus. While some
citrus is non-responsive to daylength (Warner et al., 1979), many
rootstocks and scions produce more and longer shoots in longer
daylengths (Lenz, 1969; Reuther, 1973; Warner et al., 1979).
The root and shoot relationship is measured many times as a
ratio of fresh or dry weight of shoot and root systems. Mature
citrus trees in California had shoot:root ratios of ca. 3.5
(Cameron, 1939; Cameron & Compton, 1945) and in Florida of 2.2
(Castle, 1978). This ratio varies within a species with age and
environmental or cultural conditions. Some factors that influence
shoot:root ratio of a number of crops are soil moisture levels
(Harris, 1914; Hilgeman & Sharp, 1970; Hsiao & Acevedo, 1974;
Kramer, 1983; Kriedeman & Barrs, 1981; Levy et al., 1983; Menzel et
al., 1986; Rodgers, 1939; Tinus & Owston, 1984), mineral nutrition,
especially nitrogen (Brouwer, 1962; Harris, 1914; Russel, 1977;
Turner, 1922), light intensity (Russell, 1977), and root restriction
(Hameed et al., 1987).

7
Water Relations
Growth Responses to Water Deficits
Most aspects of plant metabolism are affected by tissue water
relations, but turgor-dependent processes, such as cellular
expansion, are especially sensitive. Water deficits reduce cell
expansion and division, organ growth, or whole plant growth (Hagan
et al., 1959; Hsiao, 1973). Studies with forest trees indicate that
water deficits during bud formation can have as much influence on
shoot length as stress during shoot expansion (Kozlowski, 1983;
Doley, 1981). In addition to reduced production of new leaf area
during periods of water deficits, abscission of older existing
leaves may increase the loss in leaf area (Marsh, 1973; Kriedemann &
Barrs, 1981).
Soil water deficits may result in cessation of growth of a
flush or prolong the period of quiescence between flushes in citrus
(Cooper et al., 1969; Mendel, 1969). Ford (1964) stated that the
alternations between root and shoot growth may become less
pronounced during periods of drought.
Growth reduction in response to water stress may be immediately
followed by an increased growth rate upon relief of stress. This
has been shown with corn (Acevedo et al., 1971; Hsiao et al., 1970),
soybean (Bunce, 1977), tomato (Gates, 1955), pine (Miller, 1965),
barley and rye (Williams & Shapter, 1955) under short term stress.
Similarly, early season moisture stress suppressed dogwood shoot
growth initially, but stimulated late season growth following
regular irrigation (Williams et al., 1987).

8
Root growth is likewise affected by moisture stress, although
to a lesser degree than shoot growth. Reduced meristematic activity
and root elongation as well as reduced water and nutrient uptake due
to suberization occur in response to water deficits in most crops
(Slatyer, 1969). Working with wheat, Cole and Alston (1974) state
that root volume or weight are generally more affected by water
deficits than root length. The rate of suberization may exceed that
of elongation during periods of water deficits, substantially
reducing the non-suberized root area (Kaufmann, 1968; Kramer, 1950;
Turner & Kramer, 1980). Periods of water deficits have reduced
citrus root conductivity (Levy et al., 1983; Ramos & Kaufmann, 1979)
and root elongation (Bevington, 1983; Bevington & Castle, 1985;
Marsh, 1973). Water deficits may result in development of long
roots with restricted branching in citrus (Castle, 1978).
Resumption of root growth following rain or irrigation typically
occurs with most crops (Kozlowski, 1983), however, pine roots
reportedly matured toward the tip during water deficits and became
inactive (Kaufmann, 1968; Lesham, 1965).
Total root growth and distribution of citrus trees are often
altered by irrigation practices. Infrequent irrigation increased
root density or the proportion of total roots deep in the soil
profile when compared to frequent irrigation (Cahoon et al., 1961;
Cahoon et al., 1964; Hilgeman & Sharp, 1970; Marsh, 1973; Ruggiero &
Andiloro, 1985). Partial root zone irrigation in an arid
environment results in an increase in root growth in the wetted soil
volume (Bielorai, 1977, 1982; Bielorai et al., 1981; Cohen et al.,
1987; Rodney et al., 1977). Conversely, high root concentrations

9
within the wetted areas do not occur in regions of abundant rainfall
(Koo, 1980).
Gas Exchange Responses to Water Deficits
Water influences plant growth and development through its
effect on biochemical and physiological processes, including gas
exchange. It is clearly established that storaatal conductance
generally declines under conditions of water deficit. This stomatal
response has been considered to be a feedback response to leaf water
status, involving a reduction of leaf water potential, turgor
potential, or relative water content to a critical point, at which
time the stomata begin to close (Begg & Turner, 1976; Hsiao, 1973;
Raschke, 1975; Turner, 1979). However, correlations of bulk leaf
water status and stomatal conductance are typically weak, and the
relationship is not consistent, but is dependent on developmental
conditions. Numerous recent reports with a variety of crops show
that decreases in stomatal conductance resulting from soil moisture
depletion are not closely correlated with changes in bulk leaf water
status (Bates & Hall, 1981; Bennett et al., 1987; Black et al.,
1985; Blackman & Davies, 1985b; Cock et al., 1985; Gollan et al.,
1985; Lorenzo-Minguez et al., 1985; Osonubi, 1985; Turner et al.,
1985). Furthermore, stomata respond directly to epidermal water
relations and are only indirectly related to bulk leaf water
relations (Edwards & Meidner, 1978). The pressure probe has
recently been used to directly measure turgor of individual cells
(Shackel & Brinckmann, 1985; Frensch & Schulze, 1987) and it is now
evident that large cell turgor gradients may exist between mesophyll

10
and epidermis, illustrating the problems associated with relating
stomatal conductance with bulk leaf water status.
Stronger correlations of stomatal conductance to soil moisture
status than to leaf water status (Bates & Hall, 1981, 1982; Blackman
& Davies 1985b; Gollan et al., 1985; Gollan et al., 1986; Jones et
al., 1983; Osonubi, 1985; Turner et al., 1985) suggest that soil
water deficits may influence stomata in a manner independent of
shoot water relations. The most clear evidence of this comes from
stomatal closure in response to soil water deficits, in spite of
maintenance of leaf turgor by partial root zone irrigation (Blackman
& Davies, 1985 a,b) or by pressurizing the root system (Gollan et
al., 1986). In these cases, stomata responded directly to a signal
from roots under soil water deficits. An interruption in cytokinin
supply has been proposed as this message (Blackman & Davies 1985
a,b; Davies et al., 1986). It has been known for two decades (Itai
& Vaadia, 1965) that soil water deficits affect concentrations of
growth substances in roots and shoots. The well-known involvement
of abscisic acid (ABA) in stomatal action (Davies et al., 1981) and
the interaction of this ABA effect with other phytohormones
(Blackman & Davies, 1985 a,b; Cox et al., 1985; Snaith & Mansfield,
1982) lend support to the suggestions that plant growth substances
play a role in the stomatal response to soil water deficits.
However, stomatal closure of some plants is not related to leaf ABA
levels (Davies & Lakso, 1978; Raschke et al., 1976).
The discovery that stomata respond directly to humidity (Lange
et al., 1971) in a feed-forward manner is well-documented in many
plants. Feedforward responses are important in enabling a plant to

11
restrict excessive water loss before developing a severe water
deficit. Sharkey (1984) has shown that high transpiration may
result in reduced photosynthetic capacity in a manner similar to
soil water deficits, and suggests that stomatal responses to
humidity may guard against this reduction in photosynthetic
capacity. Stomatal responses to humidity can greatly confuse
interpretation of stomatal responses to soil water deficits. This
stomatal sensitivity to humidity or vapor pressure deficit (VPD) is
well-documented in citrus (Camacho-B., 1977; Camacho-B. et al.,
1974; Fereres et al., 1979; Hall et al., 1975; Kaufmann, 1977;
Kaufmann & Levy, 1976; Khairi & Hall, 1976b; Kriedemann & Barrs,
1981; Levy, 1980; Syvertsen, 1982) and is important in maintaining
favorable leaf water relations under arid environments. Well-
watered citrus trees in environments of drastically different
evaporative demand have been shown to have nearly equal
transpiration and equal or increased leaf water potential in arid
environments due to VPD-induced partial stomatal closure (Kaufmann,
1977; Levy, 1980; Levy & Syvertsen, 1981). Hilgeman (1966) reported
nearly equal transpiration but greater leaf water deficits of citrus
in Arizona compared to Florida. Annual evapotranspiration values
for mature citrus in different environments are quite similar,
ranging from 1.07 to 1.21 m in Florida (Koo, 1963; Rogers et al.,
1983; Rogers & Bartholic, 1975; Reitz et al., 1978; Gerber et al.,
1973), 0.91 to 1.14 m in Texas (Weigand & Swanson, 1982a), and 0.80
to 1.47 m in Arizona (Hilgeman & Sharp, 1970; Hilgeman et al., 1969;
Hoffman et al., 1982).

12
The literature concerning soil water deficit effects on
photosynthesis is extensive and has been reviewed by many [e.g.
Boyer (1976), Hsiao (1973), Lawlor (1979), Slatyer (1967)]. Early
research suggested that stomatal conductance and CC^ assimilation
are closely correlated for some species, and led to conclusions that
CO2 assimilation was reduced by soil water deficits through stomatal
closure. However, water stress also impairs non-stomatal factors
which may be causal, yet closely correlated with stomatal
conductance. The correlation could result from a concommitant yet
independent effect of water deficits on stomatal and non-stomatal
processes, or from a direct reduction in mesophyll photosynthetic
capacity, followed by stomatal closure in response to reduced CO^
assimilation (Farquhar & Sharkey, 1982; Redshaw & Meidner, 1972;
Wong et al., 1979).
Non-stomatal limitations on photosynthesis are well-documented.
Residual conductance to CC^ is reduced by water stress in many
plants (Brown & Simmons, 1979; Bunce, 1977; Collatz, 1977; Mederski
et al., 1975; O’Toole et al., 1976; Pearcy, 1983; Pellegrino et al.,
1987; Radin & Ackerson, 1981). Stress-induced reduction of
photochemical activity has been shown by using isolated chloroplasts
and artificial electron acceptors or with measurements on
fluorescence (Boyer, 1976; Boyer & Bowen, 1970; Genty et al., 1987;
Keck & Boyer, 1974; Nir & Poljakoff-Mayber, 1967; Sharkey & Badger,
1982; Von Caemmerer & Farquhar, 1981). Reduced carboxylation
capacity in response to stress has also been reported due to
decreases in activity of ribulose bisphosphate carboxylase-
oxygenase, carbonic anhydrase, phosphoenol pyruvate carboxylase,

13
fructose-1,6-bisphosphatase, or sedoheptulose-1,7-bisphosphatase
(Berkowitz & Gibbs, 1982; Boag & Portis, 1984; Huffaker et al.,
1970; Jones, 1973; O'Toole et al., 1976). In situations where
intercellular CO2 concentration is unchanged or higher in stressed
than in control plants, reduced stomatal conductance of stressed
plants is of little importance in photosynthetic reduction (Briggs
et al., 1986; Farquhar et al., 1980; Radin & Ackerson, 1981; Wong et
al., 1979).
Inhibition of leaf growth by water deficits may contribute to a
whole plant reductions in photosynthesis. Water deficits may reduce
the rate of leaf initiation (Husain & Aspinall, 1970) as well as
foliar expansion (Hagan et al., 1959; Hsiao, 1973). Additional loss
of leaf area through abscission of existing leaves may also result
from water deficits. Stress-induced foliar abscission in citrus
normally occurs at a secondary abscission zone between the leaf
blade and petiole (Schneider, 1968). These effects may be quite
damaging, since recovery does not occur until new growth replaces
the lost photosynthetic surface (Hsiao, 1973).
Effects of water deficits on photosynthesis are certainly not
fully understood. The fact that such diverse mechanisms of action
are discussed in the literature is not surprising and all mechanisms
could be applicable under certain situations.
Water deficit effects on gas exchange of citrus has received
fairly limited attention. Hilgeman (1977) stated stomatal closure
of mature citrus in Arizona occurs at progressively earlier times in
the day as soil water is depleted over several weeks. Irrigated and
nonirrigated mature sweet orange trees in Italy began days with

14
similar storaatal conductances, but conductance of nonirrigated trees
declined steadily throughout the day, while that of irrigated trees
increased to peak around midday before declining (Ruggiero &
Andiloro, 1985). Similarly, leaf conductance of severely-stressed
mature sweet orange trees increased slightly until mid-morning
before declining steadily throughout the day, while conductance of
irrigated trees increased rapidly until mid-morning and did not
decline any until mid-afternoon (Cohen & Cohen, 1983). Hilgeman et
al. (1969) reported increased transpiration of mature sweet orange
trees in Arizona at a soil matric potential of -29 kPa compared to
-79 kPa, especially during the middle part of the day. Koo (1953)
reported citrus tree transpiration in Florida was unchanged at soil
moisture levels between field capacity and depletion of two-thirds
of the readily available water. A reduction in transpiration or
storaatal conductance by soil water deficits has been reported under
many conditions (Bielorai & Mendel, 1969; Brakke et al., 1986; Cohen
& Cohen, 1983; Hilgeman, 1977; Kaufmann & Levy, 1976; Koo, 1953;
Kriedeman, 1971; Ruggiero & Andiloro, 1985; Thompson et al., 1968;
Zekri, 1984). Only controlled conditions utilizing containerized
plants have been used to illustrate a reduction of citrus
photosynthesis by soil water deficits (Bielorai & Mendel, 1969;
Brakke et al., 1986; Kriedemann, 1968, 1971; Ono & Hirose, 1984;
Thompson et al., 1968). Bielorai & Mendel (1969) reported that
sweet lime and rough lemon photosynthesis was reduced more than
transpiration with soil water deficits, while Kriedemann (1971)
reported a greater reduction of transpiration with sweet orange.

15
Irrigation of Mature Citrus
Irrigation in Florida
Although Florida's climate is characterized by an average
annual rainfall of 120-150 cm, the rainfall is distributed in an
irregular pattern. Consequently, periods of minimal and infrequent
rainfall result in unfavorable soil moisture conditions for optimum
citrus growth and production. Periods of drought combined with the
low water-holding characteristics of most Florida soils create
favorable conditions for the use of irrigation to obtain maximum
production.
Many early studies presented differing results concerning the
need to irrigate mature citrus trees (DeBusk, 1928, 1933; Hoard,
1908; Koo & Sites, 1955; Savage, 1951, 1952; Sites et al., 1951;
Staebner, 1919; Stanley, 1913, 1914, 1916; Stevens, 1909; Williams,
1909; Young, 1948; Ziegler, 1955b). Many of these reports were
based on observations and correlations between rainfall or soil
moisture content and tree response, while some were actual
irrigation experiments. Responses to irrigation and opinions on its
cost effectiveness varied widely. Ziegler (1955b) stated that
irrigation was needed in Florida only in years of light spring
rainfall, and was not essential every year, and suggested the use of
temporary wilt or "anticipated tree wilt” combined with records of
rainfall for practical irrigation timing (Ziegler, 1955a).
Results of a 3-year study were published in 1963 indicating
that irrigation based on soil moisture depletion increased yields
for a variety of scions on rough lemon rootstock (Koo, 1963).
Recommendations were to irrigate from January to June based on

16
one-third soil moisture depletion, and from July to December based
on two-thirds depletion. Since that time irrigation has ranked high
among requirements for maximum production of Florida citrus, even in
years of high rainfall. Thus, irrigation has been used in Florida
to increase yields of many cultivars on different rootstocks (Koo,
1963, 1969, 1975, 1979, 1985; Koo & Hurner, 1969; Koo & McCornack,
1965; Koo & Sites, 1955; Koo et al., 1974; Reese & Koo, 1976).
Likewise, irrigation increases vegetative growth of mature citrus
trees under Florida conditions (Koo, 1963, 1969, 1979, 1985; Koo &
Hurner, 1969; Sites et al., 1951; Zekri, 1984). Increases in citrus
yield due to irrigation may result from an increase in canopy volume
(Koo, 1969; Koo & Hurner, 1969; Levy et al., 1978). Recommendations
similar to Koo's (1963) concerning more frequent irrigation during
the spring than other parts of the year are reported for other
citrus growing regions (Hilgeman, 1956; Hilgeman & Sharp, 1970;
Mantell, 1977; Weigand & Swanson, 1982a).
Micro Irrigation of Mature Citrus
Micro irrigation systems, which direct water to only a portion
of the soil volume, have received widespread use in citrus culture
around the world. Yields of navel oranges were comparable using
trickle, basin, and dragline sprinklers in South Africa, with ca.
50% savings of applied water for the trickle systems over the other
systems (Bester et al., 1974). Similarly, water savings of 30% were
possible while using trickle Instead of flood irrigation on
grapefruit in Arizona (Roth et al., 1981). Irrigating oranges with
trickle or basin systems in Arizona reduced water application by
greater than 90% when compared with border or high volume sprinkler

17
methods (Roth et al., 1974). Increased tree growth and reduced weed
control problems also resulted from the use of trickle and basin
irrigation. Drip irrigation increased yields and fruit size of
sweet oranges by 30% over flood irrigation in Spain (Legaz et al.,
1981). Similar comparisons of trickle systems and conventional
systems with mature citrus are numerous (Alijbury et al., 1974; Cole
& Till, 1974; Leyden, 1975b; Rathwell & Leyden, 1976; Yagev &
Choresh, 1974) and include those cited in the next discussion.
Irrigation studies frequently use several types of systems or
emitters to apply water to variable percentages of soil surface.
Bielorai (1977, 1978, 1982) reported similar grapefruit yields and
tree growth resulted in Israel from wetting 30-40 as compared to 70%
of the soil surface. Similarly, irrigation of 35, 70, and 90% of
the soil surface for oranges resulted in increased or comparable
yields with the smaller wetted areas (Bielorai et al., 1981). These
results involved data from only the first 3 years following a
reduction in wetted soil volume. Following this period, however,
yields of the 90% wetting treatment were greater than for the other
treatments (Cohen et al., 1985). Another comparison of orange
performance In Israel showed a reduction in growth rate and yield
with partial compared to full wetting (Moreshet et al., 1983).
Other responses included reduced water uptake and tree hydraulic
conductance due to reduced wetted soil volume (Cohen et al., 1985,
1987). Koo and Tucker (1974) recommended in Florida that systems be
designed to wet 60% of the citrus root volume. However, irrigating
high-density oranges at 15-25%, 30-40%, and 70-78% of the under-tree
area showed that yield Increased with increasing coverage (Koo,

18
1978). Grapefruit irrigated at 14, 64, and 100% coverage of under¬
tree area also responded with increased yield and growth with
increasing coverage (Koo, 1985). Irrigation at 19, 88, and 100% of
under-tree area gave similar results (Zekri, 1984). Five to 10%,
and 28 to 51% coverage of the under-tree area increased yield of
oranges 41-44 and 65%, respectively, over nonirrigated controls in
Florida (Smajstrla & Koo, 1984, 1985).
Fertilization
Uptake and Function of Minerals
Plants require adequate supplies of essential elements for
optimum growth and development. These elements affect plant growth
through their involvement in physiological and biochemical
processes. The amount required by plants, form that is absorbed,
and function vary considerably among the essential elements.
Agricultural soils are more commonly deficient in nitrogen (N)
than any other element. Nitrogen is an essential component of amino
acids, chlorophyll, and many other organic materials, and is
involved in many enzymic processes. Various forms of N are absorbed
by roots, but nitrate and ammonium are the major forms.
Preferential absorption of ammonium (Wallace, 1954; Wallace &
Mueller, 1957) or nitrate (Hilgeman, 1941) has been suggested in
citrus. Several studies have provided estimates of the partitioning
of N throughout the citrus plant. Leaves contained 31% of the N in
a mature orange tree In Florida; with fruit, small roots, branches,
large root, and the trunk containing 27, 21, 13, 5, and 3%,
respectively (Smith, 1963). Cameron and Compton (1945) presented
data from California suggesting that leaves contain 40-50%; twigs

19
and shoots, 10%; trunk, and branches, 20-30%; and roots, 15-20% of
the total N. In contrast, leaves contained greater than 60% of the
total N in 3.5 year old orange trees (Cameron & Applemen, 1933).
Averaged over the entire season, N content was 1.86% of the total
tree dry weight.
Absorption and function of the remaining essential elements are
covered in detail by Clarkson and Hanson (1980) and Mengel and
Kirkby (1982) and are briefly outlined here. Carbon, hydrogen,
oxygen, and sulfur have metabolic functions similar to N, in that
they are structural components of essential plant compounds and are
involved in enzymic processes. Sulfur is absorbed from the soil as
sulfate, or from the atmosphere as sulfur dioxide. Carbon,
hydrogen, and oxygen are absorbed in gaseous forms from the
atmosphere. Phosphorus, boron, and silicon are involved in
esterification with alcohol groups in plants, and are absorbed as
phosphates, borate or boric acid, and silicate, respectively.
Potassium, sodium, magnesium, calcium, manganese, iron, chlorine,
copper, zinc, and molybdenum are absorbed by plants in the form of
ions or chelates. Their metabolic functions are numerous, and range
from controlling osmotic and electro-potentials and membrane
permeability to enzyme activation.
Fertilization of Mature Citrus in Florida
Plant growth and yield increase with added fertilizer up to a
critical level, after which no significant increase occurs. As this
level of maximum production is approached, the increase in response
for a specific rate increase becomes less and less. Citrus tree
responses to fertilization do not increase indefinitely with

20
increased fertilization rates, nor do trees accumulate excessive
amounts when minerals are present in large quantities (Koo et al.,
1984; Sites et al., 1953; Sites et al., 1961; Smith & Rasmussen,
1961; Stewart et al., 1961; Wallace et al., 1952). Optimum
fertilization rates depend not only on response to increased
fertilizer application, but on fertilizer and fruit prices as well.
Current recommendations for fertilizing mature citrus groves in
Florida (Koo et al., 1984) are based on many years of controlled
experiments. Mature citrus trees require from 100 to 340 kg
N/ha/year to maintain optimum production, however, rates above 225
kg/ha are rarely justifiable in terms of yield increase for most
citrus cultivars.
Many field experiments have been conducted to determine the
optimum number and timing of fertilizer applications and the effect
of elemental sources on mature citrus trees. Results generally
agree that frequency or timing are of far less importance than
fertilizer rate (Calvert & Reitz, 1964; Reitz, 1956; Reuther &
Smith, 1954; Sites et al., 1953, 1961; Smith, 1970). One to two
applications per year are adequate when recommended rates are used.
Nutrient source is also of little consequence in yield responses of
mature citrus trees (Camp, 1943; Leonard et al., 1961; Reitz, 1956;
Reitz & Koo, 1959; Sites, 1949; Sites et al., 1953; Smith, 1970;
Stewart et al., 1961).
Nitrogen Losses
Applied N may be lost from the soil in many ways. Ammonium
salts and urea may be lost by volatilization of ammonia, especially
on calcareous soils (Volk, 1959; Wahhab et al., 1957).

21
Nitrification, the bacterial oxidation of ammonia to nitrate, can
lead to increased losses of N from the root zone by leaching of
nitrate. Reduction of nitrate to volatile forms of N is carried out
by many species of bacteria in the soil. This process
(denitrification) is promoted by high soil moisture and temperatures
(Mengel & Kirkby, 1982).
About a third of the applied N was lost through leaching in a
newly-developed Florida citrus grove (Calvert, 1975; Calvert &
Phung, 1971). An average of 50 kg N/ha may be lost annually through
leaching in Florida groves (Barnette, 1936). Annual leaching losses
from some southern California citrus orchards have been estimated at
about 67 kg N/ha (Bingham et al., 1971), which is equivalent to
about 45% of the applied N. Wallace et al. (1952) estimated a 15%
recovery of applied N by citrus trees in southern California.
Similarly, annual losses of up to 67 kg/ha have been attributed to
volatilization (Chapman, 1951; Chapman et al., 1949).
Controlled-Release Fertilizer Sources
Controlled-release fertilizer sources that are N carriers can
be used to substantially reduce losses of applied N (Maynard &
Lorenz, 1979; Oertli, 1980). Some benefits from using
controlled-release fertilizers are reduced losses of applied N
through leaching, denitrification, or volatilization; the
possibility of less frequent applications; and increased efficiency
in use of materials (Allen & Mays, 1974; Oertli, 1980; Terman &
Allen, 1970). Controlled-release fertilizer sources have proven
beneficial with many horticultural crops (reviewed by Maynard &
Lorenz, 1979). Sulfur-coated urea and isobutylidene diurea have

22
increased mature orange tree yields over ammonium nitrate in Florida
(Koo, 1986).
Young Citrus Tree Care
Objectives of a young citrus tree care program center on
obtaining the greatest amount of growth in the shortest amount of
time. There is little concern for marketable yield or fruit quality
as with mature trees.
Irrigation
Information on irrigation of young citrus is limited. In
Arizona, basin and trickle irrigation systems provided greater
growth and lower water use of young orange trees through 5 years in
the field, compared to border-flood and sprinkler systems (Rodney et
al., 1977; Roth et al. 1974). The trickle system was operated
daily, basin and sprinkler were operated weekly, and flood
irrigations were at intervals of at least 2 weeks. Trees in the
trickle treatment received 11%, in the basin treatment, 13%, and in
the sprinkler treatment, 38% as much water as those in the flood
treatment. Leyden (1975a) compared strip watering, ring watering,
and drip irrigation on newly-planted grapefruit trees in Texas.
Tree growth was similar over a 2-year period, with drip requiring
about 18% and ring watering 25% of the water utilized with strip
watering. Strip and ring watering were scheduled at 30% soil water
depletion, and drip irrigation was scheduled on the basis of pan
evaporation. De Barreda et al. (1984) used drip and basin
irrigation in Spain to compare the application of varying amounts of
water at the same frequency. Applications were based on
coefficients of pan evaporation, and results suggested that

23
coefficients of 0.10-0.15, 0.20, and 0.30 for the first 3 years
maintained adequate growth. They made no comparison of basin and
drip irrigation methods. Split-root containers have been utilized
to simulate the effects of partial root wetting of young citrus
trees in Florida (Brakke et al., 1986). Preliminary results of gas
exchange measurements suggested that growth may be only slightly
reduced by irrigating 50-75% of the root volume compared to 100%.
Field lysimeters equipped with rain shelters have been used to
study irrigation requirements of young orange trees in Florida
(Aribi, 1985; Smajstrla et al., 1985). Irrigations were scheduled
at matric potentials of -10, -20, and -40 kPa, which corresponded to
available soil water depletions of 30, 45, and 55%. Water was
applied to return the upper 60 cm of soil to field capacity, thus
23, 34, and 42 liters were applied per irrigation per tree in the
-10, -20, and -40 kPa treatments, respectively. Maintaining
weed-free conditions around trees resulted in a 50% reduction in
water use compared to trees with a bahiagrass cover. Tree growth
was greatest when irrigations were scheduled at -20 kPa and the
ground was maintained weed-free.
General recommendations have been given for irrigation of
newly-planted citrus in Florida. Jackson and Ferguson (1984)
recommend that newly-planted trees irrigated by the basin method
should be watered 2 to 3 times per week for 8 weeks, and once per
week thereafter. Ziegler and Wolfe (1975) suggest applying 30-38
liters per tree each 2 weeks in the spring, with no irrigation
needed in the fall. Jackson and Lawrence (1984) recommend a
"generous supply" of water applied every 7-10 days.

24
Fertilization
Optimum nutrient levels for maximum canopy growth should be
maintained by fertilization of young citrus trees (Koo & Reese,
1971). Young tree fertilization practices in Florida have changed
over the years. Collison (1919) made a 10-year comparison of
fertilizer rates. The standard treatment used 0.136 kg N/tree/year
in year-1, and was gradually increased to 0.408 kg N/tree/year by
year-10. Some plots received one-half, twice, and four times this
amount. After several years, trees in the standard and one-half
standard plots had larger trunk diameters than two-fold and
four-fold treatments. Although not properly replicated, these data
suggest as little as 0.068 kg N/year are needed for fertilization of
young citrus trees. Bryan (1940) recommended applying 0.07 kg
N/year per meter of canopy spread for young citrus trees in Florida.
Several field experiments conducted in the 1950s suggested that
annual rates of 0.073 kg N/tree for the first 2 years were
sufficient to obtain optimum tree growth on the Ridge (Rasmussen &
Smith, 1961, 1962). These rates and the authors' recommended
application frequency of three times during the first year and two
times thereafter differed from the average grower practice. Up to
seven applications per year and rates of 0.13 and 0.25 kg N/tree for
the first 2 years, respectively, were not uncommon at the time.
Calvert (1969) reported that trees responded with greater
growth to rates of 0.22 to 0.32 kg N/tree than 0.11 kg/tree when
growing in the flatwoods in marginal soils. Furthermore, four
applications per year were superior to three. These data illustrate

25
the importance of considering soil type when evaluating
fertilization needs.
There is a wide range of adequate fertilization methods
available for growers, and considerable judgement is required to
choose the most efficient. Current recommendations (Koo et al.,
1984) call for 0.18-0.22 kg N/tree in year-1 and 0.29-0.36 kg N/tree
in year-2. Other elements should be applied in proportion with N in
the following ratio: N-l, P20,--1, K20-1, Mg-1/5, Mn-1/20, Cu-1/40,
B-l/300.
Controlled-release N sources have been used for fertilization
of young citrus trees. Controlled-release isobutylidene diurea
increased growth of young container-grown citrus compared to soluble
N sources (Khalaf, 1980; Khalaf & Koo, 1983). In the same study,
isobutylidene diurea and sulfur coated urea reduced N leaching
losses compared to soluble sources. Fucik (1974) similarly
demonstrated an increase in growth of young, container-grown citrus
trees with controlled-release compared to soluble fertilizer.
Jackson & Davies (1984) reported similar growth rates of young,
field-grown 'Orlando' tángelo trees occurred with sulfur coated urea
and a soluble fertilizer source, but application frequency was
reduced by 50% with sulfur coated urea.
Container- and Field-grown Nursery Trees
Cultural practices and growing conditions of any nursery affect
growth and development of plant material in the nursery, but also
after transplanting to the field. Production systems for field and
greenhouse citrus nurseries are well-developed, however, little
effort has gone into understanding the effect of nursery tree

26
characteristics on growth after field planting. Webber (1932), with
'Washington' navel orange, and Gardner and Horanic (1959) with
'Parson Brown' and 'Valencia' orange reported no relationship
between initial size of nursery trees and mature tree size. Effects
on precosity were not reported. Grimm (1956, 1957) stated the most
important factor affecting initial growth of bare-rooted nursery
trees was the protection of roots while they were out of the ground.
The advantages of containerized, greenhouse nursery systems are
varied and include greater control over the production system, lower
land requirement, and shorter production cycle (Castle et al., 1979;
Moore, 1966; Platt & Opitz, 1973; Richards et al., 1967). Nursery
trees produced under these conditions are much different from the
traditional field-grown nursery trees. Shoot:root ratio of nursery
trees is substantially increased in this system over the field
system (W.S. Castle, Citrus Research and Education Center, Lake
Alfred, personal communication). Very high nitrogen and water
applications, reduced light intensity, and root restriction by
containers may all contribute to this, as all of these factors have
been shown to increase shoot:root ratio. Vigorous root growth is
altered when the available soil volume is permeated, at which time
the growth pattern may be shifted to fibrous roots (Castle, 1978).
This results in a substantial increase in the proportion of fibrous
to non-fibrous roots of container-grown compared to field-grown
citrus nursery trees (W.S. Castle, Citrus Research and Education
Center, Lake Alfred, personal communication).
Initial expansion of the root system is critical for successful
establishment of any containerized transplant (Castle, 1987). The

27
potential for rapid initial root growth of containerized forest
seedlings has been studied in detail and is directly linked to
survival and tree growth after field planting. Container
characteristics such as size and shape have altered post-planting
shoot and root growth of container-grown transplants (Elam et al.,
1981; Hiatt & Tinus, 1974; Hite, 1974; Tinus & Owston, 1984; Van
Eerden & Arnott, 1974). Container medium has also affected tree
growth following field planting in a number of species (Elam et al.,
1981; Helium, 1981) including citrus (Warneke et al., 1975). Elam
et al. (1981) reported considerable variation between oak species
with respect to the effects of container and media characteristics
on growth. Less root growth of pine seedlings occurred as the
length of time plants were maintained in containers was lengthened
(Helium, 1981). Reduced irrigation frequency or nitrogen
fertilization prior to removal from the nursery has been used to
increase root and shoot growth of container-grown trees (Rook, 1973;
Timmis, 1974; Tinus & Owston, 1984).
Drying of container medium after planting in the field may
induce plant water stress in some cases. A substantial increase in
drainage out of the medium occurred following removal from a
container and placement in contact with field soil (Costello & Paul,
1975; Nelms & Spomer, 1983; Warneke et al., 1975). Low survival
rates have been attributed to severe water stress due to these
conditions.
Post-plant growth comparison of container- and field-grown
citrus nursery trees has been conducted in Texas. Leyden and Timmer
(1978) observed growth of grapefruit on sour orange trees for 2.5

28
years in the field and concluded that container-grown trees would be
less productive and smaller than field-grown trees during the early
years of bearing. Maxwell and Rouse (1980, 1984) reported that
container-grown grapefruit on sour orange trees remained smaller
than field-grown trees through 10 years after planting, but yield
did not differ. Container-grown trees were not produced under
greenhouse conditions as is the case in Florida, and field-grown
trees were transplanted as ball and burlapped stock in both studies.

CHAPTER III
MICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG 'HAMLIN'
ORANGE TREES. I. CANOPY GROWTH AND DEVELOPMENT
Introduction
Comparable or increased growth or yield and decreased water use
by mature citrus has been observed in several countries when micro
irrigation systems were compared with flood or high volume sprinkler
systems (Alijbury et al., 1974; Bester et al., 1974; Bielorai, 1982;
Bielorai et al., 1981; Legaz et al., 1981; Roth et al., 1981). Some
have found that increasing the area of coverage by irrigation
systems has increased growth and yield of mature citrus (Moreshet et
al., 1983; Koo, 1978, 1985; Zekri, 1984). Smajstrla and Koo (1984,
1985) reported an increase in yield over non-irrigated trees in
Florida when irrigating between 5 and 50% of the under-tree canopy
area of mature citrus. Koo and Tucker (1974) recommended that 60%
of a citrus tree's root volume be covered by an irrigation system.
A number of studies have been conducted comparing growth and
development of young citrus trees using various irrigation methods.
Border-flood, sprinkler, basin, and trickle irrigation methods were
compared on young 'Campbell Valencia' orange trees in Arizona
(Rodney et al. 1977; Roth et al. 1974). Tree growth and yield were
greater and water use lower in the first 5 years with the basin and
29

30
trickle systems compared with the other systems. Growth of young
navel orange trees has been evaluated using drip and basin
irrigation methods in Spain (De Barreda et al., 1984). Treatments
consisted of applying different volumes of water at the same
frequency based on coefficients of pan evaporation. Coefficients
were increased with each year of age, suggesting that coefficients
of 0.10-0.15, 0.20, and 0.30 for the first 3 years maintained
adequate growth. Leyden (1975a) compared irrigation systems on
newly-planted 'Star Ruby' grapefruit trees in Texas. Strip and ring
watering were scheduled based on 30% soil water depletion, and drip
irrigation was scheduled on the basis of pan evaporation. Drip
irrigation used 18 and ring watering used 25% of the cumulative
irrigation water required for strip watering. Tree growth was
similar between the three systems over a 2-year period.
Irrigation of young citrus trees in Florida is also necessary
to obtain optimum growth. Irrigation studies on newly-planted
'Valencia' orange trees were conducted using a series of lysimeters
under rain shelters (Aribi, 1985; Smajstrla et al., 1985).
Comparing irrigation scheduling at matric potentials of -10, -20,
and -40 kPa, tree growth was greatest when irrigations were
scheduled at -20 kPa and the ground was maintained weed-free.
Split-root containers were utilized to simulate the effects of
partial root-volume wetting of young 'Hamlin' orange trees (Brakke
et al., 1986). Preliminary results of gas exchange measurements
suggested that growth may be only slightly reduced by irrigating
50-75% of the root volume compared to 100%. Allen et al. (1985)

31
stated that frequent irrigation of containerized citrus rootstock
seedlings was required to sustain high photosynthesis.
An estimated 6-10 million young citrus trees are being planted
in Florida annually (Jackson et al., 1986), however, water
requirements, irrigation scheduling, and growth responses to
microsprinkler irrigation have not been studied under field
conditions. The purpose of this field study using 'Hamlin' orange
trees was to determine 1) the optimum level of soil water depletion
at which irrigations should be scheduled to maximize growth, 2) the
amount of irrigation time needed to replenish soil water to field
capacity, and 3) the effect of irrigation pattern on growth of
second season trees.
Materials and Methods
Site and Soil Characteristics
Three field experiments were conducted at the Horticultural
Research Unit near Gainesville, FL, in 1985, 1986, and 1987. Soil
type was Kanapaha sand (Carlisle et al., 1988) (loamy, siliceous,
hyperthermic, Grossarenic, Paleaquults) underlain by a hardpan.
Particle size distribution was 93.4% sand, 3.9% silt, and 2.7% clay.
The soil has a field capacity of ca. 11.3%; permanent wilting point,
3
ca. 2%; mean bulk density, 1.56 g/cm ; pH, 6.4; and percent organic
matter, 0.65. A desorption soil water characteristic curve was
determined using a vacuum desiccator employing undisturbed soil
cores in weighable pressure cells fitted with fritted glass plates
(Fig. 3-1). Saturated hydraulic conductivity was 9.3 cm/hr as
determined by using undisturbed cores in a constant head
permeameter.

Log, soil water potential (kPa)
32
0.1 0.2 0.3
Volumetric water content (m3*m“3)
Fig. 3-1. The relationship between soil water content
and water potential of Kanapaha sand at the Horticultural
Unit.

33
Beds (16.75 m width x 0.60-0.75 m height x 85 m length) were
constructed in March, 1985 to facilitate drainage and simulate
flatwoods growing conditions. Ground water table was monitored
through observation wells located in the tree rows and averaged 1.1
m deep between June and Sept, (rainy season) with a minimum of 0.45
m, and greater than 1.6 m during other months. A ground cover of
bahiagrass was developed between rows and was mowed as needed, while
tree rows (about 2.1 m width) were maintained weed-free with
herbicides.
Irrigation Treatments
The irrigation system was designed so that each treatment could
be monitored and controlled individually using a flow meter and
pressure gauge controlled by a gate valve. Water was conveyed from
a manifold through 2.54 cm PVC main lines and 1.91 cm black
polyethylene laterals down tree rows. Three tubes, one per
treatment, were positioned 50-55 cm west of each tree row which ran
the full length of the bed, allowing unrestricted randomization at
each experimental site. Irrigation water containing 287 mg/liter
total dissolved solids was applied via 90°, 38 liter/hr Maxijet"
microsprinkler emitters positioned 1 m northwest of each tree. This
positioning also provides optimum cold protection for young citrus
trees (Rieger et al., 1986). The area of ground covered was about 5
2
m for these emitters. Irrigation was controlled by hand regulation
of gate valves to maintain the system pressure at about 140 kPa.
Irrigation scheduling was based on soil water content as
monitored by the neutron scattering method (Hillel, 1982) using a
Troxler Model 1255 neutron probe. A calibration curve for the

34
neutron probe was developed by the gravimetric method (Fig. 3-2)
(Hillel, 1982). Access tubes constructed of 48 mm i.d. aluminum
irrigation tubing were driven into the soil 1 m from the emitters
and 35 cm from each of four randomly chosen trees per treatment in
1985 and 1986 and from each of three trees per treatment in 1987.
Soil moisture measurements were made daily or as needed during the
rainy season at a depth of 30 cm. The soil volume around the tree
was irrigated to field capacity when a prespecified level of
available soil water depletion (SWD) was reached. Irrigations were
initiated when any of the four neutron probe readings reached the
level of SWD in 1985 and 1986, and based on the average of the three
neutron probe readings in 1987. Three levels of irrigation were
designated as high (20% SWD), moderate (45% SWD), and low (65% SWD).
The amount of irrigation water needed for each treatment to
replenish soil moisture to field capacity was determined during the
first few irrigations in 1985. Soil water content at 30 cm depth
was monitored using the neutron scattering method at 15-min
intervals following an irrigation. By initially varying the length
of irrigation time, the approximate length of time needed for each
treatment to return soil moisture from the specified SWD level to
field capacity was determined.
Plant Material and Experimental Design
Commercially obtained, bare-rooted 'Hamlin' orange [Citrus
sinensis (L.) Osb.] on sour orange (£. aurantium L.) trees were
planted on double row beds at 7.6 m between and 4 m within rows.
Initial trunk diameter averaged 1.2, 1.4, and 1.5 cm in 1985, 1986,
and 1987, respectively. A randomized complete block design

35
Volumetric water content (m3*m-3)
Fig. 3-2. The relationship between neutron probe ratio and
volumetric water content in Kanapaha sand at the
Horticultural Unit. R = counts per min. in soil,
R = standard counts per min.
o r

36
consisting of four blocks was utilized in 1985 and 1986, employing
six single-tree replications per treatment per block, resulting in a
total of 24 replications per treatment. A completely randomized
design with 13 replications was utilized in 1987 because the block
effect was not significant in 1985 and 1986. Trees were planted on
10-12 May, 3-4 May, and 2 April for the 1985, 1986, and 1987
experimental periods, respectively. All trees were irrigated every
2 days during an establishment period of 10-14 days in each
experiment. Commercial fertilization rates recommended for Florida
(Koo et al., 1984) were followed using an 8N - 2.6P - 6.6K - 2Mg -
0.2Mn - 0.12Cu - 0.2Zn - 1.78Fe dry formulation. Four applications
were made at regular intervals in 1985, while five were made in 1986
and 1987.
Plant Measurements
Seasonal measurements. Shoot growth was measured throughout
the growing season during 1985 and 1986. As shoots began growth,
three per tree were randomly chosen and tagged. Shoot length was
measured with a ruler at about 7-day intervals thereafter. The
length and width of the third leaf from the shoot base was measured
at the same time. This leaf position was used because the bottom
two leaves were many times of unrepresentative size or shape. Leaf
expansion was determined by using the product (x) of length and
2
width in an equation for leaf area (area = 2.33 + 0.63x, r = 0.94).
This equation was derived from a sample of 100 leaves of various
sizes by linear regression of leaf area, determined with a LI-COR
Model LI-3000 leaf area meter, on leaf length x width (data not
shown). Final shoot length and leaf area were compared, as were

37
expansion rates. Rates were calculated on a daily basis by
subtracting the initial length or area when shoots were tagged from
the final length or area, and dividing by the number of days
required to reach the final size. Although shoot and leaf expansion
were not measured in 1987, dates of initial shoot growth were
recorded for each tree in each of the three flushes. Intervals of
about 7 days were again used. The percentage of the tree population
growing for each irrigation treatment and at each date was
calculated for all 3 years.
Canopy and trunk measurements. Canopy height and width in two
directions were measured initially and in December of each year (15
May and 10 Dec. 1985, 6 May and 7 Dec. 1986, 4 April and 18 Dec.
1987). Width measurements were averaged and canopy volume was
calculated as (4/3)(3.14)(1/2H)(1/2W)^, where H=height and W=width
(Westwood, 1978). This formula most closely approximates the canopy
shape of a young tree which is taller than it is wide. A mark was
painted with latex paint about 5 cm above the bud union where trunk
diameter was measured in two directions with a hand-held caliper.
Measurements were made on the same days as canopy measurements. The
two measurements were averaged and trunk cross sectional area
calculated. In addition total shoot length for each tree was
measured with a ruler within 1 week of planting dates.
Final measurements. Root excavation of 20, 21, and five plants
per treatment in 1985, 1986, and 1987, respectively, was conducted
by hand in Dec. of each year. Initially a circular trench was dug
about 40 cm deep at a distance of 120 cm from the trunks. The few
roots extending beyond this distance were individually recovered by

38
excavation. The entire shallow root system was recovered almost
completely intact by undercutting it to a depth of 40 cm until the
sand loosened exposing the roots. The depth was increased near the
trunk to ensure recovery of the taproots. This operation was not
difficult due to the loose, sandy nature of the soil and shallowness
of the root systems. Trees were taken indoors where roots and
canopies were separated. Root growth and distribution are discussed
in Chapter IV.
Leaves were removed from canopies for fresh weight
determination. Leaf area was calculated from the fresh weight
utilizing a linear relationship obtained from sampling eight trees.
Leaf area was measured on these trees using a LI-COR Model LI-3000
leaf area meter. The relationship shown in Fig. 3-3 was derived
from linear regression of leaf area on fresh weight. Total shoot
length was measured with a ruler following leaf removal.
A random sample of four, six, and five trees in 1985, 1986, and
1987, respectively, were separated into the three growth flushes
that had occurred during the experimental period. Shoots within
each flush were counted and measured with a ruler. Mean and total
shoot length and mean shoot number were calculated for each growth
flush.
All canopy parts were dried in an oven at 80° C to determine
canopy dry weight.
Analysis of Data
Shoot length, leaf area, and expansion rates measured
throughout the growing season were analyzed separately for each of
the three flushes in 1985 and 1986. Shoots in each flush were

Leaf area (m
39
Fig. 3-3. Relationship of leaf area and fresh weight from
eight representative 'Hamlin' orange trees after 8 months
in the field.

40
blocked into groups of shoots that had begun growth within periods
of time not exceeding 2 weeks in length. Data were analyzed by
analysis of variance as a treatment x time period factorial
experiment.
Data concerning the percentage of each treatment population
growing throughout the season were analyzed using linear, quadratic,
and cubic regression models. Models with the highest level of
significance and best fit for each treatment population were chosen.
Resulting equations were tested by analysis of covariance for
homogeneity within each growth flush.
Shoot number, mean length, and total length measured by
dividing excavated trees into different flushes were analyzed by a
split-plot analysis with treatments as main plots and flushes as
subplots.
Final plant measurements from different years were analyzed
separately. Data on dry weight and leaf area were subjected to
analysis of variance and data on canopy volume, trunk cross
sectional area, and shoot length to analysis of covariance to
standardize differences in initial plant measurements. Where
irrigation levels were significantly different, Williams' (Williams,
1971) test was used to compare means. This test is useful in cases
where curve-fitting is not desired or is difficult due to a small
number of dose levels. It was employed due to the small number of
treatments to compare irrigation levels to the most frequent
scheduling treatment to determine the level at which a significant
response occurred

41
Microsprinkler Irrigation Spray Patterns
Two experiments were conducted on the same site to test the
effect of microsprinkler irrigation spray pattern on 'Hamlin' orange
tree growth the second season in the field. Site characteristics
were as described for microsprinkler irrigation scheduling
experiments, except trees were set 3.4 m apart in the rows. Two
microsprinkler spray patterns were utilized, 90° and 180°.
Approximately 89 liters were applied per tree at each irrigation,
delivered through 89 liter/hr emitters and distributed over ca. 5.9
2
and 9.3 m of ground area for the 90° and 180° patterns,
respectively.
Treatments begun on 10 May 1986 in the first experiment on
trees planted in May 1985. The factorial experiment performed from
May 1985 - May 1986 was used to study two types of nursery trees
receiving four types of fertilizer (Chapters VI & VII). Each 2x4
treatment combination had been assigned to two trees within each of
four randomized complete blocks in 1985. Subsequently, each spray
pattern was randomly assigned to one of the two trees of each tree
type - fertilizer type combination in May 1986. Treatments were
begun on 2 May 1987 in the second experiment on trees planted in May
1986 (Chapters VI & VII). Each 2x4 treatment combination had been
assigned to two trees within each of two randomized complete blocks
in 1986. Each spray pattern was again randomly assigned to one of
the two trees of each tree type - fertilizer type combination in May
1987.
Trees were grown during the first year under 90° microsprinkler
irrigation on 20Z SWD scheduling. During the two experimental

42
periods irrigations were scheduled when 20% SWD was reached in
accompanying irrigation scheduling experiments.
Trunk cross sectional area and canopy volume were measured as
described previously. Measurements were made on 9 May and 7 Dec.
1986 for the first experiment and 30 April and 18 Dec. 1987 for the
second. Measurements were analyzed separately for the two
experimental periods. Split-plot analysis was utilized with
fertilizer type x tree type as main plots and irrigation patterns as
subplots. Analysis of covariance was used to standardize
differences in plant measurements from the beginning of the
experimental periods.
Environmental Variables
Rainfall was recorded daily with a Science Associates Model 503
rain gauge located about 200 m east of the experimental site.
Relative humidity and temperature were recorded continuously using a
hygrothermograph (WEATHERtronics Model 5021) located at the
experimental site.
Results and Discussion
Irrigation Amount and Frequency
Thirty-eight, 50, and 76 liters/tree were needed for the high,
moderate, and low irrigation treatments, respectively, to return the
soil to field capacity (Table 3-1). These durations resulted from
measurements of soil moisture content by the neutron probe at a
depth of 30 cm, and did not take into account the level of depletion
in the surface soil. Irrigation durations of 1-2 hr were used with
the 38 liter/hr, 90° emitters used in this study. An industry
survey has shown that many times growers do not take advantage of

43
Table 3-1. Duration of irrigation and amount of water
applied at each irrigation as related to soil water
depletion for young 'Hamlin' orange trees.
Soil water
Irrigation
Amount
applied
depletion
duration
liters/
(%)Z
(hr)
tree
mmy
20 (HighX)
1.0
38
7.5
45 (Mod.)
1.3
50
10.0
65 (Low)
2.0
76
15.1
2
Based on neutron probe measurements at 30 cm ^epth.
yBased on area wetted by emitters of about 5 m .
X *
High, moderate, and low refers to irrigation frequency.

44
the water savings that are possible with micro irrigation systems
due to excessive operating time (Hutcheson & Bellizio, 1974).
The number of irrigations needed to maintain soil moisture at
the specified levels differed greatly (Table 3-2). An average of
31, 11, and 2 irrigations per season were required for the high,
moderate, and low schedules, respectively. During dry periods,
irrigations were 2-3 days apart in the high treatment and 4-6 days
apart in the moderate schedule. The number of irrigations in the
moderate schedule varied from year-to-year more than the other
scheduling treatments. The 16 irrigations needed in 1986 were
nearly twice the number needed in the other years. Different
factors were probably responsible for the reduced number of
irrigations needed in 1985 and 1987 compared with 1986. In 1985,
more frequent rainfall (Fig. 3-4) did not allow SWD to reach 45%
(moderate) as often as occurred in 1986 (Fig. 3-5). Rainfall was
not responsible for the reduced number of irrigations in 1987 since
frequency and cumulative amount of rainfall was lower (Fig. 3-6)
than in 1986. Tree growth and water use was less during the 1987
season regardless of irrigation treatment, thus the plants required
fewer irrigations. Over the 3-year period, trees in the moderate
and low treatments received respectively 49 and 13% as much
irrigation water as those in the high treatment (Table 3-2).
Individual Shoot and Leaf Growth - Seasonal Measurements
During the first flush of 1985 trees under the high and
moderate treatments made significantly greater average shoot growth
(P<.0165) and leaf growth (P<.0315) than trees under the low
treatment (Table 3-3). Shoot and leaf size in the second and third

Rainfall (cm) Low(cm) Mod. (cm) High (cm)
45
Fig. 3-4. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1985. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.

46
Table 3-2. Number of irrigations and cumulative water applied
for young 'Hamlin' orange trees under scheduling treatments
based on soil water depletion.
Soil Cumulative
water Irrigations/ Cumulative water applied irrigation
depletion
(%)Z
year
(no.)
liters/
tree
/—N
o
B
and rain
(cm)
1985
20 (HighX)
31
1173.5
23.3
141.2
45 (Mod.)
8
402.8
8.0
125.9
65 (Low)
1
75.7
1.5
119.4
1986
20 (High)
35
1324.9
26.4
145.8
45 (Mod.)
16
805.5
16.0
135.5
65 (Low)
4
302.8
6.0
125.5
1987
20 (High)
27
1022.0
20.3
112.1
45 (Mod.)
10
503.5
10.0
101.8
65 (Low)
1
75.7
1.5
93.3
2
Based on neutron probe measurements at 30 cm ^Based on area wetted by emitters of about 5 m .
xHigh, moderate, and low refers to irrigation frequency.

Rainfall (cm) Low (cm) Mod. (cm) High (cm)
47
Time (months)
Fig. 3-5. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, May-Dec. 1986. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.

Rainfall (cm) Low (cm) Mod. (cm) High (cm)
48
Fig. 3-6. Distribution of rainfall and microsprinkler irrigation
at the Horticultural Unit, April-Dee. 1987. High, moderate, and
low refer to irrigation treatments based on 20, 45, and 65% soil
water depletion, respectively.

49
Table 3-3. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1985.
Soil water
depletion
(%)
Shoot
Leaf
n
length
(cm)
rate
(cm/day)
n
area
(cm )
rate
2
(cm /day)
Flush
one (5/30 -
6/25)Z
20 (High)
70
9.7
0.5
67
22.6
0.8
45 (Mod.)
69
10.8
0.6
67
23.3
0.9
65 (Low)
64
8.6
0.4
61
19.6
0.7
SEy
0.7
0.1
1.4
0.1
Flush
two (7/12 -
8/19)
20 (High)
49
21.5
1.2
49
38.7
1.4
45 (Mod.)
40
22.3
1.3
40
40.9
1.6
65 (Low)
34
17.4
1.0
33
28.1
1.0
SE
ns
ns
ns
ns
Flush three (9/11 â– 
- 11/25)
20 (High)
36
28.8
1.1
37
44.1
1.7
45 (Mod.)
28
31.5
1.2
28
43.8
1.6
65 (Low)
10
25.8
1.2
10
40.7
1.5
SE
ns
ns
ns
ns
2
Range in dates of growth initiation.
ySE = standard error of mean.

50
flush were not dependent on irrigation treatments, although the low
treatment consistently had the lowest means.
Irrigation treatment effects on growth of the first flush in
1986 were in contrast to 1985, in that trees in the moderate and low
treatments had greater individual shoot length (P<.0077) and leaf
area (P<.0005) than trees in the high treatment (Table 3-4). A
different pattern occurred in flush two, when shoot length of trees
in the high and moderate treatments was larger (P<.0117) than those
in the low treatment. Leaf size in flush two and both shoot and
leaf size in flush three were independent of irrigation treatments.
Shoot length, leaf area, and rates of growth generally increased
from flush one to flush three in both 1985 and 1986.
Shoot and leaf expansion rates averaged less in flush one than
the other flushes. Similarly, growth rates of the spring flush of
mature orange trees in Florida, estimated as gain in biomass, were
less than the summer flush (Syvertsen et al., 1981). Shoot growth
rates ranged from 0.4-1.0 cm/day in flush one and 0.7-1.4 cm/day in
the other flushes, which was considerably more than previously
reported for mature trees of various citrus species in subtropical
India (Krishnamurthi et al., 1960).
Shoot Number and Length Within Flushes - Final Measurements
Shoot number, average shoot length, and total shoot length were
not significantly affected by irrigation treatments in 1985.
Cumulative shoot length during the season (three flushes summed) was
650, 760, and 537 cm/tree in the high, moderate, and low treatments,
respectively. Lack of significant differences may have been be due
to the small number of replications (four) or to the loss in

51
Table 3-4. Shoot and leaf sizes and expansion rates for three
growth flushes of young 'Hamlin' orange trees as influenced by
irrigation based on soil water depletion, 1986.
Soil water
depletion
(%)
Shoot
Leaf
n
length
(cm)
rate
(cm/day)
n
area
(cm )
rate
2
(cm /day)
Flush one (5/25 -
6/17)Z
20 (High)
65
13.7
0.8
65
15.9
0.6
45 (Mod.)
45
17.5
1.0
44
22.0
0.9
65 (Low)
49
17.8
1.0
49
21.4
0.7
SE7
1.5
0.1
1.9
0.1
Flush
two (7/7 -
9/13)
20 (High)
61
20.3
1.1
60
31.3
1.1
45 (Mod.)
39
21.6
1.1
44
34.9
1.3
65 (Low)
40
17.4
0.7
40
30.4
1.1
SE
1.5
0.1
ns
ns
Flush three (8/30 •
- 12/3)
20 (High)
58
30.6
1.4
53
46.6
2.1
45 (Mod.)
49
31.7
1.4
46
46.3
1.8
65 (Low)
45
28.2
1.3
42
37.3
1.5
SE
ns
ns
ns
ns
ZRange in dates of growth initiation.
ySE = standard error of mean.

52
precision in the whole plot analysis of treatments in the split-plot
design. Across all treatments, however, individual shoot length
significantly increased (P<.0001) and shoot number per flush
decreased (P<.0108) successively from flush one to flush three
(Table 3-5). Flushes two and three added significantly (P<.0124)
more total shoot length than flush one during the season (Table
3-5).
In contrast, irrigation treatments significantly affected shoot
number per flush (P<.0079) and total shoot length per flush
(P<.0164) in 1986, although there was no effect on mean shoot length
(Table 3-6). A mean of 19 shoots per flush occurred on trees in the
high treatment, with 13 and 9 shoots per flush occurring on trees in
moderate and low treatments, respectively (SE=3). Total length per
flush was 335 cm for trees in the 20% SWD treatment, and 258 and 134
cm for trees in the 45 and 65% SWD treatments (SE=64). Reduction in
total length per flush by 65% SWD was more pronounced in flush three
(Table 3-6). Cumulative shoot length for the entire season was 1004
cm in high, and 774 and 402 cm in moderate and low treatments,
respectively. As in 1985, mean shoot length, pooled over the
treatments, significantly (P<.0001) increased from flush one to
flush three, while flush three had significantly (P<.0012) fewer
shoots than flushes one and two (Table 3-6). These trends balanced
total length of individual flushes such that the flushes were not
different.
Irrigation treatments did not affect mean shoot length or
number in 1987, but significantly (P<.0204) altered total shoot
length per flush, producing a total length of 408, 235, and 212 cm

53
Table 3-5. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1985.
Soil water
Shoot
Total
depletion
Shoot
length
length
(%)
z
no.
(cm)Z
(cm)Z
Flush one
(5/30 - 6/25)7
20 (High)
17
9.5
163.0
45 (Mod.)
15
7.9
118.7
65 (Low)
18
7.9
146.4
Mean
17
8.4
142.7
Flush two
(7/12 - 8/19)
20 (High)
13
20.5
261.3
45 (Mod.)
10
28.2
289.1
65 (Low)
8
25.7
211.7
Mean
10
24.8
256.4
Flush three
(9/11 - 11/25)
20 (High)
7
34.8
225.9
45 (Mod.)
11
32.2
352.4
65 (Low)
4
42.9
179.1
Mean
7
36.6
262.1
SEX
2
3.4
36.9
2
Means of 4 trees/treatment.
y
'Range in dates of initiation.
SE for comparison of means among growth flushes,
among irrigation levels within growth flushes is
Comparison
inappropriate
since irrigation x flush interaction is not significant.

54
Table 3-6. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1986.
Soil water
Shoot
Total
depletion
Shoot
length
length
(%)
z
no.
(cm)2
(cm)2
Flush one
(5/25 - 6/17)y
20 (High)
22
13.1
284.3
45 (Mod.)
16
13.0
203.7
65 (Low)
12
10.8
126.3
Mean
16
12.3
201.9
Flush two
(7/7 - 9/13)
20 (High)
23
15.9
366.4
45 (Mod.)
15
18.2
283.5
65 (Low)
12
20.1
234.1
Mean
17
18.1
301.1
Flush three
(8/30 - 12/3)
20 (High)
12
29.5
353.6
45 (Mod.)
9
33.4
286.7
65 (Low)
2
22.9
41.7
Mean
8
29.5
219.9
SEX
3
1.8
ns
2
Means of 6 trees/treatment.
yRange in dates of initiation.
XSE for comparison of means among growth flushes. Comparison
among irrigation levels within growth flushes is inappropriate
since irrigation x flush interaction is not significant.

55
of growth per flush (SE=59) in the high, moderate, and low
treatments, respectively (Table 3-7). As in 1986, the treatment
effect was most pronounced in flush three. Mean shoot length
significantly increased (P<.0001) and shoot number decreased
(P<.0001) successively from flush one to flush three, although total
length was not different among the three flushes. Cumulative shoot
length for the season averaged 1225 cm for the high, and 706 and 635
cm for the moderate and low treatments.
Severe defoliation shortly after transplanting trees from the
nursery to the field in 1987, possibly due to greasy spot, produced
a large number of short shoots during flush one. An average of 68
shoots were initiated per tree, compared to 17 and 16 in 1985 and
1986. Although these shoots in 1987 were much shorter than in the
other years, total shoot length in flush one was greater due to the
large number of shoots.
There was no uniform effect of irrigation treatment on average
shoot length or leaf size. This seems in contrast to the
well-established effect of decreased organ expansion due to water
deficits (Hsiao, 1973). However, Hanson and Hitz (1982) state that
under conditions of extreme diurnal variation in leaf water status,
expansion may be inhibited during the day, but long-term growth may
not be affected in some plants. There was little contribution of
soil water deficit to the pronounced midday decreases in xylem
potential during the early summer in this study, but a considerable
decrease in CO^ assimilation occurred with increased soil water
deficit (Chapter V). In contrast to the lack of effect on average
shoot length, treatments more consistently affected the number of

56
Table 3-7. Shoot number, average shoot length, and total shoot
length for three growth flushes of young 'Hamlin' orange trees
as related to irrigation based on soil water depletion, 1987.
Soil water
depletion
(%)
Shoot
z
no.
Shoot
length
(cm)Z
Total
length
(cm)Z
Flush one (4/12 - 5/6)^
20 (High)
77
4.9
376.4
45 (Mod.)
63
4.0
254.8
65 (Low)
64
4.2
264.5
Mean
68
4.4
297.4
Flush two (6/18 - 8/2)
20 (High)
22
16.2
362.4
45 (Mod.)
21
12.3
259.1
65 (Low)
20
11.0
216.8
Mean
21
13.1
277.4
Flush three (8/9 - 11/2)
20 (High)
15
32.4
486.6
45 (Mod.)
6
32.1
192.4
65 (Low)
6
27.4
153.5
Mean
9
30.6
271.7
SEX
6
1.5
ns
2
Means of 5 trees/treatment.
^Range in dates of initiation.
XSE for comparison of means among growth flushes. Comparison
among irrigation levels within growth flushes is inappropriate
since irrigation x flush interaction is not significant.

57
shoots that initiated growth during the later flushes of the season.
Perhaps the level of carbon reserves at the time of flush initiation
affects the number of shoots that grow.
Average shoot length consistently increased with the three
successive flushes during all years of the irrigation scheduling
experiments, and shoot number generally decreased from flush one to
flush three. This is consistent with previous studies with citrus
growth and development in both subtropical and arid regions
(Krishnamurthi et al., 1960; Mendel, 1969).
Final Measurements
Final canopy size was not different for trees in the high and
moderate treatments in 1985 or 1986. The low treatment, however,
significantly decreased canopy volume (P<.0003), trunk cross
sectional area (P<.0001), dry weight (P<.0043), total shoot length
(P<.0040), and leaf area (P<.0029) when compared to the high
treatment in 1985 (Table 3-8). Similarly, in 1986 trees in the low
treatment had significantly less canopy volume (P<.0023), trunk
cross-sectional area (P<.0017), dry weight (P<.0086), and leaf area
(P<.0217) than in the high treatment (Table 3-8).
Irrigation treatment effects on canopy growth followed a
different pattern in 1987. Both the moderate and low treatments
reduced tree growth measured as trunk cross sectional area
(P<.0029), canopy dry weight (P<.0050), shoot length (P<.0257), and
leaf area (P<.0112) when compared to the high treatment (Table 3-8).
In addition, final canopy size for all treatments was generally
smaller in 1987 when compared to the other years. This difference
in growth response may be due to the nursery trees used in 1987. As

58
Table 3-8. Canopy volume, dry weight, shoot length, leaf area
and trunk cross-sectional area of young ’Hamlin' orange trees
as related to irrigation based on soil water depletion.
Trunk
Soil
cross
Canopy
water
Canopy
sectional
dry
Shoot
Leaf
depletion
volume
area
wt.
length
area
(%)
, 3v
(m )
(cm )
(g)
(cm)
(m2)
1985
20 (High)
0.57
5.1
424.6
950.8
1.3
45 (Mod.)
0.52
4.9
425.8
950.9
1.2
65 (Low)
**
0.33
**
4.2
**
336.6
**
752.4
**
1.0
1986
20 (High)
0.51
8.0
379.2
950.4
1.3
45 (Mod.)
0.54
8.0
383.3
942.8
1.3
65 (Low)
**
0.31
**
6.8
**
300.0
894.3
**
0.9
1987
20 (High)
0.56
4.7
393.2
1428.6
1.4
45 (Mod.)
0.37
*
3.8
**
259.0
937.4*
*
0.7
65 (Low)
0.36
*
3.3
**
229.6
872.2*
*
0.5
* **
’ Response
is significant when compared with
the 20% soil
water
depletion
treatment by
the Williams’
method,
5% and 1%,
respectively.

59
discussed earlier, abscission of most leaves occurred shortly after
transplanting to the field, inducing a flush with more than three
times the number of shoots than occurred in 1985 or 1986. Newly-
emerged shoot growth in citrus is mainly dependent upon stored
reserves (Sinclair, 1984; Van Noort, 1969), and this heavy flush may
have severely depleted the level of reserves in these trees. Webb
(1981) stated that following a heavy defoliation of fir trees,
recovery was related to the level of carbohydrate reserves in the
trees, and trees with a low level of reserves died. Survival was
not a problem in this study, but a severe depletion of reserves
early in the spring may have caused reduced canopy growth in general
in 1987, especially in the moderate and low treatments. Weak trees
typically respond to more frequent irrigation with increased growth.
Seasonal Distribution of Shoot Growth
More than two-thirds of the trees in 1985 initiated their first
growth flush the last week in May, and all had begun growth by 25
June (Fig. 3-7). Initiation of the second flush occurred from 12
July to 19 Aug.. Irrigation treatments did not affect the dates of
initial growth in either of the first two flushes. Initiation of
the third flush occurred over a longer time period than for flushes
one and two. The entire population of trees receiving the high
irrigation treatment had begun growth of flush three in October.
The percentage of trees initiating this flush in the moderate and
low treatments was shifted to later in the season, and approximately
40% of the trees in the low treatment did not initiate a third
flush. The shapes of curves in flush three differed significantly
among treatments

60
J J A S O N
Time (months)
Fig. 3-7. Cumulative percentage of trees in three irrigation
treatments growing over the 1985 season. ( MMN ) â–  20% soil
water depletion (SWD), ( mm ) ■ 45% SWD, ( •#••• ) ■ 65% SWD.
ns,** indicates nonsignificance or significant at the 1% level,
respectively, according to analysis of covariance test of
homogeneity of the three equations.

61
Similarly, initiation of the first growth flush in 1986 was not
affected by irrigation treatments, and occurred between 25 May and
17 June (Fig. 3-8). The two subsequent flushes were initiated over
a longer time interval, ranging from 7 July to 13 Sept, in flush two
and 30 Aug. and Dec. in flush three. The shapes of the curves in
both flushes two and three differed significantly among treatments.
Again, the percentage of trees initiating a flush in the moderate
and low treatments shifted growth to later in the season compared to
the high treatment. Approximately 25 and 35% of the trees in the
moderate and low irrigation treatments had not initiated growth of
the third flush by the end of the season.
Initiation of flush one in 1987 occurred earlier than in the
other years due to an earlier planting date (Fig. 3-9). Trees
across all treatments began growth between 12 April and 6 May.
Flushes two and three were again spread over a longer time interval
than flush one. Treatment effects were similar to those in 1986 in
that the shapes of the curves for flushes two and three differed
significantly. All trees initiated a third flush by the end of the
season, in contrast to 1985 and 1986, possibly resulting from the
earlier planting date.
Microsprinkler irrigation of these young trees at 45 and 65%
SWD clearly prolonged the period between initiation of flushes two
and three in some cases when compared to 20% SWD. Similarly, Cooper
et al. (1969) stated that a prolonging of the period of quiescence
between growth flushes of citrus commonly occurs in response to
drought. This delay in shoot growth may result from decreased
levels of available reserves, since a considerable reduction in CO2

62
Tim* (months)
Fig. 3-8. Cumulative percentage of trees in three irrigation
treatments growing over the 1986 season. ( —I ) * 20% soil
water depletion (SWD), ( ) - 45% SWD, ( ••••• ) « 65% SWD.
ns,** indicates nonsignificance or significant at the 1% level,
respectively, according to analysis of covariance test of
homogeneity of the three equations.

63
so
100
Time (days)
1S0
L
200
I
M
Time (months)
Fig. 3-9. Cumulative percentage of trees in three irrigation
treatments growing over the 1987 season. ( ummm ) - 202 soil
water depletion (SWD), ( ) â–  452 SWD, ( eeeee ) - 652 SWD.
ns,*,** Indicates nonsignificance or significant at the 5
and 12 level, respectively, according to analysis of
covariance test of homogeneity of the three equations.

64
assimilation occurred in response to increased soil water deficit
(Chapter V). Perhaps a critical level of available reserves must be
met before subsequent shoot growth begins. The period between
flushes of citrus has been shortened with longer daylengths
(Piringer et al., 1961). Both increased irrigation frequency and
daylength would fit well into a hypothesis of available reserves
controlling the length of time between flushes, as both would
increase CO^ assimilation over time. Shoot length and number of
leaves per shoot are dependent upon available reserves before growth
begins (Van Noort, 1969), and the date of growth may be dependent as
well.
Other factors may be involved with this delay in growth. The
amount of growth inhibitors reportedly decreases during the period
between flushes (Mendel, 1969). It would be of value to understand
the interactions of growth delays caused by less frequent irrigation
with the levels of growth promoters and inhibitors. Knowledge of
the interactions of root growth periodicity would also add
understanding.
Time of shoot growth initiation was uniform from plant to plant
in the spring flush all 3 years and across all irrigation
treatments, but became more widespread in the subsequent flushes.
This pattern has been previously documented with mature citrus trees
in Australia (Sauer, 1951).
Microsprinkler Spray Patterns
2
Distributing irrigation water over 5.9 and 9.2 m by using 90°
and 180° spray patterns during the second growing season did not
result in different trunk cross sectional area or canopy volume

65
(Figs. 3-10, -11). Furthermore, measurements of root distribution
suggested that 90° emitters placed water over the majority of the
citrus tree's root system after one season of growth (Chapter IV).
When beginning with 90° spray patterns, there appears to be no
advantage in changing to a larger pattern after the first season in
the field. By directing more water on tree trunks, 90° patterns are
more efficient than larger patterns for freeze protection purposes
(Rieger et al., 1986). Maintaining 90° patterns for irrigation
purposes more than one season allows their use throughout another
winter for freeze protection.
In summary, growth of young 'Hamlin' orange trees was similar
in 2 out of 3 years with the high (20% SWD) and moderate (45% SWD)
irrigation treatments, but was reduced by the low treatment (65%
SWD). The seasonal amount of water applied to the moderate
treatment averaged about 50% of the amount applied to the high
treatment. There was a pronounced delay in summer and fall growth
flushes and in some cases a reduction in the number of shoots per
tree in the low irrigation treatment.
The optimum level at which irrigations should be scheduled
cannot be precisely determined from these studies, but is most
likely between 20 and 45% SWD. These values are in general
agreement with other reports. Smajstria et al. (1985) obtained
optimum growth of young 'Valencia' orange trees while scheduling
irrigations at 45% SWD in a field lysimeter study. Leyden (1975a)
presented no data, but suggested scheduling basin and strip watering
methods of irrigation on 30% SWD, based on field observations.
Therefore, it appears that on a per tree basis, microsprinkler

TCA (cm
66
Fig. 3-10. Trunk cross sectional area (TCA) and canopy volume
of young 'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern (90° and 180°), 1986. There were no
significant differences between patterns, 5% level.

TCA (cm
67
Fig. 3-11. Trunk cross sectional area (TCA) and canopy volume
of young 'Hamlin' orange trees as influenced by microsprinkler
irrigation spray pattern (90° and 180°), 1987. There were no
significant differences between patterns, 5% level.

68
irrigation at 45% SWD is as effective as at 20% SWD, resulting in a
considerable reduction in water use. However, on a population basis
the final growth flush of some trees will be reduced at the moderate
irrigation level probably because of inherent variability in SWD
among trees within the treatment. Variability in soil moisture
content is a particular problem in flatwoods areas of Florida where
soil type is extremely variable. Consequently, growers must be
certain to monitor soil moisture content carefully in a
representative portion of their grove to ensure that the entire
irrigated area receives adequate water.

CHAPTER IV
MICROSPRINKLER IRRIGATION AND GROWTH OF YOUNG
'HAMLIN' ORANGE TREES. II. ROOT
GROWTH & DISTRIBUTION
Introduction
Perennial tree root systems respond to a variety of
environmental and management factors, including irrigation.
Irrigation method and scheduling directly affect soil water content
and indirectly affect edaphic factors like physical impedence,
fertility, and aeration, thus altering root growth and distribution.
Increases in soil water content generally increase root growth
provided that oxygen or salinity levels are not limiting.
Irrigation has been observed to increase root growth or density of
many crops (Goode et al., 1978; Goode & Hyrycz, 1970; Ponder &
Kenworthy, 1976; Richards & Cockroft, 1975), including citrus
(Bielorai, 1982; Bielorai et al., 1981; Bielorai et al., 1984;
Hilgeman & Sharp, 1970; Rodney et al., 1977). In addition to growth
responses, irrigation also affects root distribution. Frequent
irrigation commonly increases the proportion of roots in shallow
zones (Beukes, 1984; Cahoon et al.; 1961, Goode et al., 1978; Goode
& Hyrcyz, 1964; Hilgeman et al., 1969; Hilgeman & Sharp, 1970; Layne
et al., 1986; Levin et al., 1980).
69

70
Increasing demands on water resources require that irrigation
be used more efficiently. Micro irrigation systems are becoming
widely used in Florida, however, there is little information
available on irrigation scheduling for young citrus trees under
Florida’s subtropical climate and sandy soil conditions. Therefore,
our objectives were to study the influence of three microsprinkler
irrigation treatments on root growth and distribution of young
citrus trees.
Materials and Methods
Three field experiments were conducted at the Horticultural
Research Unit near Gainesville, FL, with site and environmental
variables as described In Chapter III. Microsprinkler irrigation
was scheduled based on available soil water depletion (SWD) in the
root zone. Treatments were defined as high (20% SWD), moderate (45%
SWD), and low (65% SWD).
Root Growth and Lateral Distribution, Excavated Root Systems
Twenty, 21, and five root systems per treatment were excavated
by hand in 1985, 1986, and 1987, respectively. Excavation procedure
was described in Chapter III. Included in this group were samples
of 10 (1985, 1986) and five (1987) randomly selected plants per
treatment in which the root system was dyed prior to planting.
Dyeing was accomplished by dipping roots for 15 sec in a 1% solution
of safranin-0 as described by Kaufmann (1968). Root dry weights
were determined following oven-drying for 3 days at 80° C. Roots
which had developed during the experimental period were easily
distinguished from initial dyed roots, and were oven-dried and
weighed separately. Random samples of 10 (1985, 1986) and five

71
(1987) plants per treatment were utilized to determine the increase
in root volume. Water displaced by submersing root systems into
water-filled containers was collected to determine root volume at
planting time and after excavation.
Samples of 10 (1985) and five (1986, 1987) randomly selected,
excavated root systems per treatment were used to determine lateral
root distribution. After excavation these root systems were
separated into three concentric zones (0-40, 40-80, and >80 cm) from
the trunk. Distribution of total and fibrous (<_ 1.5 mm) roots was
determined on a dry weight and percentage basis.
Circular Trench Profiles
A modification of the trench profile method (Atkinson, 1980)
for root distribution studies was used to determine irrigation
effects on vertical root distribution at two distances from tree
trunks. The method was modified by Huguet (1973) by using a
curvilinear instead of a straight trench, based on the assumption
that roots radiate concentrically from trunks. Based on the same
assumption, circular trenches were dug by hand around each of three
randomly selected trees per treatment in Dec. 1987. The initial
trench was excavated 80 cm from the trunk to a depth of 50 cm. The
profile was smoothed with a spade, then ca. 2 cm of soil was removed
by hand with the aid of a brush to expose roots. It was not
difficult to remove a 2 cm layer of soil from the circular profile
walls due to the loose, sandy nature of the soil (Kanapaha sand).
Roots were counted at each of four depth increments (0-10, 10-20,
20-30, and >30 cm) and categorized on the basis of total root
number, and the number of fibrous (<^ 1.5 mm) and non-fibrous (> 1.5

72
mm) roots. Vertical distribution was determined on a concentration
2
(roots/m ) and percentage basis.
Roots were counted to allow calculation of root length density
immediately after recording root numbers. Lengths of exposed roots
were estimated by visually counting the number of 2-cm sections of
exposed root length as described previously (Bohm, 1976) in each
depth increment. Roots that were perpendicular to the profile wall
in situ extended ca. 2 cm, and were assigned a value of one, while
roots longer than 2 cm were given values greater than one. For
example, roots which were 3, 4, and 5 cm long were given values of
1.5, 2.0, and 2.5, respectively. Summation of these values
estimated the number of 2-cm increments of exposed roots. Root
-3
length density (mm dm ) was determined for each depth by
calculating total root length (multiplying the sum of all values by
2 cm) and volume of the 2 cm layer of soil on each profile wall. A
second circular profile wall was subsequently exposed on the same
tree at 40 cm from the trunk and the process repeated.
Analysis of Data
Data from excavated root systems were analyzed separately for
the 3 years. Data on root dry weights were subjected to analysis of
variance and data on root volumes to analysis of covariance to
standardize initial root volumes. Where measurements were
significantly different among irrigation treatments, Williams'
(Williams, 1971) test was used to compare means.
Lateral distribution of total and fibrous (<_ 1.5 mm) root dry
weights were analyzed separately by a split-plot analysis with
irrigation treatments as main plots and lateral zones as subplots.

73
Root percentage distribution among zones was analyzed in the same
manner following arc sine transformation.
Measurements from profiles in 1987 of root density and root
concentration of fibrous, non-fibrous, and total roots were analyzed
separately. Percentage distribution data were transformed using an
arc sine transformation prior to analysis. All data sets were
analyzed by a split-split plot analysis with irrigation treatments
as main plots, distance from the trunk as sub-plots, and depth
increment as sub-sub-plots.
Results and Discussion
Root Weight and Volume Measurements
Irrigation treatments had variable effects on root dry weight
and volume from year to year (Table 4-1). Dry weights were not
affected in 1985, but root volume was decreased significantly at the
low compared to the high irrigation treatment (P<.0083). Irrigation
levels significantly affected root growth in 1986 and 1987. Trees
receiving the low irrigation treatment in 1986 had less total root
dry weight (P<.0052), new root dry weight (P<.0249), and root volume
(P<.0116) than those in the high treatment. Both moderate and low
treatments in 1987 decreased total root dry weight (P<.0170), new
root dry weight (P<.0455), and root volume (P<.0345) compared to the
high treatment.
3
Root volume ranged from 850 to 1190 cm in 1985 and 1986, and
3
from 550 to 860 cm in 1987 (Table 4-1). Root dry weight ranged
from 260 to 350 g in 1985 and 1986, and from 160 to 260 g in 1987.
These values were in close agreement with those of Bevington (1983)
and Bevington and Castle (1982) for 13-month-old 'Valencia' orange

74
Table 4-1. Total root dry weight, dry weight of new roots,
and root volume of young 'Hamlin' orange/sour orange
trees as related to irrigation based on soil water
depletion.
Soil water
depletion
(%)
Total root
dry wt
(g)
New root
dry wt
(g)
Root
volume
(cm )
1985
(n=20)
20 (High)
348.7
156.3
1189.2
45 (Mod.)
333.9
158.8
1058.4
65 (Low)
297.3
139.4
920.5
1986
(n=21)
20 (High)
342.1
191.0
1180.9
45 (Mod.)
337.8
185.4
1396.5
*
*
65 (Low)
262.1
128.2
854.3
1987 (n=5)
20 (High)
257.8
126.2
858.2
*
*
45 (Mod.)
169.4
84.8
559.7
*
*
65 (Low)
159.6
81.2
554.0
Response is significant when compared with the 20%
SWD treatment by the Williams' method; 5% and 1%
levels, respectively.

75
on 'Carrizo' citrange or rough lemon trees grown in root observation
chambers. Tree age in this study was about 7 months in 1985 and
1986, and 8 months from planting in 1987. The comparable root sizes
in the field and root chamber studies despite the much shorter
growing period in the field study may be due to rootstock
differences or to the allowance of unrestricted root development
under the field conditions.
Shoot:root ratio was calculated using dry weight measurements
reported in Chapter III (Table 3-8). The SWD ranges used in this
study did not significantly influence shootrroot ratio, which ranged
from 1.16 to 1.54, with a mean of 1.30. These values are lower than
those reported for 1 1/2-year-old mandarin seedlings in India, where
healthy trees had a shoot:root ratio of 1.92 (Aiyappa & Srivastava,
1965). Comparatively, excavated mature citrus trees in California
had a ratio of about 3.5 (Cameron, 1939; Cameron & Compton, 1945),
and in Florida a ratio of about 2.2 (Castle, 1978).
Root growth was less in 1987 compared to the other years
despite a longer growing season. Furthermore, trees under moderate
irrigation grew similarly to those in the high treatment in 1985 and
1986, but had less root growth than those in the high treatment in
1987. These discrepancies may be due to the nursery trees used in
1987. Excessive leaf abscission occurred shortly after
transplanting trees to the field in 1987, inducing a growth flush
with a large number of shoots per tree (Chapter III, Table 3-7).
Such a growth flush early in the spring may have depleted available
reserves thus decreasing subsequent root growth.

76
In all 3 years root growth was decreased at the lowest soil
moisture content. Similarly, decreased root growth in response to
decreased soil moisture has been frequently reported for citrus
trees of various ages (Bevington & Castle, 1985; Bielorai, 1982;
Bielorai et al., 1981; Hilgeman & Sharp, 1970; Rodney et al., 1977).
Lateral Root Distribution
Lateral distribution of roots on a dry weight basis was
affected by irrigation treatments. The irrigation treatment x
lateral zone interaction for total root weight was significant in
1985 (PC.0238), 1986 (PC.0405), and 1987 (PC.0141). A greater
quantity of roots within 40 cm of the trunk occurred in response to
the high treatment when compared to moderate and low treatments
(Tables 4-2, -3, -4). Root weight in the 40-80 and >80 cm zones was
not affected by irrigation, but in all 3 years the high treatment
produced more roots than other treatments in the 0-40 cm zone. In
contrast, irrigation treatments did not affect lateral distribution
of fibrous (Cl.5 mm) root dry weight or distribution of both fibrous
and total roots on a percentage basis.
Across all irrigation treatments and years root dry weights and
percentages decreased significantly (PC.0001) with increased
distance from trunk (Tables 4-2, -3, -4). One exception occurred in
1986, when fibrous root weight was not different among the lateral
zones (Table 4-3). The percentage of fibrous roots within 40 cm of
the trunk ranged from 40 to 60%, between 40 and 80 cm from 26 to
39%, and >80 cm from 13 to 29%. A much larger percentage of total
root weight was located close to the tree, with 68 to 84%, 11 to

Table 4-2. Lateral dry weight and percentage distribution of fibrous (jG.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1985.
Distance
Irrigation treatment
from
20% SWD
45% SWD
65% SWD
trunk
High
Mod
Low
Mean
(cm)
(g) (%)
(g) (%)
(g) (%)
(g) (%)
SEZ
0-40
47.9
61.3
46.6
Fibrous
53.0 44.8
56.5
46.4
56.7
3.6
40-80
20.1
25.7
26.0
29.5
22.3
28.1
22.8
27.9
3.6
>80
10.2
13.0
15.4
17.5
12.2
15.4
12.6
15.4
3.6
Total
SEy
78.2
88.0
——
79.3
__ _
81.8
5.9
7.0
0-40
292.0
83.9
257.3
Total
80.0
roots
256.3
82.0
269.0
82.0
10.4
40-80
40.0
11.5
44.9
13.9
40.6
13.0
41.9
12.8
10.4
>80
16.0
4.6
19.7
6.1
15.3
5.0
17.1
5.2
10.4
Total
SEy
348.0
10.0
_
321.9
10.0
312.2
10.0
328.0
7.6
2
SE for comparison of irrigation treatment means within the same zone.
ySE for comparison of zone means within columns. Where no SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.

Table 4-3. Lateral dry weight and percentage distribution of fibrous (_<1.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1986.
Distance
Irrigation treatment
from
20% SWD
45% SWD
65% SWD
trunk
High
Mod
Low
Mean
(cm)
(g) (%)
(g) (%)
(g) (%)
(g) (%)
SEZ
0-40
40.2
40.3
32.4
Fibrous
40.3 27.0
40.1
33.2
40.2
7.9
40-80
31.2
30.7
26.6
33.1
27.4
38.8
28.4
34.2
7.9
>80
30.2
29.0
21.4
26.6
16.0
21.1
22.5
25.6
7.9
Total
SEy
101.6
80.4
__
70.4
84.1
5.6
5.9
0-40
239.8
68.3
189.2
Total
70.7
roots
163.0
69.2
197.3
68.6
20.0
40-80
72.6
20.1
53.8
19.1
51.0
21.6
59.1
20.6
20.0
>80
44.0
11.6
27.4
10.2
21.6
9.2
31.0
10.8
20.0
Total
SE7
356.4
13.1
__
270.4
13.1
__
235.6
13.1
__
287.4
4.7
2
SE for comparison of irrigation treatment means within the same zone.
ySE for comparison of zone means within columns. Where no SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.

Table 4-4. Lateral dry weight and percentage distribution of fibrous (<1.5 mm)
and total root systems of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD), 1987.
Distance
Irrigation treatment
from
trunk
(cm)
20% SWD
High
45% SWD
Mod
65% SWD
Low
Mean
SEZ
(g)
(%)
(g)
(%)
(g)
(%)
(g)
(%)
Fibrous
0-40
32.0
46.6
25.2
49.0
23.0
50.7
26.7
48.5
4.8
40-80
26.0
37.7
19.4
38.1
14.6
32.1
20.0
36.4
4.8
>80
10.4
15.7
6.8
12.9
7.8
17.2
8.3
15.1
4.8
Total
68.4
51.4
45.4
55.0
SEy
—
—
—
—
—
—
7.8
13.9
Total
roots
0-40
197.0
76.4
132.6
78.3
124.8
78.0
151.5
77.5
15.3
40-80
44.0
17.1
28.2
16.6
26.0
16.4
32.7
16.7
15.3
>80
16.8
6.5
8.6
5.1
8.8
5.6
11.4
5.8
15.3
Total
257.8
169.4
156.4
195.6
SEy
13.5
13.5
13.5
9.1
ZSE for
comparison of
irrigation
treatment
means
within the
same zone.
ySE for
comparison of
zone means
within columns.
Where no
SE is given,
comparison among zone means is inappropriate since irrigation x zone
interaction is not significant.

80
21%, and 4 to 11% of root dry weight in the three concentric lateral
zones, respectively.
The decrease in root growth with increasing distance from the
trunk has been reported for citrus trees of all ages (Aiyappa &
Srivastava, 1965; Bielorai, 1977, 1982; Bielorai et al., 1981;
Cahoon et al., 1964; Minessy et al., 1971; Yagev & Choresh, 1974).
Healthy 1 1/2-year-old mandarin trees in India had 60.0, 14.7, 13.2,
and 12.1% of their root dry weight in 0-30, 30-60, 60-90, and >90 cm
concentric zones, respectively (Aiyappa & Srivastava, 1965). These
values are similar to those from the young 'Hamlin' orange trees in
this study.
Maximum distance of lateral root spread pooled over the 3
seasons averaged 137 cm for the high treatment, compared with 127
for moderate and 121 cm for the low treatments, respectively (data
not shown). In comparison, 2-year-old 'Valencia' orange trees in
Australia had a lateral root spread of about 150 cm (Till & Cox,
1965), and 1 1/2-year-old mandarin trees in India had a spread of
160 cm (Aiyappa & Srivastava, 1965). Savage et al. (1945) observed
that 6-year-old trees budded on common rootstocks and spaced 30 x 90
cm in a Florida nursery had lateral root spreads up to 150 cm.
There is considerable discussion about the optimum irrigation
pattern to use for young citrus trees in Florida. Roots covered an
2
area of about 5.2 m , with 64% of this area covered by the 90°
emitters in this study. Furthermore, 96% of the roots on a dry
weight basis were within the wetted zones due to the high
concentration of roots within 80 cm of the trunk (Tables 4-2, -3,
-4). These data imply that 90° microsprinkler emitters cover the

81
majority of the young tree's roots after one season of growth. Tree
growth comparisons utilizing 90° and 180° emitters during the second
season of growth also suggest 90° emitters are adequate for up to 2
years in the field (Figs. 3-10, -11).
Weight of fibrous roots accounted for 27% of the total. This
value is similar to those found on root systems of rough lemon and
'Carrizo' citrange in the nursery (Bevington & Castle, 1982), but
considerably less than found on healthy 1 1/2-year-old mandarin
seedlings (37.5%) growing in India (Aiyappa & Srivastava, 1965). In
comparison, feeder roots on 8- to 9-year-old orange trees on sweet
orange rootstock accounted for less than 15% of the root system dry
weight (Cameron & Compton, 1945).
Circular Trench Profiles
Analysis of variance results showing effects of irrigation
treatments, distance from tree trunks, and depth increments on root
-3 -2
density (mm dm ) and concentration (roots m ) of total, fibrous,
and non-fibrous roots are summarized in Table 4-5. A larger
proportional increase in total root concentration occurred at 40
than at 80 cm from the trunk in response to the high treatment
(Table 4-6). This corroborates results of irrigation treatment
effects on lateral root dry weight distribution (Tables 4-2, -3,
-4). The high irrigation treatment produced a proportionally larger
increase in root number in the 10-20 cm layer than in the other
layers (Table 4-6). The greatest number of roots grew in this
layer, regardless of irrigation treatment and there was little root
growth in the 0-10 cm depth regardless of treatment, and trees under
the high treatment had the highest root concentration at all depths

82
Table 4-5. Partitioning of variance into main and interaction
effects for 1987 circular trench profile root growth variables of
'Hamlin' orange trees as described in Tables 4-6, 4-7, and 4-8.
F tests on variance (MS) ratios
Total
Fibrous
Non-fibrous
Total
root
root
root
root
Source
df
cone.
cone.
cone.
density
Irrigation (I)
2
*
5.39
6.05*
1.ions
*
5.73
Distance (Di)
1
***
55.40
***
54.88
***
44.03
***
55.60
I x Di
2
5.90*
6.40*
1.55nS
6.10*
Depth (De)
3
***
60.12
***
60.08
***
38.06
***
58.58
I x De
6
**
5.76
**
6.11
1.7lnS
5.32
Di x De
3
***
21.61
***
21.35
***
15.79
***
19.91
I x Di x De
6
2.27ns
2.43ns
0.92nS
2.2ins
* * * Nonsignificant or significant F values at the 5Z,
1%, and .1Z levels, respectively.

83
Table 4-6. Total root concentration at four depths and two distances
from trunk of young 'Hamlin' orange/sour orange trees as influenced
by irrigation based on soil water depletion (SWD) as determined
using circular trench profiles, 1987.
Irrigation treatment
Depth
(cm)
20% SWD
HiSh
45% SWD
Mod.
65% SWD
Low
Mean
Root cone, (roots m
2) at 40 cmZ
0-10
19.9
37.2
11.9
23.0
10-20
965.5
655.2
299.7
640.1
20-30
447.0
244.0
202.9
298.0
>30
291.8
156.5
70.3
172.9
Mean
431.1
273.2
146.2
283.5y
Root cone, (roots m
at 80 cmZ
0-10
13.9
6.6
0
6.8
10-20
215.5
222.8
45.1
161.1
20-30
159.1
172.4
87.5
139.7
>30
65.7
24.6
43.1
44.5
Mean
113.6
106.6
43.9
88.0y
zn=3 for irrigation x distance x depth means.
yMean for distance from trunk.

84
below 10 cm. This increase in root concentration in response to
maintaining higher soil moisture levels corroborates the dry weight
measurements made on excavated root systems (Table 4-1).
The concentration of fibrous roots ( similar to total root concentration in regard to irrigation
treatments, distance from trunk, and depth because fibrous roots
comprised the majority of the total roots (Table 4-7). Differences
in non-fibrous (>1.5 mm) root concentration were not associated with
irrigation treatments. There were four times as many large roots at
40 cm from tree trunks than 80 cm, and almost half were located
between 10 and 20 cm in depth (Table 4-8). Trees receiving the low
irrigation treatment had 10 times as many fibrous as large roots;
whereas, trees in the high and moderate treatments developed
approximately 15 times the number of fibrous as large roots.
The percentages presented in Table 4-9 were pooled over the two
lateral distances since they did not interact with percentage
distribution with depth. Greater than 50% of the root concentration
was between 0-20 cm with high and moderate irrigation, but was
deeper than 20 cm with low irrigation. Because of the small number
of replications and the variability, this difference was not enough
to be significant. Across all treatments, percentages of roots were
significantly (P<.0001) different with depth (Table 4-9). Roughly
half of the roots were 10-20 cm deep, with 35% between 20-30 cm, and
less than 15% below 30 cm. More than 85% of the roots were located
between 0-30 cm in depth.
The concentration of roots in the top 30 cm after 1 season of
growth suggests that when soil moisture monitoring is used for

85
Table 4-7. Concentration of fibrous (<1.5 mm) roots at four depths
and two distances from trunk of young 'Hamlin’ orange/sour orange
trees as influenced by irrigation based on soil water depletion
(SWD) as determined using circular trench profiles, 1987.
Depth
(cm)
Irrigation i
treatment
Mean
20% SWD
High
45% SWD
Mod.
65% SWD
Low
Root
cone, (roots
-2 z
m ) at 40 cm
0-10
19.9
33.2
11.9
21.7
10-20
903.2
614.1
263.9
593.7
20-30
415.1
228.1
181.7
275.0
>30
279.9
149.9
65.0
164.9
Mean
404.5
256.3
130.6
263.8:
Root
cone, (roots
—2 z
m ) at 80 cm
0-10
13.3
6.0
0
6.4
10-20
199.6
211.5
43.1
151.4
20-30
150.5
157.8
84.9
131.1
>30
63.0
23.9
41.8
42.9
Mean
106.6
99.8
42.5
83.0:
2
n=*3 for irrigation x distance x depth means
^Mean for distance from trunk.

86
Table 4-8. Concentration of non-fibrous (>1.5 mm) roots at four
depths and two distances from trunk of young 'Hamlin' orange/sour
orange trees as influenced by irrigation based on soil water
depletion (SWD) as determined using circular trench profiles, 1987.
Irrigation treatment
Depth
20% SWD
45% SWD
65% SWD
(cm)
High
Mod.
Low
Mean
Root
_2
cone, (roots m )
at 40 cmZ
0-10
0
4.0
0
1.3
10-20
62.3
41.1
35.8
46.4
20-30
31.8
15.9
21.2
23.0
>30
11.9
6.7
5.3
8.0
Mean
26.5
16.9
15.6
19.7y
Root
-2
cone, (roots m )
at 80 era2
0-10
0.7
0.7
0
0.5
10-20
15.9
11.3
2.0
9.7
20-30
8.6
14.6
2.7
8.6
>30
2.7
0.7
1.3
1.6
Mean
7.0
6.8
1.5
5.1
zn=3 for irrigation x distance x depth means
yMean for distance from trunk.

87
Table 4-9. Percentage distribution of root concentration with depth
of young 'Hamlin' orange/sour orange trees as influenced by
irrigation based on soil water depletion (SWD). Percentages are
means of measurements from 40 and 80 cm distances from trunk, 1987.
Irrigation treatment
Depth
(cm)
20% SWD
High
45% SWD
Mod.
65% SWD
Low
Mean
<1.5 mm
0-10
1.4
2.6
1.6
1.8
10-20
47.9
57.2
38.0
47.7
20-30
34.1
30.6
41.4
35.4
>30
16.6
9.6
19.0
15.1
>1.5 mm
0-10
0.6
3.1
0
1.2
10-20
50.8
48.4
45.2
48.2
20-30
36.3
38.9
38.5
37.9
>30
12.3
9.6
16.3
12.7
Total
0-10
1.3
2.7
1.4
1.8
10-20
48.1
56.5
38.4
47.7
20-30
34.2
31.3
41.6
35.7
>30
16.4
9.5
18.6
14.8

88
irrigation scheduling, measurements should be concentrated in this
zone. Furthermore, irrigation times should be limited to replenish
soil moisture only in these shallow zones. Irrigations of short
duration are adequate when using systems which direct applications
to a small areas on sandy soils. Soil moisture was monitored at a
depth of 30 cm in this study, and at 20% SWD the 90°, 38 liter/hr
emitters required approximately 1 hr to replenish the root zone to
field capacity.
-3
Root length density (mm dm ) at two distances from tree trunks
and four soil depths as influenced by irrigation treatment followed
patterns similar to root concentrations. The greatest difference
between treatments occurred between 10-20 cm, with differences of
less magnitude deeper in the profile (Table 4-10). Densities ranged
-3
from 0.5 mm dm at 80 cm from the trunk on trees under the low
-3
irrigation treatment to 9.8 mm dm at 40 cm from the trunk on trees
under the high irrigation treatment.
Mature citrus responded to frequent irrigation with an
increased proportion of roots in shallow root zones and a decrease
in root growth in deep root zones using flood (Cahoon et al., 1961;
Cahoon et al., 1964; Hilgeman & Sharp, 1970) and drip (Ruggiero &
Andiloro, 1984) irrigation systems. Results from this study
indicated that irrigating more often in the high treatment had no
effect on root growth in the 0-10 cm depth, and increased the
proportion of root growth in the 10-20 cm depth compared to deeper
zones. However, in contrast to the previous studies, there was no
decrease in root concentration or length density in the deeper zones
with the frequent irrigation. These results are similar to those

89
Table 4-10. Root length density at four depths and two distances
from trunk of young 'Hamlin' orange/sour orange trees as related
to irrigation based on soil water depletion (SWD) as determined
using circular trench profiles, 1987.
Irrigation treatment
Depth 20% SWD 45% SWD 65% SWD
(cm)
High
Mod.
Low
Mean
-3
Density (mm dm )
2
at 40 cm
0-10
0.2
0.4
0.1
0.2
10-20
9.8
6.7
3.1
6.5
20-30
4.9
2.7
2.2
3.3
>30
3.5
1.9
0.9
2.1
Mean
4.6
2.9
1.6
3.0'
_3
Density (mm dm )
at 80 cmZ
0-10
0.1
0.1
0
0.1
10-20
2.2
2.3
0.6
1.7
20-30
1.8
1.9
1.0
1.5
>30
0.8
0.3
0.5
0.5
Mean
1.2
1.2
0.5
1.0'
zn*3 for irrigation x distance x depth means
^Mean for distance from trunk.

90
found when mature citrus trees were irrigated at different rates,
scheduled at the same frequency (Bielorai et al., 1981; Bielorai et
al., 1984). Differences in soil characteristics, amount of
rainfall, and other growing conditions certainly interact with tree
root system responses to irrigation.
Observations on the spatial distribution of roots along the
exposed circular profile walls indicated that partial sampling
techniques such as partial excavation or core sampling could have
led to inaccurate results if they were used. Variability within the
volume of soil encompassing a root system is a widespread problem
for root investigators (Atkinson, 1980). The use of total
excavation and circular trench profiles helped minimize this
variability in our measurements.
In summary, the high irrigation treatment increased root growth
and proportionally increased growth near the trunk, based on root
dry weight, concentration, and length density. There was little
root growth between 0-10 cm in depth regardless of irrigation
treatment. The greatest response occurred between 10-20 cm in
depth, where the high treatment increased root number and length
when compared to the moderate and low treatments, but root growth
was increased even in deeper root zones by the high treatment. More
than 85% of the root growth was in the top 30 cm of soil,
concentrated almost entirely between 10 and 30 cm. The 90°
microsprinkler emitters covered 64% of the maximum root lateral
spread, but greater than 90% of the roots (dry weight basis) were
within the wetted zones.

91
Knowledge of the root distribution of young citrus trees is
useful in management decisions concerning microsprinkler irrigation
patterns and times, and placement of sensors for monitoring soil
moisture depletion. High rainfall during the summer months in
Florida moderates plant responses to field irrigation treatments and
may account for any contrast between some of our findings and root
distribution data from other areas.

CHAPTER V
SOIL MOISTURE STRESS AND LEAF GAS EXCHANGE
OF YOUNG, FIELD-GROWN 'HAMLIN' ORANGE TREES
Introduction
Transpiration, stomatal conductance, and CO^ assimilation of
citrus trees are affected by soil moisture content. Decreases in
soil moisture reduced CO2 assimilation of young, containerized
citrus trees under controlled conditions (Bielorai & Mendel, 1969;
Brakke et al., 1986; Kriedemann, 1968, 1971; Ono & Hirose, 1984;
Thompson et al., 1968) and transpiration or stomatal conductance of
various-aged citrus trees under field and controlled conditions
(Bielorai & Mendel, 1969; Brakke et al., 1986; Cohen & Cohen, 1983;
Hilgeman, 1977; Kaufmann & Levy, 1976; Koo, 1953; Kriedemann, 1971;
Ruggiero & Andiloro, 1985; Thompson et al., 1968; Zekri, 1984).
There are few studies, however, concerning soil moisture effects on
CO^ assimilation of citrus under field conditions.
Soil water deficits may influence gas exchange processes in a
number of ways. Stomatal closure in response to limited water
availability may arise from a decrease in leaf water status or from
non-hydraulic signals from roots (Blackman & Davies, 1985 a,b).
Reduced stomatal conductance in turn may limit transpiration and CC>2
assimilation. In addition, C09 assimilation may be reduced by soil
92

93
water deficits via direct reduction of dark and light reactions or
residual conductance to CC^*
This work was undertaken to provide information on the effect
of soil water deficit on CO2 assimilation, transpiration, stomatal
conductance, xylem potential, and leaf temperature of young 'Hamlin'
orange trees under field conditions in Florida.
Materials and Methods
Plant Material and Treatments
Commercially obtained, bare-rooted 'Hamlin' orange [Citrus
sinensis L. (Osb.)] on sour orange ((?. aurantium L.) trees were
planted on beds (16.75 m width x 0.60-0.75 m height x 85 m length)
at the Horticultural Unit northwest of Gainesville, Florida. Two
tree rows 7.6 m apart were used per bed with trees spaced 4 m within
rows. Site characteristics were as described in Chapter III. Soil
water content was maintained between field capacity and 20% soil
water depletion (SWD) of available soil water for high irrigation
treatment, 45% SWD for the moderate treatment, and 65% SWD for the
low treatment.
Leaf Measurements
The diurnal pattern of leaf gas exchange was determined on 18
October 1986 on trees planted 3-4 May 1986. Gas exchange fluxes are
dependent on leaf age in citrus (Erickson, 1968; Kriedemann, 1971;
Ono & Hirose, 1984; Syvertsen, 1982; Syvertsen et al., 1981), thus
only shoots initiated during the week of 13-20 Sept, were chosen,
which standardized leaf age. Preliminary data showed little
variation in gas exchange measurements within an irrigation
treatment both within trees and between trees. As a result, leaves

94
from the middle of two shoots on each of four trees per irrigation
treatment were used, which assured completion of all measurements
for a given time period within 30 min. Net assimilation (A) was
monitored with an Analytical Development Corporation (Hoddesdon,
England) portable, open-system infrared analyzer (Model LCA-2)
equipped with a Parkinson broadleaf leaf chamber (aperture = 6.25
2
cm ). Outside air was supplied to the chamber at a flow rate of 400
3
cm /min, and A was calculated as described previously (Jarvis &
Catsky, 1971). After all CO2 assimilation measurements were
completed, transpiration (E) and stomatal conductance (gg) were
determined on the same leaves using a LI-COR 1600 steady-state
diffusion porometer.
The diurnal pattern of leaf temperature, gas exchange, and
internal CO2 concentration (C^) was determined on 12, 13, and 15
June 1987 on fully-expanded spring flush leaves of trees planted 2
April 1987. Sample size was the same as in 1986, and shoots
initiating growth during the week of 23-30 April were chosen. Net
CO2 assimilation, E, gg, C^, and leaf temperature were monitored
with the Analytical Development Corporation instrumentation fitted
with a data processor. All adjustments and calculations on
measurements were made by the data processor as described previously
(Jarvis & Catsky, 1971). Instantaneous water use efficiency (WUE)
was calculated as A/E.
Leaf xylem potential was determined on 15 June 1987 using the
Scholander pressure chamber technique (Scholander et al., 1965).
One leaf from each of eight plants per treatment (based on variation

95
in preliminary measurements) was excised and placed immediately in
the chamber for measurement.
Means and standard errors were calculated for trees in each
irrigation treatment throughout photoperiods.
Environmental Measurements
Photosynthetic photon flux (PPF) and air temperature were
determined with the Analytical Development Corporation
instrumentation. Leaf to air temperature difference during June
1987 measurements was calculated by difference. Relative humidity
was monitored with a hygrothermograph (WEATHERtronics Model 5021)
located on the experimental site and vapor pressure deficit (VPD)
was calculated from relative humidity and air temperature. Soil
temperature at a 10 cm depth was monitored with a mercury
thermometer. Mean midday level of soil water depletion for trees in
each treatment was measured using the neutron scattering method.
Results and Discussion
Photosynthetic photon flux gradually increased to a peak of ca.
-1 -2
1500 pmol 8 m around 1200 hr on 18 Oct. 1986, and gradually
decreased thereafter (Fig. 5-la). Early morning air temperature was
approximately 15° C, and a broad midday maximum of 25° C occurred
before declining after 1700 hr (Fig. 5-lb). Relative humidity
declined to a midday minimum below 50Z and VPD increased to 2 kPa
(Fig. 5-1 c,d). Soil temperature was 22° C initially and increased
slowly to 24° C by 1400 hr (data not shown). Trees in the high
treatment were irrigated on 17 Oct., and the soil was at field
capacity on 18 Oct. in this treatment. The average midday SWD was

96
Tin» (EST)
Fig. 5-1. Diurnal cycle of photosynthetic photon flux (PPF),
air temperature, relative humidity (RH), and vapor pressure
deficit (VPD) on 18 Oct. 1986.

97
Time (ESTJ
Fig. 5-2. Diurnal cycle of leaf CC^ assimilation (A),
transpiration (E), and stomatal conductance (gg) of young
'Hamlin* orange trees on 18 Oct. 1986 as influenced by soil
water depletion (SWD). (A) * high treatment at field
capacity, (0) â–  moderate treatment at 39Z SWD, (â– ) â–  low
treatment at 42Z SWD. Bars represent SE of means, n-8.

98
39 and 42% for the moderate and low treatments, respectively (Table
5-1).
Diurnal patterns of leaf gas exchange on 18 Oct. 1986 were
similar to PPF, temperature, and VPD (Fig. 5-2). Maximum CO^
-1 -2
assimilation was above 9 ymol s m , and maximum E and gg were 8.9
-1 -2
and 275 mmol s m . There was no consistent influence of SWD on
A, E, or gg, and little or no differences occurred between
treatments during this time of cool temperatures (Fig. 5-2, Table
5-1). Diurnal variation of A was similar to that of mature,
field-grown grapefruit on days characterized by mild VPD as
described by Sinclair and Allen (1982). Transpiration of well-
watered containerized sour orange and sweet lime seedlings also
followed patterns similar to those reported here (Bielorai & Mendel,
1969).
Relative humidity averaged 90% in the early morning,
accompanied by 25-26° C air temperature during 12, 13, and 15 June
(Figs. 5-3b,c; -5b,c; -7b,c). Mid-afternoon air temperatures were
above 35° C, accompanied by minimum relative humidity of below 50%.
-1 -2
Photosynthetic photon flux was above 1700 pmol s m by mid¬
morning and remained high throughout the rest of the measurement
period (Figs. 5-3a, -5a, -7a). Vapor pressure deficit remained high
throughout midday, with maximum values of 2.5-3.0 kPa (Figs. 5-3d,
-5d, -7d). Soil temperature was 28-29° C initially and stabilized
at 30-31° C by 1300 hr (data not shown).
Trees in the high irrigation treatment had gone through nine
drying cycles from field capacity to 20% SWD, while trees in the
moderate treatment had gone through four drying cycles from field

99
Table 5-1. Mean and maximum leaf C0? assimilation, transpiration,
and stomatal conductance of young ‘'Hamlin' orange trees on 18 Oct.
1986 as influenced by soil water depletion.
Soil
water
C0„
assimilation
Transpiration
Stomatal
conductance
depletion
(%)
(ymol
✓“N
CM
1
a
1
CO
(mmol
-1 -2,
s m )
(mmol s *
m~2)
max.
me an
max.
mean
max.
mean
oz
9.5
4.5
8.7
4.3
267
151
39
8.0
4.2
9.5
4.9
289
154
42
10.2
4.3
8.5
4.5
270
149
2
Midday mean
soil water depletion
level
of high,
moderate, and
low irrigation treatments.

« loujrf) jdd (0) duel <%) Hd (®d>() OdA
100
600 800 1000 COO 1400 1600 1800
Tim* (EST)
Fig.'5-3. Diurnal cycle of photosynthetic photon flux (PPF),
air temperature, relative humidity (RH), and vapor pressure
deficit (VPD) on 12 June 1987.

101
600 600 1000 1200 WOO 1600 «00
Un» (EST)
Fig. 5-4. Diurnal cycle of leaf C0_ assimilation (A),
transpiration (E), stomatal conductance (g ), and water use
efficiency (WUE) of young 'Hamlin' orange trees on 12 June 1987
as influenced by soil water depletion (SWD). (A) " high
treatment 19Z SWD, (#) = moderate treatment at 28% SWD,
(â– ) * low treatment at 53% SWD. Bars represent SE of means,
n-8.

102
Fig. 5-5. Diurnal cycle of photosynthetic photon flux (PPF),
air temperature, relative humidity (RH), and vapor pressure
deficit (VPD) on 13 June 1987.

103
capacity to 45% SWD. Cumulative rainfall of 11.3 cm on 9 days had
interrupted the drying cycles so that the predetermined level of 65%
SWD for trees in the low treatment had never been reached. Dates of
last irrigation were 9 June for the moderate and 13 April for the
low scheduling treatments. Trees in the high treatment were
irrigated 12 June following diurnal measurements, thus SWD was near
20% on 12 and 15 June, and 2% on 13 June following the irrigation
(Table 5-2). Soil water depletion declined from 28% on 12 June to
45% on 15 June in the moderate treatment, and from 53% to 60% in the
low treatment (Table 5-2).
Maximum CO2 assimilation occurred from 700-800 hr on 12, 13,
and 15 June (Figs. 5-4a, -6a, -8b). Maximum daily values were quite
consistent through the 3 days, with trees in the high treatment
averaging 12.5, moderate treatment, 11.4, and low treatment, 9.2
-1 -2
pmol s m (Table 5-2). A gradual decline in CC^ assimilation
occurred throughout the day, with a partial recovery in midafternoon
with trees in the high treatment. The reduction in A was more
marked at the lower levels of soil moisture, and no recovery
occurred in the afternoon. Carbon assimilation was averaged over
all measurements (Table 5-2), and mean levels for the moderate and
low treatments were 68 and 42% of those recorded for the high
treatment. Mean daily CC^ assimilation fell gradually from 12 to 15
June in the moderate and low treatments.
Maximum CC>2 assimilation recorded under these conditions was
much higher than reported under greenhouse conditions of
containerized sour orange and sweet lime (Bielorai & Mendel, 1969),
rough lemon (Thompson et al., 1965), sweet orange (Kriedemann, 1971)

104
«00 «00 1000 1200 1400 «00 «00
Tims (EST)
Fig. 5-6. Diurnal cycle of leaf CC>2 assimilation (A),
transpiration (E), stomatal conductance (g ), and water use
efficiency (WUE) of young 'Hamlin' orange trees on 13 June 1987
as influenced by soil water depletion (SWD). (A) â–  high
treatment 2Z SWD, (#) - moderate treatment at 32Z SWD,
(â– ) - low treatment at 55Z SWD. Bars represent SE of means,
n»8.

Table 5-2. Mean and maximum leaf CC>2 assimilation, transpiration, stomatal conductance,
and mean internal C0~ concentration (C^) of young 'Hamlin' orange trees in June 1987
as influenced by soil water depletion.
Soil
water
depletion
CO 2
assimilation
(pmol s * m 2)
Transpiration
(mmol s * m )
Stomatal
conductance
(mmol s ^ m
Mean
c±
(X)
max.
mean
max.
mean
max.
mean
1 -1
(pi liter )
19Z
12.5
8.2
6.7
12
June 1987
5.0
325
189
226
28
11.5
6.0
6.6
4.2
305
136
232
53
9.4
4.5
4.1
2.7
259
99
234
2
12.7
8.5
6.3
13
June 1987
5.2
325
201
220
32
11.8
5.7
6.1
4.2
320
125
230
55
10.1
3.4
5.5
2.9
280
91
247
20
12.4
7.6
6.2
15
June 1987
4.6
316
154
223
45
11.0
4.7
5.7
3.5
306
105
231
60
8.1
2.4
4.1
1.8
181
54
250
ZMidday mean soil water depletion level of high, moderate, and low irrigation
treatments.
105

106
600 800 1000 1200 1400 «00 «00
Tim* (EST)
Fig. 5-7. Diurnal cycle of photosynthetic photon flux (PPF),
air temperature, relative humidity (RH), and vapor pressure
deficit (VPD) on 15 June 1987.

107
Un» < EST)
Fig. 5-8. Diurnal cycle of leaf xylem potential C'Fx)» co2
assimilation (A), transpiration (E), stomatal conductance (gg),
and water use efficiency (WUE) of young ’Hamlin’ orange trees
on 15 June 1987 as influenced by soil water depletion (SWD).
(A) - high treatment 20X SWD, (#) = moderate treatment at 4AZ
SWD, (â– ) - low treatment at 60% SWD. Bars represent SE of
means, n-8.

108
and lemon (Kriedemann, 1968), 'Carrizo' citrange, trifoliate orange,
'Cleopatra' mandarin, 'Swingle' citrumelo, and sour orange
(Syvertsen & Graham, 1985), and satsuma mandarin (Ono & Hirose,
1984). Containerized trees of several citrus species under
greenhouse conditions had maximum levels comparable to this study
(Khairi & Hall, 1976 a,b), as did mature orange and grapefruit trees
under field conditions (Sinclair & Allen, 1982). However, these
values were less than maximum CO^ assimilation reported for mature
satsuma mandarin trees under field conditions (Ono & Hirose, 1984).
The early daily peak in C0£ assimilation, followed by a general
decline thereafter for trees in the moderate and low treatments was
similar to that found with containerized rough lemon trees (Thompson
et al., 1965). Alternatively, the diurnal pattern of A in the high
treatment closely matched that of well-watered, containerized sweet
lime (Bielorai & Mendel, 1969) and mature, field-grown grapefruit on
days of high VPD (Sinclair & Allen, 1982). Midday depressions of
CO^ assimilation in the mature grapefruit trees were more severe
than in this study, and were considered to result from high VPD,
since days characterized by more mild conditions did not show any
depression. Similar midday depressions in CC^ assimilation of
rootstock seedlings were most severe on days with high VPD and
temperature, but occurred only when soil water became limiting
(Allen et al., 1985). In contrast, midday depressions of CC^
assimilation occurred in potted sweet lime at a wide range of soil
moisture levels and followed no consistent pattern in potted sour
orange (Bielorai & Mendel, 1969). The young, field-grown 'Hamlin'
orange trees in this study exhibited a gradual decline in CO^

109
assimilation at all levels of soil moisture. Partial recovery in
the afternoon, however, occurred only in trees in the high
treatment. The greatest decrease in midday A and gg of trees in the
high treatment occurred on 15 June (Fig. 5-8), which was also the
day of highest VPD (Fig. 5-7d). Moreover, midday changes in C^,
paralleled those of g within trees in the high treatment (data not
s
shown). As a result, reduced g was considered mostly responsible
s
for midday reduction in CO^ assimilation, which is in agreement with
the previously reported mechanism of reduced CC>2 assimilation under
high VPD conditions (Khairi & Hall, 1976 a,b; Kriedemann, 1968,
1971). There were no days characterized by mild conditions during
the middle of June.
Containerized citrus trees and controlled conditions have been
used to demonstrate decreases in CO^ assimilation in response to
decreased soil moisture content (Bielorai & Mendel, 1969; Brakke et
al., 1986; Kriedemann, 1968, 1971; Ono & Hirose, 1984; Thompson et
al., 1968). It is well-known that plant responses to stress under
field conditions are many times dissimilar to those under controlled
conditions (Begg & Turner, 1976; Davies & Lakso, 1979; Ludlow, 1976;
Ludlow & Ibaraki, 1979). The only reports of citrus CO^
assimilation under field conditions (Ono & Hirose, 1984; Sinclair &
Allen, 1982) do not indicate the level of soil moisture at the time
measurements were made, illustrating the need to document all
factors influencing gas exchange when studying these processes.
Temperature optima for citrus photosynthesis varies from 15 to
30°C (Khairi & Hall, 1976 a,b; Kriedemann, 1968; Ono & Hirose,
1984), suggesting that high temperatures throughout much of the day

110
could have contributed to decreases in CC>2 assimilation. High
temperature may limit A via direct effects on light reactions or
carboxylation capacity (Monson et al., 1982; Stidham et al., 1982),
although Khairi and Hall (1976 a,b) demonstrated the importance of
reduced residual conductance at high temperatures.
Transpiration of trees in the high treatment stabilized by late
morning to remain fairly constant thereafter (Fig. 5-4b, -6b, -8b).
Diurnal variation of E in the moderate and low treatments was
similar to A, but the decline was less precipitous after the early
morning peak. Mean levels of transpiration (Table 5-2) were 80% in
moderate and 50% in low treatments of those in the high treatment.
The highest instantaneous WUE (1.9-2.5) and g (181-316 mmol
-1 -2
s m ) occurred early in the morning, and the diurnal pattern of
each as related to SWD was similar to CC^ assimilation (Figs.
5-4c,d; -6c,d; -8d,e). Increased SWD caused a reduction in WUE,
suggesting that A was affected to a greater degree than E. Similar
results have been previously reported with sour orange and sweet
lime (Bielorai & Mendel, 1969).
Variation in A closely paralleled that of gg (Figs. 5-4, -6,
-8), and regression of A on g showed the close relationship (e.g.,
s
2
A = 0.04g + 0.85, r = 0.93, from 15 June data). A close
s
correlation between CO2 assimilation and stomatal conductance has
traditionally been Interpreted as being evidence of stomatal control
over A. However, the same factors limiting stomatal conductance may
have a direct effect on CO2 fixation, and this independent effect on
both processes could lead to the close correlation often found
between gg and A (Farquhar & Sharkey, 1982). Alternatively, the

Ill
close correlation between these processes could result from direct
stress effects on the capacity of the photosynthetic system fo fix
carbon, with stomata responding to this reduction in CC>2
assimilation (Farquhar & Sharkey, 1982; Redshaw & Meidner, 1972;
Wong et al., 1979). Non-stomatal effects were responsible for the
decreases in A due to SWD in this study as indicated by the
increases in internal C0_ concentration accompanying reduced g
L S
(Table 5-2).
Non-stomatal reduction of CO2 assimilation may be manifested
through changes in residual conductance to CO2 in the liquid phase
(Brown & Simmons, 1979; Bunce, 1977; Collatz, 1977; Mederski et al.,
1975; O'Toole et al., 1976; Pearcy, 1983; Pellegrino et al., 1987;
Radin & Ackerson, 1981), reduced photochemical activity (Boyer,
1976; Boyer & Bowen, 1970; Genty et al., 1987; Keck & Boyer, 1974;
Nir & Poljakoff-Mayber, 1967; Sharkey & Badger, 1982; Von Caemmerer
& Farquhar, 1981), or reduced carboxylation efficiency (Berkowitz &
Gibbs, 1982; Boag & Portis, 1984; Huffaker et al., 1970; Jones,
1973; O'Toole et al., 1976).
Differences in environmental factors from the fall and early
summer periods could explain the different diurnal pattern in gas
exchange exhibited by measurements from October and June.
Temperature, PPF, and VPD were lower during fall measurements than
June measurements (Figs. 5-1, -3, -5, -7). Air temperature
increased from <15° to 26° C on 18 October, and from 25° to 34° C
during June, accompanied by soil temperatures of 23° in October and
30° C in June.

112
Xylem potential gradually decreased from early morning to a
minimum at midday of -2.3 MPa on 15 June (Fig. 5-8a). The three SWD
levels exerted less influence on xylem potential measurements than
gas exchange measurements, possibly because stomatal closure
prevented a further drop in xylem potential at high SWD levels. The
decrease in xylem potential in the high treatment was 85% and the
moderate treatment 90% of the decrease in the low treatment.
Microclimatic factors controlling diurnal variation exerted much
more influence on xylem potential than did SWD. This relative lack
of change in xylem potential in response to large changes in soil
moisture, accompanied by a closer correlation between gas exchange
processes and soil moisture content, has been reported with other
crops (Bates & Hall, 1981; Cock et al., 1985; Lorenzo-Minguez et
al., 1985; Osonubi, 1985; Reicosky et al., 1976).
Maximum leaf to air temperature difference occurred in late
morning and ranged from 1.1 to 2.1° C, depending on SWD level (data
not shown). Average difference between leaf and air temperature was
twice as large for trees in the low as compared to the high
treatment. Increases in leaf temperature result from a reduction in
evaporative cooling due to reduced transpiration (Hanson & Hitz,
1982). Leaf to air temperature difference has been used as an
indicator of water stress in several crops (Clark & Hiler, 1973;
Idso et al., 1981; Sojka & Karlen, 1984; Stark & Wright, 1985),
including citrus (Syvertsen & Levy, 1982).
The relatively large changes in gas exchange processes with
each measurement period throughout the photoperiod illustrate the
need to limit gas exchange measurements to small time frames. Under

113
these conditions, factors independent of SWD levels would certainly
confound results if measurements were made over time periods
exceeding 2 hr in length.
In conclusion, SWD exerted little influence on CO^
assimilation, transpiration, or stomatal conductance during cool
days in the early fall, when these processes gradually increased
before and decreased after a midday peak. However, increasing SWD
resulted in decreased A, E, gg, and instantaneous WUE of young
'Hamlin' orange trees throughout early summer days characterized by
high VPD and air temperature. Reduced CC^ assimilation of trees in
the low irrigation treatment during spring and early summer may have
contributed to a reduction in seasonal biomass accumulation and
possibly a retardation of summer and fall growth flushes (Chapter
III) when compared to trees in the high treatment.

CHAPTER VI
GROWTH OF YOUNG 'HAMLIN' ORANGE TREES USING STANDARD
AND CONTROLLED-RELEASE FERTILIZERS
Introduction
Efficiency of fertilizer use can be expressed as the percent of
applied nutrients recovered by the crop. Nitrogen is the most
important nutrient in a citrus fertilization program and the
nutrient with the most variability in efficiency of recovery.
Nitrogen losses due to erosion, leaching, denitrification, and
volatilization reduce N availability for plant uptake. Sandy soils
and heavy rainfall in Florida are frequently associated with
substantial N losses, especially through leaching. The problem is
greater in areas where high water tables limit rooting depth.
Concerns over energy conservation and groundwater pollution,
combined with the competitive pressure to reduce production costs in
the Florida citrus industry (Fairchild & Brown, 1986) make reduction
of fertilizer losses desirable.
Controlled-release fertilizers potentially reduce N losses,
improving efficiency of plant recovery (Khalaf & Koo, 1983; Maynard
& Lorenz, 1979). Fewer applications are needed (Jackson & Davies,
1984; Koo, 1986; Maynard & Lorenz, 1979), which reduces labor and
equipment costs and soil compaction by equipment.
114

115
Controlled-release fertilizers have been used on many
horticultural crops (Maynard & Lorenz, 1979), including citrus.
These sources increased fruit production on mature citrus (Koo,
1986) and growth of young containerized citrus (Fucik, 1974; Khalaf
& Koo, 1983) when compared to more soluble fertilizer sources. In
contrast, growth of young 'Orlando' tángelo trees was comparable for
controlled-release sulfur-coated urea and soluble sources, but
frequency of application was reduced by 50% (Jackson & Davies,
1984). Nevertheless, acceptance of controlled-release fertilizers
by the Florida citrus industry has been limited (Jackson et al.,
1986), primarily because of higher fertilizer costs and lack of
grower experience with these materials.
Current fertilizer recommendations (Koo et al., 1984) for young
citrus trees have been based on previous studies (Calvert, 1969;
Rasmussen & Smith, 1961), observations, and industry trends.
Recommendations call for 0.03 to 0.05 kg N/tree for newly-planted
citrus, applied five to six times per year (Koo et al., 1984).
Fertilizer is usually broadcast evenly in a 0.9 m diameter circle,
which translates to more than 3000 kg N/year/treated ha. Young
citrus trees require an adequate supply of nutrients to optimize
growth; however, this rate is 10 times more than recommended levels
for mature trees and may be excessive under some circumstances.
Rasmussen and Smith (1961) also expressed concern that young trees
were being over-fertilized and recommended reduced application
frequency and rates.
Objectives of this study were to compare the effects of
commonly available controlled-release and standard fertilizers on

116
growth of young citrus trees, and to determine the effects of three
rates of application of standard fertilizer on leaf nutrient levels
and tree growth.
Materials and Methods
Four field experiments were conducted at the Horticultural Unit
northwest of Gainesville, Florida, using 'Hamlin' orange on sour
orange rootstock. Double row beds, 16.75 m wide and 0.60-0.75 m in
height, were constructed in March, 1985. Soil type was Kanapaha
sand (loamy, siliceous, hyperthermic, Grossarenic, Paleaquults)
underlain by a hardpan. Two tree rows 7.6 m apart were used on each
bed with trees set 3.4 m apart. Irrigation was applied by 90
degree, 38 liter/hr microsprinklers located 1 m northwest of tree
trunks. Available soil moisture was maintained above 20% soil water
depletion in the root zone, based on results of irrigation
experiments on the same site (Chapter III).
Controlled-release Fertilizers
Experiment one. Containerized and bare-rooted trees obtained
from commercial nurseries were planted in May, 1985. Trunk diameter
averaged 1.2 cm for bare-rooted and 0.8 cm for containerized trees.
Fertilizer treatments consisted of standard fertilizer broadcast
four times in the first year at 0.45 kg/tree/application and four
times in the second year at 0.91 kg/tree/application (Koo et al.,
1984), and isobutylidene diurea and Wonder Gro" applied twice each
year at 0.91 kg/tree/application in year 1 and 1.81 kg/tree/
application in year 2. All trees received equal amounts of
fertilizer using the same formulation (8N - 2.6P - 6.6K - 2Mg -
0.2Mn - 0.12Cu - 0.2Zn - 1.78Fe). There were 12 single-tree

117
replications per treatment combination (three fertilizer types x two
tree types) arranged as a randomized complete block, resulting in 24
trees per fertilizer type.
Trunk diameter at 5.1 cm above the bud union and canopy height
and width were measured on 16 May 1985, 10 Dec. 1985, and 7 Dec.
1986. Trunk cross-sectional area was calculated from diameter and
canopy volume as (4/3)(3.14)(1/2 H)(1/2 W)^, where H=height and
W=width (Westwood, 1978). This formula is used with canopies that
are taller than they are wide.
Four trees per fertilizer type were carefully excavated as
described in Chapter III in Dec. 1985 for measurements of fresh
weight, dry weight, new root growth, and total shoot length. Roots
were stained with safranin-0 dye prior to planting, which allowed
roots that developed in the field to be distinguished from those
present at planting time.
Data were analyzed by analysis of variance. Due to lack of
interactions among treatments, only data comparing fertilizer
sources are discussed in this chapter. Comparison of nursery tree
type is made in Chapter VII.
Experiment two. A second, similar study was begun in May, 1986
using trees from commercial nurseries different from those used in
experiment one. Trunk diameters averaged 1.0 cm and 0.8 cm for
bare-rooted and containerized trees, respectively. The same
fertilizer sources were used and treatments were replicated six
times per treatment combination (three fertilizer types X two tree
types), resulting in 12 trees per fertilizer source. Trunk cross
sectional areas and canopy volumes were determined on 6 May 1986, 7

118
Dec. 1986, and 20 Dec. 1987. Four trees per fertilizer type were
excavated in December, 1986 and measurements similar to those of
1985 were made on these 8-month-old trees.
Fertilizer Rates
Experiment three. Experiments three and four were designed to
determine if less than recommended fertilizer rates could be
utilized to obtain adequate growth of young citrus trees. Three
rates were compared on bare-rooted trees obtained from a commercial
nursery (trunk diameter 1.2 cm) and planted in May, 1985. Standard
fertilizer (8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu - 0.2Zn -
1.78Fe) was applied four times throughout the year at 0.23, 0.45, or
0.68 kg/tree/application, the average recommended amount being 0.45
kg per tree (Koo et al., 1984). There were 16 single-tree
replications per treatment arranged in a randomized complete block.
Canopy volumes and trunk cross sectional areas were measured on
16 May and 10 Dec. 1985. Eight trees per treatment were excavated
in Dec. 1985 for plant fresh and dry weight determination. Leaf
samples were collected from spring flush growth of non-fruiting
shoots in August for mineral analysis. Each sample consisted of 30
leaves taken from two trees. Eight samples were collected for each
fertilizer treatment. Data were analyzed by analysis of variance.
Experiment four. A study similar to experiment three was begun
in March, 1987, but using container-grown trees (trunk diameter 1.3
cm) grown in plastic bags (15 cm diameter). Fertilizer was applied
five times/year to 11 single-tree replications/treatment using the
same three rates. Trunk cross sectional area and canopy volume were
measured on 30 March and 20 Dec. 1987.

119
Results and Discussion
Controlled-release Fertilizers
Trees from experiment one, averaged over both classes of tree
2
types, were initially 3.0 cm in trunk cross sectional area. There
were no differences in tree growth among fertilizer sources 8 and 20
months after planting (Table 6-1), when measured as trunk cross
sectional area, canopy volume, fresh and dry weight, new root
growth, and total shoot length. Trunk cross sectional area averaged
2
4.3 and 14.8 cm after 8 and 20 months, respectively.
Smaller trees were obtained for experiment two with initial
2
trunk cross sectional area averaging 2.6 cm . This initially
smaller size was reflected in ultimate tree size after 8 and 18
months when compared to experiment one (Table 6-1). Again, no
differences among fertilizer sources were found. Trunk cross
2
sectional area averaged 3.5 and 9.5 cm after 8 and 20 months,
respectively. Measurements of canopy volume, fresh and dry weight,
new root growth, and total shoot length followed a similar pattern
to experiment one, with no differences among fertilizers.
These results indicate that isobutylidene diurea and Wonder
Gro" may be used to reduce application frequency by 50% without
decreasing growth. Sulfur-coated urea has been used on young citrus
trees with similar results (Jackson & Davies, 1984). The
feasibility of using controlled-release fertilizers in a young tree
care program should be determined on a case-by-case basis, since
controlled-release materials are more expensive than more soluble
fertilizers. However, reduction in application frequency and costs

120
Table 6-1. Effects of controlled-release fertilizers on growth
of young 'Hamlin' orange trees in the field.
Tree
age
(month)
Fertilizer
y
source
.X
TCA
, 2\
(cm )
Canopy
volume
(m3)
Total
fresh wt
(kg)
Total
dry wt
(kg)
New
root
dry wt
(kg)
Total
shoot
length
(cm)
Experiment one
8
Standard
4.3
0.32
1.49
0.54
0.10
744.2
WG
4.6
0.30
1.52
0.57
0.11
678.4
IBDU
3.9
0.26
1.72
0.57
0.10
877.1
20
Standard
13.7
1.78
WG
13.7
2.32
—
—
—
—
IBDU
16.8
1.97
Experiment two
8
Standard
3.4
0.26
1.33
0.44
0.09
641.6
WG
3.5
0.32
1.29
0.40
0.08
656.6
IBDU
3.5
0.33
1.51
0.47
0.10
732.8
20
Standard
9.6
1.82
WG
8.9
1.57
—
—
—
—
IBDU
9.9
1.90
zNo significant differences among treatments. Blank spaces indicate
measurements were made only on 8-month-old trees.
^WG = Wonder Gro, IBDU = isobutylidene diurea.
TCA - trunk cross sectional area.

121
could be realized for a limited number of replants in bearing
groves.
Fertilizer Rates
Application of 0.23, 0.45, or 0.68 kg of fertilizer/tree four
times throughout the season resulted in no difference in growth of
the bare-rooted trees in experiment one (Table 6-2). Trunk cross
2
sectional area increased from 1.1 to 5.4 cm from May to Dec. 1985.
3
Canopy volume, fresh weight, and dry weight averaged 0.55 m , 2.34
kg, and 0.90 kg, respectively. Fertilizer rate had little influence
on leaf analyses, as no consistent relationship among treatments
existed for all elements (Table 6-3). Levels of most elements were
in the optimum or high range (Koo et al., 1984) in all cases except
potassium for the lower two rates and zinc for all three rates.
Leaf N was optimum for the lower two rates, and ranged between
optimum and high for the 0.68 kg/tree rate.
Fertilizer rate did not significantly affect tree growth in
experiment four (Table 6-2). These container-grown trees, although
originally larger in trunk diameter than the bare-rooted trees of
experiment three, had limited growth from March to Dec. 1987. This
slow initial growth of some container-grown trees was not related to
fertilizer rate and has been seen in other experiments on the same
site (Chapter VII). Trunk cross sectional area increased from 1.4
2 3
to 1.7 cm and final canopy volume was 0.06 m . These data indicate
a reduction in currently recommended fertilizer rates (Koo et al.,
1984) the first 2 seasons in the field may be possible in some
situations without any reduction in plant growth or leaf nutrient

122
Table 6-2. Effects of standard fertilizer rate on growth of young
'Hamlin' orange trees in the field.
Rate per tree
Fertilizer
N
Canopy
Total
Total
per applic.X
per yr
TCAy
volume
fresh wt
dry wt
(kg)
(kg)
, 2\
(cm )
(m3)
(kg)
(kg)
Experiment three
(bare-rooted)
0.23
0.07
5.2
0.65
2.40
0.88
0.45
0.15
5.5
0.48
2.14
0.85
0.68
0.22
5.5
0.53
2.50
0.96
Experiment four
(container)
0.23
0.09
1.7
0.06
0.45
0.18
1.6
0.06
—
—
0.68
0.27
1.7
0.06
z
No significant differences among treatments.
yTCA = trunk cross sectional area.
fertilizer analysis was 8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu -
0.2Zn - 1.78Fe.

Table 6-3. Influence of standard fertilizer rate on leaf analysis of young
'Hamlin' orange trees in the field.
„ z
Rate
(kg/tree)
Dry wt (%)
mg/liter
N
P
K
Ca
Mg
Fe
Zn
Mn
Cu
0.23
2.56
0.13
0.87
4.48
0.51
91.88
14.63
36.25
4.81
0.45
2.58
0.14
1.06
4.11
0.50
106.25
18.00
45.44
5.31
0.68
2.78
0.14
1.20
3.70
0.51
100.63
17.00
48.56
4.50
SEy
0.09
0.01
0.05
0.19
0.02
4.36
1.09
6.05
0.25
z
y
Fertilizer analysis was 8N - 2.6P - 6.6K - 2Mg - 0.2Mn - 0.12Cu - 0.2Zn -
1.78Fe.
SE =* standard error, n =» 8 samples of 15 leaves from each of two trees.

124
status. The low rate of 0.23 kg fertilizer/tree/application (0.07
to 0.09 kg N/year) was adequate under these circumstances.
Rasmussen and Smith (1961) suggested that 0.07 kg N/year for
the first 2 years after planting was adequate for young citrus.
Their study was conducted in Lake and Pasco Counties using large,
bare-rooted trees with trunk diameters of over 5 cm after 1 year.
In contrast, Calvert (1969) reported that trees responded more
favorably to 0.22-0.33 kg N/year than 0.11 kg when grown on raised
beds on marginal soil, illustrating the importance of location and
soil type in determining fertilizer rates for young trees.
Many times a 1-year-old tree in the southern portion of the
state may be as large as a 2-year-old tree in more northerly regions
where fertilization is discontinued in September to reduce the
possibility of cold damage. There is also considerable variation in
size of currently-available nursery trees as well as initial growth
of young citrus trees due to factors independent of fertilizer
rates. These observations suggest that fertilization
recommendations of non-bearing citrus may be more accurately based
on tree size, as suggested by Bryan (1940), instead of being
categorically based on age of trees after planting.

CHAPTER VII
GROWTH OF BARE-ROOTED AND CONTAINER-GROWN 'HAMLIN'
ORANGE TREES IN THE FIELD
Introduction
Florida nurserymen have been producing bare-rooted citrus trees
in field nurseries for many years. Recently, however, many citrus
trees have been produced in various types of containers in
greenhouses (Castle et al., 1979). Advantages of greenhouse systems
include greater control over the production system, shorter growing
cycles, and reduced transplant shock (Moore, 1966; Platt & Opitz,
1973; Richards et al., 1967). Much controversy exists, however,
concerning growth and survival rates of container-grown compared to
bare-rooted trees. Some Florida growers feel that bare-rooted trees
grow faster because of their spreading, extensive root system, while
others believe that containerized trees are superior due to the
water-holding properties of the medium surrounding the roots.
Unfortunately, many of these observations have arisen from
unreplicated tests with variable soil, cultural, and environmental
conditions.
Replicated comparisons of container- and field-grown grapefruit
trees on sour orange rootstock in Texas have shown that size of
field-grown trees is greater than container-grown trees for up to 10
125

126
years (Leyden & Timmer, 1978; Maxwell & Rouse, 1980; Maxwell &
Rouse, 1984). A survey of growers in 1983-84 suggested the need for
information on this subject under Florida conditions ranked second
only to rootstock selection (Larry K. Jackson, Fruit Crops Dept.,
Univ. of Fla., personal communication).
The objective of this study was to compare establishment and
initial canopy and root growth of container-grown and bare-rooted
citrus trees under the same cultural, climatic, and edaphic
conditions. In addition, the effect of removal of container medium
on growth of containerized trees was studied.
Materials and Methods
Four field experiments were conducted at the Horticultural Unit
located northwest of Gainesville using 'Hamlin' orange trees on sour
orange rootstock. Beds (16.75 m width x 0.60-0.75 m height x 85 m
length) were constructed in March, 1985. Soil type was Kanapaha
sand (loamy, siliceous, hyperthermic, Grossarenic, Paleaquults)
underlain by an impervious hardpan. Two tree rows 7.6 m apart were
used on each bed with trees set 3.4 m apart. Irrigation water was
applied by 90 degree, 38 liter/hr microsprinklers located 1 ra
northwest of tree trunks. Available soil moisture was maintained
above 20% soil water depletion in the root zone, based on results of
irrigation experiments on the same site (Chapter III).
Experiment one. Greenhouse-grown trees in 10.2 x 10.2 x 35.6
cm plastic containers (Citripots") and field-grown, bare-rooted
trees were obtained from commercial nurseries in May, 1985 and
planted as part of a study comparing tree types and fertilizer
sources (Chapter VI). Typical nursery trees were obtained, with

127
bare-rooted trees being larger than containerized trees. Trunk
diameter ca. 5 cm above the bud union averaged 1.2 and 0.8 cm for
bare-rooted and containerized trees, respectively. Twelve single
tree replications per treatment combination (three fertilizer types
X two tree types) were used, resulting in 36 trees per tree type.
A mark was painted on each tree ca. 5 cm above the bud union.
Trunk diameter at this mark and canopy height and width were
measured on 16 May 1985, 10 Dec. 1985, and 7 Dec. 1986. Trunk
cross-sectional area was calculated from diameter and canopy volume
was calculated as (4/3)(3.14)(1/2H)(1/2W)^, where H = height and W
= width (Westwood, 1978). This formula is used when a canopy is
taller than it is wide.
Six trees for each tree type were carefully excavated as
described in Chapter III in Dec. 1985 for growth measurements and
root examination. Total plant fresh and dry weight, new root
growth, and total shoot length were measured after 8 months in the
field. Roots of bare-rooted trees were stained with safranin-0 dye
prior to planting, which allowed roots that developed in the field
to be distinguished from those present at planting time. Roots
extending out of the media in containerized trees were considered to
have developed in the field.
Weight and shoot length measurements were subjected to analysis
of variance and canopy volume and trunk cross sectional area to
analysis of covariance to standardize differences in initial plant
measurements. Due to lack of interactions among treatments, only
data comparing nursery tree type are discussed in this chapter.
Comparisons of fertilizer sources are made in Chapter VI.

128
Experiment two. A similar study was begun in May, 1986 using
trees from commercial nurseries different from those used in
experiment one. Trees of more uniform size than those used in
experiment one were selected, with trunk diameter averaging 1.0 and
0.8 cm for bare-rooted and containerized, respectively. The same
fertilizer sources were compared (Chapter VI) and six single tree
replications were used per treatment combination (three fertilizer
sources X two tree types), resulting in 18 trees per tree type.
Trunk cross sectional area and canopy volume were determined on 6
May 1986, 7 Dec. 1986, and 20 Dec. 1987. Six trees per tree type
were excavated in December, 1986 and measurements similar to those
of 1985 were made on these 8-month-old trees. Analysis of data was
as described in experiment one.
Experiment three. Poor root growth was observed on some
containerized trees in experiment one, therefore a third experiment
was designed to compare the effect of removing different amounts of
container media on subsequent growth. Greenhouse-grown trees
produced in 10.2 cm citripots and averaging 0.8 cm in trunk diameter
were treated by using water from a garden hose to rinse away all,
the bottom half, or no media (control) prior to planting in May,
1986. Treatments were replicated nine times in a randomized
complete block. Standard fertilizer was applied using recommended
fates (Koo et al., 1984) and irrigation was as described previously.
Trunk cross sectional areas and canopy volumes were measured on 6
May and 7 Dec. 1986. All tree root systems were excavated in Dec.
1986 for root examination and growth measurements. Plant fresh and
dry weight, and dry weight of new roots were measured after

129
excavation. Data on weights were subjected to analysis of variance
and data on trunk cross sectional area and canopy volume to analysis
of covariance.
Experiment four. A study combining aspects of all three
preceding experiments was begun in April 1987 using a new bed
constructed in January 1987. Greenhouse-grown trees in 15.2 cm
plastic bags and averaging 1.35 cm in trunk diameter were treated
prior to planting by rinsing away all of the media, by breaking up
the root ball by hand, or with no pre-plant treatment. These
container-grown trees, planted by the three methods, were compared
to bare-rooted trees averaging 1.34 cm in trunk diameter.
Treatments were replicated 12 times in a randomized complete block.
Irrigation and fertilizer rates were as described previously. Trunk
cross sectional area and canopy volume were determined on 14 April
and 5 Dec. 1987. All tree root systems were excavated in Dec. and
plant fresh and dry weight, dry weight of new roots, and total shoot
length were measured after excavation. Data were analyzed as in
experiment three. Where treatments were significantly different,
Dunnett's t test (Dunnett, 1964) was used to compare each container
planting method against the bare-rooted treatment.
Results and Discussion
Tree size, expressed as trunk cross sectional area, canopy
volume, plant fresh and dry weights, total shoot length, and dry
weight of new roots, were significantly less for container-grown
than bare-rooted trees after 8 months of growth in experiment one
(Fig. 7-1, Table 7-1). Canopy volume was affected most, as
container-grown trees were 25% the size of bare-rooted trees (Fig.

TCA (cm ) Canopy volume (m3)
130
Time (months)
Fig. 7-1. Effect of 'Hamlin' orange nursery tree type on
increase in trunk cross sectional area (TCA) and canopy volume
from May 1985 to Dec. 1986. Bars represent standard errors
of means, n*36. (•) * bare-rooted, (■) ** container-grown.

131
Table 7-1. Effect of nursery tree type on growth of 'Hamlin'
orange trees after 8 months in the field.
Tree type
Fresh
wt
(kg)
Dry
wt
(kg)
Dry wt
new
roots
(kg)
Total
shoot
length
(cm)
Experiment
one (May-Dec.
1985)
Bare-rooted
2.18
0.76
0.15
1056.4
Container
0.86
0.31
0.05
439.9
**
**
**
**
Experiment
two (May-Dec.
1986)
Bare-rooted
1.55
0.49
0.11
792.0
Container
1.17
0.36
0.06
613.9
*
*
**
ns
ns,*,** Nonsignificant or significant at the 5% and 1% levels,
respectively, by F test. Mean of six trees/treatment.

132
7-1). Size of bare-rooted trees, measured as trunk cross sectional
area and canopy volume, remained significantly greater than that of
container-grown trees 20 months after planting (Fig. 7-1). Trunk
cross sectional area of container-grown trees averaged 51% and
canopy volume 37% the size of bare-rooted trees.
Bare-rooted trees in experiment two were also significantly
larger than container-grown trees after 8 months in the field, as
determined by trunk cross sectional area, canopy volume, plant fresh
and dry weight, and dry weight of new roots (Fig. 7-2, Table 7-1).
Total shoot length of the tree types was not significantly
different, however. Averaged over all measurements, container-grown
trees were 68% as large as bare-rooted trees. After 20 months in
the field, container-g own trees averaged 84% the size of
bare-rooted trees based on trunk cross sectional area and canopy
volume.
Leyden and Timmer (1978) concluded that container-grown citrus
trees would be smaller and less productive initially than
field-grown trees based on a 2.5 year comparison of grapefruit/sour
orange trees. Maxwell and Rouse (1980, 1984), however, compared
grapefruit/sour orange trees through 10 years in the field, and
reported field-grown nursery trees were larger than container-grown
trees, but that yield did not differ. These studies are not
comparable to Florida conditions because container stock was not
grown under greenhouse conditions, container size (up to 5.8 liter)
was larger than used in Florida, and field-grown trees were
transplanted ball and burlapped.

133
Time (months)
Fig. 7-2. Effect of 'Hamlin' orange nursery tree type on
Increase in trunk cross sectional area (TCA) and canopy volume
from May 1986 to Dec. 1987. Bars represent standard errors
of means, n-18. (#) - bare-rooted, (â– ) - container-grown.

134
Webber (1932) found that after roguing off-type rootstocks,
initial 'Washington' navel orange tree size was not correlated with
tree size after 8 years. Similarly, Gardner and Horanic (1959)
found no relationship between initial and ultimate tree size with
17-year-old 'Parson Brown' and 'Valencia' orange trees.
Unfortunately, neither of these reports discussed the influence of
initial tree size on precosity and size during the early years of
bearing. The data presented here indicate that nursery tree size
strongly influences growth of citrus trees during the first 20
months in the field.
Initial size difference may not have been the only factor
determining the large difference in growth of bare-rooted and
container-grown trees in experiment one. Roots of many
container-grown trees after 8 months in the field were limited to a
small volume of soil surrounding the media. Container-grown trees
in experiment two, however, did not respond similarly, as all
excavated trees had root growth greater than two feet beyond the
container media. This is not reflected in root dry weight (Table
7-1), which indicates that initial root extension into field soil
may be as important as total root growth measured as biomass
accumulation. These observations also indicate that initial root
growth from newly-planted containerized trees is highly variable
from year-to-year or nursery-to-nursery.
Removal of potting medium prior to planting container-grown
trees in experiment three significantly improved tree growth,
measured as plant fresh and dry weight, new root dry weight, canopy
volume, and trunk cross sectional area (Table 7-2). Differences

135
Table 7-2. Effect of removing medium prior to planting on growth of
containerized 'Hamlin' orange trees from May to Dec., 1986.
Amount of
Dry wt
container
Fresh
Dry
new
Canopy
medium
wt
wt
roots
volume
TCAZ
removed
(kg)
(kg)
(kg)
(m3)
(cm2)
All
1.46
0.47
0.12
0.38
3.10
1/2
1.40
0.45
0.09
0.24
3.16
Control
1.17
0.36
0.06
0.17
2.77
SEy
0.13
0.04
0.01
0.06
0.19
2
Trunk cross sectional area
y
SE = standard error, n = 9

136
between the control and treatment with all medium removed were
two-fold or greater for root growth and canopy volume.
Growth of bare-rooted trees in experiment four was
significantly greater than container-grown trees in all three
planting methods, measured as plant fresh and dry weight, new root
dry weight, total shoot length, canopy volume, and trunk cross
sectional area (Table 7-3). Size of bare-rooted trees was more than
three times greater than the mean size of all container-grown trees,
averaged over all measurements. Statistical comparison of the
container planting methods was not made, however, from the means in
Table 7-3, it is evident that medium removal prior to planting did
not increase tree growth that year. Limited lateral root growth of
container-grown trees occurred as in the trees planted in 1985
(experiment one).
Studies from California suggest that water movement from field
soil to organic media around the roots may be slow (Richards et al.,
1967). This is particularly true as the medium dries and its
hydraulic conductivity decreases. One advantage of a California mix
was that it was easily shaken loose prior to planting (Platt &
Opitz, 1973). Drying of container medium after planting due to
evapotranspiration, drainage (Nelms & Spomer, 1983; Warneke et al.,
1975), and difficulty in rewetting could lead to increased plant
stress and decreased growth. Death of containerized landscape
plants has been attributed to these phenomena (Costello & Paul,
1975). Survival of container-grown and bare-rooted citrus trees was
similar in our study, but slow initial growth occurred for

137
Table 7-3. Growth of bare-rooted and container-grown ’Hamlin' orange,
trees as influenced by planting procedure (April-Dee., 1987).
Dry wt
Total
Fresh
Dry
new
shoot
Canopy
wt
wt
roots
length
volume
TCAy
z
Treatment
(kg)
(kg)
(kg)
(cm)
(m3)
(cm2)
BR
1.35
0.45
0.09
844.3
0.42
3.45
C-l
**
0.51
**
0.21
**
0.03
**
446.6
**
0.07
**
1.74
C-2
**
0.33
**
0.15
**
0.02
**
314.3
**
0.04
**
1.53
C-3
**
0.58
kk
0.23
**
0.03
kk
498.4
**
0.10
kk
1.99
ZBR = bare-rooted; C-l = container-grown, all media removed;
C-2 = container-grown, root ball broken up; C-3 = untreated
container-grown.
yJrunk cross sectional area.
Different from BR according to Dunnett's t test, 1% level.
Mean of 11 trees/treatment.

138
containerized trees and has been observed in other plantings on the
same site.
Poor root growth accompanied and may have contributed to slow
initial growth of many container-grown trees. Kaufmann (1968)
reported that pine root tips matured in dry soils, resulting in
reduced elongation. Severe drying of container medium following
planting could cause the same results. Root growth of pine
seedlings grown in a medium high in peat and planted in sandy loam
to clay loam was much less than when grown in soil that closely
resembled that in which they were planted (Helium, 1981). These
factors may have contributed to reduced root growth in our study,
except in the case of experiment four, where medium removal did not
enhance root growth. Other factors associated with post-plant root
growth of forestry species that are produced in containers are
season of planting (Lavender & Cleary, 1974; Tinus, 1974), container
characteristics (Elam et al., 1981; Hiatt & Tinus, 1974; Hite, 1974;
Tinus & Owston, 1984; Van Eerden & Arnott, 1974), and acclimation to
field conditions prior to transplanting (Rook, 1973; Timmis, 1974;
Tinus & Owston, 1984). Furthermore, less post-plant root growth
occurrs as the length of time seedlings are maintained in containers
is lengthened (Helium, 1981). Elam et al. (1981) reported a large
variation in the effects of container and media characteristics on
growth of oak species.
Lack of acclimation to field growing conditions prior to
planting may play a role in the poor initial growth seen in some
containerized greenhouse-grown citrus trees. Optimum conditions for
growth in the greenhouse, combined with little concern for

139
acclimation to field conditions, resulted in poor survival rates for
containerized forestry seedlings (Tinus & Owston, 1984). Potted
plants transferred from a shaded, humid environment to full sun
typically suffer stress even with optimum soil moisture conditions
in the field (Kramer & Kozlowski, 1979). Moreover, vigor of
container-grown ornamental plants frequently declines rapidly
following removal from a high liquid N fertilization program (T.H.
Yeager, Ornamental Hort. Dept., Univ. of Fla., personal
communication). The problem is lessened by lowering N rates near
the end of the production cycle or by utilizing controlled-release
fertilizers instead of a liquid fertilizer program. Acclimation is
also promoted by reduced irrigation frequency in the nursery (Rook,
1973).
Season of planting may affect the degree of acclimation needed
prior to transplanting greenhouse-grown citrus trees. The dry windy
conditions that frequently occur in Florida during early spring
months may be much harsher conditions for the greenhouse-grown
transplants than would occur at other times. Windy days with
relative humidity below 302 occurred in 1987 (experiment four)
shortly after transporting the greenhouse-grown trees to the field.
Many container-grown citrus trees are being produced and
planted throughout Florida with a high degree of success. However,
most of these trees are initially smaller than bare-rooted trees and
have distinctively different rooting and branching patterns.
Inconsistencies in the results of this series of field trials and in
performance of container-grown citrus trees in the commercial
industry indicate that the question of how well container-grown

140
trees perform in the field cannot be answered categorically.
Initial growth of containerized trees depends largely on the
morphological and physiological condition of the stock, condition of
the site, planting operation, care after planting, weather at time
of and shortly after planting, container and medium characteristics,
length of time in the nursery, and rootstock/scion choices. It is
imperative that nurserymen, planting crews, and grove managers work
closely together to ensure widespread successful establishment and
growth of containerized, greenhouse-grown citrus trees.
In conclusion, our data suggest that bare-rooted trees will
grow faster than container-grown trees for the first 2 seasons if
compared under the same cultural, edaphic, and environmental
conditions. Survival is excellent for both types of nursery trees
if proper care is given after planting. It is important to choose
healthy, unstressed, uninjured trees initially, whether bare-rooted
or containerized, to improve growth after planting.

CHAPTER VIII
CONCLUSIONS
Young citrus tree care is an important and costly effort in the
Florida citrus industry. Understanding the influence of nursery
conditions and field management decisions on young tree growth and
development is vital to maximize growth and survival.
The first portion of this research demonstrated the influence
of irrigation and soil water deficits on growth of young 'Hamlin'
orange trees. Canopy growth was similar with high (20% soil water
depletion [SWD]) and moderate (45% SWD) irrigation treatments in 2
out of 3 years, but was reduced with the low treatment (65% SWD).
Summer and fall flushes were also delayed sometimes by the moderate
and low treatments. The low treatment did not decrease individual
shoot and leaf growth rates or ultimate size, but sometimes
decreased shoot number. Trees in the high treatment received an
average of 31; in the moderate treatment, an average of 11; and in
the low treatment an average of two irrigations annually. Trees in
the moderate treatment received approximately 50% less water than
those in the high treatment. Irrigation frequency during dry
periods was 2-3 days for the high and 4-6 days for the moderate
treatments.
141

142
Total root growth in relation to treatments followed patterns
similar to shoot growth. More than 90% of the roots (dry weight
basis) were covered by the 90° microsprinkler emitters after one
season of growth. This observation, combined with the lack of
growth difference in second season trees irrigated with 90° and 180°
emitters, suggests that 90° emitters have sufficient coverage to be
used more than one season. Most root growth occurred between 10 and
30 cm in depth. Irrigation times with young citrus trees should be
limited to replenish soil moisture only in these shallow zones.
Soil water deficit influenced xylem potential relatively little
compared to gas exchange processes during early summer days
characterized by high vapor pressure deficit and temperature.
Increasing soil water deficit decreased CO^ assimilation (A),
transpiration (E), stomatal conductance (g ), and instantaneous
8
water use efficiency of the young citrus trees. While A and gg were
highly correlated, stomatal limitation of A was assumed to be of
little importance in soil water deficit influence on A, based on
internal CO^ concentrations. Midday depressions of A, however, were
assumed to result from stomatal closure, as internal CC^
concentration decreased with g and A during these midday
s
measurements. Soil water deficits had little influence on A, E, or
g during the fall when temperatures were cooler.
8
The precise level of SWD at which microsprinkler irrigation
should be scheduled cannot be determined from these experiments, but
is most likely between 20 and 45%, depending on tree age, soil type,
and location in Florida

143
The second portion of this research demonstrated that currently
used fertilization rates may be excessive and that controlled-
release fertilizers may be a viable alternative to standard methods.
Applying granular 8% N fertilizer four to five times per season at
rates of 0.23, 0.45 (average recommended rate), and 0.68 kg/tree
resulted in similar growth of young 'Hamlin' orange trees for the
first year, suggesting that commonly used rates may be substantially
reduced in many situations. Wonder Gro* and isobutylidene diurea
applied half as many times as standard granular fertilizer at the
same annual rate also resulted in similar tree growth to the
standard treatment. Controlled-release sources may be beneficial in
certain situations, especially with a limited number of scattered
resets where application costs are high. Large variations in tree
size due to nursery tree type, location within the state, and other
factors not associated with fertilization rate suggest that
recommendations for non-bearing trees may be more accurately based
on tree size instead of categorically based on the number of years
in the field.
The third part of this project demonstrated a substantial
influence of nursery tree type on initial growth of young 'Hamlin'
orange trees in the field. Bare-rooted, field-grown trees grew
faster than containerized, greenhouse-grown trees for at least 2
seasons when compared under the same cultural, edaphic, and
environmental conditions. Removal of potting medium prior to
planting container-grown trees improved tree growth in one year, but
had no effect in another. Considerable variability occurred in
growth rates of container-grown trees, and further research is

144
needed to identify potential factors controlling the initial
performance of these trees in the field. Conditions of the nursery
operation and the field at time of planting, as well as certain
morphological and physiological factors of the tree that may
influence initial growth need to be identified and correlated with
post-plant growth.
These studies suggest that microsprinkler irrigation may be
optimized by using 90° emitters to maintain soil water content above
20-45% SWD (3-6 day frequency under these conditions) and limiting
irrigation times to 1-2 hr. To increase efficiency of young citrus
fertilization programs, these experiments suggest that emphasis
should be placed mainly on rate reduction and in some cases the use
of controlled-release sources. Nursery stock condition may affect
initial growth after field-planting citrus trees, and care should be
taken to choose healthy, unstressed, and uninjured trees.

APPENDIX A
MEAN MONTHLY PAN EVAPORATION AND MAXIMUM AND MINIMUM AIR
TEMPERATURES FOR MAY-DEC. 1985, MAY-DEC. 1986, AND APRIL-DEC.
1987. DATA WERE OBTAINED FROM THE IFAS AGRONOMY FARM WEATHER
STATION LOCATED ABOUT 14 KM FROM THE HORTICULTURAL UNIT.
Month
A
M
J
J
A
S
0
N
D
Pan
E (mm day *)
6.9
5.7
1985
4.5
4.2
4.4
4.1
2.9
2.3
Max
temp (°C)
33.5
33.6
32.8
32.0
30.9
30.2
26.9
22.7
Min
temp (°C)
17.7
21.9
21.4
22.1
20.6
19.6
16.2
9.4
Pan
E (mm day *)
7.2
5.4
1986
6.2
4.7
4.8
3.4
2.3
2.7
Max
temp (°C)
30.4
33.3
34.1
32.6
32.7
29.1
26.9
22.7
Min
temp (°C)
15.8
21.0
21.7
21.5
20.9
17.1
17.1
13.0
Pan
E (mm day *)
6.2
5.8
5.6
1987
4.9
5.4
4.2
3.1
4.5
2.4
Max
temp (°C)
29.1
30.1
33.0
33.8
33.9
32.1
26.5
24.4
21.5
Min
temp (°C)
12.5
18.0
21.1
22.3
22.6
20.8
13.1
12.4
7.5
145

APPENDIX B
WHOLE PLANT FRESH AND DRY WEIGHTS AND SHOOT .-ROOT
RATIO OF YOUNG 'HAMLIN' ORANGE TREES AS RELATED
TO IRRIGATION BASED ON SOIL WATER DEPLETION.
Soil
water
depletion
(%)
Whole plant
fresh
weight
(g)
Whole plant
dry
weight
(g)
Shoot:
root
ratio
1985
o
CM
II
a
20 (High)
2176.4
773.3
1.16
45 (Mod.)
1991.0
759.7
1.28
**
**
65 (Low)
1645.6
633.9
1.21
1986
(n=21)
20 (High)
2197.0
721.3
1.18
45 (Mod.)
2195.8
721.1
1.16
65 (Low)
1786.0
562.1
1.12
1987
(n»5)
20 (High)
1857.2
651.0
1.54
*
*
45 (Mod.)
1183.4
428.4
1.54
*
*
65 (Low)
1050.0
389.2
1.49
* Response is significant when compared with the
20% soil water depletion treatment by the Williams'
method, 5% and 1%, respectively.
146

APPENDIX C
REGRESSION EQUATIONS AND COEFFICIENTS OF DETERMINATION
(r ) REPRESENTING THE CUMULATIVE PERCENTAGE OF TREES IN
THREE IRRIGATION TREATMENTS GROWING OVER THE SEASON.
Flush
Equation
2
r
one
Y
20%
s
1985
-31.36 + 9.57day - 0.1678day2
O
0.92
Y45%
=
-28.07 + 9.37day - 0.1662day
2
0.89
Y65%
=
-27.33 + 10.58day - 0.2019day
2
0.80
two
Y
20%
s
-447.70 + 13.63day - 0.0840day
2
0.97
Y45%
=
-316.05 + 8.78day - 0.0458dayil
2
0.95
Y65%
=
-197.06 + 6.03day - 0.0314day
2
0.84
three
Y20%
=
-521.10 + 7.31day - 0.0214day
2
0.82
Y45%
=
-221.00 + 2.85day - 0.0062day
2
0.89
Y
65%
*
-98.30 + 1.17day - 0.0018day
0.85
one
Y
*20%
-
1986
-17.4 + 10.52day - 0.2265day2
2
0.99
Y45%
=
-9.21 + 7.11day - 0.1141dayz
2
0.99
Y65%
3
-26.93 + 9.01day - 0.1610day
2
0.99
two
Y20%
=
-231.33 + 7.56day - 0.04l5day
2
0.86
Y45%
3
-153.77 + 4.48day - 0.0200day^
2
0.98
Y65%
=â– 
-96.28 + 2.47day - 0.0068day
2
0.97
three
Y20%
3
-560.18 + 8.04day - 0.0240day*
2
0.95
Y45%
*
-360.99 + 5.06day - 0.0145day*
2
0.97
Y
o5%
-
-217.06 + 2.86day - 0.0072day
0.99
one
Y20%
3
1987
-39.43 + 4.27day
0.94
Y45%
=
-39.14 + 4.19day
0.93
Y65%
3
-33.7 + 3.71day
2
0.78
two
Y20%
3
-1054.05 + 22.57day - 0.1085day
2
0.93
Y45%
3
-758.64 + 15.55day - 0.700dayz
2
0.95
Y65%
=
-509.48 + 9.88day - 0.0401dayz
2
0.92
three
Y20%
3
-767.17 + 9.85day - 0.0275day
0.82
Y45%
=
-816.57 + 9.73day - 0.0256day2
2
0.97
Y65%
3
-618.5 + 7.08day - 0.0174day
0.96
147

APPENDIX D
MEAN AND MAXIMUM LEAF TO AMBIENT AIR TEMPERATURE
DIFFERENCE OF 'HAMLIN' ORANGE TREES IN JUNE
1987 AS INFLUENCED BY SOIL WATER DEPLETION.
Soil
water
depletion
(%)
Leaf to
air temp,
diff. (°C)
maximum minimum
12 June 1987
19Z 1.3 0.8
28 1.5 0.9
53 2.1 1.4
13 June 1987
2 1.1 0.6
32 1.3 0.8
55 2.0 1.3
15 June 1987
20 1.2 0.7
45 1.4 0.8
60 2.0 1.4
ZMidday mean soil water depletion level of high,
moderate, and low irrigation treatments.
148

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BIOGRAPHICAL SKETCH
Thomas Marler was born in Seoul, Korea in February of 1959.
After living in Korea, Guam, and Mississippi he graduated from high
school in 1977 in Jackson, MS. Tommy attended Mississippi College
in Clinton, MS for three semesters prior to becoming a horticulture
major at Mississippi State University in Starkville, MS. He
received his Bachelor of Science degree in 1982 and Master of
Science degree in 1984, and subsequently entered the University of
Florida to begin working on a Doctor of Philosophy degree. He will
be awarded the Ph.D. degree in 1988 and become the curator of
tropical fruit crops at Fairchild Tropical Gardens in Miami, FL.
173

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.
Xs)
Frederick S. Davies, Chair
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.
ison
^sociate 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. JJ ¡
Larry JC/ Jaofcfion
Profeasor or 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.
‘ c
Robert C. J. Koo
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.
AULft
Allen G. Smajstria'
Professor of Agricultural
Engineering

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 1988
Dean,
£
ege of Agriculture
Dean, Graduate School

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