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Title:
Effect of flooding duration, periodic flooding, season, and temperature on growth, development, and water relations of young rabbiteye blueberry (Vaccinium ashei Reade) plants
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x, 110 leaves : ill. ; 28 cm.
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English
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Crane, J.H ( Jonathan Henry ), 1952-
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s.n.
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Subjects / Keywords:
Blueberries -- Effect of floods on -- Southern States   ( lcsh )
Blueberries -- Growth -- Southern States   ( lcsh )
Horticultural Science thesis Ph. D
Dissertations, Academic -- Horticultural Science -- UF
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bibliography   ( marcgt )
non-fiction   ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 98-109.
Statement of Responsibility:
by Jonathan Henry Crane.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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oclc - 17886165
notis - AFA2029
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Full Text












EFFECT OF FLOODING DURATION, PERIODIC FLOODING,
SEASON, AND TEMPERATURE ON GROWTH, DEVELOPMENT,
AND WATER RELATIONS OF YOUNG RABBITEYE BLUEBERRY
(Vaccinium ashei Reade) PLANTS






By

JONATHAN HENRY CRANE


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


1987





































To my wife, Nancy, and our two children, Laurel and Colin

and to my grandmother, Mabelle (Nana) C. Besler.















ACKNOWLEDGEMENTS

Special appreciation and thanks are extended to Dr. Frederick S.

Davies, chairman of my committee, for his invaluable aid, encouragement,

and support during the past 3 years. His advice, guidance,

encouragement, and friendship have been of great importance in the

development and success of the student and project.

I would also like to thank my friends L. W. (Rip) Rippetoe, for his

invaluable technical help with experiments, and Mark W. Rieger and Dr.

L. K. Jackson for their advice and encouragement during these past 3

years. I would also like to thank the other members of the committee,

Drs. P. C. Andersen, D. A. Graetz, K. E. Koch, and J. M. Bennett for

their advice and support during this project.

Special appreciation goes to my parents, Marjorie F. Crane and

Carlyle W. Crane, and in-laws, Manuel and Dinorah Rodriguez, for their

encouragement and support the past 8 years I have been in school. Most

of all I want to express my love and appreciation for the love and

support given to me by my wife, Nancy, and our children, Laurel and

Colin.
















TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ...............................................iii

LIST OF TABLES ......................................... ...... vi

LIST OF FIGURES ............................... ....... .........vii

ABSTRACT ..... ................................................... ix

CHAPTERS

I INTRODUCTION ............................................. 1

II REVIEW OF THE LITERATURE ................................. 3

Effect of Flooding on Soil .............................. 3
Effect of Flooding on Plant Survival and Growth .......... 8
Efffect of Flooding on Plant Physiology .................. 9
Flooding and Plant Hormones .............................. 11
Flooding and Plant Metabolism ............................ 13
Adaptations to Flooding ................................ 15
Flooding and Plant Disease ............................... 16

III FLOODING DURATION AND SEASONAL EFFECTS ON GROWTH AND
DEVELOPMENT OF YOUNG RABBITEYE BLUEBERRY PLANTS .......... 18

Introduction ............................................. 18
Materials and Methods .................................... 20
Results and Discussion .................................. 22

IV PERIODIC AND SEASONAL FLOODING EFFECTS ON SURVIVAL,
GROWTH, AND STOMATAL CONDUCTANCE OF YOUNG RABBITEYE
BLUEBERRY PLANTS ........................................... 40

Introduction ............................................. 40
Materials and Methods ............................... 41
Results and Discussion ............................... 44

V HYDRAULIC CONDUCTIVITY, ROOT ELECTROLYTE LEAKAGE, AND
STOMATAL CONDUCTANCE OF FLOODED AND UNFLOODED RABBITEYE
BLUEBERRY PLANTS ........................................... 58

Introduction ............................................. 58
Materials and Methods ................................... 60
Results and Discussion ................................... 62









VI SOIL TEMPERATURE AND FLOODING EFFECTS ON YOUNG RABBITEYE
BLUEBERRY PLANT SURVIVAL, GROWTH, AND RHIZOSPHERE ETHYLENE
EVOLUTION ................................................ 70

Introduction .......................................... 70
Materials and Methods .................................... 72
Results and Discussion ................................. 75

VII CONCLUSIONS ................. ......................... 93

LITERATURE CITED .............. ................................... 98

BIOGRAPHICAL SKETCH .............................................. 110















LIST OF TABLES


Table Page

3-1. Flooding duration and survival of 'Woodard'
rabbiteye blueberry plants ..........................29

3-2. Flooding duration and leaf area of 'Woodard'
rabbiteye blueberry plants ..........................34

3-3. Flooding duration, flower bud number, fruit set,
and yields of 'Woodard' rabbiteye blueberry plants ..36

4-1. Flooding treatments and dates used for studies on
periodic flooding of young 'Tifblue' rabbiteye
blueberry plants .....................................43

4-2. Effect of periodic flooding on survival of 'Tifblue'
rabbiteye blueberry plants ..........................49

4-3. Effect of periodic flooding on leaf area of 'Tifblue'
rabbiteye blueberry plants .........................51

4-4. Effect of periodic flooding on fruit set, yields,
and flower bud number of 'Tifblue' rabbiteye
blueberry plants ........................... .........53

6-1. Effect of flooding temperature on plant survival
of 'Woodard' rabbiteye blueberry plants .............85

6-2. Effect of flooding on dry weight of 'Woodard'
rabbiteye blueberry plants ............................86

7-1. Time course of physiological and growth responses of
rabbiteye blueberry plants to flooding duration .....96















LIST OF FIGURES


Figure Page

3-1. Soil oxygen diffusion rates of flooded and unflooded
plots planted to 'Woodard' rabbiteye blueberry plants
during summer, 1984 ..................................23

3-2. Soil oxygen diffusion rates of flooded and unflooded
plots planted to 'Woodard' rabbiteye blueberry plants
during spring, 1985 .......................... .... ..26

3-3. Soil redox potentials of flooded and unflooded plots
planted to 'Woodard' rabbiteye blueberry plants
during spring, 1985 ........................... .......27

3-4. Effect of flooding duration on stem elongation and
leaf expansion of 'Woodard' rabbiteye blueberry
plants during spring, 1985 .........................31

3-5. Effect of flooding duration on stem elongation and
leaf expansion of 'Woodard' rabbiteye blueberry
plants during summer, 1985 ...........................32

3-6. Effect of flooding duration on stomatal conductance
and transpiration of 'Woodard' rabbiteye blueberry
plants during summer, 1985 ............... .........38

4-1. Soil redox potentials of flooded and unflooded plots
planted to 'Tifblue' rabbiteye blueberry plants at
the 15-cm soil depth during spring, 1985 .............45

4-2. Soil redox potentials of flooded and unflooded plots
planted to 'Tifblue' rabbiteye blueberry plants at
the 30-cm soil depth during spring, 1985 .............46

4-3. Effect of periodic flooding on stomatal conductance
and transpiration of 'Tifblue' rabbiteye blueberry
plants during summer, 1985 .........................55

5-1. Effect of flooding on root hydraulic conductivity of
'Tifblue' (expt. 1) and 'Woodard' (expt. 1 and 2)
rabbiteye blueberry plants ..........................64

5-2. Effect of flooding on stem hydraulic conductivity of
'Tifblue' (expt. 1) and 'Woodard' (expt. 1 and 2)
rabbiteye blueberry plants ...........................65









5-3. Effect of flooding on root electrolyte leakage of
'Tifblue' (expt. 1) and 'Woodard' (expt. 1 and 2)
rabbiteye blueberry plants ...........................66

5-4. Effect of flooding on stomatal conductance of
'Tifblue' (expt. 1) and 'Woodard' (expt. 1 and 2)
rabbiteye blueberry plants ..........................68

6-1. Soil oxygen diffusion rates and redox potentials of
flooded and unflooded Myakka fine sand planted to
'Woodard' rabbiteye blueberry plants (expt. 1) .......77

6-2. Soil oxygen diffusion rates and redox potentials of
flooded and unflooded Myakka fine sand planted to
'Woodard' rabbiteye blueberry plants (expt. 2) .......79

6-3. Soil oxygen diffusion rates and redox potentials of
flooded and unflooded Myakka fine sand planted to
'Woodard' rabbiteye blueberry plants (expt. 3) .......81

6-4. Soil oxygen diffusion rates and redox potentials of
flooded and unflooded peat:perlite media planted to
'Woodard' rabbiteye blueberry plants .................83

6-5. Ethylene evolution from the rhizosphere of flooded
'Woodard' rabbiteye blueberry plants grown in Myakka
fine sand (expt. 1) at 200C, 250C, and 300C soil
temperatures .........................................89

6-6. Ethylene evolution from the rhizosphere of flooded
'Woodard' rabbiteye blueberry plants grown in Myakka
fine sand (expt. 2) at 200C, 250C, and 3000 soil
temperatures ...................................... 91


viii
















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


EFFECT OF FLOODING DURATION, PERIODIC FLOODING,
SEASON, AND TEMPERATURE ON GROWTH, DEVELOPMENT,
AND WATER RELATIONS OF YOUNG RABBITEYE BLUEBERRY
(Vaccinium ashei Reade) PLANTS

By


Jonathan Henry Crane


August 1987


Chairman: Dr. Frederick S. Davies
Major Department: Horticultural Science (Fruit Crops)

Flooding is a problem for southeastern blueberry growers because

many plantings are on flatwoods-type soils underlain by an impervious

hardpan. Field, laboratory, and combination field-laboratory

experiments were used to study the effect of flooding on young rabbiteye

blueberry (Vaccinium ashei Reade) plant survival, growth, development,

and water relations.

Two- to 3-year-old rabbiteye blueberry plants were used to study

the interactions of flooding duration and season (spring and summer) and

periodic flooding of different durations and seasons (spring and summer)

under field conditions. Survival of rabbiteye blueberry plants appears

to be a function of season and the total number of days of flooding,

regardless of whether flooding is accumulated periodically or

continuously. Generally, the longer soil redox potentials were below









200 mV, the more detrimental flooding was to rabbiteye blueberry growth

and survival. Plant survival in spring significantly decreased after 25

days of continuous or 3 periods of 15-day periodic flooding; however,

it decreased significantly after only 15 days of continuous or two 7-day

flooding periods in summer.

Flooding effects on fruit set and yields were variable, while

number of flower buds formed was significantly decreased after 5 days of

continuous or two 2-day summer flooding periods. Stomatal conductance

significantly decreased after 2 days of periodic or continuous flooding

and was slow to recover after protracted periods of flooding.

The relationship of root and stem hydraulic conductivity, root

electrolyte leakage and stomatal conductance for flooded and unflooded

containerized plants was investigated. Stomatal conductance and root

hydraulic conductivity of flooded plants decreased concomitantly after 4

to 6 days of flooding. After 6 to 10 days of flooding, stem hydraulic

conductivity decreased and electrolyte leakage increased for flooded

plants, possibly indicating xylem plugging and irreversible root damage,

respectively.

The effect of flooding, soil media, and root temperature (200C,

250C, and 300C) on plant survival and growth was investigated under

controlled laboratory conditions. Number of days of plant survival of

flooded plants grown in Myakka fine sand decreased as soil temperature

increased, but soil temperature did not affect survival of plants grown

in peat:perlite media.
















CHAPTER I
INTRODUCTION


Rabbiteye blueberry (Vaccinium ashei Reade) plants are native to

the riverbanks of the southeastern United States. Although rabbiteye

blueberry plants are considered moderately flood tolerant, growers have

observed poor plant vigor and yields from plantings on poorly drained

sites. Moreover, shoot growth, stomatal conductance to water and carbon

dioxide, and carbon assimilation are reduced for flooded containerized

rabbiteye blueberry plants. Flooding of containerized rabbiteye

blueberry plants during the early spring and fall has been found to be

less detrimental to plant growth than spring or summer flooding.

Florida's blueberry industry is expanding rapidly with about 428 ha

currently planted (32). Older plantings which are predominately U-pick

operations were planted to rabbiteye cultivars, while newer plantings,

aimed at the fresh fruit market, consist of a combination of rabbiteye

cultivars and earlier ripening hybrids.

Many of Florida's blueberry plantings are on acid flatwoods type

soils characterized by poor drainage and an impervious soil layer, which

may flood periodically after heavy rainfall or excessive irrigation.

Plantations with low-lying areas, improperly sloped drainage ditches,

and insufficient planting bed height add to the flooding problem.

The objective of the first part of this research was to investigate

the effect of flooding rabbiteye blueberry plants under field

conditions. Two experiments were designed, one to investigate the









interaction of flooding duration and season (spring and summer) and the

second to examine the effect of periodic flooding of different durations

during the spring and summer on the growth and development and water

relations of young rabbiteye blueberry plants.

Combination field-laboratory research concentrated on the effect of

short-term flooding on rabbiteye blueberry plant water relations. The

objective of the research was to examine the relationships between root

and stem hydraulic conductivity, stomatal conductance, and root

electrolyte leakage of flooded and unflooded rabbiteye blueberry plants.

Experiments under controlled laboratory conditions had two objectives:

1) to investigate the effect of flooding soil temperature and soil type

on rabbiteye blueberry plant survival, and; 2) to determine if ethylene

was evolved from a common field soil (Myakka fine sand) found at many

blueberry plantings and if so, whether it was associated with plant

response to flooding.















CHAPTER II
REVIEW OF THE LITERATURE


Flooding affects soil microbiology, structure, and chemistry.

Displacement of the soil atmosphere by water and the subsequent

depletion of dissolved oxygen by soil microorganisms and plant roots

changes the soil from an aerobic to a hypoxic or anaerobic condition.

Aerobic microbial respiration is replaced by anaerobic or fermentative

respiration and the once oxidized soil becomes reduced.

Mesophytic plants are obligate aerobes and cannot live indefinitely

under anaerobic soil conditions. Flooding tolerance varies greatly

among and within mesophytic plant species and is affected not only by

the inherent genetics of the plant, but by the physiological state of

the plant (e.g., dormant or active), presence of pathogens, and

environmental parameters such as soil temperature, and the duration and

season of flooding. Flooding may drastically alter plant growth,

development, anatomy, physiology, and biochemistry.

Effect of Flooding on Soil

The oxygen content of unflooded soils ranges from about 21% by

volume (v/v gas:gas) to less than 5% (105, 109), depending on soil

depth, porosity, and moisture content, as well as microbial and plant

root respiratory demands (47, 109). Low oxygen content of flooded soils

is due to the following: 1) initial displacement of oxygen from large

soil pores upon flooding; 2) utilization of remaining oxygen by aerobic,

microaerophyllic, and facultatively anaerobic microbes and plant roots;









3) the dramatically reduced oxygen diffusion rate through water (10,000

times slower than through air); and 4) the reoxidation of reduced

compounds in the thin oxidized layer of soil at the soil-water interface

(105, 109).

Gas exchange between the soil and atmosphere occurs by mass action

and diffusion (105). The driving force for mass action is variation in

temperature, pressure, and rainfall, while oxygen diffusion is driven by

concentration gradients. Under well drained soil conditions both forces

influence soil aeration, however, under saturated or flooded soil

conditions diffusion is the predominant process responsible for gas

exchange (105).

During the 1950s, Lemon and Erickson (87, 88) adopted polarographic

techniques to characterize soil aeration status. Theoretically,

electric current produced from reduction of oxygen at the surface of a

platinum electrode is proportional to and governed by the rate at which

oxygen diffuses to the electrode surface from the surrounding soil

solution (88). When a voltage is applied between the platinum and

reference electrode oxygen reduction occurs at the platinum electrode

surface and produces a current. By convention, the oxygen diffusion

rate (ODR) is converted from amperes to flux (ug 02 cm min ) (87,

88). Typically, ODR values range from 0.7 (unflooded but moist soil
-2 -1
conditions) to 0.2 ug cm min (submerged soil conditions) (13).

Several problems are encountered in the practical use of the

polarographic method for determining ODR. Theoretically, ODR should

increase as soil moisture content (SMC) decreases (87, 133); however,

readings taken as a function of SMC demonstrated ODR values increased as

SMC decreased to a point, beyond which ODR values decreased with









decreasing SMC (8, 92). This is because in unsaturated media the

platinum electrode surface may not be completely covered by water, which

causes the oxygen reduction rate, not the oxygen diffusion rate to be

the current controlling process (92). Other problems with the

polarographic method include contamination of the platinum electrode

surfaces with iron and aluminum oxides, calcium and magnesium

carbonates, and high concentrations of phosphorus, sulfur, copper, and

arsenate (92). These substances reduce ODR values by restricting oxygen

diffusion to the electrode surface and reducing the electrode surface

area exposed to the soil (92).

Despite the problems associated with ODR measurements, they have

proven useful in associating soil oxygen flux with plant responses under

moderately moist to submerged soil conditions. Root growth of

snapdragon (134), cotton, sunflower (90), and barley (91) ceased at ODR

values of 0.2 yg cm-2 min-1 or below. Stomatal conductance of numerous

herbaceous (129) and woody (6) plants have been found to decrease in
-2 -1
response to ODR values below 0.15 to 0.30 pg cm min Plant uptake

of potassium and phosphorus decreased with decreasing soil ODR in pea

(26), snapdragon (89), and barley (91).

Besides oxygen depletion, flooding induces numerous microbiological

and electrochemical changes in the soil. Aerobic soils are

characterized by adequate oxygen for microbial, plant, and chemical

oxidative processes (105), while facultative and obligate anaerobes

predominate under anoxic or hypoxic soil conditions because of their

ability to utilize electron acceptors other than oxygen in their

respiration (47, 105). During anaerobic respiration organic matter is









oxidized and oxidized soil components are reduced in a sequence

determined by the thermodynamics of the redox system (104).

A useful measure of the oxidation-reduction status of a soil is

redox potential (Eh). Redox potentials are a qualitative measure of the

electrochemical state (oxidized or reduced) of all the redox couples

present in the soil solution (47, 104). Platinum electrodes inserted in

soil acquire a potential determined by the oxidation-reduction state of

the soil. This potential is measured against a reference electrode of

known potential on a microvoltmeter (104). Intrinsic errors of redox

potential measurements include pH effects, absence of true equilibrium,

and poor soil-water-electrode contact (104). Aerobic soils have Eh in

the range of 200 to 800 mV, while anaerobic soils range from less than

200 to -450 mV (104, 120). The rate and magnitude of Eh decrease upon

flooding increased with increased organic matter content, concentration

of electron acceptors, temperature, and flooding duration (25, 104).

The reduction of submerged soils roughly follows a sequence predicted by

thermodynamics, but is microbially mediated. The first to last soil

components to be reduced are as follows: oxygen, nitrate, manganese,

ferric iron, sulfate, and carbon dioxide (104, 105).

Generally, soil nutrient availability increases after submergence,

reaches a maximum, and then declines as flooding duration increases

(25, 104). Increased nutrient availability of submerged soils is due to

the solubilization of calcium, iron or manganese phosphates (104),

increased solubility of reduced forms of iron and manganese, and

displacement of potassium, calcium, magnesium, and sodium from soil

colloids by reduced manganese and iron (105). Organic nitrogen may

undergo a complex cycle of transformations which includes








consecutive years increased radial trunk growth by about 50% compared to

unflooded trees. Apple trees survived 3 consecutive 6-week periods of

flooding during the spring, summer, or fall (96), but growth and plant

vigor decreased with successive floodings.

Flooding adversely affects growth and development of mesophytic

plants. Dry weights of numerous herbaceous (43, 65, 85, 141), hardwood

(95, 137, 138), and fruit tree (86, 96, 127) species decreased after

flooding. Likewise flooding has been shown to inhibit shoot elongation

(1, 2, 6, 39, 65, 149) and leaf expansion (2, 43, 56, 58, 65, 95, 137,

138, 141) of numerous plants. Flooding may induce increased flowering

as in apple (24, 96) or decreased flowering as in highbush blueberry

(3), while fruit set and yields decreased for both apple (24, 96) and

highbush blueberry (3).

Flooding may induce numerous morphological and anatomical changes

in plants such as stem and lenticel hypertrophy, leaf epinasty, root and

stem aerenchyma formation, adventitious rooting, and replacement root

formation (61). Other symptoms of flooding include leaf wilting (61,

119, 123), chlorosis (6, 61), reddening (6, 61), and abscission (6, 61,

83), small fruit size and color (24), premature fruit drop (3, 61), and

stem dieback (6).

Effect of Flooding on Plant Physiology

Water uptake under submerged soil conditions decreases for numerous

herbaceous (16, 83) and woody (5, 7, 35, 95, 100, 135, 137, 138) plant

species. This has been attributed to decreased permeability of roots to

water induced by low oxygen and high carbon dioxide concentrations found

in submerged soils (21, 51, 82, 83), and to xylem blockage (5).








A rapid and common response of many plants to flooding is reduced

stomatal conductance and transpiration (6, 36, 100, 106, 129), despite

the fact leaf water potentials may be similar for flooded and unflooded

plants (6, 16, 36, 100). Stomatal closure of flooded plants has been

attributed to reduced root hydraulic conductivity (5, 34, 35, 136)

inducing leaf desiccation and decreased water potentials (28, 81, 84),

potassium deficiency caused by reduced absorption (81, 143), decreased

production and translocation of cytokinins (14, 15, 19), and

gibberellins (81, 111) produced in the roots, increased ethylene

concentration (99, 139), and toxic substances produced by anaerobic

microorganisms (42, 81) or plant roots (42, 81).

Carbon assimilation of mesophytic plants decreases under flooded

conditions (14, 23, 35, 36, 37, 99, 101, 103, 110, 146). Davies and

Flore (37) showed carbon assimilation of flooded rabbiteye blueberry

plants decreased to 64% of unflooded plant values after 1 day. The

initial reduction in carbon assimilation after short-term flooding has

been attributed to decreased stomatal conductance (23, 35, 36, 81, 103),

while reduced carbon assimilation after long-term flooding has been

attributed to decreased carboxylation efficiency (35), decreased quantum

yield (37), increased photorespiration or dark respiration (146), and

disruption of carbohydrate transport from leaves (146). Decreased

chlorophyll content of leaves (72, 141, 143), premature senescence (43,

141), and decreased leaf area (43, 95, 137, 138, 141) would also

contribute to lowered carbon assimilation of flooded compared to

unflooded plants.

Nutrient absorption may decrease under flooded conditions (81)

despite the increased nutrient availability of submerged soils (25,








105). Nitrogen, potassium, and phosphorus content of agronomic crops

such as wheat (141, 142, 143), barley (43), rice (21), and corn (21),

vegetable crops like tomato (65) and pea (72), and fruit crops like

citrus (86), apple (57), avocado (127), and highbush blueberry (58)

decreased under flooded conditions. Iron, manganese, and boron

absorption may also decrease under flooded conditions (58, 65, 86, 114,

127), while sodium (65, 86) and chlorine uptake (86, 148) may increase.

Flooding may also reduce translocation of photosynthates from

source to sink regions of plants (81). Anaerobic conditions have been

shown to inhibit translocation of sugars from source leaves (131) and

uptake at the sink tissue (root) (48) of beets and along the

translocation pathway of squash (126) and Saxifraga sarmentosa (109).

Reduced translocation has been attributed to decreased membrane

permeability (49), increased cytoplasmic viscosity (49), reduced

metabolic energy levels (48, 131), and blockage of sieve tube elements

(107) under anaerobic root conditions.

Flooding and Plant Hormones

Flooding may affect the synthesis, metabolism, and translocation of

plant hormones. This is reflected in the numerous changes in plant

growth such as leaf epinasty, aerenchyma development, stem hypertrophy,

and reduced shoot and root growth which many plants undergo due to

flooding stress (111).

The concentration of ethylene produced in flooded soils and plants

is a function of the relative rates of production, catabolism, and

diffusive efflux to the atmosphere (42, 66). Biosynthesis of ethylene

from methionine has been well characterized for tomato (4, 153), but








alternative precursors and modes of production may exist in other plant

species and microorganisms (66).

Ethylene has been shown to be involved in many morphological and

physiological plant responses to flooding (66), including leaf and

petiole epinasty (17, 67, 68), leaf wilting (123), senescence and

abscission (44), inhibition of root extension (33, 41), initiation of

adventitious rooting (40, 74), aerenchyma development (40, 41), and stem

and lenticel hypertrophy (80, 111). Ethylene may also be involved in

stomatal closure of some flooded plants (16, 98, 99, 139), although the

response is variable (16, 98, 99) and the mechanism unknown (99).

Several studies with corn (70) and tomato (66, 75) have demonstrated

ethylene gas applied to the soil solution moves rapidly into roots and

shoots to initiate petiole epinasty.

Shoots may accumulate auxin under flooded conditions (102, 145) and

an auxin-ethylene interaction may be involved in adventitious root

formation of sunflowers (102, 145) and stem hypertrophy of woody plants

(61). Yu and Yang (155) demonstrated auxin stimulated ethylene

production in mung bean roots by inducing or activating

l-amino-cyclopropane-l-carboxylic acid (ACC) synthase, which catalyzes

the conversion of S-adenosylmethionine to ACC in flooded tomato plants

(4). Wample and Reid (145) found ethylene alone did not stimulate

adventitious root formation in sunflower and proposed ethylene-induced

auxin accumulation was involved. Auxin accumulation in shoots may be

caused by decreased auxin oxidation in roots and an ethylene or

anoxia-induced inhibition of basipetal auxin movement (102, 111).

Abscisic acid (ABA) content of leaves of flooded plants has been

shown to increase (60, 124, 152) and induce stomatal closure (15, 60).








However, the stimulus and source of increased leaf ABA content is

unknown because leaf turgor pressure is generally maintained in flooded

plants (16, 81, 100). Proposed stimuli for increased ABA content of

flooded plants include transient leaf wilting, translocation of ABA from

flood-stressed roots, and induction of ABA production from root or soil

produced toxins (60, 111).

Gibberellic acid (GA) and cytokinin content have been shown to

decrease in flooded tomato plants (15, 19, 68, 112, 113) and this was

characterized by reduced stem elongation (68, 112), increased leaf

epinasty (68), and reduced dry weight (68). Appearance of adventitious

roots, thought to be a source of GA (68, 112), or exogenous application

of GA to flooded tomato plants decreased leaf epinasty and increased the

rate of stem elongation (68, 112).

Reduced cytokinin production and export from flooded roots of

tomato plants resulted in decreased leaf chlorophyll content (20),

stomatal conductance (15, 68), and photosynthetic capacity (15).

Symptom expression was slowed and/or prevented by exogenous application

of cytokinin (15, 19) or cytokinin plus GA (68).

Flooding and Plant Metabolism

Higher plants are obligate aerobes and under aerobic conditions

oxidation of 1 mol of glucose to carbon dioxide and water via the

electron transport system yields 38 mol of adenosine triphosphate (ATP).

In contrast, the oxidation of glucose is blocked under anaerobic

conditions and only 2 mol of ATP are produced by substrate

phosphorylation via the glycolytic pathway which does not require oxygen

to function.








Mitochondria are less numerous and larger in oxygen deficient

tissues (97). After several hr of anaerobiosis, mitochondria of flood

tolerant (144) and intolerant (97, 144) plants developed tubular

inclusions, possessed fewer matrix ribosomes and cristae, and developed

irregular form. Exogenous glucose feeding of anaerobically treated rice

and pumpkin roots prevented mitochondrial ultrastructural changes and

prolonged function and survival up to 4 days (144); however, untreated

mitochondria were irreversibly damaged after 24 hr or more of

anaerobiosis (97).

Intimately dependent upon mitochondrial function is the ATP

concentration of plant tissues, which in turn is affected by the

environmental conditions of the cells (69). Adenylate energy charge

(AEC) is a useful means of comparing the energy status of plant tissues

and the activity of enzymes involved in various respiratory pathways and

is a ratio of the various adenylate pools in plant tissues (69).

Actively respiring aerobic tissues had an AEC of about 0.90, while the

AEC for root tips of maize (121), river birch (140), and European birch

(140) under hypoxic conditions was 0.20, 0.61, and 0.70, respectively.

Decreased AEC was correlated with increased alcohol dehydrogenase enzyme

(ADH) activity in river birch (140) and increased concentrations of

ethanol in maize roots (121).

Early investigations into respiratory alterations under flooded

conditions suggested flooding tolerance was greatest for those plants

demonstrating reduced ADH activity and low production or accumulation of

ethanol (30, 93). This led McManmon and Crawford (93) to propose a

metabolic theory of flood tolerance based on the control of metabolic

rate and production of glycolytic end products other than ethanol, which









was thought to be toxic. Later work, however, cast doubt on this theory

because both flood tolerant (63, 64, 71, 73) and intolerant (30, 31,

103) plant species produced ethanol under anaerobic conditions.

Furthermore, ethanol was not greatly toxic to numerous herbaceous (42,

71) and woody plants (42, 62) when applied at above endogenous

concentrations.

Anaerobic respiration of submerged roots and plant organs may lead

to several different fermentative end-products such as ethanol, malate,

lactate, shikimate, alanine, and glutamine (69). However, only

formation of ethanol, lactate, and alanine result in a net gain in ATP

thought necessary for maintaining cell integrity (69). More recently,

avoidance of cytoplasmic acidosis via ethanol production has been linked

to short-term tolerance of hypoxia (115, 116, 117) by soybeans, pumpkin,

and maize. Plants such as blackeye peas and navy beans (115) with low

ADH activity and ethanol production underwent cytoplasmic acidosis and

were intolerant of hypoxic conditions.

Hydrogen cyanide, produced by peach, apricot and plum roots (119),

phenols released by walnut roots (20), and hydrogen sulfide produced by

anaerobic bacteria in the rhizosphere of flooded citrus roots (46) have

been shown to decrease flooding tolerance of these plants.

Adaptations to Flooding

Flooding tolerance of mesophytic plants often depends upon a

combination of factors including anatomical, physiological, and

biochemical adaptations (61). Physiological and biochemical changes

include control of metabolic rate, i.e., glycolysis, diversification of

glycolytic end-products, and tolerance, detoxification, or efflux of

glycolytic end-products (10, 31, 93). Anatomical adaptations which may








increase the aeration of submerged plant parts include adventitious

roots, aerenchymatous tissue, lenticel and stem hypertrophy, and

development of aerenchymatous replacement roots (76).

Whether anatomical changes are an adaptation or a symptom of

flooded conditions is debated (61). This is because the correlation

between anatomical changes (e.g., aerenchyma) and flooding tolerance is

not always good (6, 61). However, there is indisputable evidence for

oxygen transport from the atmosphere to the root system of woody (11,

29, 62) and herbaceous (12, 61, 154) plants. Vartapetian et al. (142)

attributed the difference in flood tolerance of rice and pumpkin to the

plants' inherent ability to form aerenchyma tissue, since no differences

in mitochondrial function were found for either plant's root tissue

under anoxic conditions.

Flooding and Plant Disease

Flooding stress may predispose plants to pathogen invasion. This

depends, however, on the tolerance of the pathogens to flooded

conditions, activities of antagonistic microbes to the pathogen, and

host resistance to infection (42, 135). High soil water content is

necessary for the germination or sporulation, mobility and dispersal,

and growth of many plant pathogens associated with flooded soil

conditions (42, 135).

Roots exposed to hypoxic or anoxic soil conditions increase

exudation of soluble sugars, amino acids, and organic acids (135).

These substances released in the rhizosphere are known to attract

pathogenic microorganisms (135). Increased root exudation combined with

nutrient deficiencies and decreased carbon assimilation and

translocation may predispose many plants to pathogen invasion (42, 135).








Blueberry species are reported to vary in their tolerance of

Phytophthora infection (45, 94). Generally, rabbiteye blueberry plants

attract fewer zoospores and are less affected by Phytophthora than

highbush blueberry plants (45, 94). However, these studies were done on

either excised root pieces or non-stressed whole plants and cannot be

extrapolated to field conditions.















CHAPTER III
FLOODING DURATION AND SEASONAL EFFECTS ON GROWTH
AND DEVELOPMENT OF YOUNG RABBITEYE BLUEBERRY PLANTS

Introduction

Flooding is a periodic problem for blueberry growers in the

southeastern United States because rabbiteye blueberries are often

planted into poorly drained flatwoods soils underlain by an impervious

hardpan. Consequently, flooding is a common occurrence during the rainy

season in late spring and summer. Growers have observed reduced plant

vigor and yields, and in some cases plant mortality due to flooding (77,

F.S. Davies, personal communication).

Plant survival under flooded conditions varies greatly among and

within mesophytic species ranging from 1-2 days for tobacco (84), 5-20

days for peach (6, 119), 7-19 days for different walnut species (20), 12

months for apple (6), and over 20 months for Pyrus betulaefolia Bunge

(6). In contrast, various mangrove, cypress (10), willow (6, 9), tupelo

and ash (62, 63) species live and thrive under waterlogged conditions.

Rabbiteye blueberry plants grown in containers survived up to 2 months

of flooding (39), while containerized highbush blueberry plants grown in

a peat:perlite:sand media survived up to 30 months of flooding (2).

Flooding adversely affects plant and fruit growth. Davies and

Wilcox (39) found stem number and length of flooded rabbiteye blueberry

plants decreased compared to unflooded plants. Flooding inhibited stem

growth and induced stem dieback of highbush blueberry plants (1, 2).

Similarly, flooding reduced stem elongation in apple (6, 96), quince,









and pear (6) and leaf expansion of American sycamore (137), elm (95),

paper birch (80), apple (56), and highbush blueberry (2, 58). Childers

and White (24) reported increased flowering for a 'Winesap' apple tree

flooded 5 weeks the previous spring; however, fruit were small and

dropped prematurely.

Time of year when flooding occurs also affects flooding tolerance.

Numerous hardwood species like sweetgum, green ash, and sycamore

tolerate dormant season flooding but may be damaged by flooding during

the growing season (55). Childers and White (25) and Olien (96)

reported summer flooding was more detrimental to apple growth than fall

flooding. Containerized rabbiteye (38) and highbush blueberry (1, 2)

plants are more flood tolerant during the fall, winter and early spring

than during the summer.

Based on previous studies with containerized rabbiteye blueberry

plants, stomatal closure (gs ) is an early (3 to 7 days) and rapid

response to flooding stress (36, 39). Davies and Wilcox (39) showed

stomatal closure of rabbiteye blueberry plants occurred more rapidly in

summer than spring. Generally, however, this is not the result of

desiccation stress (36, 39) as leaf water potentials of flooded and

unflooded plants are similar.

Rabbiteye blueberry plants are considered moderately flood tolerant

based on studies with potted plants (38, 39); however, little work has

been done to determine tolerance of young bushes in the field. Our

objectives were to determine the effect of different flooding durations









during spring and summer on rabbiteye blueberry survival, growth, fruit

set, yields, and water relations under field conditions.

Materials and Methods

Plant Material

Three-year-old 'Woodard' rabbiteye blueberry plants were planted in

the field during Feb. 1984 and 1985 at the Horticultural Research Unit

near Gainesville, Florida. A 9.1 x 9.1 m and 9.1 x 21.9 m plot of land

were used in 1984 and 1985, respectively. Soil type was a Myakka fine

sand (sandy, siliceous, hyperthermic, Aeric Haplaquod) characterized by

a pH of 5.0 and a hardpan at about a 0.9 m soil depth. An initial soil

test revealed Phytophthora cinnamomi was not present prior to planting.

However, subsequent tests of roots revealed some plants were infected

with Phytophthora cinnamomi (Dr. D. J. Mitchell, personal

communication). Bare-rooted plants were placed into square planting

holes (about 0.6 m on a side) spaced 1.8 x 1.8 m apart lined with 0.15

mm plastic and backfilled with Myakka fine sand soil. One 3-year-old

bush was planted per hole.

Treatments consisted of flooding for 0, 5, 15, 25, and 35 days

during the spring and summer-fall growth periods. Treatments were

laid out in a completely randomized design, with 3 single plant

replications per treatment per season in 1984 and 6 single plant

replications per treatment per season in 1985. Flooding during the

spring began 13 Mar. 1984 and 23 Mar. 1985, while summer-fall flooding

started 1 Sept. 1984 and 23 Sept. 1985. Plots were flooded using a

garden hose at irregular intervals to maintain 3 to 5 cm of standing

water in treatment plots. Flooding treatments were discontinued by

puncturing the plastic-lined plots.









Soil Oxygen Measurements

In the summer of 1984, soil oxygen diffusion rate (ODR) was

determined at 15- and 30-cm soil depths using 25-gauge platinum

electrodes attached to an Oxygen Diffusion Ratemeter (Jensen

Instruments, Model C). A Ag /AgCl reference electrode was used.

Measurements were taken at an applied voltage of 650 mV after a 5-min

equilibration period. In spring and summer of 1985, soil oxygen

diffusion rate was determined as previously described and in addition

soil redox potential was determined using platinum electrodes at 15- and

30-cm soil depths attached to a Keithley microvoltmeter (Model 177 DMM).

A Ag /AgCl reference electrode was used and a value of 222 mV was added

to all redox potential readings to adjust for the potential of the

reference electrode. Readings were taken after a 5- to 10-min

equilibration period and electrodes were removed from the soil between

sampling dates. Soil characterization data were analyzed within a

season by a split-plot analysis with treatments as main plots and soil

depth as subplots.

Plant Measurements

Plant survival was determined by visual assessment of stem bark and

roots (yellow-orange = alive and brown = dead) based on previous

experiments with rabbiteye blueberry. Leaf area was determined for all

plants at termination of the spring (15 July 1984, 13 July 1985) and

summer (10 Nov. 1984, 11 Nov. 1985) treatments using a LI-COR leaf area

meter (Model LI-3000). Leaf expansion was determined in 1985 on 3

leaves per bush by measuring leaf length and width with a ruler and

using the product (x) in an equation for leaf area (area=0.369+0.660(x),
20
r =0.97) derived from linear regression of leaf area determined with the









leaf area meter on leaf length x width. Shoot growth was determined

with a ruler by measuring all new shoots arising from 3 main stems in

1984 and 1 new shoot arising from each of 3 main stems in 1985. Flower

counts were made on 16 Mar. 1984 and 21 Mar. 1985. Percent fruit set

and yields were determined between 20 June and 13 July in 1984 and 29

May and 15 July in 1985, while flower buds were counted on all shoots on

10 Nov. in 1984 and 6 Nov. in 1985. Data were analyzed by Williams'

method which compares flooding treatments with an unflooded control

(22). This method assumes the responses (means) to the treatments are

monotonically ordered and involves ranking treatment means, taking an

average of treatment means not following the monotonic order,

calculating the standard error of a difference, and testing the null

hypothesis that there is no response to the treatment using a t-test.

This method more accurately describes the relationship between

treatments than regression analysis.

Water Relations Measurements

Abaxial stomatal conductance was determined between 1100 and 1230

hr on 2 randomly selected mature leaves per plant using a LI-COR Steady

State Porometer (Model 1600) as described previously (39). Leaf water

potentials were determined between 1230 and 1330 hr on 2 randomly

selected leaves per plant using the pressure chamber method (122). Data

were analyzed by Williams' method (22).

Results and Discussion

Soil ODR and Redox Potential

Soil ODR of flooded plots at the 15-cm soil depth decreased to 0.20
-2 -1
jg cm min or less within 2 days of flooding and remained below those

of unflooded plots during the 1984 treatment period (Fig. 3-1). The









0.7


0.6 l


0.5 Flooding .S
duration













e 0.5 .
(Days)












'9 0. 30 cm depth


0.4
C)












%0.3
2 0.2
0 0.1














0. 0 5 10 15 20 25 30 35cm depth











significance among treatments for a given day.
significance among treatments for a given day.









situation was similar in 1985 during the first 15 days of the study

(Fig. 3-2). However, frequent rainfall during the last part of the

study decreased ODR values of unflooded plots. Soil ODR generally was

lower and more similar among treatments at the 30- compared to the 15-cm

soil depth for both seasons (Figs. 3-1, 3-2). Oxygen diffusion rates
-2 -1
below 0.20 yg cm min have been reported to inhibit or slow root

growth of snapdragon (134), and barley (91) and shoot growth of pea

(26). The minimum ODR for rabbiteye blueberry is unknown; however,

Korcak (79) found little difference in respiration rate of rabbiteye

blueberry root segments between 2.5 and 21% oxygen.

Redox potentials of flooded plots decreased to negative values

within 1 day after flooding and were significantly more negative than

those of unflooded plots (Fig. 3-3). Redox potential was generally

higher at the 15- compared to the 30-cm soil depth. Redox potentials

greater than about 300 mV are indicative of aerobic conditions, while

potentials below 200 mV suggest anaerobic conditions (105, 120).

Following termination of the flooding treatments, redox potentials

recovered to control levels within 10 days. Rapid increases and

decreases in redox potentials in response to soil oxygen status have

been demonstrated under laboratory conditions (108).

No consistent relationship between ODR and redox potential values

was found (Figs. 3-2, 3-3). Oxygen diffusion rates and redox potentials

decreased rapidly in response to flooding as found by others (105, 120).

However, while redox potentials increased within 10 days of flooding

termination at both sampling depths, ODR values increased only slightly

at the 15-cm depth and were variable at the 30-cm depth. Similarly,

Armstrong (8) found a large increase in redox potential associated with




















*K II

O+


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.C 4 (o
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4-) 0W *U -

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ca


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- 000 -


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O b )
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Oc
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IH (*0^00
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400 uraion *- -- -.
(Days) ns


300


300 --15
025
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200 -


Z100


CL 0
0

S-100 *


-200

ns
300 30 cm depth


200


100
0sns








-200


-300
0 5 10 15 20 25 30 35 40
Time (Days)






Fig. 3-3. Soil redox potentials of flooded and unflooded plots planted
to 'Woodard' rabbiteye blueberry plants during spring, 1985. Each
point is the mean of 3 plots (10 reps per plot) taken at the 15- and
30-cm soil depths. Symbol **, indicates significance 4 to the 1%
level and ns = non-significance among treatments for a given day.









only a small increase in ODR. Oxygen diffusion rates of flooded plots

never recovered to control levels after flooding termination (Figs. 3-1,

3-2), while redox potentials indicated the soil was reoxidized. Poor

recovery of ODR in plots where flooding was ended may have been due to

decreased soil porosity caused by solubilization of aggregate forming

soil constituents and oxygen diffusing into the soil being consumed in

reoxidation of reduced soil components (105).

Plant Survival

All plants survived 25 days of flooding in 1984, with some losses

resulting from 35 days of flooding during the spring (Table 3-1). In

contrast, percentage survival decreased significantly with flooding

duration in the spring and summer of 1985. Previous studies have shown

that rabbiteye blueberry plants survive more than 58 days of flooding

when grown in containers (39). These differences in survival for

different experiments and years may be due to preflood environmental

conditions on plant vigor and plant-to-plant variability which is

prevalent in most flooding studies (6, 39, 80), or to greater incidence

of Phytophthora root rot (Dr. D. J. Mitchell, personal communication)

and/or soil produced toxins (42, 105, 135). Flooding stress may

predispose plants to pathogen invasion (135), and although most

rabbiteye cultivars have been reported to be tolerant to root rot (45,

94) current work (Dr. D. J. Mitchell, personal communication) with

flooded rabbiteye blueberry plants suggests that root rot is a problem

particularly when flooding occurs during hot summer months.

Field observations indicate that flooded rabbiteye blueberry plants

died either slowly over a 3- to 5-week period or rapidly within one

week. Symptoms of plant death due to physiological causes include leaf










Table 3-1. Flooding duration and survival of
'Woodard' rabbiteye blueberry plants.



Flooding Survival (%)
Year duration (days) Spring Summer


1984


100
100
100
100
83


1985 0 100 100
5 100 67*
15 100 33*
25 83*7 17*
35 67* 0*


z
n=3 plants in 1984, 6 plants in 1985.

Y*Indicates that the treatment differs from
unflooded control by Williams' method, 5%
level.








chlorosis, reddening, necrosis, and abscission followed by stem dieback.

This is in contrast to the rapid desiccation, browning, and death of

stems and leaves, and lack of leaf abscission of plants damaged by root

rot.

More rapid plant death observed during summer than spring flooding

suggests that soil and air temperatures may be important determinants of

flooding tolerance. Flooded soil temperatures reached 220C (15-cm

depth) in spring, and 330C (15-cm depth) in summer, while air

temperatures reached 270C in spring and over 380C during summer flooding

in 1984 and 1985. Abbott and Gough (1, 2) suggested that increased soil

and air temperatures increased the extent of stem and root damage and

the rate of highbush blueberry plant death. Survival of potted

rabbiteye blueberry plants was much reduced at 300C compared with 200C

in growth chamber studies (Table 6-1). Davies and Flore (36) found that

carbon assimilation of flooded rabbiteye blueberry plants became

negative above 280C, possibly contributing to the more rapid death of

plants at high temperatures.

Stem Elongation

The effect of flooding on stem elongation was variable in 1984, but

stem elongation decreased in the spring and summer of 1985 (Figs. 3-4,

3-5). Spring and summer stem elongation was variable among treatments

for the first 15 and 7 days, respectively, whereafter growth was

inhibited for flooded compared to unflooded plants. Some stem growth

occurred during and after spring flooding for plants flooded 5, 15, and

25 days. In contrast, stem elongation ceased for plants flooded in the

summer and little difference was found among plants flooded 5, 15, 25,

and 35 days. This resulted from wilting of immature stems, increased









20.0


cu 15.0
E


aM10.0



-5.0



0
Flooding
20.0 duration



15.0 15





0 025





0 5 10 15 20 25 30 35 40 45 50 55
Time (Days)




Fig. 3-4. Effect of flooding duration on stem elongation and leaf
expansion of 'Woodard' rabbiteye blueberry plants during spring,
1985. Each point is the mean of 3 leaves or stems of 6 plants per
treatment+SD.










ri iouing
5.0 / duration
NE (Days)
o/ 0

130.0 A n 115
O / 025
S/ 35
5.0
-W%


0

E10.0

-c

a 5.0



0
0 5 10 15 20 25 30 35
Time (Days)
Fig. 3-5. Effect of flooding duration on stem elongation and leaf
expansion of 'Woodard' rabbiteye blueberry plants during summer,
1985. Each point is the mean of 3 leaves or stems of 6 plants per
treatment+SD.









stem die-back, and plant death. Davies and Wilcox (39) found decreased

stem elongation of flooded compared to unflooded containerized 'Woodard'

blueberry plants. Similarly, prolonged flooding also reduced stem

elongation in highbush blueberry (2), apple, quince, and several Pyrus

species (6).

Leaf Area and Expansion

Total plant leaf area decreased after 35 days of spring flooding in

1984 and 1985 (Table 3-2). This decrease can be attributed to increased

leaf abscission and decreased individual leaf area. Total leaf area was

not significantly decreased by summer flooding treatments in 1984. In

the summer of 1985, total plant leaf area decreased significantly after

5, 15, 25, and 35 days of flooding, mostly due to decreased plant

survival (Table 3-1). Individual leaf area significantly decreased

after 35 days in 1984 and 25 and 35 days of spring flooding in 1985

(Table 3-2). This was probably due to wilting of immature leaves.

Individual leaf areas were not affected by summer flooding treatments in

1984, but were significantly decreased after 5, 15, 25, and 35 days of

summer flooding in 1985. This can be attributed to leaf wilting and

plant death. Similarly, individual leaf area decreased for flooded

paper birch (80), American elm (95), apple (56), and highbush blueberry

(2 58). Individual leaf area increased after 35 days of summer

flooding in 1984. This is considered a anomoly not consistent with what

was found in 1985 (Table 3-2) or with flooded 'Tifblue' rabbiteye

blueberry plants (Chapter IV, Table 4-3). Leaf expansion was similar

among spring flooding treatments for 16 days, after which leaf areas of

unflooded plants continued to increase while leaf expansion stopped for











































en -K4










**
It -




M c0C4 0M-4-4

-4-4 ~4 .-







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-K


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C -.4 *-4 -4 -







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*'
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O


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flooded plants (Fig. 3-4). During the summer, leaf expansion stopped

for flooded plants after 7 days, but continued for unflooded plants for

an additional 8 days (Fig. 3-5).

Flower Bud Number, Fruit Set and Yield

The number of flower buds formed decreased significantly with as

little as 5 days of summer flooding in 1984 and 1985 (Table 3-3). This

possibly results from hormonal imbalances (111), plant or soil produced

toxins (69, 105), or decreased carbon assimilation (36, 37), leaf area

(Table 3-2), and water uptake (Chapter V; 34) for flooded compared with

unflooded plants. In contrast, flooding significantly decreased the

number of highbush blueberry flower buds per shoot only after 4 to 12

months of flooding (3).

Percent fruit set decreased by 13% and 29% after 25 days of

flooding in 1984 and 1985, respectively (Table 3-3). Similarly, Abbott

and Gough (3) reported poor fruit set for flooded highbush blueberry

plants.

Yields decreased with 35 days of spring flooding in 1984 (Table

3-3). Low yield for unflooded plants was probably due to drought stress

as described previously (39). Yields were variable in 1985, but

decreased significantly after 25 days of flooding, with increased plant

death a contributing factor (Table 3-1). Fruit shrivelling and

abscission occurred for plants flooded 25 days or more in both years.

Flooding for 15 days or more also delayed fruit development and

ripening. Likewise, Haas (54) reported flooding increased avocado fruit









Table 3-3. Flooding duration, flower bud number,
fruit set, and yields of 'Woodard' rabbiteye
blueberry plants.



Spring Summer
Flooding Fruit Yield Flower buds
duration set dry wt formed
Year (days) (%) (g) (% of control)


1984 0 48 31.65 100
5 49 61.76 69*
15 58 58.20 48*
25 35*x 41.80 42*
35 30* 24.20* 49*

1985 0 48 25.80 100
5 38 9.67 33*
15 42 14.36 1*
25 19* 5.54* 0*
35 19* 5.29* 0*


zArcsin transformation of percent data before
analysis.

Yn=3 in 1984, n=6 in 1985.


X*Indicates that the treatment differs
unflooded control by Williams' method,


from the
5% level.









abscission and Childers and White (25) reported premature fruit drop

from a 'Winesap' apple tree flooded for 5 weeks during the spring.

In contrast to rabbiteye blueberry plants, some highbush blueberry

plants flooded for over 1 year, bloomed and set fruit the following

spring (1). Childers and White (25) and Olien (96) found a large

increase in the number of flowers on apple trees flooded the previous

spring. It is possible surviving rabbiteye blueberry bushes in this

study would have bloomed and set fruit the following spring. However,

the flower bud data (Table 3-3) indicated that potential yields obtained

would have been drastically reduced.

Water Relations

Stomatal conductance decreased significantly for flooded plants

after 2 days of summer flooding and remained below those of unflooded

plants throughout the treatment period (Fig. 3-6). Transpiration (E)

was more variable, but followed a similar trend as gs (Fig. 3-6). Leaf

water potentials (T ) were similar among flooded and unflooded

treatments for the first 2 weeks of treatments (-0.50 to -1.50 MPa).

However, 'F of plants flooded 25 days or more decreased to about -2.5

MPa indicating leaf water stress. This may have been due to reduced

plant water uptake (Figs. 5-1, 5-2).

In conclusion, the longer the soil Eh was below about 200 mV, the

more detrimental flooding was to plant growth and survival. Because ODR

values were variable among and within treatments, no clear association

was found between plant response to flooding and ODR measurements.

Flooding during the spring and summer adversely affected the survival

and growth of young rabbiteye blueberry plants under field conditions.

Plant survival, although variable among years, decreased in 1985 after











200


cO 150


cE
o

O E
EE 50
4-,


4.0


5 1


Flooding
duration
(Days)
S0
Os 5
A 15
025
035


15 20
Time (Days)


Fig. 3-6. Effect of flooding duration on stomatal conductance and
transpiration of 'Woodard' rabbiteye blueberry plants during summer,
1985. Each point is the mean of 6 plants (2 leaves per plant) +SE.
Symbol *, indicates the treatment differs from the unflooded control
by Williams' method, 5% level.









25 days of spring flooding, but after only 5 days of flooding in late

summer. This may be due to pathogen infection, but demonstrates that

even short-term flooding may kill plants thought to be moderately flood

tolerant. Stem elongation decreased with flooding duration, confirming

previous research on highbush (1, 2) and rabbiteye blueberry plants

(39). Plant survival and growth were more severely affected by flooding

during summer than spring. This may be attributed to two factors: 1)

the adverse effect high soil and air temperatures have on carbon

assimilation and plant respiration (36, 37); and 2) the possibility of

increased damage by Phytophthora root rot at elevated temperatures.

Flooding 25 days or longer during the spring reduced fruit set and

yields, while as little as 5 days of summer flooding significantly

decreased flower bud formation. Stomatal conductance and E of flooded

plants decreased significantly after 2 to 4 days of flooding, supporting

previous research with containerized rabbiteye and highbush blueberry

plants (36, 39).
















CHAPTER IV
PERIODIC AND SEASONAL FLOODING EFFECTS
ON SURVIVAL, GROWTH, AND STOMATAL CONDUCTANCE
OF YOUNG RABBITEYE BLUEBERRY PLANTS

Introduction

Flooding is often a problem for southeastern blueberry growers

because many blueberry plantings are on flatwoods type soils underlain

by an impervious hardpan. Growers have observed and we have documented

(Chapter III) the adverse effects of continuous flooding on rabbiteye

blueberry survival, growth, and yields under field conditions. Plant

survival and growth of rabbiteye blueberry plants decreased with

flooding durations greater than 35 days and flooding also reduced

percent fruit set, yields, and flower bud number (Chapter III).

Flooding adversely affects many physiological processes in

rabbiteye blueberry plants, such as stomatal conductance (36, 39),

transpiration (36), and root hydraulic conductivity (Chapter V) within 4

to 6 days of flooding. Carbon assimilation is reduced within 9 to 19

days and may become negative as flooding duration increases and at

temperatures above 280C (36). Root electrolyte leakage increased and

stem hydraulic conductivity decreased after 6 to 10 days of flooding

indicating damage to cell membranes and decreased water movement into

stems (Chapter V).

Relatively little research has been performed on plants exposed to

alternating flooded and unflooded conditions (periodic flooding). Two

days of flooding once a week for 6 weeks reduced tomato growth and









stomatal conductance as much as continuous flooding (106). Apple trees

survived 6 weeks of spring, summer, or fall flooding for 3 consecutive

years (96); however, the adverse effects of flooding on yields and plant

growth intensified with succeeding years. Periodic flooding decreased

plant survival (53, 55, 78) and growth (53) of trees in mixed hardwood

stands. Moreover, grower observations suggest that periodic flooding

may be more damaging to blueberry plants than continuous flooding under

field conditions.

Time of year and flooding duration also affect flooding tolerance.

Numerous hardwood tree species survived long-term dormant season

flooding, but succumbed to short-term flooding during the growing season

(55). Apple (56), highbush (1, 2) and rabbiteye (38) blueberry plants

tolerated moderately protracted dormant season flooding but were damaged

by short periods of flooding during the growing season.

Flooding may occur periodically during the growing season in

the southeastern United States. Rabbiteye blueberry plants survived up

to 58 days of continuous flooding (39); however, the effect of periodic

flooding of different durations on rabbiteye blueberry has not been

investigated. The objectives of this study were to determine the

effects of periodic flooding of different durations and seasons on

survival, growth, yields, and water relations of young rabbiteye

blueberry plants under field conditions.

Materials and Methods

Plant Material

Dormant 'Tifblue' rabbiteye blueberry plants were planted in the

field on a 9.1 x 21.9 m plot during Feb. 1985 and 1986 at the

Horticultural Research Unit near Gainesville, Florida. Soil type was a









Myakka fine sand (sandy, siliceous, hyperthermic, Aeric Haplaquod)

characterized by a pH of 5.0 and a hardpan at about a 0.9 m soil depth.

An initial soil test revealed Phytophthora spp. were not present prior

to planting. Three-year-old bare-rooted plants in 1985 and 2-year-old

plants in 1986 were placed into square planting holes (about 0.6 m on a

side) spaced 1.8 x 1.8 m apart, lined with 0.15 mm plastic and

backfilled with soil. Previous studies suggest that 2- and 3-year-old

rabbiteye blueberry plants respond similarly to flooding (38, 39).

Treatments consisted of periodic flooding for various durations

during the spring and summer-fall growing periods (Table 4-1).

Treatments were laid-out in a completely randomized design, with 6

single plant replications per season in both years. Plots were flooded

with a garden hose at irregular intervals to maintain 3 to 5 cm of

standing water in treatment plots during flooding periods. Holes were

punched in the plastic lining of the unflooded plant treatment plots to

allow drainage, while flooding treatment plots were flooded and

unflooded by raising and lowering the plastic lined side-walls.

Soil Oxygen Measurements

During the spring of 1985 soil redox potentials (Eh) were

determined at the 15- and 30-cm soil depths using 25-gauge platinum

electrodes attached to a Keithley microvoltmeter (Model 177 DMM). A

Ag+/AgCl reference electrode was used and a value of 222 mV was added to

all redox potential readings to adjust for the potential of the

reference electrode. Readings were taken after a 5 to 10 minute

equilibration period and electrodes were removed from the soil between

sampling dates. Data were analyzed within a season by a split-plot

analysis with treatments as main plots and soil depths as subplots.
































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Plant Measurements

Leaf area was determined for all plants at termination of the

spring (18 July 1985, 28 July 1986) and summer-fall (26 Nov. 1985, 18

Nov. 1986) treatments using a LI-COR leaf area meter (Model LI-3000).

Flower counts were made on 27 Mar. 1985 and 2 April 1986. Percent fruit

set and yields were determined between 13 June and 18 July 1985, and 13

June 1986 and 29 July 1986, while the number of flower buds was counted

on all shoots on 6 Nov. 1985 and 18 Nov. 1986. Data were analyzed by

Williams' method which compares various levels of flooding treatments

with an unflooded control (22). This method more accurately describes

the relationship among each treatment and the control than regression

analysis.

Stomatal Conductance

During the summer of 1985 stomatal conductance (g ) and
s
transpiration (E) were determined for all treatments between 1100 and

1300 hr on 2 mature leaves per plant using a LI-COR Steady State

Porometer (Model 1600) as described previously (39). Data were analyzed

by Williams' method (22) for each sampling date.

Results and Discussion

Soil Redox Potentials

Soil redox potentials (Eh) at the 15- (Fig. 4-1) and 30-cm (Fig.

4-2) soil depths responded similarly to periodic spring flooding. Upon

flooding Eh at both depths decreased to about -200 and -300 mV,

respectively, within 2 days. Eight to 10 days after release from

flooding Eh recovered to preflood levels at both depths. A similar

length of time for recovery of Eh to preflood conditions was observed

previously (Fig. 3-3). Redox potentials of the 106-day flooding
















































Time (Days)


Fig. 4-1. Soil redox potentials of flooded and unflooded plots planted
to 'Tifblue' rabbiteye blueberry plants at the 15-cm soil depth
during spring, 1985. Each point is the mean of 3 plots (10 reps per
plot). Arrows indicate onset (+) and release (+) of flooding
periods. Symbols + and ** indicate significance 4 to the 10% and 1%
level, respectively and; ns = non-significance among treatments for a
given day.












300 *
+ a*
200
100


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300 b
200
100



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-100
-200
-300
-400
200 c
100 *








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-200
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300 Flooding duration
200 (days) (no.)
0 0
100 2 4
0 15 3
-100 .106 1 O
-200
-300

0 10 20 30 40 50 60 70 80
Time (Days)




Fig. 4-2. Soil redox potentials of flooded and unflooded plots planted
to 'Tifblue' rabbiteye blueberry plants at the 30-cm soil depth
during spring, 1985. Each point is the mean of 3 plots (10 reps per
plot). Arrows indicate onset (+) and release (+) of flooding
periods. Symbols + and ** indicate significance < to the 10% and 1%
level, respectively and; ns = non-significance among treatments for a
given day.









treatment also decreased to below -100 mV within 2 days and remained

below 0 mV for the duration of the study (Fig. 4-le, 4-2e). Redox

potentials of unflooded plots were always greater than 200 mV at the

15-cm depth, but on 4 occasions decreased to 0 to -100 mV at the 30-cm

depth. This is attributable to heavy rains and slow drainage on

occasion through the perforated plastic lining of unflooded plots.

Redox potentials were not monitored with the second 2-day flooding

period at the 15- or 30-cm soil depths (Figs. 4-lb, 4-2b). However,

with the third 2-day (Figs. 4-lb, 4-2b) and the second 15-day (Figs.

4-ld, 4-2d) flooding treatments, Eh decreased upon flooding and

recovered after release. Redox potentials at both depths of the 2-day

(4 periods) treatment did not decrease to previous levels, which may

have been due to a decrease in available organic matter and reducible

soil constituents with short-term periods of flooding and unflooding as

observed by others (108). Redox potentials of the 15-day (3 periods)

treatment at the 15-cm depth (Fig. 4-id) decreased to a level similar to

that of the first flooding which may have been due to an increase in

available organic matter as flooding duration increased (105, 108).

However, Eh did not decrease to prior levels at the 30-cm depth for the

15-day (3 periods) (Fig. 4-2d) treatment, which may indicate the soil

was low in available organic matter at this depth (105, 108). Redox

potentials again decreased in response to the final flooding periods of

the 2-, 7-, and 15-day treatments (Figs. 4-lbcd, 4-2bcd). At the 15-cm

depth, Eh did not decrease to the extent it did with the first flooding,

while Eh at the 30-cm depth decreased to values similar to those of the

first flooding period.









Plant Survival

All plants survived periodic 2-day (4 periods) and 7-day (2

periods) flooding treatments during the spring of both years (Table

4-2). Some plant losses occurred after the 15-day (3 periods) flooding

treatment in 1986, but not in 1985 (Table 4-2). Most (83%) of the

plants survived 106 and 117 days of continuous spring flooding in 1985

and 1986, respectively. All plants survived the 2-day (2 periods) and

7-day (2 periods) summer flooding treatments in 1985, while a

significant number of plants died after the 15-day (2 periods) and

78-day flooding treatments. In contrast, no plants survived the 7-day

(2 periods), 15-day (2 periods), and 90-day flooding treatments in the

summer of 1986. Similarly, a lower percentage survival of 'Woodard'

rabbiteye blueberry plants continuously flooded during summer than

spring (Table 3-1) was observed previously probably due to higher soil

temperatures. In addition, under controlled laboratory conditions,

survival of rabbiteye blueberry plants decreased as flooded soil

temperatures increased (Chapter VI).

Flooding stress may predispose plants to pathogen invasion (135)

and although most rabbiteye blueberry plants are considered tolerant of

Phytophthora root rot (94), recent work with 'Woodard' rabbiteye

blueberry plants under field conditions suggests root rot can be a

problem especially during hot summer months. In contrast, highbush

blueberry plants growing in Rhode Island survived up to 30 months of

continuous flooding (2). This may have been due to the artificial media

used having higher Eh (less reduced) or to lower soil and air

temperatures found in Rhode Island compared to Florida.





































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Survival of rabbiteye blueberry plants appears to be a function of

season of flooding (soil temperature) and the total number of days of

flooding, regardless of whether accumulated periodically or

continuously. For example, plant survival decreased after 25 days or

more of continuous spring flooding (Table 3-1) or 3 periods of 15-day

spring flooding (Table 4-2). Plant survival during the summer decreased

after two 7-day flooding periods -- 14 days total (Table 4-2) -- or 15

days of continuous flooding (Table 3-1).

Leaf Area

Total plant leaf areas significantly decreased after the 2-day (4

periods) spring flooding treatment in 1985 and 7-day (2 periods)

treatment in 1986 (Table 4-3). In contrast, 35 days of continuous

spring flooding significantly decreased total plant leaf area as

previously reported (Table 3-2). The 15-day (3 periods) spring flooding

treatment decreased total plant leaf area by 52 to 72% in 1985 and 1986,

respectively. Continuous spring flooding (106 days in 1985 and 117 days

in 1986) decreased leaf area by 81% in 1985 and 66% in 1986. In 1985,

plant leaf area was not significantly affected by two periods of summer

flooding for 2, 7, or 15 days (Table 4-3). In contrast, all leaves

abscised during the 7-day, 15-day, and 78-day summer flooding treatments

in 1986 due to plant mortality (Table 4-2). Previous studies indicate

that leaf area of highbush (56) and rabbiteye blueberries (Table 3-2)

decreases in response to flooding.

Individual leaf areas decreased significantly after the 7-day (2

periods), 15-day (3 periods), 106-day, and 117-day spring flooding

treatments (Table 4-3). Summer flooding significantly decreased

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all leaves abscised after the 7-day (2 periods), 15-day (2 periods) and

90-day flooding treatments in summer of 1986 due to plant mortality

(Table 4-2). Similarly, individual leaf areas of highbush blueberry

plants decreased in response to flooding (2, 58).

Fruit Set, Yields, and Flower Bud Development

Percent fruit set and yields were variable in 1985, but decreased

significantly compared to unflooded plants after 106 days of continuous

spring flooding (Table 4-4). In 1985, low fruit set and yield of the

7-day (2 periods) treatment and high fruit set and yield of the 15-day

(3 periods) treatment are considered anomalies not consistent with

previous findings (Table 3-3). Fruit set significantly decreased after

the 15-day (3 periods) flooding treatment in 1986, and yields after the

2-day (4 periods) flooding treatments. Relatively high fruit set and

low yields for the 2-day (4 periods) and 7-day (2 periods) flooding

treatments indicate berry size decreased due to flooding. Differences

in fruit set and yields among years and experiments (Table 3-3) may be

due to yearly differences in environmental factors such as temperature

and rainfall, and bee activity during early spring.

Fruit shrivelling and abscission were observed for all flooding

treatments in 1985 and 1986, suggesting water uptake was restricted

during flooding and that susceptibility to drought stress increased

after flooding was released. Similarly, rabbiteye blueberry fruit

shrivelling and abscission were observed after 15 to 25 days of

continuous flooding (Chapter III). This effect may have been due to

decreased uptake and transport of water under flooded conditions

(Chapter V). In contrast, highbush blueberry fruit abscission was

observed only after 4 or more months of continuous flooding (2). These




































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differences are likely due to different soil and air temperatures in

Rhode Island and Florida during late spring.

The number of flower buds per plant was significantly decreased

compared to unflooded plants in 1985 and 1986 after the 2-day (2

periods) summer flooding treatments (Table 4-4). This probably resulted

from decreased leaf area (Table 3-2) and carbon assimilation as

suggested by others (36, 37). Similarly, 5 days of continuous summer

flooding significantly reduced the number of flower buds on 'Woodard'

(Table 3-3) rabbiteye blueberry plants. In contrast, Abbott and Gough

(3) found the number of highbush blueberry flower buds decreased 61% to

77% after 4 months of flooding. Further decreases in flower bud number

occurred after the 7-day (2 periods), 15-day (2 periods), and 78-day

flooding treatments in 1985, while no flower buds formed after the 7-day

(2 periods), 15-day (2 periods), and 90-day treatments in 1986 due to

plant death (Table 4-2).

Stomatal Conductance and Transpiration

There was a concomitant increase and decrease in g with the onset

and release of the 2-day (2 periods) and 7-day (2 periods) summer

flooding treatments (Fig. 4-3). A similar but more varied response

occurred with transpiration (E) (Fig. 4-3) perhaps as a result of

day-to-day temperature and vapor pressure deficit fluctuations (39). In

contrast, gs and E remained greatly reduced for the 15-day (2 periods)

and 78-day summer flooding treatments. Fluctuations in response to

flooding and release have been reported for rabbiteye blueberry plants

under greenhouse and laboratory conditions (36). Reduced gs decreases

water loss but also decreases carbon assimilation needed for growth and

flower bud formation.










250


200 7 2
^ | 15 2 H
5 E 78 1 O
C 5 150 s
0I





0 50



4.0















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transpiration of 'Tifblue' rabbiteye blueberry plants during summer,
1985. Each point is the mean of 6 plants (2 leaves per plant) +SE.
Arrows (+) indicate start of flooding periods. Symbol *, indicates
the treatment differs from the unflooded control by Williams' method,
5% level.
-o

















5% level.










Rabbiteye blueberry gs recovered from both short-term periodic

(Fig. 4-3) or continuous flooding (Fig. 3-6) after termination of

flooding, which supports findings of Davies and Flore (35) and Davies

and Wilcox (39) that blueberry gs responds to flooding in stages based

on flooding duration. During short-term flooding (2 to 7 days) g and E

of flooded plants are less than those of unflooded plants, at

intermediate flooding durations (10 to 25 days) gs and E approach zero

but stomata remain responsive to environmental changes, and after long

term flooding (30 or more days) gs remains depressed, does not respond

to environmental changes, and may not recover to preflood levels after

flooding is terminated.

Generally, the longer the soil Eh was below 200 mV, the more

detrimental the effect of flooding on rabbiteye blueberry survival and

growth. The flooding response of rabbiteye blueberry plants may depend

on the total number of days of flooding and not whether the time flooded

was accumulated periodically or continuously. For example, flower bud

development was consistently and dramatically decreased after short-term

continuous (Table 3-3) or periodic flooding (Table 4-4), while fruit

set, yields, and plant survival were affected by longer continuous

(Tables 3-1, 3-3) or accumulated flooding durations (Tables 4-2, 4-4).

There does not seem to be more damage resulting from periodic versus

repeated flooding as reported by others (80, 118).

Plant growth and survival decreased more after summer flooding than

spring flooding, supporting previous findings with rabbiteye (Table 3-1;

39) and highbush (1, 2) blueberry plants. This may be attributed to the

temperature dependence of plant and soil microbe metabolism (69, 135), a

more rapid decrease in plant water uptake (Figs. 5-1, 5-2) and vigor,





57



and an increased plant predisposition to plant pathogens with increased

soil and air temperatures (135) during summer compared to spring.
















CHAPTER V
HYDRAULIC CONDUCTIVITY, ROOT ELECTROLYTE LEAKAGE,
AND STOMATAL CONDUCTANCE OF FLOODED AND UNFLOODED
RABBITEYE BLUEBERRY PLANTS

Introduction

Flooding decreases water uptake of herbaceous plants such as tomato

(16), sunflower, and tobacco (83) and woody plants like pear (5), citrus

(136), and highbush blueberry (35). Davies and Wilcox (39) observed a

rapid increase in stomatal conductance (gs) after root excision of

rabbiteye blueberry plants flooded for 25 to 45 days, suggesting that

root hydraulic conductivity (RLp) was limiting water uptake and

controlling gs.

Reduced water uptake under flooded conditions has been attributed

to the adverse effects of high carbon dioxide and low oxygen

concentrations on permability of robt cells to water (21, 51, 82, 83).

High carbon dioxide and low oxygen concentrations reduced water uptake

by excised sunflower (51, 82), pea (51), and tomato (82) and intact

wheat, corn, and rice plants grown in solution culture (21). Increased

carbon dioxide brought a more rapid and pronounced response than oxygen

deficiency alone.

Anaerobic conditions increase root exudation and leakage of organic

and inorganic substances presumably via the adverse affects of

anaerobiosis on cell metabolism, membrane permeability, and mechanisms

of active ion uptake (69, 81, 135). Hiatt and Lowe (59) reported

increased efflux of amino acids and potassium ions from barley roots










under anaerobic conditions.

Decreased gs is an early and rapid response to flooding in many

plant species (36, 39, 100). Generally, however, this is not the result

of desiccation stress (16, 36, 39, 100) as leaf water potentials (V )

are comparable for flooded and unflooded plants (6, 16, 36, 100).

Stomatal conductance generally decreased in 5 to 10 days for flooded

peach, quince, apple, pear (6), 4 to 8 days for citrus (136), and 3 to 7

days for highbush and rabbiteye blueberry plants (36, 39).

Numerous explanations have been proposed to explain flood-induced

stomatal closure including decreased RLp (5, 35, 136), decreased

production and translocation of root produced cytokinins and/or

gibberellins (109, 111, 112), accumulation of ABA in leaves (15, 60, 81,

110, 111), potassium deficiency caused by reduced absorption (81, 111),

and translocation of toxic substances from the anaerobic root system

(16, 34, 35, 81, 111) or rhizosphere (42, 81).

Rabbiteye blueberry plants are moderately flood-tolerant during the

spring (Chapter III, IV; 38, 39). They appear to adapt to flooded

conditions by stomatal closure which reduces transpiration (35, 36, 39)

and delays damaging decreases in water potentials (36, 81). Although

previous work with highbush blueberry plants showed that RLp decreased

with flooding duration (35), no one has studied the effect of flooding

on root electrolyte leakage (EL), RLp, and stem hydraulic conductivity

(SLp) of rabbiteye blueberry plants. Our objectives were to determine

the sequence of some physiological changes occurring during flooding and

to investigate relationships among RLp, SLp, EL, and gs for flooded and

unflooded rabbiteye blueberry plants.










Materials and Methods

Plant Material

Two-year-old 'Woodard' and 'Tifblue' rabbiteye blueberry plants

were used for 3 similar experiments. Plants were grown in Myakka fine

sand (sandy, siliceous, hyperthermic, Aeric Haplaquod) (pH 5.0) in 6

liter containers and pruned to one main stem 5 to 6 months prior to each

experiment. Soil media was treated with the systemic fungicide

metalaxyl about 2-weeks before each experiment to prevent plant losses

due to Phytophthora root rot. A 3.1 x 4.6 x 0.3 m pool was excavated in

the field and lined with plastic in order to flood the plants. Plants

were slowly flooded with a garden hose at irregular intervals to

maintain 5 to 8 cm of standing water above the soil surface in the

containers. Containers of unflooded plants were placed in 19 liter

pails to prevent flooding. Flooding treatments were released by

removing plants from the pool. Flooding treatments were imposed from 13

Aug. to 28 Aug. 1986 (15 days) for 'Tifblue' ('Tifblue' expt. 1), 29

Sept. to 19 Oct. 1986 (20 days) for 'Woodard' ('Woodard' expt. 1), and

15 Oct. to 5 Nov. 1986 (21 days) for 'Woodard' ('Woodard' expt. 2), in a

completely randomized design with 3 unflooded and 3 to 5 flooded bushes

sampled at various flooding durations.

Soil Redox Potential Measurements

Soil redox potentials (Eh) were monitored periodically throughout

the experiment at a 12-cm depth to indicate the oxidation status of the

soil (105, 120). Two 25-gauge platinum electrodes and a Ag /AgC1l

reference electrode were attached to a Keithley microvoltmeter (Model

177 DMM). Readings were taken after a 5 to 10 min equilibration time

and a value of 222 mV was added to all Eh readings to compensate for the










potential of the reference electrode. Stomatal conductance was

determined between 1100 and 1230 hr on 2 randomly selected mature leaves

per plant using a LI-COR Steady State Porometer (Model 1600) as

described previously (39). After gs was measured, plants were removed

from the pool and root systems were washed free of soil.

Plant Measurements

Following outdoor measurements plants were transported to the

laboratory for water uptake and root electrolyte leakage measurements.

Plants were then detopped and either stem sections or entire root

systems were placed into a modified pressure chamber for water uptake

measurements as described previously (35). The severed stem was pushed

through a rubber circular opening in the chamber lid to make a tight

seal and the root system or base of the stem section submerged in 200C

to 220C water in the chamber which was sealed. A constant pressure of

0.50+0.025 MPa was applied to the chamber and root exudate collected for

1 hr, after which the volume of the root system was determined by

displacement. Preliminary studies found oxygen status of the deionized

water had no affect on RLp within the measurement time period. Volume

flow through the root system was linear between 0.1 and 1.5 MPa and no

diurnal fluctuation in RLp was found.

Stem sections 12- to 15-cm long consisting of about one-half stem

above and one-half below the flood-water line were used in determining

SLp. Measurement procedures were similar to root measurements except a

constant pressure of 0.30+0.025 MPa was applied to the chamber and

exudate was collected for 15 min. Stem volume was determined by taking

the mean of the calculated stem cylinder volume using top and bottom

radii and length of the stem section.










Electrolyte leakage from root tissue was used to indicate root

damage and was determined as described previously (132). Two fibrous

root samples (about 0.5 g fresh weight per sample) were cut from each

plant, rinsed in deionized water and placed in test tubes containing 15

ml of deionized water. Test tubes were shaken for 24 hr at about 220C

and electrical conductance (EC) of the effusate was measured using a

Copenhagen Conductivity Meter (Model CDM3). Root pieces were then

frozen at -800C for 1 hr, reshaken with effusate for 24 hr, and EC taken

again. Electrolyte leakage was expressed as: sample EC (pS) / total EC

(PS) x 100 (130).

Flooding stress responses among rabbiteye blueberry plants are

variable especially during the first 10 days of flooding (Chapters III,

IV; 39). Therefore 3 unflooded and 3 to 5 flooded plants were required

per sampling time. Data were subjected to weighted regression analysis;

however, no regression equation satisfactorily described the abrupt

physiological changes associated with the first 0 to 10 days of flooding

stress. Therefore data are presented as means+SE.

Results and Discussion

Soil Redox Potentials

Soil redox potentials of flooded containers typically decreased to

about -225 mV within 4 to 10 days of flooding, while Eh of unflooded

containers remained greater than 400 mV throughout all experiments (data

not shown). Redox potentials below 200 mV indicated little or no oxygen

was present and that the soil was in a highly reduced state (105, 120).

Root and Stem Hydraulic Conductivity

Root hydraulic conductivity decreased by 44 to 60% compared to that

of unflooded plants after 4 to 6 days of flooding in 2 of 3 experiments










(Fig. 5-1). It was not clear why RLp of unflooded 'Tifblue' plants was

depressed on day 10. Increased RLp of flooded 'Tifblue' between 10 and

15 days of flooding may have been due to root damage. The RLp of

flooded 'Woodard' plants in expt. 1 remained below that of unflooded

plants throughout the experiment. In 'Woodard' expt. 2, RLp of flooded

plants also decreased in 2 days but was similar to that of unflooded

plants on days 4 and 10. Low RLp of unflooded plants may have resulted

from low soil temperatures (16-190C) prior to RLp measurements, as has

been observed in other plants (7, 125, 151). Nevertheless, RLp of

flooded plants was generally decreased compared to unflooded plants

supporting previous findings with highbush blueberry (35) and pear (5).

Stem hydraulic conductivity of flooded 'Woodard' plants generally

decreased 4 to 6 days after RLp decreased (Fig. 5-2). In contrast, SLp

of flooded 'Tifblue' decreased after 6 days of flooding and remained

below that of unflooded plants throughout the experiment. Stem

hydraulic conductivity was comparable among flooded and unflooded

'Woodard' plants for the first 6 days of flooding (Fig. 5-2) in expt. 1;

however, by day 10 SLp of flooded plants had decreased to 57% of that of

unflooded plants. A similar trend in SLp occurred in 'Woodard' expt. 2,

however, the decrease in SLp of flooded plants was less than in expt. 1.

This was probably due to cooler water and air temperatures prior to

these measurements (Oct.-Nov.). Andersen et al. (5) also observed

decreases in SLp with flooding of pears which they attributed to xylem

plugging.

Electrolyte Leakage

Electrolyte leakage was generally similar between treatments up to

6 days of flooding (Fig. 5-3). Thereafter, EL for flooded plants





64







S0.4



S0.2 Flooded
Unflooded

> Tifblue expt.-1


Co
E 0.4

O
C\I

E
0.2
o Woodard expt.-1
> 0
o 0.6



0.4


Co
50.2


0 Woodard expt.-2
C 0
0 2 4 6 8 10 12 14 16 18 20
Time(Days)


Fig. 5-1. Effect of flooding on root hydraulic conductivity of
'Tifblue' (expt. 1) and Woodard' (expt. 1 and 2) rabbiteye
blueberry plants. Data points are means+SE. Standard error
bars not shown are within symbols.










100

7C 80

a. 60

40
~- Flooded
> Unflooded (
E 20
4- Tifblue expt.-1

E
0
60
O
04
I 40

O 20
Woodard expt.-1

080

0 60

40

-v 20 *
.= Woodard expt.-2
E 01 --I -II--I V I
^ 0 2 4 6 8 10 12 14 16 18 20
w Time (Days)





Fig. 5-2. Effect of flooding on stem hydraulic conductivity of
'Tifblue' (expt. 1) and 'Woodard' (expt. 1 and 2) rabbiteye
blueberry plants. Data points are means+SE. Standard error
bars not shown are within symbols.











60 -

50

40
40 Flooded -
30 Unflooded O
2 TTifblue expt.-1
^ 20
0
460


CO


3 30
e Woodard expt.-l


50

40

30
Woodard expt.-2
20
0 2 4 6 8 10 12 14 16 18 20
Time (Days)




Fig. 5-3. Effect of flooding on root electrolyte leakage of 'Tifblue'
(expt. 1) and 'Woodard' (expt. 1 and 2) rabbiteye blueberry plants.
Data points are means+SE. Standard error bars not shown are within
symbols.










increased compared to unflooded plants. Similarly, anoxic root

conditions increased efflux of amino acids and potassium ions

from barley roots (59). Electrolyte leakage from flooded and unflooded

plants was nearly identical on day 20 for 'Woodard' expt. 1. This is

considered an anomaly not representative of the general trends in the

other experiments. Generally, EL increased and SLp decreased

concomitantly (Figs. 5-2, 5-3). Reduced RLp occurred prior to increases

in EL indicating flooding affected membrane hydraulic permeability

before electrolytes leaked from the roots (Figs. 5-1, 5-3).

Stomatal Conductance

Stomatal conductance of flooded 'Tifblue' decreased below that of

unflooded values after 6 days of flooding (Fig. 5-4). In contrast, gs

of flooded 'Woodard' plants in expt. 1 and 2 decreased after 4 days of

flooding and remained below that of unflooded plants throughout the

experiments (Fig. 5-4). Based on previous research (35, 36, 39), T' of

flooded rabbiteye blueberry plants is similar to or greater than

unflooded plants; therefore it is unlikely stomatal closure in our

experiments was due to decreased Y The decrease in gs and RLp

occurred concomitantly for flooded plants in all 3 experiments (Figs.

5-1, 5-4) further supporting the relationships between these factors as

suggested for citrus (136) and pear (5). Since gs probably decreased

without a change in T (35, 36, 39) it is likely that early effects of

flooding on stomata are due to the presence or absence of a signal

produced in the root (34, 67, 72, 111) as previously proposed rather

than to a turgor-mediated response as seen in long-term flooding (35,

39).












350 Tifblue expt.-1

300

250
\ Flooded
1 200 Unflooded O








o J Woodard expt.-1





co 200 -
-150

100






0
E 5

















(expt. 1) and 'Woodard' (expt. 1 and 2) rabbitexpt.-e blueberry plants.
0C-200
150

o 100

4-

0
Woodard expt.-2















Data points are means+SE. Standard error bars not shown are within
symbols.






69



Rabbiteye blueberry is considered moderately flood tolerant based

on its survival in field (Chapters III, IV) and greenhouse (36) tests.

Nevertheless, reduced gs, RLp, and carbon assimilation (36, 37) and

increased EL may occur after 4 to 6 days of flooding, depending on soil

conditions.

















CHAPTER VI
SOIL TEMPERATURE AND FLOODING EFFECTS ON YOUNG
RABBITEYE BLUEBERRY PLANT SURVIVAL, GROWTH,
AND RHIZOSPHERE ETHYLENE EVOLUTION

Introduction

Plant survival is affected by soil temperature and oxygen status

under flooded conditions. Plant survival of containerized tomato (83),

sunflower (90), and several Prunus (119) and Juglans (20) species

decreased as flooded soil temperature increased under controlled root

temperature conditions. Under field conditions, high soil temperatures

during summer flooding coincided with decreased plant survival of

numerous hardwood tree species (55) and rabbiteye blueberry plants

(Tables 3-1, 4-2).

Soil and air temperatures may affect plant growth under low oxygen

conditions. Letey et al. (91) found no effect of soil temperature on

root growth of flooded cotton and sunflower, while greater shoot growth

occurred at an air temperature of 310C for cotton and 230C for

sunflower. Tomato (83, 85) shoot and root dry weight decreased as

flooded soil and air temperatures increased, while wheat dry weight

(130) decreased at low soil oxygen levels regardless of soil

temperature. Rabbiteye blueberry shoot elongation (Figs. 3-4, 3-5) and

leaf expansion (Figs. 3-4, 3-5) decreased more after summer than spring

flooding. Similarly, Olien (96) found less apple tree growth after

summer compared to spring or fall flooding.











Temperature of flooded soils affects the rate of oxygen depletion,

diffusion, and decrease in redox potential. Root and soil microbial

respiration increase with temperature causing a more rapid decrease in

the limited oxygen supply under flooded conditions (25, 105). Although

the diffusion rate of oxygen is about 10,000 times slower in water than

through air (147), oxygen diffusion through water increases by 3 to 4%

per OC increase, while oxygen solubility decreases about 1.6% per OC

increase (147). Letey et al. (91) demonstrated a 1.8% increase in ODR

per OC increase in a silt loam soil. Cho and Ponamperuma (25) showed

that the rate and magnitude of the decrease in redox potential upon

flooding increased with flooding temperature and organic matter content

of the soil.

The type of rooting media also affects the flooding response of

plants. Tomato, tobacco, and yellow poplar plant injury and mortality

were greater for plants flooded in soil compared to those grown in

unaerated water or sand culture (83). This may be due to differences in

microbial populations, microbial end-products, and concentrations of

free oxygen remaining in different media after flooding (83).

Ethylene is produced by living, injured and dying microbes (42) and

plants (66). Under well aerated conditions, ethylene diffuses rapidly

out of the soil (66) or is metabolized by aerobic microbes (27).

Ethylene production has been shown to increase with increasing soil

temperature (66, 128) and organic matter content (52). Under soil

conditions where gas exchange is restricted but not completely

anaerobic, ethylene accumulates due to reduced microbe metabolism and

drastically reduced diffusion out of the soil (42, 66). Ethylene is

quite soluble in water (1:9 v/v at 250C) (150) and has been shown











capable of diffusing into corn (70) and tomato (67) roots, stems, and

leaves when bubbled into solution culture.

Field studies (Chapters III, IV) have shown plant survival and

growth of rabbiteye blueberry plants decreased more after summer

flooding when soil and air temperatures were high than in spring when

soil and air temperatures are low. However, no one has studied

rabbiteye blueberry plant survival and growth under controlled

temperature conditions. Our objectives were: 1) to determine the effect

of soil flooding temperature on rabbiteye blueberry plant survival and

growth and; 2) determine whether ethylene is evolved from the

rhizosphere of common soil (Myakka fine sand) found at many blueberry

plantings in Florida.

Materials and Methods

Plant Material

Thirty-six 12- to 18-month-old 'Woodard' rabbiteye blueberry plants

were transplanted into 3.9 liter glass containers filled with Myakka

fine sand (MFS) for 3 similar experiments or into 2.5 liter plastic pots

filled with peat:perlite (P:P) (1:1 v/v) media for 1 experiment. Myakka

fine sand (sandy, siliceous, hyperthermic, Aeric Haplaquod) is

characterized by a pH of 5.0, 2% organic matter content, and a low

cation exchange capacity. Peat:perlite (P:P) media had a pH of about

4.4 and was 50% organic matter. Plants used for experiments in glass

containers (experiments 1, 2, and 3 in MFS) were pruned to 1 main stem 3

to 4 months prior to transplanting. Plants used for experiments 2 and 3

in MFS were treated with the systemic fungicide metalaxyl 2 to 3 weeks

before transplanting into glass containers with untreated soil to

prevent plant losses due to Phytophthora root rot. Glass containers











were covered with aluminum foil to exclude light and prevent algal

growth. Plants used in all experiments were transferred to the growth

chambers 7 to 10 days before flooding treatments were started.

Experiments using MFS were carried-out from 3 June to 5 Aug. 1985 (expt.

1), 22 May to 1 Sept. 1986 (expt. 2), and 21 Oct. 1986 to 15 Feb. 1987

(expt. 3). The experiment using P:P media was conducted from 12 Oct. to

21 Dec. 1985.

A 3 x 2 factorial set of treatments consisting of constant 200C,

250C, or 300C root temperatures and flooded or unflooded soil was

laid-out in a split-plot design with root chamber temperature as main

plots and flooding or unflooding as subplots. Three flooded and

unflooded plants were used per chamber with 2 chambers at each root

temperature. Plants within a chamber for a given treatment were

considered subsamples, therefore there were 2 replications (root

chambers) per treatment.

Growth Conditions

Six computer controlled root growth chambers were used to maintain

constant soil temperatures of 200C, 250C, or 30+1.500C as described

previously (50). Artifical lighting was provided for each root chamber

by one 400-watt mercury vapor lamp surrounded by 4 90 to 100 watt

incandescent light bulbs. Photon flux ranged from 300 to 800 ymol s-1
-2
m at 15 cm from the surface of the root chamber. Air temperatures in

the growth room fluctuated from 220C to 330C and relative humidity was

between 30 to 45%.

Soil Rhizosphere Measurements

Plexiglass lids with 4 port holes, one for the main stem of the

plant, one for a serum cap used for drawing gas samples from the head











space of glass containers, and 2 ports for soil oxygen diffusion rate

(ODR) and redox potential (Eh) measurements were used in expt. 1 and 2

with MFS. Oxygen diffusion rates (ODR) were monitored at a 10-cm soil

depth using platinum electrodes attached to a Jensen ODR meter (Model C)

and a Ag /AgC1 reference electrode. Measurements were taken at an

applied voltage of 650 mV and an equilibration time of 3 min. Redox

potentials were taken using platinum electrodes and a Ag /AgCl reference

electrode after a 5 to 10 min eqiulibration period. A value of 222 mV

was added to all Eh values to compensate for the potential of the

reference electrode.

Ethylene gas samples were taken from the head space of glass

containers 24 hr after sealing the plexiglass lids onto glass containers

with modeling clay. One ml gas samples were taken with a 1 ml syringe

from the head space above the soil. Gas samples were then injected into

a Hewlett-Packard (Model 5730A) gas chromatograph using a

flame-ionization detector with a 1.5 m x 3.2 mm column of activated

alumina, 70-100 mesh, 2500C column temperature with air, N2 and H2 at

300, 50, and 50 ml min-1, respectively. Data were analyzed by

split-plot analysis with soil temperature as main plots and flooding

treatments as subplots.

Plant Measurements

Initial plant fresh weights were taken prior to experiments using

MFS media. At termination of experiments, all plants were divided into

roots, stems, and leaves and dried at 800C for about 4 days. Covariant

analysis using initial plant fresh weight as the covariant did not alter

conclusions, therefore data was analyzed by split-plot analysis with

soil temperatures as main plots and flooding treatments as subplots.











Results and Discussion

Soil Oxygen Measurements

Generally, soil temperature had no significant effect on soil ODR

and Eh of MFS and P:P media and therefore only the effect of flooding

treatments are shown (Figs. 6-1, 6-2, 6-3, 6-4). This may be due to the

low organic matter content (2.0%) of MFS and low microbial populations

in P:P media. Similarly, Syvertsen et al. (136) found no effect of soil

temperature on Eh. Oxygen diffusion rates of MFS in expt. 2 and expt. 3
-2 -1
decreased below 0.2 yg cm min within 1 day of flooding and remained

below unflooded MFS values throughout the experiments. In contrast, ODR
-2 -1
in expt. 1 fluctuated between 0.18 and 0.26 yg cm min during the

first 18 days of the experiment. Oxygen diffusion rates of P:P media

decreased much more slowly than those of MFS, decreasing to about 0.2 pg
-2 -1
cm min after about 42 days of continuous flooding (Fig. 6-4). This

may be due to trapped oxygen in the media, greater porosity of P:P media

compared to MFS soil, and low microbial consumption of oxygen.

Redox potentials of flooded MFS in expt. 2 and 3 and P:P media

decreased from about 600 mV before flooding to 300 to 400 mV after 1 to

5 days of flooding; decreasing further to about 150 to 200 mV as

flooding duration increased (Figs. 6-2, 6-3, 6-4). In contrast, Eh of

MFS in expt. 1 (Fig. 6-1) indicated reduced conditions for unflooded MFS

and highly reduced conditions for flooded MFS. This may be due to

overwatering and wet soil conditions at the bottom of undrained glass

containers. Consequently, the Eh recorded in expt. 1 may not reflect Eh

of the soil above the bottom of the glass containers.























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Plant Survival

The LD50 (number of days at which 50% of the plants died) of

flooded plants in MFS decreased as soil temperatures increased (Table

6-1). More than 50% of the plants flooded at 200C soil temperature

survived 102 and 117 days of continuous flooding in expt. 2 and 3,

respectively. Similarly, rabbiteye blueberry plants survived 106 to 117

days of continuous flooding in the field during the spring under cooler

air and soil temperatures (Table 4-2). The LD50's of plants in expt. 1

(MFS) were less at all soil temperatures than in expt. 2 and 3, which

may have been due to Phytophthora infection (Chapter III) and highly

reduced soil conditions (Fig. 6-1).

Soil temperatures appeared to have little or no effect on survival

of plants grown in P:P media (Table 6-1). Similarly, Kramer (82) found

more plants survived flooding in solution and sand culture than in soil.

This may be due to a more rapid decline in ODR of MFS compared to P:P

(Figs. 6-2, 6-3, 6-4) and differences in microbial end products (42, 66,

83). Rate and magnitude of Eh decrease was similar for MFS and P:P

(Figs. 6-2, 6-3, 6-4), suggesting Eh potentials were not responsible for

differences in plant survival among media tested. However, Eh in mixed

systems such as soil or artificial media are indicative of what elements

are reduced but not their concentration (104, 120), therefore

differences in the quantity of reduced inorganic elements may have been

a factor in differences in LD50's between MFS and P:P.

Plant Growth

Plant growth was not significantly affected by soil temperature and

inconsistently affected by flooding treatments in MFS and therefore only

the effect of flooding treatments are reported (Table 6-2).






85




Table 6-1. Effect of flooding temperature on plant
survival of 'Woodard' rabbiteye blueberry plants.



Rhizosphere
Experiment temperature
Soil media number (00) LD50

Myakka fine sand 1 20 46
25 28
30 16

2 20 x
25 64
30 33

3 20 x
25 117
30 56

Peat:perlite 1 20 68
25 x
30 61


n=4 to 6 plants per treatment temperature.

YNumber of days at which 50% of the plants died.

XLess than 50% mortality by termination of the
experiment.





































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Consequently, no clear effect of flooding or temperature on plant growth

from the experiments using MFS can be made. In contrast, low soil

oxygen status decreased plant growth of numerous herbaceous (83, 85, 91,

130) and woody plants (Figs. 3-4, 3-5; 80). Poor plant growth of

unflooded plants under our laboratory conditions may result in part from

low stomatal conductance (5.98 to 30.53 mmol s1 m 2) and high air

temperatures (30-330C) limiting carbon dioxide assimilation as found by

others (36). Only in the second experiment using MFS and the experiment

using P:P were unflooded plant weights greater than flooded plants.

Phytophthora root rot may have infected plants in expt. 1 with MFS

(Chapter III) confounding the effects of flooding on plant growth. Leaf

symptoms of plants flooded in MFS or P:P include leaf chlorosis,

reddening, and abscission.

Rhizosphere Ethylene

Generally, extremely small but statistically greater quantities of

ethylene were evolved from the rhizosphere of plants in flooded MFS

(expt. 1 and 2) (Figs. 6-5, 6-6), compared to those in unflooded MFS.

Soil temperature had no consistent effect on the amount of ethylene

evolved from the rhizosphere of plants flooded in MFS. Ethylene

concentration was expressed on a container (rhizosphere) basis because

no distinction could be made between soil- and root-produced ethylene.

The amount of ethylene evolved from the rhizosphere of flooded plants

ranged from 0 to 0.12 pl rhizosphere- This is equivalent to a range

of approximately 0 to 0.0036 mg liter-1 on a soil volume basis alone.

In contrast, Smith and Dowdell (128) reported 1 to 15 mg liter-1

ethylene was evolved from several soils under flooded conditions.

Ethylene evolution in expt. 1 with MFS may have been confounded by




























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