Citation
Effect of flooding duration, periodic flooding, season, and temperature on growth, development, and water relations of young rabbiteye blueberry (Vaccinium ashei Reade) plants

Material Information

Title:
Effect of flooding duration, periodic flooding, season, and temperature on growth, development, and water relations of young rabbiteye blueberry (Vaccinium ashei Reade) plants
Creator:
Crane, J.H ( Jonathan Henry ), 1952-
Publisher:
[s.n.]
Publication Date:
Language:
English
Physical Description:
x, 110 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Blueberries -- Effect of floods on -- Southern States ( lcsh )
Blueberries -- Growth -- Southern States ( lcsh )
Dissertations, Academic -- Horticultural Science -- UF
Horticultural Science thesis Ph. D
Floods ( jstor )
Blueberries ( jstor )
Soil science ( jstor )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

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

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
021065455 ( ALEPH )
17886165 ( OCLC )
AFA2029 ( NOTIS )
AA00004837_00001 ( sobekcm )

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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




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.
iii


TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT ix
CHAPTERS
IINTRODUCTION 1
IIREVIEW 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
IIIFLOODING DURATION AND SEASONAL EFFECTS ON GROWTH AND
DEVELOPMENT OF YOUNG RABBITEYE BLUEBERRY PLANTS 18
Introduction 18
Materials and Methods 20
Results and Discussion 22
IVPERIODIC 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
VHYDRAULIC 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
iv


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
v


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
vii


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 20C, 25C, and 30C soil
temperatures 89
6-6. Ethylene evolution from the rhizosphere of flooded
'Woodard' rabbiteye blueberry plants grown in Myakka
fine sand (expt. 2) at 20C, 25C, and 30C 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
ix


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 (20C,
25C, and 30C) 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.
x


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
1


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


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
-2 -1
rate (ODR) is converted from amperes to flux (pg 0^ cm min ) (87,
88). Typically, ODR values range from 0.7 (unflooded but moist soil
-2 -1
conditions) to 0.2 pg 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 yg 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


mineralization, followed by nitrification in the thin aerobic soil-water
interface, followed by denitrification in the reduced soil below this
interface, and possibly volatilization as ammonia (104), Sulfur which
in reduced form (H^S) is highly toxic to plants, may be present in
organic form or as sulfate. Sulfur undergoes a sequence of changes
similar to those undergone by nitrogen under flooded soil conditions
(104).
Decomposition of organic matter in submerged soil occurs slowly and
many transitory substances may accumulate to toxic concentrations before
being metabolized to carbon dioxide, methane, and humic acids (42, 105).
These transitory compounds include gases (hydrogen sulfide, ethylene,
butane, methane), alcohols (ethanol, methanol), carbonyls
(acetaldehyde), fatty acids (acetate), phenols (coumaric), and volatile
sulfur compounds (mercaptans) (105). How toxic these substances are to
plants depends upon their concentration, susceptibility of the host, and
duration of exposure.
Alternating aerobic and anaerobic soil conditions have been shown
to decrease organic matter content and increase nitrogen loses more than
continuous flooding (110). This was attributed to increased
solubilization of organic matter during flooding, subsequent increase in
organic matter available for decomposition during the unflooded period,
and stimulation of the nitrification-denitrification process. As the
duration of the aerobic-anaerobic cycles decreased the loss of organic
matter and nitrogen increased (108). Changes in redox potentials have
been shown to correlate with alternating aerobic and anaerobic soil
conditions (108).


Effect of Flooding on Plant Survival and Growth
Flooding tolerance varies within and among plant species. Flooding
tolerance of woody fruit species ranges from 5-20 days for peach (6,
118), 7-19 days for various Juglans species (20), 12 months for apple
(6), and over 20 months for quince and various Pyrus species (6). In
one report (80), mango and guava trees were tolerant of flooding, while
citrus, loquat and pawpaw were sensitive. Rowe and Beardsells (118)
review on flooding of fruit trees lists a wide range of flooding
sensitivity for various peach, plum, pear, apple, and citrus rootstocks.
Containerized rabbiteye (39) and highbush blueberry (1, 2) plants
survived up to 2 and 30 months of continuous flooding, respectively.
Hardwood forest species also show variable flooding tolerance, ranging
from 12 days for poplar (83) to indefinitely for various willow (6, 9),
mangrove and cypress (80) species.
Flooding duration and season also affect flooding tolerance.
Numerous hardwood species such as sweetgum, green ash, and sycamore
tolerated long periods of dormant season flooding, but died after
short-term flooding during the growing season (55). Apple (96) and
rabbiteye (38) and highbush blueberry (1, 2) plants tolerated dormant
season flooding but were damaged by flooding during the growing season.
Alternating periods of flooded and unflooded conditions may also
affect flooding tolerance. Hall and Smith (55) observed that survival
of hardwood species to periodic flooding over an 8-year period depended
upon the percent of time plants were flooded during the growing season.
In contrast, growth of some hardwood species increased after release
from short-term flooding (80). Broadfoot (18) reported flooding a
mature mixed hardwood stand for 5 months during the growing season for 3


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 Saxfraga 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
1-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), tpelo
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,
18


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 (g^) 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, Aerie 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 L1-C0R 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),
2
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
raonotonically 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 OPR and Redox Potential
Soil ODR of flooded plots at the 15-cm soil depth decreased to 0.20
-2 -1
yg 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


Oxygen diffusion rateugcm 2m¡n 1)
0.1 AJ i il i i
0 5 10 15 20 25 30 35
Time (Days)
Fig. 3-1. Soil oxygen diffusion rates of flooded and unflooded plots
planted to 'Woodard' rabbiteye blueberry plants during summer, 1984.
Each point is the mean of 3 plots (10 reps per plot) taken at the 15-
and 30-cm soil depths. Symbols +, *, and ** indicate significance 4
to the 10%, 5%, and 1% level, respectively and; ns = non
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 pg 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


Fig. 3-2. Soil oxygen diffusion rates 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. Symbols +,
and ** indicate significance 4 to the 10%, 5%, and 1% level, respectively and; ns
non-significance among treatments for a given day.


Oxygen diffusion rate (pg cm-2 min- 1)
0 5 10 15 20 25 30 35 40
Time (Days)


-300
20 25
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.
Year2
Flooding
duration (days)
Survival
Spring
(%)
Summer
1984
0
100
100
5
100
100
15
100
100
25
100
100
35
83
100
1985
0
100
100
5
100
67*
15
100
33*
25
83 *y
17*
35
67*
0*
Zn=3 plants in 1984, 6 plants in 1985.
^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 22C (15-cm
depth) in spring, and 33C (15-cm depth) in summer, while air
temperatures reached 27C in spring and over 38C 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 30C compared with 20C
in growth chamber studies (Table 6-1). Davies and Flore (36) found that
carbon assimilation of flooded rabbiteye blueberry plants became
negative above 28C, 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
cm 15.0
E
o
CO
10.0
co
CO
CD
' 5.0
0
20.0
E
o
-15.0
sz.
O)
c
a>
E
10.0
5.0
0
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.


O 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


Table 3-2. Flooding duration and leaf area of 'Woodard'
rabbiteye blueberry plants.
Flooding Spring Summer
duration
Total
Individual
Total
Individual
V 2
Year
(days)
f 2v
(cm )
Co,2
(cm2)
(cm2)
1984
0
2500
4.04y
1707
4.49
5
2402
4.03
1990
4.33
15
1974
3.73
1586
4.11
25
1965
3.75
1324
3.99
35
672*X
2.82*
1292
5.48
1985
0
1579
2.15*
1706
2.44
5
1063
1.71
913*
1.49*
15
1126
1.71
15*
0.27*
25
1044
1.20*
1*
0.21*
35
744*
0.82*
0*
0.00*
Zn=3 plants in 1984, 6 plants in 1985.
^n=90 leaves in 1984, all leaves per plant in 1985.
^Indicates that the treatment differs from the unflooded
control by Williams' method, 5% level.


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 decribed 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
Year^
Flooding
duration
(days)
Fruit
z
set
(%)
Yield
dry wt
(g)
Flower buds
formed2
(% 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*
2
Arcsin transformation of percent data before
analysis.
^n=3 in 1984, n=6 in 1985.
X*Indicates that the treatment differs from the
unflooded control by Williams' method, 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 gg (Fig. 3-6). Leaf
water potentials (f ) were similar among flooded and unflooded
treatments for the first 2 weeks of treatments (-0.50 to -1.50 MPa).
However, V of plants flooded 25 days or more decreased to about -2.5
w
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


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 28C (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
40


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, Aerie 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.


Table 4-1. Flooding treatments and dates used for young rabbiteye blueberry
plants.
Season/
zy
year J
Flooding
duration
(days)
Flooding
periods
(no.)
Cumulative
flooding
duration Dates of
(days) flooding treatments
Spring
0
0
0
Unflooded control
1985
2
4
8
3-5 Apr., 19-21 Apr.,
29 Apr.-l May,
13-15 June
7
2
14
3-10 Apr., 13-20 June
15
3
45
3-18 Apr., 29 Apr.-14
May, 13-28 June
106
1
106
Continuously flooded,
3 Apr. to 18 July
Spring
0
0
0
Unflooded control
1986
2
4
8
3-5 Apr., 18-20 Apr.,
27-29 Apr.,
13-15 June
7
2
14
3-10 Apr., 13-20 June
15
3
45
3-18 Apr., 27 Apr.-12
May, 13-28 June
117
1
117
Continuously flooded,
3 Apr. to 29 July
Summer
0
0
0
Unflooded control
1985
2
2
4
8-10 Sept., 1-3 Oct.
7
2
14
8-18 Sept., 1-8 Oct.
15
2
30
8-23 Sept., 1-16 Oct.
78
1
78
Continuously flooded,
8 Sept, to 26 Nov.
Summer
0
0
0
Unflooded control
1986
2
2
4
20-22 Aug., 6-8 Oct.
7
2
14
20-27 Aug., 6-16 Oct.
15
2
30
20 Aug.-3 Sept., 6-21
Oct.
90
1
90
Continuously flooded,
20 Aug. to 18 Nov.
zn=6 plants.
^3-year-old plants in 1985 and 2-year-old plants in 1986.


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 (gg) and
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


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.


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 (t) 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.


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-ld) 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 artifical media
used having higher Eh (less reduced) or to lower soil and air
temperatures found in Rhode Island compared to Florida.


Table 4-2. Effect of periodic flooding on survival of 'Tifblue'
rabbiteye blueberry plants.
Spring
Summer
Flooding
Flooding
Survival54
Flooding
Flooding
Survival54
duration
periods
duration
periods
Year2-^ (days)
(no. )
(%)
(days)
(no.)
(%)
1985
0
0
100
0
0
100
2
4
100
2
2
100
7
2
100
7
2
100
15
3
100
15
2
67*
106
1
83*
78
1
33*
1986
0
0
100
0
0
100
2
4
100
2
2
83
7
2
100
7
2
0*
15
3
83*
15
2
0*
117
1
83*
90
1
0*
Zn=6 plants.
^3-year-old plants in 1985 and 2-year-old plants in 1986.
x*Indicates that the treatment differs from the unflooded
control by Williams' method, 5% level.
C 4-,


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
individual leaf areas of the 78-day flooding treatment in 1985, while


Table 4-3. Effect of periodic flooding on leaf area of 'Tifblue' rabbiteye
blueberry plants.
Spring Summer
Leaf
area
Leaf
area
Flooding
duration
Year2^ (days)
Flooding Total
periods ^
(no.) (cm )
Indiv.
(cm2)
Flooding
duration
(days)
Flooding Total
periods ^
( no.) ( cm )
Indiv.
, 2.
(cm )
1985
0
0
1735
1.86
0
0
831
2.30
2
4
1250*X
2.05
2
2
594
2.41
7
2
1168*
1.18*
7
2
564
2.66
15
3
825*
0.47*
15
2
566
2.05
106
1
326*
0.47*
78
1
128*
1.44*
1986
0
0
825
3.07
0
0
330
2.57
2
4
665
2.64
2
2
216
2.18
7
2
540*
2.32*
7
2
0*
0*
15
3
231*
1.72*
15
2
0*
0*
117
1
278*
1.80*
90
1
0*
0*
u
i-
2
n=6 plants.
y
3-year-old plants in 1985 and 2-year-old plants in 1986.
x*Indicates the treatment differs from the unflooded control by Williams'
method, 5% level.


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


Table 4-4. Effect of periodic flooding on fruit set, yields, and flower bud
number of 'Tifblue' rabbiteye blueberry plants.
Year2^
Flooding
duration
(days)
Spring
Flooding
periods
(no.)
Summer
Fruit Flooding Flooding Flower buds
setX Yield duration periods formed
(%) (% of control) (days) (no.) (% of control)
1985
0
0
40
100
0
0
100
2
4
30
84
2
2
62*
7
2
14
42
7
2
56*
15
3
39
105
15
2
11*
106
1
23*w
36*
78
1
0*
1986
0
0
45
100
0
0
100
2
4
38
41*
2
2
30*
7
2
39
63*
7
2
0*
15
3
22*
14*
15
2
0*
117
1
15*
23*
90
1
0*
Zn=6 plants.
y
3-year-old plants in 1985 and 2-year-old plants in 1986.
Arcsin transformation of percent fruit set data before analysis.
Lr
u
w.
*Indicates that the treatment differs from the unflooded control by
Williams' method, 5% level.


54
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
s
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, g^ 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 g decreases
s
water loss but also decreases carbon assimilation needed for growth and
flower bud formation.


55
Fig. 4-3. Effect of periodic flooding on stomatal conductance and
transpiration of 'Tifblue' rabbiteye blueberry plants during summer,
1985. Each point is the mean of 6 plants (2 leaves per plant) +SE.
Arrows (f) indicate start of flooding periods. Symbol *, indicates
the treatment differs from the unflooded control by Williams' method,
5% level.


56
Rabbiteye blueberry gg 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 gg responds to flooding in stages based
on flooding duration. During short-term flooding (2 to 7 days) gg and E
of flooded plants are less than those of unflooded plants, at
intermediate flooding durations (10 to 25 days) gg and E approach zero
but stomata remain responsive to environmental changes, and after long
term flooding (30 or more days) g^ 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 storaatal conductance (gg) 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 g .
s
Reduced water uptake under flooded conditions has been attributed
to the adverse effects of high carbon dioxide and low oxygen
concentrations on permability of root 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 presumeably 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
58


59
under anaerobic conditions.
Decreased 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 (Â¥ )
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 gg for flooded and
unflooded rabbiteye blueberry plants.


60
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, Aerie 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+/AgCl
reference electrode were attached to a Keithley microvoltraeter (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


61
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 gg 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 20C
to 22C 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 0.3041).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.


62
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 22C
and electrical conductance (EC) of the effusate was measured using a
Copenhagen Conductivity Meter (Model CDM3). Root pieces were then
frozen at -80C for 1 hr, reshaken with effusate for 24 hr, and EC taken
again. Electrolyte leakage was expressed as: sample EC (uS) / total EC
(US) 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 meansj^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


63
(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 (16-19C) 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
O 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.


65
^ 100
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80
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CO
CO
E
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O
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£
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sz
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40
20
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Flooded
Unflooded
o
Tifblue expt.-1
60 -
40
20
0
80
60 -
- 40 -
20
Woodard expt.-2
I i i i
X
x
X
X
X
0 2 4 6 8 10 12 14 16 18 20
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.


66
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.


67
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, gg
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), ¥ of
w
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 V The decrease in g and RLp
w s
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 gg probably decreased
without a change in (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).


68
Time (Days)
Fig. 5-4. Effect of flooding on stomatal conductance 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.


69
Rabbiteye blueberry is considered moderately flood tolerant based
on its survival in field (Chapters III, IV) and greenhouse (36) tests.
Nevertheless, reduced gg, 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 31C for cotton and 23C 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.
70


71
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 C increase, while oxygen solubility decreases about 1.6% per C
increase (147). Letey et al. (91) demonstrated a 1.8% increase in 0DR
per C 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 25C) (150) and has been shown


72
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 peatrperlite (P:P) (1:1 v/v) media for 1 experiment. Myakka
fine sand (sandy, siliceous, hyperthermic, Aerie 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


73
were covered with aluminum foil to exclude light and prevent algal
growth. Plants used in all experiments were transfered 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 20C,
25C, or 30C 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 20C, 25C, or 3(H1.5C 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 ^
-2
m at 15 cm from the surface of the root chamber. Air temperatures in
the growth room fluctuated from 22C to 33C 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


74
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+/AgCl 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, 250C column temperature with air, and at
300, 50, and 50 ml min 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 80C for about 4 days. Covariant
analysis using inital 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.


75
Results and Discussion
Soil Oxygen Measurements
Generally, soil temperature had no significant effect on soil ODR
and Eh of HFS 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 yg
-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.


Fig. 6-1. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 1) planted to 'Woodard* rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at the 10-cm soil depth. Symbols
+ *, and ** indicate significance 4 to the 10%, 5%, and 1% level, respectively among
treatments for a given day.


Oxygen diffusion rate
0.5
0
Flooded
Unflooded
ODR
o
Eh

0
5 10
Time (Days)
15
100
0
-100
-200
-300
-400
Redox Potential (mV)


Fig. 6-2. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 2) planted to 'Woodard' rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at the 10-cm soil depth. Symbols
* and ** indicate significance 4 to the 5% and 1% level, respectively and; ns =
non-significance among treatments for a given day.


0
0
C
O
cx)
13
I
C
E
£: c\j
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Time (Days)
Redox Potential (mV)


Fig. 6-3. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 3) planted to 'Woodard' rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at a 10-cm soil depth. Symbol
**, indicates significance 4 to the 1% level and ns = non-significance among
treatments for a given day.


Oxygen diffusion rate
Redox potential (mV)


Fig. 6-4. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
peat:perlite media planted to 'Woodard' rabbiteye blueberry plants. Each point is
the mean of 6 replications (12 subsamples) at a 10-cm soil depth. Symbols +, *, and
** indicate significance 4 to the 10%, 5%, and 1% level, respectively and ; ns =
non-significance among treatments for a given day.


Oxygen diffusion rate
(yg cm_2rriin_ 1)
opo p o o o
o a bt b> '-M
£8


84
Plant Survival
The LD^q (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 20C 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 artifical 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 LD^'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 number2 (C) LD^q^
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.
^Number of days
at which 50% of
the plants
died.
^ess than 50% mortality by termination of the
experiment.


Table 6-2. Effect of flooding on dry weight of 'Woodard' rabbiteye
blueberry plants.
Experiment Dry weight (g)
, z ; _
Soil media
number
Treatment
Stems
Roots
Leaves
Total !
Shoot/root
Myakka fine sand
1
Flooded
6.54
2.72
5.05
14.31
2.58
Unflooded
5.82
2.85
4.87
13.53
2.38
Significance^ ns
ns
ns
ns
ns
2
Flooded
7.60
4.79
3.49
15.88
1.72
Unflooded
10.46
8.97
6.55
25.98
1.22
Significancey **
**
**
**
>'c -k
3
Flooded
17.55
18.34
7.94
43.83
0.98
Unflooded
17.07
19.53
8.49
45.09
0.95
Significance^ ns
ns
ns
ns
ns
Peat:perlite
1
Flooded
15.56
7.69
14.09
37.36
2.07
Unflooded
19.57
10.01
18.96
48.60
2.08
Significancey **
*
*
*
ns
2
n=14 to 18 plants per treatment,
y
Split-plot analysis of variance where symbols and ** indicate significance
at the 5 and 1% levels, respectively and; ns = non-significant.


87
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
_1 _2
low stomatal conductance (5.98 to 30.53 mmol s m ) and high air
temperatures (30-33C) 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 pi rhizosphere This is equivalent to a range
of approximately 0 to 0.0036 mg liter ^ on a soil volume basis alone.
In contrast, Smith and Dowdell (128) reported 1 to 15 mg liter *
ethylene was evolved from several soils under flooded conditions.
Ethylene evolution in expt. 1 with MFS may have been confounded by


Fig. 6-5. Ethylene evolution from the rhizosphere of flooded 'Woodard' rabbiteye
blueberry plants grown in Myakka fine sand (expt. 1) at 20C, 25C, and 30C soil
temperatures. Each point is the mean of 2 replications (6 subsamples).


c 0.08
=i-0.05
0
10 15
Time (Days)


Full Text

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INGEST IEID EJO63JCLW_6FWLDI INGEST_TIME 2011-11-03T16:45:04Z PACKAGE AA00004837_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES


UNIVERSITY OF FLORIDA
3 1262 08554 1570


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.
iii

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT ix
CHAPTERS
IINTRODUCTION 1
IIREVIEW 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
IIIFLOODING DURATION AND SEASONAL EFFECTS ON GROWTH AND
DEVELOPMENT OF YOUNG RABBITEYE BLUEBERRY PLANTS 18
Introduction 18
Materials and Methods 20
Results and Discussion 22
IVPERIODIC 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
VHYDRAULIC 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
iv

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
v

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
vii

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 20°C, 25°C, and 30°C soil
temperatures 89
6-6. Ethylene evolution from the rhizosphere of flooded
'Woodard' rabbiteye blueberry plants grown in Myakka
fine sand (expt. 2) at 20°C, 25°C, and 30°C 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
ix

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 (20°C,
25°C, and 30°C) 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.
x

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
1

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

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
-2 -1
rate (ODR) is converted from amperes to flux (pg 0^ cm min ) (87,
88). Typically, ODR values range from 0.7 (unflooded but moist soil
-2 -1
conditions) to 0.2 pg 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 yg 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

mineralization, followed by nitrification in the thin aerobic soil-water
interface, followed by denitrification in the reduced soil below this
interface, and possibly volatilization as ammonia (104). Sulfur which
in reduced form (l^S) is highly toxic to plants, may be present in
organic form or as sulfate. Sulfur undergoes a sequence of changes
similar to those undergone by nitrogen under flooded soil conditions
(104).
Decomposition of organic matter in submerged soil occurs slowly and
many transitory substances may accumulate to toxic concentrations before
being metabolized to carbon dioxide, methane, and humic acids (42, 105).
These transitory compounds include gases (hydrogen sulfide, ethylene,
butane, methane), alcohols (ethanol, methanol), carbonyls
(acetaldehyde), fatty acids (acetate), phenols (coumaric), and volatile
sulfur compounds (mercaptans) (105). How toxic these substances are to
plants depends upon their concentration, susceptibility of the host, and
duration of exposure.
Alternating aerobic and anaerobic soil conditions have been shown
to decrease organic matter content and increase nitrogen loses more than
continuous flooding (110). This was attributed to increased
solubilization of organic matter during flooding, subsequent increase in
organic matter available for decomposition during the unflooded period,
and stimulation of the nitrification-denitrification process. As the
duration of the aerobic-anaerobic cycles decreased the loss of organic
matter and nitrogen increased (108). Changes in redox potentials have
been shown to correlate with alternating aerobic and anaerobic soil
conditions (108).

Effect of Flooding on Plant Survival and Growth
Flooding tolerance varies within and among plant species. Flooding
tolerance of woody fruit species ranges from 5-20 days for peach (6,
118), 7-19 days for various Juglans species (20), 12 months for apple
(6), and over 20 months for quince and various Pyrus species (6). In
one report (80), mango and guava trees were tolerant of flooding, while
citrus, loquat and pawpaw were sensitive. Rowe and Beardsell's (118)
review on flooding of fruit trees lists a wide range of flooding
sensitivity for various peach, plum, pear, apple, and citrus rootstocks.
Containerized rabbiteye (39) and highbush blueberry (1, 2) plants
survived up to 2 and 30 months of continuous flooding, respectively.
Hardwood forest species also show variable flooding tolerance, ranging
from 12 days for poplar (83) to indefinitely for various willow (6, 9),
mangrove and cypress (80) species.
Flooding duration and season also affect flooding tolerance.
Numerous hardwood species such as sweetgum, green ash, and sycamore
tolerated long periods of dormant season flooding, but died after
short-term flooding during the growing season (55). Apple (96) and
rabbiteye (38) and highbush blueberry (1, 2) plants tolerated dormant
season flooding but were damaged by flooding during the growing season.
Alternating periods of flooded and unflooded conditions may also
affect flooding tolerance. Hall and Smith (55) observed that survival
of hardwood species to periodic flooding over an 8-year period depended
upon the percent of time plants were flooded during the growing season.
In contrast, growth of some hardwood species increased after release
from short-term flooding (80). Broadfoot (18) reported flooding a
mature mixed hardwood stand for 5 months during the growing season for 3

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 Saxífraga 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
1-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), túpelo
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,
18

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 (g^) 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, Aerie 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 L1-C0R 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),
2
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
raonotonically 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 OPR and Redox Potential
Soil ODR of flooded plots at the 15-cm soil depth decreased to 0.20
-2 -1
yg 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

Oxygen diffusion rateíugcm 2m¡n 1)
Ojl-j— i i » ■ . ; i i
0 5 10 15 20 25 30 35
Time (Days)
Fig. 3-1. Soil oxygen diffusion rates of flooded and unflooded plots
planted to 'Woodard' rabbiteye blueberry plants during summer, 1984.
Each point is the mean of 3 plots (10 reps per plot) taken at the 15-
and 30-cm soil depths. Symbols +, *, and ** indicate significance 4
to the 10%, 5%, and 1% level, respectively and; ns = non¬
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 pg 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

Fig. 3-2. Soil oxygen diffusion rates 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. Symbols +,
and ** indicate significance 4 to the 10%, 5%, and 1% level, respectively and; ns
non-significance among treatments for a given day.

Oxygen diffusion rate (pg cm-2 min- 1)
0 5 10 15 20 25 30 35 40
Time (Days)

-300
20 25
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.
Year2
Flooding
duration (days)
Survival
Spring
(%)
Summer
1984
0
100
100
5
100
100
15
100
100
25
100
100
35
83
100
1985
0
100
100
5
100
67*
15
100
33*
25
83 *y
17*
35
67*
0*
Zn=3 plants in 1984, 6 plants in 1985.
^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 22°C (15-cm
depth) in spring, and 33°C (15-cm depth) in summer, while air
temperatures reached 27°C in spring and over 38°C 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 30°C compared with 20°C
in growth chamber studies (Table 6-1). Davies and Flore (36) found that
carbon assimilation of flooded rabbiteye blueberry plants became
negative above 28°C, 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
cm 15.0
E
o
CO
©10.0
k_
CO
co
a>
' 5.0
0
20.0
E
o
-15.0
sz.
O)
c
a>
E
CO
10.0
5.0
0
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.

O 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

Table 3-2. Flooding duration and leaf area of 'Woodard'
rabbiteye blueberry plants.
Flooding Spring Summer
duration
Total
Individual
Total
Individual
V 2
Year
(days)
f 2v
(cm )
Co,2»
(cm2)
(cm2)
1984
0
2500
4.04y
1707
4.49
5
2402
4.03
1990
4.33
15
1974
3.73
1586
4.11
25
1965
3.75
1324
3.99
35
672*X
2.82*
1292
5.48
1985
0
1579
2.15^
1706
2.44
5
1063
1.71
913*
1.49*
15
1126
1.71
15*
0.27*
25
1044
1.20*
1*
0.21*
35
744*
0.82*
0*
0.00*
Zn=3 plants in 1984, 6 plants in 1985.
^n=90 leaves in 1984, all leaves per plant in 1985.
^Indicates that the treatment differs from the unflooded
control by Williams' method, 5% level.

flooded plants (Fig. 3-4). During the summer, leaf expansion stopped â– 
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 decribed 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
Year^
Flooding
duration
(days)
Fruit
z
set
(%)
Yield
dry wt
(g)
Flower buds
formed2
(% 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*
2
Arcsin transformation of percent data before
analysis.
^n=3 in 1984, n=6 in 1985.
X*Indicates that the treatment differs from the
unflooded control by Williams' method, 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 gg (Fig. 3-6). Leaf
water potentials (’f ) were similar among flooded and unflooded
w
treatments for the first 2 weeks of treatments (-0.50 to -1.50 MPa).
However, V of plants flooded 25 days or more decreased to about -2.5
w
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

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 28°C (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
40

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, Aerie 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.

Table 4-1. Flooding treatments and dates used for young rabbiteye blueberry
plants.
Season/
zy
year J
Flooding
duration
(days)
Flooding
periods
(no.)
Cumulative
flooding
duration Dates of
(days) flooding treatments
Spring
0
0
0
Unflooded control
1985
2
4
8
3-5 Apr., 19-21 Apr.,
29 Apr.-l May,
13-15 June
7
2
14
3-10 Apr., 13-20 June
15
3
45
3-18 Apr., 29 Apr.-14
May, 13-28 June
106
1
106
Continuously flooded,
3 Apr. to 18 July
Spring
0
0
0
Unflooded control
1986
2
4
8
3-5 Apr., 18-20 Apr.,
27-29 Apr.,
13-15 June
7
2
14
3-10 Apr., 13-20 June
15
3
45
3-18 Apr., 27 Apr.-12
May, 13-28 June
117
1
117
Continuously flooded,
3 Apr. to 29 July
Summer
0
0
0
Unflooded control
1985
2
2
4
8-10 Sept., 1-3 Oct.
7
2
14
8-18 Sept., 1-8 Oct.
15
2
30
8-23 Sept., 1-16 Oct.
78
1
78
Continuously flooded,
8 Sept, to 26 Nov.
Summer
0
0
0
Unflooded control
1986
2
2
4
20-22 Aug., 6-8 Oct.
7
2
14
20-27 Aug., 6-16 Oct.
15
2
30
20 Aug.-3 Sept., 6-21
Oct.
90
1
90
Continuously flooded,
20 Aug. to 18 Nov.
zn=6 plants.
â– ^3-year-old plants in 1985 and 2-year-old plants in 1986.

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 (gg) and
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

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.

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 (t) 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.

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-ld) 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 artifical media
used having higher Eh (less reduced) or to lower soil and air
temperatures found in Rhode Island compared to Florida.

Table 4-2. Effect of periodic flooding on survival of 'Tifblue'
rabbiteye blueberry plants.
Spring
Summer
Flooding
Flooding
SurvivalX
Flooding
Flooding
SurvivalX
duration
periods
duration
periods
Year2-^ (days)
(no. )
(%)
(days)
(no.)
(%)
1985
0
0
100
0
0
100
2
4
100
2
2
100
7
2
100
7
2
100
15
3
100
15
2
67*
106
1
83*
78
1
33*
1986
0
0
100
0
0
100
2
4
100
2
2
83
7
2
100
7
2
0*
15
3
83*
15
2
0*
117
1
83*
90
1
0*
Zn=6 plants.
^3-year-old plants in 1985 and 2-year-old plants in 1986.
x*Indicates that the treatment differs from the unflooded
control by Williams’ method, 5% level.
C 4-,

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
individual leaf areas of the 78-day flooding treatment in 1985, while

Table 4-3. Effect of periodic flooding on leaf area of 'Tifblue' rabbiteye
blueberry plants.
Spring Summer
Leaf
area
Leaf
area
Flooding
Flooding Total
Indiv.
Flooding
Flooding
Total
Indiv.
duration
Year2^ (days)
periods ^
(no.) (cm )
(cm2)
duration
(days)
periods
(no.)
(cm2)
( cm2)
1985
0
0
1735
1.86
0
0
831
2.30
2
4
1250*X
2.05
2
2
594
2.41
7
2
1168*
1.18*
7
2
564
2.66
15
3
825*
0.47*
15
2
566
2.05
106
1
326*
0.47*
78
1
128*
1.44*
1986
0
0
825
3.07
0
0
330
2.57
2
4
665
2.64
2
2
216
2.18
7
2
540*
2.32*
7
2
0*
0*
15
3
231*
1.72*
15
2
0*
0*
117
1
278*
1.80*
90
1
0*
0*
u
i-
2
n=6 plants.
y
3-year-old plants in 1985 and 2-year-old plants in 1986.
x*Indicates the treatment differs from the unflooded control by Williams'
method, 5% level.

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

Table 4-4. Effect of periodic flooding on fruit set, yields, and flower bud
number of 'Tifblue' rabbiteye blueberry plants.
Year2^
Flooding
duration
(days)
Spring
Flooding
periods
(no.)
Summer
Fruit Flooding Flooding Flower buds
setX Yield duration periods formed
(%) (% of control) (days) (no.) (% of control)
1985
0
0
40
100
0
0
100
2
4
30
84
2
2
62*
7
2
14
42
7
2
56*
15
3
39
105
15
2
11*
106
1
23*w
36*
78
1
0*
1986
0
0
45
100
0
0
100
2
4
38
41*
2
2
30*
7
2
39
63*
7
2
0*
15
3
22*
14*
15
2
0*
117
1
15*
23*
90
1
0*
Zn=6 plants.
y
3-year-old plants in 1985 and 2-year-old plants in 1986.
Arcsin transformation of percent fruit set data before analysis.
Lr
u
w
*Indicates that the treatment differs from the unflooded control by
Williams' method, 5% level.

54
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
s
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, g^ 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 g decreases
s
water loss but also decreases carbon assimilation needed for growth and
flower bud formation.

55
Fig. 4-3. Effect of periodic flooding on stomatal conductance and
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.

56
Rabbiteye blueberry gg 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 gg responds to flooding in stages based
on flooding duration. During short-term flooding (2 to 7 days) gg and E
of flooded plants are less than those of unflooded plants, at
intermediate flooding durations (10 to 25 days) gg and E approach zero
but stomata remain responsive to environmental changes, and after long
term flooding (30 or more days) g^ 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 storaatal conductance (gg) 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 g .
s
Reduced water uptake under flooded conditions has been attributed
to the adverse effects of high carbon dioxide and low oxygen
concentrations on permability of root 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 presumeably 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
58

59
under anaerobic conditions.
Decreased 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 (Â¥ )
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 gg for flooded and
unflooded rabbiteye blueberry plants.

60
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, Aerie 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+/AgCl
reference electrode were attached to a Keithley microvoltraeter (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

61
potential of the reference electrode. Storaatal 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 gg 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 20°C
to 22°C 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 0.3041).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.

62
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 22°C
and electrical conductance (EC) of the effusate was measured using a
Copenhagen Conductivity Meter (Model CDM3). Root pieces were then
frozen at -80°C for 1 hr, reshaken with effusate for 24 hr, and EC taken
again. Electrolyte leakage was expressed as: sample EC (uS) / total EC
(US) 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

63
(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 (16-19°C) 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
O 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.

65
^ 100
I
80
i
CO
0- 60
o
>
£
CD
CO
CO
E
o
O
OvJ
I
CO
£
o
>
o
•O
c
o
o
o
3
cc
x>
>>
sz
£
CD
CO
40
20
0
Flooded
Unflooded
o
Tifblue expt.-1
60 -
40
20
0
80
60 -
- 40 -
20
Woodard expt.-2
JL— i i « i
X
X
X
X
X
0 2 4 6 8 10 12 14 16 18 20
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.

66
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.

67
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, gg
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), ¥ of
w
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 V . The decrease in g and RLp
w s
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 gg probably decreased
without a change in (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).

68
Time (Days)
Fig. 5-4. Effect of flooding on stomatal conductance 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.

69
Rabbiteye blueberry is considered moderately flood tolerant based
on its survival in field (Chapters III, IV) and greenhouse (36) tests.
Nevertheless, reduced gg, 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 31°C for cotton and 23°C 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.
70

71
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 °C increase, while oxygen solubility decreases about 1.6% per °C
increase (147). Letey et al. (91) demonstrated a 1.8% increase in 0DR
per °C 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 25°C) (150) and has been shown

72
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 peatrperlite (P:P) (1:1 v/v) media for 1 experiment. Myakka
fine sand (sandy, siliceous, hyperthermic, Aerie 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

73
were covered with aluminum foil to exclude light and prevent algal
growth. Plants used in all experiments were transfered 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 20°C,
25°C, or 30°C 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 20°C, 25°C, or 3(H1.5°C 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 ^
-2
m at 15 cm from the surface of the root chamber. Air temperatures in
the growth room fluctuated from 22°C to 33°C 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

74
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+/AgCl 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, 250°C column temperature with air, and at
300, 50, and 50 ml min 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 80°C for about 4 days. Covariant
analysis using inital 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.

75
Results and Discussion
Soil Oxygen Measurements
Generally, soil temperature had no significant effect on soil ODR
and Eh of HFS 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 yg
-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.

Fig. 6-1. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 1) planted to 'Woodard* rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at the 10-cm soil depth. Symbols
+ , *, and ** indicate significance 4 to the 10%, 5%, and 1% level, respectively among
treatments for a given day.

Oxygen diffusion rate
0.5
0
Flooded
Unflooded
ODR ©
o
Eh Ü
â–¡
0
5 10
Time (Days)
15
100
0
-100
-200
-300
-400
Redox Potential (mV)

Fig. 6-2. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 2) planted to 'Woodard' rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at the 10-cm soil depth. Symbols
* and ** indicate significance 4 to the 5% and 1% level, respectively and; ns =
non-significance among treatments for a given day.

o
03
c
O
cx)
13
I
C
E
£: c\j
c o
<1> O)
co zi_
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X
O
Time (Days)
Redox Potential (mV)

Fig. 6-3. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
Myakka fine sand (expt. 3) planted to 'Woodard' rabbiteye blueberry plants. Each
point is the mean of 6 replications (12 subsamples) at a 10-cm soil depth. Symbol
**, indicates significance 4 to the 1% level and ns = non-significance among
treatments for a given day.

Oxygen diffusion rate
Redox potential (mV)

Fig. 6-4. Soil oxygen diffusion rates and redox potentials of flooded and unflooded
peat:perlite media planted to 'Woodard' rabbiteye blueberry plants. Each point is
the mean of 6 replications (12 subsamples) at a 10-cm soil depth. Symbols +, *, and
** indicate significance 4 to the 10%, 5%, and 1% level, respectively and ; ns =
non-significance among treatments for a given day.

Oxygen diffusion rate
(yg cm_2rriin_ 1)
opo p o o o
o -ifou a cn b> '-M
£8

84
Plant Survival
The LD^q (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 20°C 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 artifical 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 LD^'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 number2 (°C) LD^q^
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.
^Number of days
at which 50% of
the plants
died.
^ess than 50% mortality by termination of the
experiment.

Table 6-2. Effect of flooding on dry weight of 'Woodard' rabbiteye
blueberry plants.
Experiment Dry weight (g)
, z _ . . — — ; —_
Soil media
number
Treatment
Stems
Roots
Leaves
Total !
Shoot/root
Myakka fine sand
1
Flooded
6.54
2.72
5.05
14.31
2.58
Unflooded
5.82
2.85
4.87
13.53
2.38
Significance^ ns
ns
ns
ns
ns
2
Flooded
7.60
4.79
3.49
15.88
1.72
Unflooded
10.46
8.97
6.55
25.98
1.22
Significancey **
**
**
**
>'c -k
3
Flooded
17.55
18.34
7.94
43.83
0.98
Unflooded
17.07
19.53
8.49
45.09
0.95
Significance^ ns
ns
ns
ns
ns
Peat:perlite
1
Flooded
15.56
7.69
14.09
37.36
2.07
Unflooded
19.57
10.01
18.96
48.60
2.08
Significancey **
*
*
*
ns
2
n=14 to 18 plants per treatment,
y
Split-plot analysis of variance where symbols * and ** indicate significance
at the 5 and 1% levels, respectively and; ns = non-significant.

87
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
_1 _2
low stomatal conductance (5.98 to 30.53 mmol s m ) and high air
temperatures (30-33°C) 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 pi rhizosphere This is equivalent to a range
of approximately 0 to 0.0036 mg liter ^ on a soil volume basis alone.
In contrast, Smith and Dowdell (128) reported 1 to 15 mg liter *
ethylene was evolved from several soils under flooded conditions.
Ethylene evolution in expt. 1 with MFS may have been confounded by

Fig. 6-5. Ethylene evolution from the rhizosphere of flooded 'Woodard' rabbiteye
blueberry plants grown in Myakka fine sand (expt. 1) at 20°C, 25°C, and 30°C soil
temperatures. Each point is the mean of 2 replications (6 subsamples).

0.12
0.1 1
_0.10
0.09
c 0.08
| 0.07
8 0.06
"H-0.05
'w'
0.04
c
0.03
£ 0.02
LU
0.01
0
Time (Days)

Fig. 6-6. Ethylene evolution from the rhizosphere of flooded 'Woodard' rabbiteye
blueberry plants grown in Myakka fine sand (expt. 2) at 20°C, 25°C, and 30°C soil
temperatures. Each point is the mean of 2 replications (6 subsamples).

O 5 10 15 20
25 30
Time (Days)
Temp
(°C)
20 0
25 £
30 A
SE |

92
pathogen invasion. No correlation was observed between ethylene
evolution, plant symptoms such as leaf abscission and chlorosis or plant
survival in either expt. 1 or 2. Ethylene is not considered toxic to
plants (42) and leaf symptoms observed may have been due to nutritional
deficiencies as observed by others (141).
Soil temperature and type were found to affect plant survival under
flooded conditions. Blueberry growers can expect a more rapid rate of
blueberry plant mortality as observed under field conditions (Chapters
III, IV) during summer flooding when soil temperatures are high than
during spring when temperatures are lower. Although temperature had no
consistent effect on growth of flooded plants in MFS under laboratory
conditions, summer flooding in the field decreased shoot elongation and
leaf area more than spring flooding (Figs. 3-4, 3-5, Table 4-2). Plants
flooded in artifical media (P:P) were less sensitive to media
temperature than plants in field soil, but did succumb to flooding
durations greater than about 60 days. Ethylene evolution from flooded
MFS was low compared to other soil types (42, 128) and may not greatly
affect the flooding response of rabbiteye blueberry plants.

CHAPTER VII
CONCLUSIONS
Plant survival of rabbiteye blueberry plants appears to be based on
soil temperature and the number of days soil Eh is below 200 mV
regardless of whether flooding is accumulated continuously or
periodically. This was seen under both laboratory and field conditions
where plant survival decreased as soil temperatures increased. Although
variable year to year, plant survival decreased after 25 to 35 days of
continuous or three 15-day spring flooding periods under field
conditions. In contrast, plant survival during the summer decreased
after 15 days of continuous or two 7-day (14 days total) flooding
periods. This suggested soil temperature affected rabbiteye blueberry
plant survival under flooded soil conditions. Laboratory studies showed
the of flooded plants decreased with increasing soil temperature,
confirming what was observed in the field. Similarly, walnut (20),
peach, plum, and apricot (119) plant survival decreased with increasing
soil temperature.
Flooding duration also affected plant growth, yields, and flower
bud number. Total plant leaf area decreased after 35 days of continuous
and two 7-day periodic spring flooding periods. The slightly greater
sensitivity of spring leaf area to periodic rather than continuous
flooding may be due to the adverse affect of repeated flooding stress
followed by repeated drought stress during unflooded periods. The
effect of spring flooding on plant yields was variable from year to

94
year, but tended to decrease with flooding durations greater than 25 to
35 days regardless of whether accumulated continuously or periodically.
Fruit shrivelling and premature drop were observed after continuous or
periodic flooding and may have been caused by reduced conductance (RLp)
and movement (SLp) of water. Flower bud development was very sensitive
to flooding stress and decreased dramatically after just 5 days of
continuous and two 2-day periodic summer flooding treatments. This
probably was the result of decreased carbon assimilation as found by
Davies and Flore (34, 37) and/or reductions in plant growth hormones
involved in flower bud development (111).
Root hydraulic conductivity decreased after 4 to 6 days of
flooding, which supports previous findings with flooded pear (5), citrus
(136), and highbush blueberry plants (36). Whether this is due to the
adverse effect of high carbon dioxide and low oxygen levels (21, 51),
flood-induced hormonal imbalances (19, 111, 112), or plant and soil
produced toxins on root cell permeability (69, 81, 105) to water is
unknown. Flooding durations greater than 6 to 10 days decreased SLp and
increased EL suggesting root damage and blockage of the xylem vessels
occurred concomitantly. Stomatal conductance of flooded plants
decreased significantly below control values within 2 to 4 days,
although ¥ were probably comparable for flooded and unflooded plants
during this time (35, 36, 37). Reductions in g^ and RLp occurred
concomitantly supporting the relationship between these factors found
for citrus (136) and pear (5). Since the initial decrease in gg
probably occurred without a significant reduction in ¥ (35, 36), it is
likely the early effects of flooding on rabbiteye blueberry plant gg are
due to a root-produced signal as proposed by Davies and Flore (34)

95
rather than to a turgor-mediated response as seen after long-term
flooding (39).
In conclusion, this research confirms and extends previous findings
found with containerized rabbiteye blueberry plants flooded outdoors
(39), and under greenhouse (36), and laboratory conditions (35, 37). A
likely sequence of rabbiteye blueberry flooding-stress response with
time is as follows (Table 7-1): 1) after 1 to 4 days of flooding
stomatal conductance to carbon dioxide water is decreased (35), root
hydraulic conductivity and carbon assimilation is reduced (35, 37), and
flower bud development is decreased; 2) after 6 to 14 days of flooding
stem hydraulic conductivity residual conductance to carbon dioxide (35,
37), and quantum yield are reduced (37), shoot elongation and leaf
expansion are inhibited, and the appearance of fruit shrivelling and
premature leaf senescence and abscission occur, and; 3) after about 25
to 45 days of flooding fruit set and yields are decreased and stem
desiccation and death begin. This sequence of events may be shortened
by high soil temperatures and the presence of disease.

96
Table. 7-1. Time course of physiological and growth responses of
rabbiteye blueberry plants to flooding duration.
Days
Plant response
0
4
4 g (+), g ,(+), A(4), Q(4)
4 RLp (4 ), Y (no change) (ref. 36, 37)
4 W
5 Flower bud number (4) (after summer flooding)
4
4 Leaf expansion and shoot elongation cease (high temp.)
4
4- SLp (4), EL (+), gr,(+), Q(4)
10 A(4), g less responsive to environment (ref. 36, 37)
4- Very new shoots and/or leaves may wilt
4
4 Premature leaf senescence and abscission (high temp.)
4 g (44), A (44), V (no change) (ref. 36, 37)
15 Llaf expansion an? shoot elongation ceases (mod. temp.)
4 RLp (44), SLp (44), E (4+),
4 Fruit shrivelling may begin
4
4
20 g (near zero), Y (no change) (ref. 37), A (zero - may
4 bicorne negative a¥ high temp.) (ref. 36)
4
4 g is slow to recover to preflood levels (ref. 36)
4 ^
25 Premature leaf senescence and abscission (mod. temp.)
4 Fruit set begins to decline (spring)
4 Yields begin to decline (some fruit drop may occur)
4
4
30 Continued decline in plant vigor, growth, fruit set,
4 yields, and leaf area
4 Stem desiccation and dieback may begin especially at
4 high temp.
+
35 Continued decline in fruit set and yields - fruit drop
4 may increase
4
4
4
40 Continued decline in plant vigor, growth, fruit set,
4 yields, and leaf area
4
4
4
45 Continued decline in plant vigor, growth, fruit set,
yields, and leaf area, ¥ (4) (ref. 37, 40)
w
Stem desiccation and dieback may intensify and some
plants may succumb to flooding stress

Table 7-1
continued
Symbol
Meaning
gc
Stomatal conductance to water
g_.
Stomatal conductance to carbon
dioxide
AS
Carbon assimilation
Q
Quantum yield
RLp
Root hydraulic conductivity
C
Leaf xylem water potential
Sip
Stem hydraulic conductivity
EL
Electrolyte leakage
gr'
Residual conductance to carbon
dioxide
+r
Decrease
Greater decrease
+
Increase
Greater increase
2
Disease infection (i.e., Phytophthora) may rapidly accelerate
flood-stress symptoms and loss in plant vigor leading to an even
more rapid plant death.

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110
BIOGRAPHICAL SKETCH
Jonathan Henry Crane was born in Plainfield, New Jersey, on October
10, 1952, of Marjorie L. Fountain and Carlyle W. Crane. He attended
grade school and high school with his twin brother David in Plainfield,
graduating from Plainfield High School in 1971. After traveling
througout the continental United States for about 1 year he attended
Union College (Cranford, New Jersey) and Franklin Pierce College
(Rindge, New Hampshire) for 1 year each. Jonathan then worked for the
Lehigh Valley Rail Road as a track laborer for approximately 3 years
(1974-1977) after which, he and his wife, Nancy (married on March 12,
1976), moved to Oregon. In 1978 he entered Oregon State University in
Corvallis, Oregon, and received his Bachelor of Science degree in
horticulture in 1981. In June, 1981, he and his family (Nancy, Laurel,
and Colin) moved to Florida, where he began graduate work for a Master
of Science degree in horticultural science in the Department of Fruit
Crops at the University of Florida in Gainesville, Florida. In August,
1984, he received his master's degree from the Fruit Crops Department
and continued his graduate work for a Doctor of Philosophy degree in
horticultural science. Jonathan will receive his Ph.D. in horticultural
science from the Department of Fruit Crops at the University of Florida
in August, 1987.

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Frederick S. Davies, Chairman
Associate Professor of
Horticultural Science
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
£l
L
J
-k
Peter C. Andersen
Assistant Professor of
Horticultural Science
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Karen E. Koch
Associate Professor of
Horticultural Science
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Associate Professor of
Soil Science

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Bennett
Associate Professor of
Agronomy
This dissertation was submitted to the Graduate Faculty of the College
of Agriculture and to the Graduate School and was accepted as partial
fulfillment of the requirements for the degree of Doctor of Philosophy.
August, 1987
Já iL
^Dean, College of Agriculture
Dean, Graduate School

UNIVERSITY OF FLORIDA
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