Group Title: role of environment during seed development on subsequent seed quality of cowpea (Vigna unguiculata) /
Title: The role of environment during seed development on subsequent seed quality of cowpea (Vigna unguiculata) /
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 Material Information
Title: The role of environment during seed development on subsequent seed quality of cowpea (Vigna unguiculata) /
Alternate Title: Vigna unguiculata
Physical Description: xiii, 145 leaves : ill. ; 28 cm.
Language: English
Creator: Tang, An-Ching Cheng, 1952-
Publication Date: 1982
Copyright Date: 1982
 Subjects
Subject: Cowpea -- Seeds   ( lcsh )
Seeds -- Viability   ( lcsh )
Horticultural Science thesis Ph. D
Dissertations, Academic -- Horticultural Science -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1982.
Bibliography: Bibliography: leaves 121-128.
Statement of Responsibility: by An-Ching Cheng Tang.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00099366
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000297772
oclc - 08512263
notis - ABS4147

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THE ROLE OF ENVIRONMENT DURING SEED DEVELOPMENT ON SUBSEQUENT
SEED QUALITY OF COWPEA (Vigna unguiculata)








By

AN-CHING CHENG TANG


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








UNIVERSITY OF FLORIDA




















To my parents.













ACKNOWLEDGEMENTS


The author wishes to express her gratitude to Dr. Daniel J.

Cantliffe, chairman of the supervisory committee, for his support,

guidance and patience throughout the course of this research.

She extends appreciation to Dr. C.B. Hall, Dr. T.E. Humphreys,

Dr. T.A. Nell and Dr. I.K. Vasil for serving as her committee.

Special thanks go to Dr. T.A. Nell for his assistance in the

greenhouse experiments. Appreciation is extended to Frank Woods

and Jeanne Fischer for their technical assistance and to Miss Rena Herb

for her typing of this manuscript.

The author would also like to thank those in the Vegetable Crop

Department for their friendship and help during her study.

Most of all, thanks go to the author's family. Without their

support and encouragement this study would not have been possible.














TABLE OF CONTENTS

Page

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

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

LIST OF FIGURES . . . . . . . ... . . ix

ABSTRACT . . . . . . . . . . . . xii

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

Chapter

I. LITERATURE REVIEW . . . . . . . . . 3


Influence of Light Stress on Seed Vigor
Influence of Water Stress on Seed Vigor
Influence of Temperature Stress on Seed


of the Progeny
of the Progeny
Vigor of the


Progeny . . . . . . . . . . .
Influence of Nutritional Status of the Mother Plants on
Seed Vigor of the Progeny . . . . . .

II. SEED YIELD AND VIGOR IN COWPEA SEEDS WHICH DEVELOPED
UNDER DIFFERENT PHOTOPERIODS AND LIGHT INTENSITIES.


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


III. REDUCTION IN SEED YIELD AND VIGOR OF COWPEA DUE TO WATER
STRESS DURING SEED DEVELOPMENT . . . . .. 33


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


IV REDUCTION IN SEED YIELD AND VIGOR OF COWPEA DUE TO
TEMPERATURE STRESS DURING SEED DEVELOPMENT . . .


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


. . . . . 17
. . . . . 19
. . . . . 32


. . . . .


. . . . .










Chapter Page

V. CHANGES IN SEED VIGOR OF COWPEA DUE TO NUTRITIONAL
TREATMENTS IMPOSED AT DIFFERENT GROWTH STAGES . 89

Materials and Methods . . . . . . .... 90
Results and Discussion . . . . . . .... 92
Summary . . . . . . . . ... . . . 113

SUMMARY AND CONCLUSIONS . . . . . . . .... . 115

LITERATURE CITED . . . . . . . ... .... . 121

APPENDIX . . . . . . . . ... . . . . . 130

BIOGRAPHICAL SKETCH . . . . . . . ... . 145














LIST OF TABLES


Table Page

1 Seed Germination and Seedling Growth of 'Texas Cream 40'
and 'Pinkeye Purple Hull' Cowpeas after Accelerated
Aging . . . . . . . . . . . 21

2 Effect of Photoperiod during Seed Development on Seed
Yields of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpea . . . . . . . ... .. . .22

3 Effect of Photoperiod during Seed Development on Subsequent
Seed Germination of 'Texas Cream 40' and 'Pinkeye Purple
Hull' Cowpeas with the Standard Germination Test and the
Accelerated Aging Test . . . . . .... .24

4 Effects of Photoperiod during Seed Development on Subsequent
Seedling Growth of 'Texas Cream 40' and 'Pinkeye Purple
Hull' Cowpeas with the Standard Germination Test . 26

5 Effect of Photoperiod during Seed Development on Subsequent
Seedling Growth of 'Texas Cream 40' and 'Pinkeye Purple
Hull' Cowpeas after Accelerated Aging . . ... 27

6 Effect of Light Intensity during Seed Development on Seed
Yield Measurement of 'Texas Cream 40' and 'Pinkeye
Purple Hull' Cowpeas . . . . . . . . 28

7 Effect of Light Intensity during Seed Development on
Subsequent Seed Germination and Seedling Growth of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas
with the Standard Germination Test . . . . . 30

8 Effect of Light Intensity during Seed Development on
Subsequent Seed Germination and Seedling Growth of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas
after Accelerated Aging . . . . . . . 31

9 Effect of Water Stress during Seed Development on Seed Size
Distribution of 'Texas Cream 40' and 'Pinkeye Purple
Hull' Cowpeas. . . . . . . . . 47

10 Embryo Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas in Media Containing Different Sucrose
Concentrations . . . . . .. . . 53

11 Embryo Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas in Different Culture Media . . . ... 55









Table Page

12 Effect of Water Stress during Seed Development on Embryo
Size at Seed Maturity and Its Growth in Culture of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas . . 56

13 Effect of Different Temperatures During Seed Development
on Embryo Size at Seed Maturity and Its Growth in Culture
of 'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas . 77

14 Effect of Different Temperatures during Seed Development
on the Percentage Changes in Weight and Nutrient Composition
in the Axis and Cotyledons of 'Texas Cream 40' Cowpea
during Germination . . . . . . . . . 80

15 Effect of Different Temperatures during Seed Development on
Nutrient Concentrations in the Axis and Cotyledons of
'Texas Cream 40' Cowpea after, 0, 1, 3 and 5 days of
Germination . . . . . . . . . . 82

16 Effect of Different Temperatures during Seed Development on
the Percentage Changes in Weight and Nutrient Composition
in the Axis and Cotyledons of 'Pinkeye Purple Hull' Cowpea
during Germination. . . . . . . . . 83

17 Effect of Different Temperatures during Seed Development on
Nutrient Concentrations in the Axis and Cotyledons of
'Pinkeye Purple Hull' Cowpea after 0, 1, 3 and 5 days of
Germination . . . . . . . . . . . 85

18 Effect of Different Temperatures during Seed Development on
Changes in Weight and Nutrient Composition of Axis and
Cotyledons of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpea during a 5 Day Germination Period . . . .. 86

19 Nutritional Treatments Imposed at the Seedling Stage of the
Parental Plant . . . . . . . . . 91

20 Effect of Nutritional Treatments Imposed at the Seedling
Stage of the Parent Plant on Seed Yields and Weights of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas . 93

21 Effect of Nutritional Treatments Imposed at the Seedling
Stage of the Parent Plant on Seed Germination and Seedling
Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas with the Standard Germination Test . . . 95

22 Effect of Nutritional Treatments Imposed at the Seedling
Stage of Parent Plant on Seed Germination and Seedling
Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas after Accelerated Aging . . . . . . 97








Table Page
23 Effect of Nutritional Treatments Imposed at the Seedling
Stage of the Parent Plant on Seed Composition of 'Texas
Cream 40' and 'Pinkeye Purple Hull' Cowpeas ...... 99

24 Effect of Nutritional Treatments Imposed at Anthesis of
Parent Plant on Seed Yields of 'Texas Cream 40' and
'Pinkeye Purple Hull' Cowpeas . . . . . ... 103

25 Effect of Nutritional Treatments Imposed at Anthesis on the
Pod Development Period and Seed Weights of 'Texas Cream 40'
and 'Pinkeye Purple Hull' Cowpeas . . . . .... 104

26 Effect of Nutritional Treatments Imposed at Anthesis of the
Parent Plant on Seed Germination and Seedling Growth of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas with
the Standard Germination Test . . . . . ... 105

27 Effect of Nutritional Treatments Imposed at Anthesis of the
Parent Plant on the Average Days to Germination of 'Texas
Cream 40' and 'Pinkeye Purple Hull' Cowpeas with the
Standard Germination Test . . . . . . .... 107

28 Effect of Nutritional Treatments Imposed at Anthesis of the
Parent Plant on Seed Germination and Seedling Growth of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas after
Accelerated Aging . . . . . . . .... 108

29 Effect of Nutritional Treatments Imposed at Anthesis of the
Parent Plant on Seed Composition of 'Texas Cream 40' and
'Pinkeye Purple Hull' Cowpeas . . . . . . . 110

A-i Seed Coat Color, Seed Weight and Relative Maturity of
Ten Cowpea Cultivars . . . . . . . . . 130

A-2 Seed Germination and Seedling Growth of Ten Cowpea Cultivars
with the Standard Germination Test . . . .... 131

A-3 Seed Germination and Seedling Growth of Ten Cowpea Cultivars
after Accelerated Aging at 410C and 100% RH for 5 days 132

A-4 Seed Coat Thickness of Hard and Nonhard 'Pinkeye Purple Hull'
Seeds Which Developed under SD . . . . .... 133

A-5 Partitioning of Seedling Weight and Nutrient Composition
during Germination of 'Texas Cream 40' Cowpea as affected
by Temperatures during Seed Development . . . .. 134

A-6 Partitioning of Seedling Weight and Nutrient Composition
during Germination of 'Pinkeye Purple Hull' Cowpea as
Affected by Temperatures during Seed Development . . 135

A-7 Effect of Developmental Temperature on Changes in Weight
and Nutrient Composition in the Axis of 'Pinkeye Purple
Hull' Seeds during Germination . . . . . .. 136














LIST OF FIGURES


Figure Page

1 Effect of water stress during seed development on seed
yield measurements of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas . . . . . . 37

2 Effect of water stress during seed development on seed
germination, germination rate and seedling lengths of
the progeny of 'Texas Cream 40' with the standard
germination test and after accelerated aging . . 39

3 Effect of water stress during seed development on seedling
weight of the progeny of 'Texas Cream 40' with the
standard germination test and after accelerated aging 40

4 Effect of water stress during seed development on seed
germination, germination rate and seedling lengths of
the progeny of 'Pinkeye Purple Hull' with the standard
germination test and after accelerated aging . 42

5 Effect of water stress during seed development on seedling
weight of the progeny of 'Pinkeye Purple Hull' with
the standard germination test and after accelerated
aging . . . . . . . . .. . 43

6 Final size and appearance of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas as affected by water
stress during seed development . . . ... 48

7 Effect of water stress during seed development on seed
composition (% dry weight basis) of 'Texas Cream 40'
and 'Pinkeye Purple Hull' cowpeas . . . ... 49

8 Effect of water stress during seed development on seed
composition (mg per seed) of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas . . . . .. 52

9 Effect of different temperatures during seed development on
seed yield measurements of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas . . . . . . 63

10 Final size and appearance of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas as affected by
temperature during seed development . . . ... 64








Figure Page

11 Effect of different temperatures during seed development
on seed germination, germination rate and seedling
lengths of the progeny of 'Texas Cream 40' cowpea with
the standard germination test and after accelerated
aging . . . . . . . . . . 66

12 Effect of different temperatures during seed development
on seedling weight of the progeny of 'Texas Cream 40'
cowpea with the standard germination test and after
accelerated aging . . . . . . . . 67

13 Effect of different temperatures during seed development
on seed germination, germination rate and seedling
lengths of the progeny of 'Pinkeye Purple Hull' cowpea
with the standard germination test and after
accelerated aging . . . . . . . . 69

14 Effect of different temperatures during seed development
on seedling weight of the progeny of 'Pinkeye Purple
Hull' cowpea with the standard germination test and
after accelerated aging . . . . . .... 70

15 Effect of different temperatures during seed development
on seed composition (% dry weight basis) of 'Texas
Cream 40' and 'Pinkeye Purple Hull' cowpeas . . 74

16 Effect of different temperatures during seed development
on seed composition (mg per seed) of 'Texas Cream 40'
and 'Pinkeye Purple Hull' cowpeas . . . ... 75

17 Effect of nutritional treatments imposed at anthesis of
the parent plant on seed composition of 'Texas Cream 40'
and 'Pinkeye Purple Hull' cowpeas . . . ... 112

A-1 Effect of water stress during seed development on seed
germination, germination rate and seedling lengths of
the progenies of 'Texas Cream 40' and 'Pinkeye Purple
Hull' cowpeas with the standard germination test 137

A-2 Effect of water stress during seed development on seedling
weights of the progenies of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas with the standard germination
test . . . . . . . . ... .. . .. 138

A-2 Effect of water stress during seed development on seed
germination, germination rate and seedling lengths
of the progenies of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas after accelerated aging . . 139

A-3 Effect of water stress during seed development on seedling
weights of the progenies of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas after accelerated
aging . . . . . . . . ... .. . 140










Figure


page


A-5 Effect of different temperatures during seed development
on seed germination, germination rate and seedling
lengths of the progenies of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas with the standard
germination test . . . . . . . . . 141

A-6 Effect of different temperatures during seed development
on seedling weights of the progenies of 'Texas Cream 40'
and 'Pinkeye Purple Hull' cowpeas with the standard
germination test . . . . . . . . . 142

A-7 Effect of different temperatures during seed development
on seed germination, germination rate and seedling
lengths of the progenies of 'Texas Cream 40' and
'Pinkeye Purple Hull' cowpeas after accelerated aging .143

A-8 Effect of different temperatures during seed development
on seedling weights of the progenies of 'Texas Cream 40'
and 'Pinkeye Purple Hull' cowpeas after accelerated
aging . . . . . . . . . . . . 144














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

THE ROLE OF ENVIRONMENT DURING SEED DEVELOPMENT ON
SUBSEQUENT SEED QUALITY OF COWPEA (Vigna unguiculata)

By

An-Ching Cheng Tang

May, 1982

Chairman: Daniel J. Cantliffe
Major Department: Vegetable Crops

Seed yields of 'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas

were significantly reduced when seeds were produced under high temper-

ature, drought stress, low light intensity, or deficient levels of N or

P in the growth media, but were unaffected by variation in photoperiod.

Seed vigor of either cultivar was unaffected by photoperiod or

light intensity. Hardseededness developed in 'Pinkeye Purple Hull'

under shortdays.

Drought stress of -4 or -8 bars during seed development reduced

subsequent seedling axis growth of 'Texas Cream 40' compared to a

nonstressed treatment. At seed maturity smaller embryo size and less

nutrient accumulation, especially starch, in the cotyledons accounted

for this reduction in seed vigor. Under similar conditions, 'Pinkeye

Purple Hull' produced seeds as vigorous as the unstressed control.

At day/night temperatures of 280/200C and 330/250C, seeds of both

cultivars had higher vigor characteristics than seeds which developed

at 380/300C or 230/150C. In 'Texas Cream 40', heat stress at 380/30 C








appeared to reduce the ability of seedlings to utilize food reserves

effectively during germination. Reduced seed vigor of 'Pinkeye Purple

Hull' at 380/300C was related to a shortage of reserve food, especially

starch, at seed maturity. Vigor of seeds of both cultivars produced at

230/150C was less than seeds produced at 280/200C and 330/250C but was

not as reduced as those produced at 380/300C.

In nutritional experiments, seed vigor of 'Texas Cream 40' was

unaffected by various nutritional treatments imposed from sowing to

seed maturity. 'Pinkeye Purple Hull' plants grown on a deficient level

of N, P or S had reduced seed vigor compared with those grown in complete

Hoagland's solution. Seed vigor of this cultivar was also reduced by

doubling the N level in the nutrient media. When imposed after anthesis,

increased N and P levels in the nutrient media of the parent plant did

not alter seed vigor of either cultivar. However, reducing N or P

levels to one-quarter of those in a complete Hoagland's solution

enhanced axis growth of the progenies of both cultivars.















INTRODUCTION


There is a great demand for increased research emphasis on high

protein species to meet the food needs of the ever increasing world

population. Although cowpea is nutritionally poor in S-containing amino

acids, it is a main source of dietary protein in many tropical countries

(Summerfield et al., 1974). This is because cowpea normally contains

20 to 25% protein in the mature seed and is more widely adapted compared

to other legumes.

Low and inconsistent yields of cowpea have led to the initiation of

more research to gain a better understanding of yield components for this

species. Photoperiod, light intensity, temperature, soil moisture, and

soil fertility have been shown to affect plant development and the final

seed yield of cowpea. Rapid loss of viability during storage and

extremely poor seedling establishment in the field constantly plague

cowpea growers. Because cowpea is usually grown as a rainfed crop, the

environment can greatly alter yields through a reduction in seed vigor.

Thus, the importance of optimizing seed vigor in order to achieve higher

yields is indisputable.

Although genetically determined, seed vigor is often altered

by external factors. This includes the environment during seed

production, seed maturity, field weathering, pathogen and insect

attack, processing, and storage conditions. Altering any of these factors

could reduce seed vigor. Compared to other factors, environment during

seed development is beyond human control in the field. Variation in








seed vigor due to different growing seasons or locations has been shown

in several species including cowpea.

For many species there is abundant literature on the influence of

altered environmental conditions during seed development on subsequent

seed quality. However, most of the literature for cowpea is restricted

to studies on plant development, N fixation, and seed yields. Little

is known about the influence of environment on subsequent seed vigor.

Therefore, research concerning the role of environment during seed

development on seed vigor of the progeny of cowpea was conducted. The

objectives of the present experiments are (1) to investigate the influence

of different photoperiods, light intensities, water stresses, temperatures,

and nutritional media during cowpea seed development on subsequent seed

vigor, and (2) to examine closely the portion of the seed in which vigor

is affected. The results should be of benefit for the improvement of

cowpea seed production practices to ultimately produce higher quality

seed.














CHAPTER I
LITERATURE REVIEW



Research on seed vigor has drawn great attention in the past

several decades. An understanding of what seed vigor is and the

cause of low vigor may help to improve the yields of high quality

crops which may have a significant impact on economical returns to

farmers. Seed vigor is defined, according to the Association of

Official Seed Analysts (A.O.S.A.), as "the sum total of all those

properties in seeds which, upon planting, result in rapid and

uniform production of healthy seedlings under a wide range of

environment including both favorable and stress conditions"

(Woodstock, 1976, p. 4). Seedlings from high vigor seeds develop

more rapidly and yield more than those from low vigor seeds.

An accurate assessment of seed vigor cannot be made before a

clear understanding of the cause of low vigor is understood.

Several vigor tests have been established for certain species in

the A.O.S.A. seed vigor testing handbook (Woodstock, 1976). They

are the accelerated aging test, cold test, conductivity test, cool

germination test, seedling growth rate test, seedling vigor

classification test, and tetrazolium test. Other methods based on

biochemical metabolism during germination, such as respiration and

enzyme activity, have been correlated with seedling growth, but are

not recommended as routine procedures for vigor testing because of

variability in the tests (Woodstock, 1976).






Although genetically determined, seed vigor is often modified by

external factors. Several of these can affect seed vigor during seed

development. These include the environment and nutritional status under

which the mother plant grows, the stage of seed maturity at harvest,

field deterioration, and pathogen attack in the field (Austin, 1972;

Delouche, 1980). Environmental and nutritional stress can affect seed

vigor early in the life of the seeds. If seeds which develop under

adverse environments are low in vigor, they will be more susceptible

to field deterioration, pathogen attack and improper handling during

harvest and storage than seeds which develop under optimum conditions.

Therefore, understanding how seed vigor will be affected by the environ-

ment and nutritional status with which the seed develops is highly

important. Yet, research on this area is rather limited compared with

the other external factors which affect vigor.



Influence of Light Stress on Seed Vigor of the Progeny

Effect of photoperiod on seed vigor. Although the inductive effect

of photoperiod on flower initiation has been intensively studied for many

years, the influence of daylength during seed maturation on seed quality

was not reported until the 1960's (Koller, 1962). The photoperiodic

response of flower induction and seed quality is not always the same.

For example, prickly lettuce (Lactuca scariola) which flowered only

under 16 hours daylength produced high vigor seeds when the seeds

developed under 8 hours daylength compared with seeds developed under

16 hours daylength (Gutterman et al., 1975).

When seeds developed under 8 hours daylength, the seed weights

of soybean (Glycine max) and restharrow (Ononis sicula) decreased,




5



while those of lettuce (Lactuca sativa) and lamb's quarter (Chenopodium

album) increased, compared with their counterparts which developed

under longdays (16 to 20 hours daylength) (Koller, 1962; Wentland,

1965; Evenari et al., 1966; Patterson et al., 1977). A change in

daylength during the last 12 days of seed ripening affected the germin-

ability of 'Grand Rapids' lettuce (Gutterman, 1973). Seeds which

developed under continuous light had increased tolerance to germination

at high temperature both in continuous darkness and after a short

light break, compared with seeds developed under 8 hours daylength.

Conversely, pigweed (Amaranthus retroflexus) seeds which developed

under 8 hours daylength had higher percentage of germination in the

dark and at 300C after a short illumination or low temperature

(5 to 100C) pretreatment than seeds which developed under 16 hours

daylength (Kigel et al., 1977).

Seed dormancy can also be affected by photoperiod during seed

development. Both restharrow grown under 20 hours daylength and

lamb's quarter grown under 16 hours daylength produced a higher

proportion of dormant seeds than those under 8 hours daylength

(Wentland, 1965; Evenari et al., 1966; Karssen, 1970; Gutterman

and Evenari, 1972). The dormant seeds, which gave rise to scattered

stands upon sowing, possessed water impermeable seed coats.

Seedling growth following germination was not reported in the species

discussed above.

Purslane (Portulaca oleracea) seeds which matured under 8 hours

daylength reached 50% germination 4 days earlier than those matured

under 11 or 13 hours daylengths (Gutterman, 1974). The reduction in

daylength from 16 to 8 hours during the last 8 days of seed maturation








increased germination rate. Continuous light during seed development

and maturation of red beet (Beta vulgaris) increased the proportion

of empty seed balls and reduced the germinability of normally

developed seeds by 12% (Heide et al., 1976). The progeny yield

of roots was decreased by 10% compared with those grown under 8 hours

daylength. Similarly, seeds of prickly lettuce which developed

under 8 hours daylength were larger in size and had higher germination

percentage and rate than those developed under 16 hours daylength

(Gutterman et al., 1975). Seedling growth, measured as hypocotyl

length and cotyledon size, in seeds which developed under 8 hours

daylength was 25% greater thanin seeds which developed under 16 hours

daylength. Progeny plants flowered earlier in the former situation.

The high seed vigor of prickly lettuce which developed under 8

hours daylength was related to an increase in gibberellic acid

content of the seeds.

Effect of light intensity and quality on seed vigor. Little

is known about the relationship of light intensity and quality

during seed development to subsequent seed vigor. A reduction in

light intensity to 27% of incident radiation during pigweed seed

development affected its germination. Seed germination in the

dark or after a low temperature pretreatment was less in longday

shaded seeds than a longday control, but was higher in shortday

shaded seeds than the shortday control (Kigel et al., 1977).

In mouse-ear cress (Arabidopsis thaliana), germination in

the dark from seeds grown under cool white fluorescent lamps was

45%, under cool white plus incandescent lamps, 12%, and under

incandescent lamps, 0% (McCullough and Shropshire, 1970).








The phytochrome mediated system was thought to control this germination

response because the incandescent lamps provided a larger proportion

of light energy at wavelengths greater than 700 pm than cool white

fluorescent lamps. This mechanism was not apparent in purslane

because continuous red illumination during seed maturation did not

improve germination in the dark (Gutterman, 1974). However, red-far

red interruption during maturation under shortday affected germinability

of purslane. Red interruption increased seed germination at 400C,

while far-red interruption increased germination at 250 to 300C.



Influence of Water Stress on Seed Vigor of the Progeny

Rainfall during seed maturation of cotton (Gossypium hirsutum)

accounted for 27% of the variability in seed vigor (Peacock and

Hawkins, 1970). Higher rainfall led to progeny with better seedling

growth than seeds produced in drier weather. The percentage of

abnormal seedlings increased from 7% to 16% in peanut (Arachis

hypogaea) when seeds developed without irrigation (Cox et al., 1976).

Excessive irrigation of bean (Phaseolus vuloaris) plant bearing

mature seeds reduced seed vigor (Siddique and Goodwin, 1980b).

The number of normal seedlings was reduced by 10% from plants

watered with overhead irrigation every day, compared with those

with surface watering every other day.

Seed vigor of 'Moapa' alfalafa (Medicago sativa) was unaffected

by water stress of up to -10 bars imposed during the reproductive

stage (Walter and Jensen, 1970). Another cultivar 'Ranger' emerged

earlier and had greater seedling weight and volume when seeds

developed at soil moistures of -1 to -10 bars compared with -1/6

to -1/3 bars. Varietal differences in seed vigor due to drought








stress during seed development were also reported for peanut

(Pallas et al., 1977). The large-seeded Virginia type 'Florigiant'

was the most sensitive cultivar, the Spanish type 'Tifspan' the

least and the runner type 'Florunner' intermediate. The number of

mature seeds of 'Florigiant' was decreased by 34%, seed weight

reduced by 20%, and germination of mature seeds was 20% less when

the mother plant was grown at -15 bars soil water tension compared

with those grown above -2 bars. 'Tifspan' grown at -15 bars had

similar seed size and germinability as those grown above -2 bars.

Lettuce seed germination was high (97-100%) regardless of

water stress during seed development (Izzeldin et al., 1980b).

Seed wieght, however, was 40% higher when seeds developed at -5 bars

than seeds which developed at -0.3 or -0.8 bars. The percentage of

abnormal seedlings doubled and radicle length was 20% less in seeds

which developed at -0.3 bars compared with -5 bars. A similar

change in seed vigor by drought stress was reported for Indian

ricegrass (Oryzopsis miliacea) (Whalley et al., 1966). Seed weight

and subsequent seedling growth of the progeny increased by 10%

when the mother plant was grown in severe water stress (-15 bars)

compared with an adequate water supply (-1 bar).

'Luther Hill' sweet corn (Zea mays) tolerated drought stress

up to -5 bars during seed development without influencing seed size

distribution or viability (El-Forgany and Makus, 1979). A seed

vigor index, the combination of a cold test, germination rate and

the accelerated aging test, in the progeny was the same regardless

of water stress, but decreased significantly when -3 or -5 bars

water stress were imposed during silking. Those plants exposed to







water stress 3 to 6 weeks after silking produced seeds as vigorous

as the control.


Influence of Temperature Stress on Seed Vigor of the Progeny

The influence of temperature during seed development on subsequent

seedling growth was observed in spring wheat (Triticum aestivum) in the

1930's (Kostjucenko and Zarubailo, 1937). Exposure of the plants to low

temperatures (<140C) during the milk-ripe stage of seed development

significantly reduced the requirement for vernalization. Progeny plants

grew and developed more rapidly when seeds matured at 15.50C instead

of 26.50C (Riddell and Gries, 1958).

Day/night temperature treatments (250/200C, 200/150C and 150/10 C)

during seed development of Italian ryegrass (Lolium multiflorum),

perennial ryegrass (Lolium perenne) and meadow fescue (Festuca pratensis)

had a significant influence on subsequent seed germination and vigor

(Akpan and Bean, 1977). The germination percentage was the highest

in seeds which developed at 250/200C, especially at germination

temperatures of 130 to 200C. Seeds from the 150/10C treatment germin-

ated 2 to 3 days slower, but produced seedling with 20 to 24% more dry

weight than those which developed at higher temperatures (250/200C and

200/150C). In pearl millet (Pennisetum americanum), seed viability and

field emergence were unaffected by the temperatures (330/280C, 300/250C,

270/220C and 210/160C) at which the seeds developed (Fussell and

Pearson, 1980). However, grains which developed at 210/160C produced

seedlings with 40 to 60% more hieght and dry weight than those at

330/280C.

Similar improvements of seed vigor after maturation under low

temperatures were reported for several legumes (Green et al., 1965;







Harris et al., 1965; Walter and Jensen, 1970; Harrison and Perry,

1973; Perry and Harrison, 1973). Soybean plants which matured in

hot weather ( >32C) produced seeds with a lower percentage of germin-

ation and field emergence than those produced in cool weather

(Green et al., 1965). Seedling growth and seed yields in the progeny

of soybean were reduced by high temperature (270C) during the last

45 days of seed maturation compared with those produced under low

temperature (21C) (Harris et al., 1965). Seed germination was

delayed and seedling growth was reduced in pea (Pisum sativum) when

the mother plant encountered high temperatures (>350C) during the

10 day period after the pods had started to wrinkle compared with those

which developed at 300C (Harrison and Perry, 1973; Perry and Harrison,

1973). Seed yields in the progeny were reduced when the seeds were

originally matured at high temperature. In alfalfa, seedling weights

and leaf numbers were greater in 'Ranger' when the seeds matured at

13C than at 24C (Walter and Jensen, 1970). Seed vigor of another

cultivar 'Moapa' was unaffected by the temperatures at which seeds

had matured. When 10 genotypes of bean were subjected to six

temperature regimes (330/280C to 180/13C) from anthesis to seed

maturity, all produced seeds of higher vigor at the lower temper-

atures (210/160C and 180/130C) than at the higher temperatures

(330/280C and 300/250C) (Siddique and Goodwin, 1980a). Resistance

to mechanical injury increased in seeds which developed under low

temperatures. Seed size and vigor of 'Apollo' bean decreased

linearly as temperature increased from 210/160C to 330/280C during

seed development and maturation (Siddique and Goodwin, 1980b).

This adverse effect of high temperature on seed vigor was observed








even on plants with well-developed seeds at the yellow fleshy pod

stage.

Germination of tobacco (Nicotiana tabacum) was similar regardless

of the temperatures (220/180C, 260/220C and 300/260C) during seed

maturation (Thomas and Raper, 1975a). Rate of emergence, however,

was 1 to 2 days slower and seedling growth, determined as dry weight,

fresh weight and leaf size, was reduced by 17% when seeds matured

at 330/260C compared with 220/180C or 260/220C. When a low temperature

(180/140C) was imposed on tobacco plants during vegetative growth,

seed germination of the progeny was 14% less than those grown at

the higher temperatures (220/180C to 330/260C) (Thomas and Raper,

1975b). Seedling growth following germination was not reported.

Contrary to the above, seeds of bracted plantain (Plantago

aristata) which matured at warm temperature (270C) germinated 1 to

2 days earlier and produced seedlings of 25% greater growth than

seeds matured at cool temperature (160C) (Stearns, 1960). The diff-

erence in seedling growth persisted for 120 days. Similarly, seed

germination of red beet decreased by 50% and the plants yielded 13%

less when the mother plant was exposed to low temperature (120C)

during seed development compared with those exposed to higher

temperatures (180 and 240C) (Heide et al., 1976). Controversial

results, due to different experimental conditions, were reported

for cotton (Peacock and Hawkins, 1970; Quisenberry and Gipson, 1974).

Longer seedlings and higher yields were obtained when the mother

plant matured at a cool minimum temperature (150C) in the field

compared with a warm temperature (210C) (Peacock and Hawkins, 1970).

When plants matured at night temperatures of 110, 150, 210 and 270C








in the growth chamber, 20 to 70% less seedlings emerged and the resultant

plants yielded 20% less cotton in the field from seeds which matured at

110 and 150C than at 210 and 270C (Ouisenberry and Gipson, 1974).


Influence of Nutritional Status of the Mother
Plants on Seed Vigor of the Progeny

Seed viability of pepper (Capsicum annuum), carrot (Daucus

carota) and lettuce were similar regardless of the N levels (ranging

from 23mM to 0.23 mM) in which the mother plants were grown

(Harrington, 1960). Seedling growth, however, was not measured in

these studies. A 10 fold increase in the level of N in the culture

media of the mother plant did not affect seed germination and seedling

growth of pea (Austin, 1966b), nor did N application at 141 Kg/ha

to the mother plant influence subsequent progeny seed germination

and root yield of carrot (Austin and Longden, 1965, 1966). In tobacco,

an increase of N application from 50 to 94 Kg/ha to the mother plant

reduced the time to germination and enhanced seedling uniformity

of the progeny (Thomas and Raper, 1979). Seedling dry weights in

the progeny of wheat and Indian ricegrass were also increased by the

previous N supplement (Whalley et al., 1966; Ries and Everson, 1973).

A high percentage of seed germination was reported in pepper,

carrot and lettuce regardless of the P levels (ImM to 0.0078mM)

in the culture media of the mother plant (Harrington, 1960). Red

cotyledon, a physiological disorder in lettuce, increased by 50%

when the mother plant was grown under P deficiency conditions.

These seeds germinated 2 to 3 days slower and plant development,

as demonstrated by the formation of new leaves, accumulation of leaf








area, and vegetative maturity, was delayed 1 to 2 weeks compared with

those grown in complete Hoagland's solution. Formation of a firm

commercial lettuce head in the progeny was significantly reduced by

P deficiency in the mother plant (Izzeldin et al., 1980a). A reduction

in seed vigor due to P deficiency in the mother plant was observed in

pea, watercress (Rorippa nasturtium aquaticum) and carrot (Austin and

Longden, 1965, 1966; Austin, 1966a, 1966b). Seed germination and

emergence of these species were 95 to 100% regardless of the P levels

imposed on the mother plants. Seedling weight and pea yields were

25% less from seeds grown in 0.4mM P than in 4.0mM P (Austin, 1966b).

In watercress, early seedling dry weight and seed yields were 20 to

30% less in seeds produced at low P levels than those produced at

high P levels (Austin, 1966a). A 10% increase in carrot root yield

was related to an increase in seed P content which resulted from the

P supplement (133 Kg/ha) to the mother plants (Austin and Longden, 1966).

Phosphorus application to the mother plant of Indian ricegrass also

improved subsequent seedling growth (Whalley et al., 1966).

When K from 6mM to 0.094mM or Ca from 5mM to 0.078mM was applied

to the mother plants, the germination percentages of pepper, carrot and

lettuce seeds were not altered (Harrington, 1960). Seed longevity,

however, was significantly reduced when seeds developed on K or Ca

deficient plants. A high proportion of red cotyledon developed in

lettuce and the subsequent seedling growth was adversely affected

when deficient levels of Ca were imposed on the mother plant

(Harrington, 1960; Izzeldin et al., 1980a). Calcium content of the

peanut seeds was reduced from 420ppm to 200ppm when the mother plant

was grown without Ca supplement (Cox et al., 1976). Seeds low in Ca








germinated 40% less and produced seedlings with a higher incidence

of abnormality, such as dark plumule and watery hypocotyl, than seeds

high in Ca. Another abnormality, hollow heart, developed in peanuts

when the mother plant was grown in low B soil (Harris and Brolmann,

1966). Similarly, soybeans grown in Mo deficient soil were low in

seed Mo content which led to a reduction in seedling growth and seed

yield (Harris et al., 1965).

In brief, the nutritional status of the mother plant had little

effect on seed germinability, but seed vigor was either reduced by

nutrient deficiencies or improved by additional applications of

nutrients to the mother plant. Some of the 'carry-over' effects

were via changes in chemical composition (P or Ca) of the seeds, as

observed in carrot, watercress and peanut (Austin, 1966a; Austin and

Longden, 1966; Cox et al., 1976).

An interesting relationship between seed protein content and

seed vigor was reported in bean, wheat and oat (Avena sativa)

(Schweizer and Ries, 1969; Ries, 1971; Ries and Everson, 1973).

Supplemental N (50 to 100 Kg/ha) to the mother plant increased seed

protein content in bean which was highly correlated with subsequent

seedling size (r=0.95***) and yield (r=0.61**) (Ries, 1971).

Protein content was increased in wheat seeds by the application of

herbicide or N to the mother plant (Ries et al., 1970; Ries and

Everson, 1973). Progeny plants grown from high protein seeds were

more advanced in morphological development and yielded more seeds

than those from low protein seeds. A 21 to 42% increase in oat

grain yield was related to an increase in seed protein content by

herbicide applications to the mother plant (Schweizer and Ries, 1969).




15



In conclusion, seed vigor was altered by environmental and

nutritional stress imposed on the mother plant. In order to

understand how the environment alters seed development and subsequent

seed vigor, the present research was undertaken.













CHAPTER II
SEED YIELD AND VIGOR IN COWPEA SEEDS
WHICH DEVELOPED UNDER DIFFERENT PHOTOPERIODS
AND LIGHT INTENSITIES


Variation in light duration, intensity or spectral quality can

affect plant development. Light duration (photoperiod) is well-known

for its influence on flower initiation, while light intensity controls

photosynthesis, and light quality affects phytochrome mediated

processes (Leopold and Kriedemann, 1975). Thus, changes in light

during seed development might affect seed viability and vigor.

A shortday (SD) treatment during seed development of red beet

increased yield in the progeny compared with a longday (LD) treatment

(Heide et al., 1976). Improvements in seedling growth due to SD

treatments during seed development of the mother plant were reported

for purslane (Gutterman, 1974) and prickyly lttuce (Gutterman et al.,

1975).

Diverse responses in seed yield of cowpeas to SD (11 hours

40 minutes) and LD (13 hours 20 minutes) treatment were reported

for 61 cultivars examined (Huxley and Summerfield, 1974). A 50%

reduction in light intensity decreased seed yield of 'Prime' cowpea,

but increased seed weights (Summerfield et al., 1976). The effect

of light on seed germinability and vigor in the progeny was not

reported. The objective of the present experiments was to investi-

gate the influence of different photoperiods and light intensities

during seed development on seed vigor of cowpea.








Materials and Methods

Biological materials

Ten cowpea cultivars were provided by Dr. R. L. Fery, USDA,

Charleston, South Carolina (Table A-i). All seeds were propagated

at the University of Florida, Horticulture Unit in Gainesville in

the spring of 1979. The maturity date and seed weight were recorded

at the time of harvest (Table A-i). After preliminary tests which

included a standard germination test and an accelerated aging test

(Tables A-2, A-3), 'Texas Cream 40' and 'Pinkeye Purple Hull' were

chosen for the experiments which follow.

Experiment I. Effect of photoperiod during seed development on seed vigor

A greenhouse experiment was conducted in the winter of 1980 at a

temperature of 240 + 3C day and 190 + 3C night under natural irradiation.

'Texas Cream 40' and 'Pinkeye Purple Hull' were grown in 8 liter pots

filled with 3:1 peat:perlite mixture. All pots were watered daily

to field capacity and fertilized once a week with N:P:K (2:1:2).

Insecticides and fungicides were applied as needed. At anthesis, a SD

treatment was initiated by covering the plants with black plastic to

reduce daylength to 8 hours. One-half of these plants were subjected

to a 1 hour light break (50 aE/m2/s) in the middle of the 16 hours

dark period. This constituted the LD treatment. Daylength treatments

were continued until all seeds were harvested. The experimental

design was a split plot with four replications.

Experiment II. Effect of light intensity during seed development
on seed vigor

'Texas Cream 40' and 'Pinkeye Purple Hull' were grown in the

greenhouse in the summer of 1980, using the same cultural procedures

as Experiment I. At anthesis, plants were transferred to growth








chambers (Conviron E-15) where light intensities varied accordingly:

275, 475, or 775 pE/m2/s. Photoperiod and temperature were maintained

constant in all chambers: 12 hours day and 12 hours night at a

temperature of 250C day and 200C night. The experimental design

was a split plot with four replications within each cultivar in the

growth chamber.

Evaluation of seed vigor

Seed yield. In both experiments flowers were tagged at anthesis

and the maturity date of individual pods was recorded. Seeds were

harvested as the pods dried. The pod development period (Pod Dev. Per.),

pod number per plant, seed number per pod, seed yield, and seed weight

were recorded. After cleaning by hand, seeds were stored at 100C

and 50% RH.

Standard germination test. Twenty-five or 50 seeds (dependent

on the quantity of seeds available) were germinated on moist paper

towels at 250C (A.O.S.A., 1970). Germination, defined as radicle

protrusion, was counted daily. At the end of the fifth day, the total

number of germinated and abnormal seedlings were counted, hypocotyl

and radicle lengths were measured, and fresh and dry weights of the

axis and cotyledons were taken. An abnormal seedling was classified

according to the A.O.S.A. rules for testing seeds (A.O.S.A., 1970).

A formula EGiTi/XGi was used to calculate the average days to

germination (ADG), where G. was the number of germinated seeds at

day T., and T. was the i-th day of germination (Quisenberry and

Gipson, 1974).

Accelerated aging test. A preliminary test was conducted with

field grown cowpeas to determine the appropriate aging period








(Table 1). Fifty seeds were aged at 410C and 100% RH for 0, 4 and

5 days, then germinated as in the standard germination test (Woodstock,

1976). Measurements of seedling growth were taken as above.

Cold test. A cold test was verified for cowpea as recommended

in the A.O.S.A. seed vigor testing handbook (Woodstock, 1976). Fifty

seeds were sown in unsterilized soil, then imbibed at 100C for seven

days, and finally placed at 250C for seven days after which germination

percentage was determined.

Determination of seed coat thickness. Seed coat thickness was

measured in 'Pinkeye Purple Hull' seeds which developed under SD

treatment. The hard and nonhard seeds were separated and dried in

a 300C oven for three days after imbibing in water for 24 hours.

A cross-section was made through the hilum portion with a single

edged razor. Sections of the seeds were then mounted on aluminum

stubs with tube coat (G.C. Electronics Co., Rockford, Illinois),

and coated with 50nm of gold-paladium using a Hammers V sputter coater.

Samples were viewed in a Hitachi scanning electron microscope,

Model-450, with an accelerated voltage of 20 Kv and seed coat thickness

was measured.

Statistical analysis. Analysis of variance for the experimental

data was performed with the aid of the Northeast Regional Data Center

in Gainesville. Analysis of variance was used to determine whether

the differences between mean values of the treatments were significant.



Results and Discussion

'Texas Cream 40' and 'Pinkeye Purple Hull' were used because

both had similar maturity dates (21 months) and seed weights

(Table A-i). Seed germination and seedling growth were similar in








both cultivars with the standard germination test and after aging at

410C and 100% RH for four days (Table 1). Seed vigor of 'Pinkeye Purple

Hull' was unaffected by five days of accelerated aging, but that of

'Texas Cream 40' was dramatically reduced. Only 32% of the seedlings

were classed as normal in 'Texas Cream 40' after this period.

Germination rate and radicle lengths of these seedlings from the five

day aging treatment were significantly less than those from unaged

seeds. Because of the severity of the five day treatment on

'Texas Cream 40', an aging period of 4 days was used to compare seed

vigor of both cultivars after the various environmental treatments. The

c cold test was excluded as a vigor index for cowpea because no seed
of either cultivar was viable after exposure to 100C for seven days

(data not presented).

Experiment I. Effect of photoperiod during seed development on seed vigor

A greater number of heavier seeds of 'Pinkeye Purple Hull' led to a

50% increase in seed yield compared with 'Texas Cream 40' (Table 2).

Although the period of seed development in 'Texas Cream 40' was five

days longer than 'Pinkeye Purple Hull', seed weight of the former was

40% less than that of the latter. Differences in photoperiod had no

effect on seed yield, weight or the total time of pod development

in these two cultivars of cowpea. Robertson et al. (1962) reported

that peas which developed under 16 hours daylength after flowering

produced heavier seeds than those under eight hours daylength.

In this case, seed weight differences might have reflected plant

growth differences due to the longer light period during seed

development. Seed numbers per pod were similar in 'Pinkeye Purple Hull'

under both daylength treatments, but fewer seeds per pod were produced



























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in 'Texas Cream 40' when plants developed under SD than under LD

(Table 2). Huxley and Summerfield (1976) reported an increase in

seed number per pod of 'K2809' cowpea under a LD (13 hours 20 minutes)

compared with a SD (11 hours 40 minutes) treatment.

Germinability of 'Texas Cream 40' was the same regardless of

the photoperiod treatment during seed development (Table 3). However,

germination was 20% less in 'Pinkeye Purple Hull' seeds which developed

under SD than under LD. This difference was due to hardseededness,

since germination was 100% after scarification of the seed coat.

Seed coat thickness of both hard and nonhard types of 'Pinkeye Purple

Hull' developed under SD were similar, 60 to 701pm (Table A-4).

Hardseededness might be caused by the change in seed coat structure

and/or composition. Gutterman and Heydecker (1973) found that the

cuticle layer in the thinner part of restharrow seed coat was well-

developed and thickened in hard seeds whichmatured under 20 hours

daylength. Werker et al. (1979) observed that hardseeded pea species

had a continuous and very hard pectinaceous layer on caps of the

palisade cells. The presence of quinone in a continuous layer around

the seed coat, either in the palisade cells or osteosclereids,

correlated with water impermeability.

Hard seeds of 'Pinkeye Purple Hull' grew as well as nonhard seeds;

thus, seed vigor was not reduced. Hardseededness in cowpea which

developed under SD might serve as a survival mechanism for this

tropical species. Oropeza (1976) observed that rainfall combined

with high temperature during seed maturation reduced germinability

of 'Mognolia' cowpea by 20% within several days. Potts et al. (1978)

reported that hardseededness in soybean was beneficial in preventing



































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viability loss during field deterioration. The hardseeded line

'D67-5677-1' was able to maintain viability for up to 9 weeks in the

field when seeds were exposed to warm and humid weather, while a normal

nonhard line 'Dare' lost viability by 50% during the same period.

Seedling size of 'Texas Cream 40' from both photoperiod treatments

was generally larger than that of 'Pinkeye Purple Hull' after the

standard germination test (Table 4), but these differences, except

hypocotyl length, were not significant after accelerated aging

(Table 5). Seedling growth of both cowpea cultivars was unaffected

by photoperiod. Heide et al. (1976) observed that root yield in the

progeny increased in red beet when seeds developed under SD (8 hours)

than under LD (24 hours). Guttennan et al. (1975) reported that larger

seedlings developed in pricklylettuce from seeds which matured under

SD (8 hours) than those under LD (16 hours). In this case, the

improvement of seed vigor in seeds which matured under SD was related

to an increase in gibberellic acid content of the seeds.

Experiment II. Effect of light intensity during seed development
on seed vigor

Average seed yield of 'Texas Cream 40' was 41% greater than that

of 'Pinkeye Purple Hull' (Table 6). More pods per plant were produced

in 'Texas Cream 40' compared with 'Pinkeye Purple Hull', while the

number of seeds per pod and seed weight were similar for both cultivars.

'Texas Cream 40' had a slightly longer seed development period than

'Pinkeye Purple Hull'.

Yields of seeds which developed under 275 uE/m2/s were 66% of

those which developed under the higher light intensities (Table 6).

This appeared to be due to a reduction in the number of pods produced

at this light intensity. A 50% decrease in full daylight at the
























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reproductive stage of 'Prima' cowpea led to a 20% reduction in seed

yield (Summerfield et al., 1976). In this case, rapid leaf senescence

and possibly a reduction in photosynthetic rate under low light

intensity decreased available nutrients for seed development, which

in turn reduced seed yield. Pod maturity of 'Texas Cream 40' and

'Pinkeye Purple Hull' was delayed two to three days when seeds

developed under the lowest light intensity compared with seeds which

developed under the higher two light intensities. Sofield et al. (1977)

observed that the seed development period in wheat was unaffected by

light intensities ranging from 8.1 to 48.4 Klux after anthesis.

However, seed size decreased under shading. In 'Prima' cowpea,

Summerfield et al. (1976) reported that seed weight increased 14%

when plants developed under a 50% reduction of normal sunlight.

Robertson et al. (1962) found that seed weight decreased when peas

developed under 1000 f.c. compared with 1500 f.c. The capacity

of nutrient redistribution and sink size (seed number per plant)

probably contributed to the variation of seed weight in the species

discussed above.

Seeds of 'Texas Cream 40' germinated faster and generally

developed into larger seedlings than 'Pinkeye Purple Hull' in both

the standard germination test (Table 7) and the accelerated aging

test (Table 8). Seed germinability and vigor of both cultivars

before and after aging were unaffected by the intensity of light

imposed after flowering. A decrease in radicle length and axis

weights from seeds which developed under 475 vE/m2/s with the

standard germination test was not thought to be of practical

significance (Table 7). This was possibly related to a slight but














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non significant decrease in individual seed weight of seeds which developed

under this light intensity.

From these results, it appeared that seed vigor in two cultivars

of cowpea was unaffected when the mother plants encountered a change

in daylength or a reduction in light intensity during seed development.

Seeds produced under different light conditions should be of high

vigor although seed yields might be somewhat reduced by the lower

light intensities.



Summary

'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas were subjected

to different photoperiods and light intensities during seed development

to investigate the influence of these environmental factors on seed

yield and vigor.

Photoperiods, shortday (8 hours day/16 hours night) versus

longday (8 hours day/16 hours night with a 1 hour light break in

the middle), did not influence seed yield, size or vigor of either

cultivar. Hardseededness was observed in 'Pinkeye Purple Hull' when

seeds developed under shortdays. Once the coat of such seeds was

ruptured, germination and seedling growth were normal. Differences

in seed coat thickness were not observed between hard and nonhard

seeds; thus, it appeared that seed coat composition was altered.

Seed yields of both cultivars decreased significantly when

light intensity was reduced from 775 to 275 vE/m2/s. However,

neither seed weight nor seed vigor was affected by light intensity

differences imposed during seed development.














CHAPTER III
REDUCTION IN SEED YIELD AND VIGOR OF COWPEA
DUE TO WATER STRESS DURING SEED DEVELOPMENT



Water stress during seed development can affect seedling growth

in the progenies of lettuce (Izzeldin et al., 1980b), Indian ricegrass

(Whalley et al., 1966), peanut (Cox et al., 1976), alfalfa (Walter

and Jensen, 1970), and sweet corn (El-Forgany and Makus, 1979).

Seed vigor was reduced in peanut and alfalfa, improved in lettuce

and Indian ricegrass, and unaffected in sweet corn by drought stress.

An increase in seed weight under drought conditions was related to

an improvement in seed vigor of lettuce (Izzeldin et al., 1980b)

and Indian ricegrass (Whalley et al., 1966). Decreased seed vigor of

Spanish peanuts was associated with reduced ethylene production during

early seed germination from plants grown under low soil moisture

stress (Ketring et al., 1978). When 'Florigiant' peanuts matured

under a drought condition, seeds low in Ca were produced which

gave rise to seedlings with a physiological disorder (Cox et al.,

1976). Seed chemical composition was altered when water stress was

imposed during seed development of sorghum (Eck and Musick, 1979),

wheat (Terman et al., 1969; Campbell and Davidson, 1979) and oat

(Chinnici and Peterson, 1979); however, the relation of this change

in composition to seed vigor was not reported.

Water stress, either a deficit or excess, adversely affected

nodule growth in cowpea (Doku, 1970; Hong et al., 1977). Seed yield








varied with water stress levels and the growth stage at which the

water stress was imposed. Treatments that decreased the number of

mature pods reduced cowpea seed yields (Hiler et al., 1972; Kamara,

1976; Minchin et al., 1978; Turk et al., 1980). The influence of

water stress on progeny seed vigor was not observed. The present

research was conducted to investigate the influence of water stress

during seed development on subsequent seed vigor of cowpea. Since a

decrease in vegetative growth by water stress might severely affect

seed development and vigor, plants were grown with similar quantities

of available water until flowering. Active embryo metabolism and

nutrient supply from the cotyledons determine seedling growth at the

early stages of plant development. Thus, the chemical composition

of the seeds was analyzed to determine changes induced by water

stress and its possible relationship with changes in seed vigor.

Also, in order to determine if there was drought injury to the

embryos themselves, embryos were cultured to eliminate the influence

of cotyledons during early seedling development.



Materials and Methods

Experimental water stress treatments during seed development

A greenhouse experiment was conducted in the summer of 1980 at

a temperature of 330 + 3 C day and 250 + 3 C night under natural

light intensity and photoperiod. 'Texas Cream 40' and 'Pinkeye

Purple Hull' were germinated in flat trays filled with a 3:1

peat:perlite mixture. Seedlings with fully-expanded primary leaves

were rinsed under tap water to remove debris on the root surfaces,

then were transferred to 1-liter jars filled with complete Hoagland's








solution (Hoagland and Arnon, 1938). The pH of the solution was adjusted

to 5.5-6.0. The jars were wrapped with aluminum foil to eliminate algae

growth and the solutions were aerated with an air pump.

At anthesis, polyethylene glycol-6000 (PEG-6000) was added at

the rate of 160 or 250 g/Kg water to the culture solutions to reduce

water potential to -4 or -8 bars, respectively (Michel and Kaufmann,

1973). A control, Hoagland's solution, was maintained for comparison.

The amount of PEG-6000 was increased slowly so that water potential

dropped 1 to 2 bars every 3 to 4 days. The stress conditions were

maintained until seed harvest. The experiment was a completely

randomized design with four replications.

Evaluation of seed vigor

Influence of water stress on seed yield and vigor. Seed yield

measurements and seed vigor tests were conducted as previously

reported (Chapter II).

Influence of water stress on seed chemical composition. Mature

seeds were ground in a Wiley mill to pass through a 40 mesh sieve.

Total seed nitrogen (N) was determined by the micro-Kjeldahl method

(A.O.A.C., 1975). Total inorganic phosphorus (P) was determined with

a dry-ashed sample by the Fiske-Subbarow method (Fiske and Subbarow,

1925). Total protein was extracted with 0.5N NaOH in a homogenizer

(Sorvall Omni mixer) at high speed for 3 minutes. After trichloroacetic

acid precipitation, the pellet was resuspended in 0.1N NaOH and an

aliquot was taken to determine protein content by Lowry's method

(Lowry et al., 1951). Bovine albumin was used as a standard.

Starch was analyzed by a modified method of Dekker and Richards (1971).

Starch was extracted with 0.5N NaOH over a boiling water bath








for 15 minutes, then neutralized with 0.5N acetic acid. An aliquot

was taken and hydrolyzed with amyloglucosidase (Sigma, Rhizopus genus)

at 550C for 30 minutes. The starch concentration in glucose equivalents

was measured in a glucose analyzer (YSI Model 27).

Embryo culture. A preliminary test was conducted to determine

the optimum culture medium. Sterilization of the seeds was found to

be unnecessary. Embryos were excised from the dry mature seeds,

transferred to sterilized media containing 1.5% agar, 2% sucrose,

and 0.1% casein hydrolysate. Petri dishes with embryos were set

upright at an 800 angle. The embryos were grown at 250C for 5 days,

then the axis fresh weight, hypocotyl and radicle lengths were

measured. Dry weights were determined after drying at 700C for 24 hours.

Statistical analysis. Analysis of variance was performed as

previously described (Chapter II). Regression analysis was followed,

if significant differences were observed, to determine the trend of

changes in vigor characteristics due to water stress treatments.

Lines shown in the figures were drawn from the regression equation

for each measurement.



Results and Discussion

Influence of water stress on seed yield and vigor

Seed yields of 'Pinkeye Purple Hull' decreased linearly as

water stress intensified during seed development, while that of

'Texas Cream 40' decreased to half of the control (-0.8 bars)

under both moderate (-4 bars) and severe (-8 bars) stress (Fig. 1A).

Reduction in seed yield was directly correlated with a decrease in

pod number per plant (r=0.90***) (Fig. 1B). Seed number per pod












60


S 48


S36
_J

- 24
Q 1


--Texas Cream 40

N ----Pinkeye Purple Hulll3
\



)
V N -
Ns. /
N^_--


0
B
26


22

18
18 N


14


10 I
-0.8 -4.0 -8.0
WATER PC


I I,


51 1I I
-0.8 -4.0 -8.0
ITENTIAL (bars)


Fig. 1 Effect of water stress during seed development on seed yield
measurements of 'Texas Cream 40'and 'Pinkeye Purple Hull' cowpeas.

A. seed yield, B. pod number per plant, C. seed number per pod,
D. weight of 100 seeds.


--
--------







decreased in both cultivars under drought stress (Fig. 1C). A similar

reduction in seed yields by drought stress during seed development

was reported for 'Bungundy' (Hiler et al., 1972), 'Temne' (Kamara, 1976),

'California No. 5', and 'Chino 3' (Turk et al., 1980) cowpeas.

Decreases in pod number per plant and/or seed number per pod also

accounted for the reduction in seed yields of these other cowpea

cultivars. When imposed after anthesis, drought stress did not influence

the length of time to pod maturation of 'Texas Cream 40' and 'Pinkeye

Purple Hull' (19 days under all water stress levels) or seed weight

of 'Pinkeye Purple Hull' (Fig. 1D). A 23% reduction in seed weight was

observed in 'Texas Cream 40' when plants developed under both moderate

and severe stress compared with the control (Fig. 10). Kamara (1976)

reported that seed weight of 'Temne' cowpea decreased by 70% after

withdrawing irrigation at the podding stage compared with a control

watered to field capacity. The average seed yield of 'Pinkeye Purple

Hull' was 54% more than that of 'Texas Cream 40', due to a greater

number of heavier seeds produced by 'Pinkeye Purple Hull'.

The percentage and rate of seed germination under optimum conditions

were unaffected in 'Texas Cream 40' by drought stress imposed on the

mother plant (Figs. 2A, B). Hypocotyl length was also unaffected by

water stress, but radicle length was slightly reduced in seeds which

developed under severe water stress (Figs. 2C, D). The fresh and dry

weights of the axis and whole seedling declined linearly as water

stress intensified (Figs. 3A, B, D, E). A decrease in cotyledon

weights (25 to 30%) from seeds which developed under -4 and -8 bars

water stress was highly correlated (r=0.91***) with weight decrease in

the mature seeds (23%) (Figs. 3C, F).












TOTAL
A-----------

-- Standard

--Aging


20 ABNORMAL


-0.8 -4.0


-8.0


E
u 9


IJ
-j

- 7
-i
Co

6
cn


-15


I13


11
-j

S9


WATER POTENTAIL (bars)


D


I II I
-0.8 -4.0 -8.0


Fig. 2 Effect of water stress during seed development on seed germination,
germination rate and seedling lengths of the progeny of 'Texas
Cream 40' with the standard germination test and after accelerated
aging.

A. germination percentage, B. average days to germination,
C. hypocotyl length, D. radicle length.


1.6


~1.4


1.2


1.0


. I













960


720


480


240


[I


960


S720
i-
480
Lu

L 240
LL-


-- -Standard


*- -- Aging


104


I 78
E

52


26


960 -


720 -


480-


0 1'
-0.8


-8.0


I I


I I I


52 -


26


0-4.0 -8.
-0.8 -4.0 -8.0


WATER POTENTIAL (bars)


Fig. 3 Effect of water stress during seed development on seedling
weight of the progeny of 'Texas Cream 40' with the standard
germination test and after accelerated aging.

Fresh weight: A. seedling, B. axis, C. cotyledon.
Dry weight: 0. seedling, E. axis, F. cotyledon.


O


4








The percentage of germinated and abnormal seedlings, and

hypocotyl lengths of 'Texas Cream 40' after accelerated aging were

unaffected by water stress (Figs. 2A, C). Germination was delayed

in seeds which developed under -4 and -8 bars water stress, however

(Fig. 2B). Radicle length (Fig. 20), axis weights and seedling

fresh weight decreased linearly as water stress that was imposed on the mother

plant intensified (Figs. 3A, B, E). Cotyledon weight and seedling

dry weight were correlated with the original seed weight (r=0.96***)

(Figs. 3C, D, F).

The influence of water stress on seed germination and seedling

growth in the progeny of 'Texas Cream 40' were similar in both

the standard germination test and the accelerated aging test

(Figs. 2, 3). More abnormal seedlings arose after aging. Compared

with the unstressed control, seeds which developed under -4 and -8

bars water stress germinated slower and had shorter radicle lengths

after aging. This did not occur after the standard germination

test, however. The overall radicle growth of aged seeds was 21%

less than that of unaged seeds, while hypocotyl length was reduced

by 1.3 cm after aging. Aging also led to a 10 to 15% decrease in

axis weight and the corresponding increase in cotyledon weight,

when compared with the standard germination test.

Seed germination and seedling growth of 'Pinkeye Purple Hull'

in the standard germination test were unaffected by water stress up

to -8 bars (Figs. 4, 5). After accelerated aging, germination

percentage and axis lengths of 'Pinkeye Purple Hull' were also

unaffected by water stress (Fig. 4). Seeds which developed under

the most severe water stress, however, germinated slower and had











100 TOTAL 10-
A ----------- C

80 9
---- Standard
60 -
60 -- Aging 8 -
.U.-
z40 7


s 20 6_
ABNORMAL
0 --------- ---- 5
B D
1.8 -15-


1.6S- -13- ----------
SuC
Ui
l.4 -1
U.J11
-J

1 .2 .


1.0 I 7 1 I
-0.8 -4.0 -8.0 -0.8 -4.0 -8.0
WATER POTENTIAL (bars)



Fig. 4 Effect of water stress during seed development on seed germination,
germination rate and seedling lengths of the progeny of
'Pinkeye Purple Hull' with the standard germination test and
after accelerated aging.


A. germination percentage, B. average days to germination,
C. hypocotyl length, D. radicle length.












-- --- -- --- --


---Standard

Aging


ED


0
E

-104


78 -

52
I 52
55 .- -- -


960 -


720 .


480 -


240 -------


-0.8 -4.0


0 I
F

104


I 01 I I
-8.0 -0.8 -4.0 -8.0
WATER POTENTIAL (bars)


Fig. 5 Effect of water stress during seed development on seedling weight
of the progeny of 'Pinkeye Purple Hull' with the standard
germination test and after accelerated aging.

Fresh weight: A. seedling, B. axis, C. cotyledon
Dry weight: D. seedling, E. axis, F. cotyledon


960


720


480


240


0


S960

a720






CX,
I-u_
S480


S240
h-


iI I







lower axis weights after aging when compared with the control

(Figs. 5B, E). Therefore, severe water stress reduced seed vigor

slightly but significantly. Compared with the standard germination

test, aging did not alter seed germinability or the development of

abnormal seedlings, but reduced hypocotyl lengths by 11%, radicle

lengths by 17.5%, and axis dry weights by 8.4% in 'Pinkeye Purple Hull'.

Overall, axis growth of 'Pinkeye Purple Hull' after aging

decreased linearly as water stress levels increased; but the rate

was much slower in 'Pinkeye Purple Hull' (-0.64 mg per plant per bar)

than in 'Texas Cream 40' (-1.28 mg per plant per bar) (Figs. A-2, A-4).

Average differences in axis dry weights between aged and unaged seeds

were smaller in 'Pinkeye Purple Hull' (5%) than in 'Texas Cream 40'

(12%). Thus, seed vigor of 'Texas Cream 40' was significantly

reduced by moderate to severe water stress. Vigor was further reduced

if seeds germinated under aging stress. Seed vigor of 'Pinkeye

Purple Hull' was only reduced in seeds which developed under the most

severe water stress and then germinated in an adverse condition.

Compared with the control, decrease in axis dry weight of seeds

which developed under -8 bars water stress was less in 'Pinkeye

Purple Hull' (9%) than in 'Texas Cream 40' (19%). Thus, 'Pinkeye

Purple Hull' appeared to be more tolerant to drought stress than

'Texas Cream 40'.

Drought sensitive 'Texas Cream 40' produced small and low

vigorous seeds under -4 and -8 bars water stress without any apparent

influence on seed germinability and germination rate. Pallas et al.,

(1977) reported similar decreases in seed size and normal seedling

development in Virginia type peanuts by drought stress up to -15 bars








during seed development. The close relationship between seedling

weights and seed weight in 'Texas Cream 40' (r=0.96) was also

reported in lettuce by Izzeldin et al., (1980b) and in Indian ricegrass

by Whalley et al., (1966). Heavy seeds produced seedlings with longer

radicles in lettuce and seedlings of greater length and weight in

Indian ricegrass than light seeds. However, the change in seed

weight by drought stress in 'Texas Cream 40' cowpea was opposite to

those in lettuce and Indian ricegrass. Stresses up to -5 bars in

lettuce and -15 bars in Indian ricegrass reduced seed yields of both

crops, but compensatorily increased seed weight by 40% in lettuce and

9% in Indian ricegrass. In 'Texas Cream 40' cowpea, seed weight was

reduced 23% under water stress conditions. Hence, drought stress

during seed development did not appear to affect vigor in lettuce

and Indian ricegrass to the extent it did with 'Texas Cream 40' cowpea.

'Pinkeye Purple Hull' was drought tolerant and able to produce

seeds with comparable size and vigor as the control under -4 bars

water stress. A similar observation was made in 'Luther Hill' sweet

corn by El-Forgany and Makus (1979). Seed size and vigor index from

a cold test, an accelerated aging test, and germination rate were

unaffected by soil moisture depletion up to -5 bars during corn

seed development. Severe drought stress (-8 bars), however, reduced

seed vigor of 'Pinkeye Purple Hull' slightly after aging.

Because seedling weight of cowpea was well-correlated with seed

weight (r=0.96), an attempt to separate the effect of drought from

the effect of seed size was made. Little variation was found in size

distribution of 'Pinkeye Purple Hull' with or without water stress,

while more small seeds were produced in 'Texas Cream 40' under water








stress (Table 9). Most seeds of 'Texas Cream 40' from drought stress

treatments appeared less-filled and most had a wrinkled seed coat

(Fig. 6). A further separation on seed fullness was performed with

a slot sieve. The proportion of less-filled seeds increased from

1% to 13% as drought stress intensified. Since weight differences

still existed after final sieving, water stress apparently reduced

nutrient deposit in the developing seeds of 'Texas Cream 40'.

Comparison of seed vigor with the same seed weight between water

stress treatments was not feasible in this experiment.

Influence of water stress on seed chemical composition

Water stress during seed development significantly altered the

chemical composition of mature seeds (Fig. 7). Compared with

unstressed seeds, N concentration increased 13% in 'Texas Cream 40'

and 7% in 'Pinkeye Purple Hull' seeds under both moderate and severe

water stress (Fig. 7A). The protein concentration of 'Texas Cream 40'

increased from 17.5% to 20.5% as drought levels intensified, while

that of 'Pinkeye Purple Hull' increased significantly over the

control only under the -8 bars water stress (Fig. 7B). Similar

increases in seed N and protein concentrations due to water stress

during seed development were reported for wheat by Terman et al. (1969),

for oat by Chinnici and Peterson (1979), and for sorghum by Eck and

Musick (1979). Ries (1971) observed that seed vigor in bean was

associated with seed protein concentration. Larger seedlings and

higher bean yield were obtained from high protein seeds compared with

low protein seeds of the same size. This relationship was also

reported in wheat by Lowe and Ries (1972). However, in the present

experiment with 'Texas Cream 40' cowpea drought stress led to the





47















E E
-- E E*


2



C-)
0 V













5- L- (0 F0 (0 LO C N
o 0 e
0 0 (O N EE C --


4 u r, to .
N a o m m
s- 'AD-IE o r^ u s o : ~- -
t/ 2 E In c\j C i

cm ID CD _
(v 0 ico co r-
NW! CD
C* 0. A
V) 3
3

0-
Q)


tm -E

00 On 0 c
0j 1-) 5- *- (o E

0 0 Q. r- 0* *
,a E S- C C: ) 5-
I- 03 A A c -
0 -
SCC i E







41 ai a1

. C)- -





a) * I I S.- c
O 0 i


3 s -
4 0 0 c
4-CO O O O E
- 5 3 .. .. .-












Sa)) a) -)
+5 5- C (0 C C C C C* 0 c

4-' (U D







41 S-
'4-. 5






U ra)

3 5IU 5l- 00

























































Fig. 6 Final size and appearance of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas as affected by water stress during
seed development
















5.1 I-


0.65


A -- Texas Cream 40


-- Pinkeye Purple Hull 0.61


aQ.57


0.53
a-

-0.49


0.45


16 I I 1
-0.8 -4.0 -8.0


20 L'
-0.8


WATER POTENTIAL (bars)


Fig. 7 Effect of water stress during seed development on seed composition
(% dry weight basis) of 'Texas Cream 40' and 'Pinkeye Purple Hull'
cowpeas.
A. nitrogen, B. protein, C. phosphorus, D. starch.


-4.7


c4.3
C
S-~
o-
Z:^


L


. rF


*


I I I







high protein but low vigor seeds regardless of seed size. Similar

responses were observed in seeds of 'Pinkeye Purple Hull' developed

under severe water stress, but the reduction in vigor was less in

'Pinkeye Purple Hull' than in 'Texas Cream 40'.

Seed P concentration increased linearly in 'Texas Cream 40'

but remained unchanged in 'Pinkeye Purple Hull' as water stress

intensified (Fig. 7C). Eck and Musick (1979) reported that the

concentration of P in sorghum seeds was unaffected by drought stress

up to a leaf water potential of -24 bars during seed development.

Austin (1966a, 1966b) and Austin and Longen (1966) reported that

seedling growth and crop yields of carrot, watercress and pea were

related to seed P content. The higher the P concentration of the seed,

the higher the seed vigor of the progeny. A similar relationship

was not observed in the present work with 'Texas Cream 40' cowpea.

In this case, seeds with higher P concentrations after water stress

had reduced seedling vigor.

The starch concentration of cowpea seeds increased 9% in

'Pinkeye Purple Hull', but decreased 14% in 'Texas Cream 40',

under drought stress compared with unstressed seeds (Fig. 70).

Unlike seed N, protein and P concentrations, which were negatively

correlated with the axis growth in both cultivars (r=-0.61**,

-0.53** and -0.75** for N, protein and P, respectively), starch

concentration was positively correlated with axis growth in 'Texas

Cream 40' (r=0.61**);

Vegetative growth of the cowpea plants was similar before

treatment in the present experiment. After imposing drough stress

with PEG-6000, leaf senescence occurred and new pods were no longer








produced. The increases in seed N, protein and P concentrations

in seeds of both cultivars under water stress might simply be the

result of the decrease in the sink (seed number and weight) compared

with the available nutrient source (current metabolism and vegetation)

for seed development. The decrease in starch concentration of

'Texas Cream 40' seeds under water stress indicated that

photosynthesis or carbohydrate metabolism might be reduced in this

more drought sensitive cultivar compared with the N and P metabolism.

Total nutrient quantities on a per seed basis were directly

correlated with the seed weight in both cultivars (r=0.95***,

0.91***, 0.96*** and 0.97*** for N, protein, P and starch,

respectively). Therefore, water stressed seeds of 'Texas Cream 40'

contained less reserve food than unstressed seeds (Fig. 8). The total

quantity of N decreased-from 13 to 14%, protein 10 to 12%, P 15 to

18% and starch 33%, in 'Texas Cream 40' seeds which developed

under water stress compared with the control. In 'Pinkeye Purple

Hull', however, total quantities of N, protein, P and starch in

water stressed seeds varied to within +5% of unstressed seeds

(Fig. 8). The composition of N, protein, P and starch accounted

for 30 to 50% variation in seed vigor of both cultivars. This

indicated that factors other than nutrient supply from the cotyledons

might possibly be involved in vigor determination.

Embryo culture

Selection of a medium for embryo culture was conducted with

media containing different sucrose concentrations (Table 10).

Hypocotyl length of both cultivars decreased as the sucrose

concentration increased from 2% to 6%. Radicle length and axis











A --Texas Cream 40

---Pinkeye Purple
-


1.00

HuL0.92
,J
S0.84

0.76

0.68
S0.68
0_


9

S8














30
E
z
oZ











25

!!20
4

35

530







15
vl


-0.8 -4.0


---- 20 1 I !
-8.0 -0.8 -4.0 -8.0
WATER POTENTIAL (bars)


Fig. 8 Effect of water stress during seed development on seed composition
(mg per seed) of 'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas.

A. nitrogen, B. protein, C. phosphorus, D. starch.


0.601 ---


i !


ji












Table 10

Embryo Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas in Media Containing Different Sucrose Concentrations.




Axis length Axis wt
Cultivar Sucrosez hypocotyl radicle fresh dry
% m-------- mg/axis-

Texas Cream 40 0 0.9by 1.8b 28.9c 2.0d
2 1.7a 3.la 44.3b 3.4c
4 1.2a 3.6a 49.5a 4.7b
6 1.0b 3.4a 42.9b 5.5a


Pinkeye Purple Hull 0 1.0a 1.6b 36.0c 2.3d
2 1.9a 3.8a 53.0b 3.8c
4 1.8a 4.3a 57.7a 5.0b
6 1.3b 3.9a 54.0b 6.3a

ZAll media contained 0.1% casein hydrolysate and 1.5% agar.

YValues followed by the same letter within cultivar column are not
significantly different at 5% level of probability according to
Duncan's Multiple Range Test.








fresh weight reached maximum in 4% sucrose, then declined as sucrose

concentration increased to 6%. Embryonic axis dry weight increased

linearly as the sucrose concentration in the media increased.

Because of a possible osmotic effect leading to a reduction in axis

length, a medium containing 2% sucrose was used. Combinations of

sucrose and casein hydrolysate in 1.5% agar media were also examined

(Table 11). Embryos of both cultivars grew better, shown by increases

in axis length and weight, with sucrose in the media than without.

Casein hydrolysate in the presence of sucrose increased radicle

length of 'Texas Cream 40' and axis weight of 'Pinkeye Purple Hull'.

Therefore, both sucrose and casein hydrolysate were used in the medium

for the cowpea embryo culture experiment.

Embryos from 'Texas Cream 40' seeds which developed under -4 and

-8 bars water stress were smaller in size and had shorter radicle lengths

and reduced axis weights after 5 days culture at 250C compared with the

unstressed control (Table 12). Hypocotyl length of 'Texas Cream 40'

was unaffected by water stress treatments to the mother plant.

Although embryo size of 'Pinkeye Purple Hull' was unaffected by

water stress treatments, and axis dry weight and hypocotyl length

were similar after five days of culture, radicle lengths and axis

fresh weights declined as the water stress treatment to the mother

plant intensified.

Anatomically, embryos of drought stressed seeds of both cultivars

were the same as unstressed seeds, regardless of embryo size (data

not presented). Seed development of 'Pinkeye Purple Hull' seemed

unaffected by drought stress. A decrease in embryonic axis fresh

weight and radicle lengths of culture embryos of this cultivar












Table 11

Embryo Growth of 'Texas Cream 40' and 'Pinkeye Purple Hull'
Cowpeas in Different Culture Media


Mediaz Axis length Axis wt
Cultivar SU CH hypocotyl radicle fresh dry
-%- -m --mg/axis-

Texas Cream 40 0 0 0.8ay 2.0c 13.9b 0.96b
2 0 1.0a 2.9b 16.3a 1.33a
0 0.1 0.8a 2.0c 12.7b 0.96b
2 0.1 l.la 4.0a 15.7a 1.31a


Pinkeye Purple Hull 0 0 0.9a 2.3b 15.6b 1.llb
2 0 1.0a 4.la 15.4b 1.32ab
0 0.1 1.0a 2.5b 16.3b 1.17b
2 0.1 1.la 4.3a 18.la 1.56a

SU: sucrose, CH: Casein hydrolysate.

YValues followed by the same letter within cultivar column are not
significantly different at 5% level of probability according to
Duncan's Multiple Range Test.























M m m rQ o
c O La )i- U) -
oo- c- ~-


5C, I0





r- e






-J i- em
L


'C





(W Cj C c I m
zt. m v U Ul c
S. 5. . c. .



:3 C L -







o =

0( 0 U- (U

4-' * 0-O







C--
o C
Ve CL 4
















oa 10 100 -
I^o r o moo -3
f i mm m o ." 3-
LL U















o Wu




> s oc
V- x- L




E c



o 0 W d




S *- !

0+10

Lo W

(0 C.. )

I (0 -I (04-
4,- 0 M

) C) 3- 4-
cu> 0 D- 12
4- 7; i




vi C
'r I I IX
a) 0








indicated that severe drought stress during seed development might

exert damage on the growth of the embryo. Since embryo morphology

and size of 'Pinkeye Purple Hull' were unaffected, internal injury

in ultrastructure or metabolism was possible. Thus, drought stress

imposed after anthesis might have adversely affected cowpea embryo

development. The embryo size and growth of 'Texas Cream 40' and

embryo growth of 'Pinkeye Purple Hull' were significantly reduced

by water stress. Ketring et al. (1978) reported that low soil

moisture during maturation of Spanish type peanuts reduced ethylene

production during early germination which resulted in reduced

seedling growth. The embryo, being the major site of ethylene

production (Ketring and Morgan, 1969), was possibly impaired by

drought stress although it was not investigated in Ketring's work.

The influence of water stress on embryo growth of the progeny

of 'Texas Cream 40' in culture (Table 12) was simliar to that in

the whole seed vigor test (Figs. 2, 3). In both, radicle lengths

and axis weights decreased linearly, while hypocotyl lengths

remained unchanged, as drought stress treatments intensified during

seeds development. Apparently, axis growth of 'Texas Cream 40'

was a reflection of embryo size at seed maturity and thus its

potential to grow. Compared with unstressed seeds, axis growth of

'Texas Cream 40' from seeds which developed under moderate and severe

stress was 6% and 14% less, respectively, in embryo culture. A further

decrease in axis growth was observed in the whole seed vigor test:

14% and 19% less under moderate and severe stress, respectively,

compared with the control. The decrease in seed nutrient content in

'Texas Cream 40' under water stress (Fig. 8) might have led to the








further reduction in axis growth in the whole seed. Since axis dry

weight increase in culture was proportional to available sucrose

(Table 10), the 33% decrease in available starch of 'Texas Cream 40'

seeds which developed under water stress might be a major contributing

factor that led to the reduction of axis growth.

In conclusion, water stress during seed development reduced

cowpea seed yield and vigor. The influence was more detrimental

to 'Texas Cream 40' than to 'Pinkeye Purple Hull'. When developed

under drought stress during seed development, 'Pinkeye Purple Hull'

produced vigorous seeds although a reduction in seed yield resulted.

In the same situation, adequate water supply was necessary for

'Texas Cream 40' to produce more vigorous seeds.



Summary

'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas were subjected

to water stresses of -4 and -8 bars during seed development to

investigate the influence of drought stress on subsequent seed vigor

in the progeny. Seed yields of both cultivars were significantly

reduced by water stress imposed after anthesis. When developed

under water stress, drought sensitive 'Texas Cream 40' produced

seeds which were smaller in size than the unstressed control.

Decreases in embryo size and available nutrient reserves, especially

starch, in the water stressed seeds accounted for the reduction in

seed vigor of 'Texas Cream 40'. Droughttolerant 'Pinkeye Purple Hull'

was able to produce seeds of the same size and vigor under water

stress as the unstressed control. Embryo size and available nutrients

in 'Pinkeye Purple Hull' seeds which developed under drought conditions

were similar to the unstressed seeds.














CHAPTER IV

REDUCTION IN SEED YIELD AND VIGOR OF COWPEA
DUE TO TEMPERATURE STRESS DURING SEED DEVELOPMENT


Temperature stress during seed development can affect subsequent

seed germinability, stand uniformity and early seedling growth.

Seed viability of alfalfa (Walter and Jensen, 1970), lettuce (Koller,

1962), tobacco (Thomas and Raper, 1975a, 1975b), barley (Khan and

Laude, 1969) and ryegrass (Wiesner and Grabe, 1972) increased when

seeds matured at high temperature. Improvement of seedling growth

due to low temperature during seed development was reported in

pearl millet (Fussell and Pearson, 1980), pea (Perry and Harrison,

1973), soybean (Harris et al., 1965), bean (Siddique and Goodwin,

1980a, 1980b) and alfalfa (Walter and Jensen, 1970). However, little

is known regarding how temperature affects seed vigor in the progeny.

An increase in seedling weight of pearl millet was related to an

increase in size of seeds which developed at low temperature (Fussell

and Pearson, 1980). A physiological disorder, hollow heart, developed

in pea seeds which matured at high temperature (Perry and Harrison,

1973). This abnormality led to poor seedling growth (Harrison and

Perry, 1973).

Night temperature can have a profound influence on cowpea

reproductive ontogeny (Huxley and Summerfield, 197d, 1976).

Plants flowered only at night temperatures above 190C; the higher

the temperature, the earlier the plant flowered. Seed vigor of cowpea









has been reported to be similar at temperatures of 330/24C and

270/190C during seed development (Ndunguru et al., 1978). In the

present experiments temperatures were varied during seed development

to determine theireffect on seed vigor of cowpea. The composition of

mature seeds was analyzed to determine changes induced by temperature

stress during seed development and the possible relationship of this

to seed vigor. Embryos were cultured in vitro to determine possible

injury to the axis by temperature stress during seed development.

Seedling growth and changes in the major storage components of

starch and protein in the axis and cotyledons during germination

were studied to evaluate the relationship between food utilization

and seedling vigor.



Materials and Methods
Experimental treatments during seed development

'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas were grown in

the greenhouse in the spring of 1980 using cultural practices as

previously described (Chapter II). At anthesis, plants were transferred

to growth chambers (Coviron E-15) in which the day/night temperatures

were set at 380/300C, 330/250C, 280/200C and 230/150C. Light intensity,

775 pE/m2/s, and photoperiod, 12 hours day and 12 hours night, were

the same in all chambers. The experiment was a split plot design

with four replications within each cultivar in the growth chamber.

Evaluation of seed vigor

Influence of temperature stress on seed yield and vigor. Cowpea yield

measurements and vigor tests were evaluated as previously described

(Chapter II).








Influence of temperature stress on seed composition. Total seed

N, protein, P and starch contents were analyzed as previously

described (Chapter III).

Embryo culture. Embryos were excised from mature seeds and

cultured as previously described (Chapter III).

Partitioning of seedling weight and nutrient composition during

germination. Ten to 15 axes were excised from cotyledons after 0, 1,

3 and 5 days of germination. Both axis and cotyledons were weighed,

then dried in a VirTis freeze-drier for two days. After dry weights

were taken, each plant part was ground and assayed for amino acid

and soluble carbohydrate contents. The tissues were extracted three

times with 80% alcohol over a water bath at 82C for 20 minutes.

The alcohol extract was evaporated to near dryness, then diluted

with distilled water. Total amino acid content was determined by

the ninhydrin method (Moore and Stein, 1954; Moore, 1968) and

total soluble carbohydrate by the phenol-sulfuric acid method (Dubois

et al., 1956). Both starch and protein were dissolved in 0.5N NaOH

by heating the residue of the alcohol extraction over a boiling

water bath (Cruz et al., 1970). After centrifugation, total protein

and starch contents in the supernatant were determined as previously

described (Chapter III).

Statistical analysis

Analyses of variance and regression were analyzed as previously

described (Chapter II, III).



Results and Discussion

Influence of temperature stress on seed yield and vigor

Seed yield of 'Texas Cream 40' decreased from 49g to 14g per plant as







temperature increased from 230/150C to 330/250C (Fig. 9). In 'Pinkeye

Purple Hull', seed yield was comparable at temperatures of 230/150C and

280/200C, then declined from 23g to 5g per plant as temperature increased

from 280/200C to 380/300C. Seed yields of both cultivars were directly

correlated (r=0.91***) with pod number per plant (Fig. 9B).

Stewart et al. (1980) reported that seed yield of 'K2809' cowpea

was reduced by 27% when plants were grown at 330/240C compared with

270/190C, and lower pod numbers also led to the lower seed yield.

The delay of pod maturation at low temperature appeared to be related

to an increase in seed weight of both 'Texas Cream 40' and 'Pinkeye

Purple Hull' (Fig. 10), since these factors were highly correlated

(r=0.81***) (Figs. 9C, 0). Seed weights at 230/150C were double

those at 380/300C.

Wien and Ackah (1978) reported a similar relationship between

pod development period, maturation temperature and seed weight of

several cowpea cultivars. Roberts et al. (1978) reported that

'K2809' cowpea had higher seed growth rate and heavier seeds when

plants were grown at 270/190C than at 330/240C. Huxley and Summerfield

(1976) observed that day-night temperatures affected seed development

of 'K2809' cowpea differently. Seed weight was 19% less at a night

temperature of 24C compared with 19C, but was 18% more at a day

temperature of 330C compared with 270C. Siddique and Goodwin (1980b)

reported that seed maturity in bean was delayed 11 days but that

seed size increased 2.5 times when developmental temperatures decreased

from 330/280C to 180/130C. A similar delay in seed development and

increase in seed weight at low maturation temperature was reported

in pea (Robertson et al., 1962). Egli and Wardlaw (1980) reported












50


-40


30
-J


S20
LU

10


0


24


18
i-
12
L

06


N,
N
N
N


S --Texas Cream 40

----Pinkeye Purple
SHull


.,_


N\

I I I I

B


50


40


30
or
C-
20


S10


0


21


V 17
LU

S13


9


N,
N
N
N


0 1 I I I 51 I I I I
23/15 28/20 33/25 38/30 23/15 28/20 33/25 38/30

DAY/NIGHT TEMPERATURE (oC)


Effect of different temperatures during seed development on
seed yield measurements of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas.

A. seed yield, B. pod number per plant, C. pod development
period, D. weight of 100 seeds.


I I I


Fig. 9






















































Fig. 10 Final size and appearance of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas as affected by temperature during
seed development.







that the duration of soybean seed growth was unaffected by temperatures

of 240/190C to 300/25C, but was 3 days less at 330/280C. Seed weight

of mature soybean was 25% less at both 330/280C and 180/130C than

at temperatures between this range.

Seed germinability, abnormal seedling development, and radicle

length of 'Texas Cream 40' with the standard germination test were

unaffected by temperature during seed development (Fig. 11). Germination

rate in the progeny was slower in seeds from the 230/150C treatment

and hypocotyl length was shorter in seeds from the 380/300C treatment

compared with those from 280/200C and 330/25C. Although seed weight

increased at low temperature, seedling fresh weight and axis weight in

the progeny of 'Texas Cream 40' were the greatest in seeds which devel-

oped at 280/200C, followed by 230/150C and 330/250C, and the least at

380/300C (Figs. 12 A, B, E). Seedling dry weight and cotyledon

weights were related to the original seed weight (r=0.97***)

(Figs. 12 C, D, F).

After accelerated aging, germinability of 'Texas Cream 40' was

reduced in seeds which developed at 230/150C compared with seeds which

developed at the higher temperatures (Fig. 11). Germination rate,

normal seedling development and axis growth in the progeny were the

greatest at developmental temperatures of 330/250C and 280/200C,

and the least at 380/300C (Figs. 11, 12). Hypocotyl length was 30%

less, radicle length 25% less, and axis weight 42% less in seeds

which developed at 380/300C compared with 280/200C and 330/250C.

Low temperature (230/150C) incurred during seed development led to

a reduction in hypocotyl length by 20% and axis weight by 11% in

the progeny compared with optimum temperatures. Seedling dry weight

and cotyledon weight after aging were related to seed weight at

maturity (r=0.97***).












TOTAL
A


--Aging

-- Standard




ABNORMAL ^


60 .


0, I I


2.0 k


1.5
-0
1.0 -

0.5


N ,,


I 1 I I
23/15 28/20 33/25 38/30
DAY/NIGHT


11


S9
-I

7

S5



D
220





10
10 -

e 5

I I I I

23/15 28/20 33/25 38/30
TEMPERATURE (C)


Fig. 11 Effect of different temperatures during seed development on
seed germination, germination rate and seedling lengths of
the progeny of 'Texas Cream 40' cowpea with the standard
germination test and after accelerated aging.

A. germination percentage, B. average days to germination,
C. hypocotyl length, D. radicle length.


100



















-Aging
._Standard


0 1 I I I


- -


0120


1 90


E


601-


30
Q


1041-


521-


23/15 2820 33/25 38/
23/15 28/20 33/25 38/30


0
0 -- 1 --- -- 1 --- 1

F
120


90


60


30



23/15 28/20 33/25 38/30


DAY/NIGHT TEMPERATURE (oC)
Fig. 12 Effect of different temperatures during seed development on
seedling weight of the progeny of 'Texas Cream 40' cowpea with
the standard germination test and after accelerated aging.


Fresh weight:
Dry weight:


A. seedling, B. axis, C. cotyledon.
D. seedling, E. axis, F. cotyledon.


130 r-


' 104


78
E

52
U

S26
0L
U-
0


78


52 L


I I I I








'Texas Cream 40' seeds which developed at 230/150C were larger in

size butproduced smaller seedlings before or after aging than seeds

which developed at 280/200C. Axis growth from seeds which developed

at 380/300C were 42% less after aging and 31% less before aging,

compared with seeds which developed at temperatures of 280/200C and

330/250C. Heat stress (380/300C) during seed development of 'Texas

Cream 40' led to the production of seeds with the lowest vigor.

Cool temperature (230/150C) led to seeds of intermediate vigor, and

temperatures in between led to seeds with the highest vigor.

In 'Pinkeye Purple Hull', seed viability, abnormal seedling

development and radicle length with the standard germination test were

unaffected by temperature treatments imposed during seed development

(Fig. 13). Seed germination in the progeny was delayed at developmental

temperatures of 230/150C and 380/300C compared with 280/200C and

330/250C. Hypocotyl length was 20% less from seeds which developed

at 230/150C than at the higher temperatures. The optimum temperatures

for subsequent high seedling fresh weight and axis weights of

'Pinkeye Purple Hull' were 280/200C and 330/250C (Fig. 14).

Temperatures below or above these values reduced axis growth in the

progeny by 20%. Seedling dry weight and cotyledon weights were

correlated with the initial seed weight (r=0.97***).

After accelerated aging, germination of 'Pinkeye Purple Hull'

was reduced by 20% and more abnormal seedlings were produced when

seeds developed at 230/150C compared with 280/200C and 330/250C

(Fig. 13). Seeds from 280/200C and 330/250C treatments during

seed development germinated faster and had longer hypocotyls than

seeds from 230/150C and 380/300C treatments. After aging, radicle











100 TOTAL 13
A C
80- 11 -
--Aging
60 -
----Standard -
o -J
S40 7 -
^ G-

20 N AL 5

0 3
B D
2.0 20-
U



115
1.5 15 -
-v1
1.0 10


0.5 5


0.0 I I I
23/15 28/20 33/25 38/30 23/15 28/20 33/25 38/30
DAY/NIGHT TEMPERATURE (oC)

Fig. 13 Effect of different temperatures during seed development on
seed germination, germination rate and seedling lengths of
the progeny of 'Pinkeye Purple Hull' cowpea with the standard
germination test and after accelerated aging.

A. germination percentage, B. average days to germination,
C. hypocotyl length, D. radicle length.











A
104

78
--Aging

52 -- Standard


120 -


0- I I I I


N


'* 120

90
| gO
E
I..-
| 60


S30
0:


0 1 I I I


120 -


60 -


26


23/15 28/20 33/25 38/30
DAY/NIGHT


23/15
TEMPERATURE ( C)


\
\



`-2-t \
1\


28/20 33/25 38/30


Fig. 14 Effect of different temperatures during seed development on
seedling weight of the progeny of 'Pinkeye Purple Hull' cowpea
with the standard germination test and after accelerated aging.

Fresh weight: A. seedling, B. axis, C. cotyledon.
Dry weight: D. seedling, B. axis, C. cotyledon.


Z104
f0

78
E

S52
26
= 26
U4


n1


------~







length, axis weight and seedling fresh weight were greater from seeds

which developed at 280/200C than at 330/250C or 230/150C (Figs. 13, 14).

These values were the least from seeds which developed at 380/300C.

Seedling dry weight and cotyledon weight followed the same trend as

initial seed weight, namely, weights increased as temperatures decreased.

Axis growth of 'Pinkeye Purple Hull' seeds which developed at

230/150C, 330/250C and 380/300C were 80%, 91% and 60%, respectively, of

those which developed at 280/200C after aging, compared with 80%, 100%

and 80% before aging. Thus, seedling growth after aging was reduced

from seeds which developed at 330/250C and 380/300C. Overall, temp-

eratures of 280/200C and 330/250C led to the highest quality seeds of

'Pinkeye Purple Hull', while temperature of 380/300C led to the lowest

quality seeds.

The influence of temperature treatments during seed development

on seed vigor was similar in both cowpea cultivars. Both produced small

and low vigor seedlings when seeds developed at 380/300C. The heaviest

seeds were produced in both cultivars at 230/150C, but axis growth

in the progeny from these seeds was reduced compared with those

which developed at 280/200C. The reduction in axis growth of

both cultivars which developed at 230/150C was less than that from

seeds which developed at 380/300C. Although axis growth in seeds

which developed at the lower temperatures was greater in 'Texas

Cream 40' than in 'Pinkeye Purple Hull' with the standard germination

test (Fig. A-6), this difference was not significant after aging

(Fig. A-8). This indicated that 'Texas Cream 40' seeds which

developed at low temperature appeared to be more sensitive to stress

during germination than 'Pinkeye Purple Hull'.








Seeds with the highest vigor were produced in 'Texas Cream 40'

and 'Pinkeye Purple Hull' when developed at 280/200C and 330/250C.

A similar result was reported by Ndunguru et al. (1978) for 'K2809'

cowpea. Plant development, final seed yield and weight were unaffected

by developmental temperatures of 330/240C and 270/190C imposed on the

mother plant. Large seeds produced larger seedlings at the early stages

of development than small seeds from both temperature treatments.

The adverse effect that high temperature (>320C) during seed

development had on seed vigor was also observed by Green et al. (1965)

in soybean. Compared with low temperature (210C), high temperature

(270C) during the last 45 days of seed maturation in soybean reduced

subsequent seedling growth and seed yield (Harris et al., 1965).

Siddique and Goodwin (1980a, 1980b) reported that normal seedling

development in bean decreased linearly as temperature during seed

development and maturation increased from 210/160C to 330/280C.

Walter and Jensen (1970) reported that seeds with similar vigor were

produced in 'Moapa' alfalfa at both 130C and 24 C, while seedling

weight and leaf number in the progeny were greater in 'Ranger' when

seeds developed at 130C instead of 240C. If seedling weight of

cowpea was taken as a vigor index, the influence of temperature during

seed development on vigor would be similar to those of bean and alfalfa;

for example, the lower the developmental temperature, the higher the

seed vigor. However, axis weight from seeds which developed at

230/150C was less than that from 280/200C (Figs. 12, 14). Because larger

seeds had a less axis weight, the use of seedling weight as a vigor index

in the above case was concealed by the influence of temperature on

seed size.







Influence of temperature stress on seed composition

Temperature variation during seed development had a profound

influence on the chemical composition of the seeds produced (Fig. 15).

Seed N and protein concentrations of 'Texas Cream 40' were similar at

280/200C and 330/250C, 6% higher than those values at 380/300C, and

13.5% lower at 230/150C. In 'Pinkeye Purple Hull', seed N and protein

concentrations decreased as temperature increased from 230/150C to

330/250C, then increased to 5.6% total N and 22% protein as temperature

increased to 380/300C. Seed P concentration increased linearly in

'Texas Cream 40' and quadratically in 'Pinkeye Purple Hull' as

developmental temperature increased from 230/150C to 380/300C.

Starch concentrations, however, decreased in both cultivars as

temperature increased. On a per seed basis, all these nutrients

decreased as temperature increased (Fig. 16). Thus, seed size

determined the total available nutrients.

Robertson et al. (1962) reported that protein and starch

synthesis in pea seeds was delayed by low maturation temperature;

however, starch content on a fresh weight basis increased at low

temperature due to prolonged duration for accumulation. Sofield et al.

(1977) observed in wheat that although seed N and P concentrations

increased at high developmental temperature (300/250C) the seeds

produced were smaller in size than those which developed at the lower

temperatures. This led to the reduced total N and P quantities in

the seed (mg per seed). Similar results of low N and P contents in

seeds produced at high temperature were reported by Chowdhury and

Wardlaw (1978) in wheat, oat and sorghum. High production temperatures

led to reduction in seed size, starch, and protein content in wheat













-Texas Cream 40
/
--Pinkeye Purpl$
Hull


0.60

'0.55


0.50

0.45


0.401 1 I I I


/ 55 -


50 -


S45

in 40

I I I I 35
23/15 28/20 33/25 38/30 23/15 28/20 33/25 38/30
DAY/NIGHT TEMPERATURE (oC)


Fig. 15 Effect of different temperatures during seed development on seed
composition (% dry weight basis) of 'Texas Cream 40' and 'Pinkeye
Purple Hull' cowpeas.

A. nitrogen, B. protein, C. phosphorus, D. starch.


-5.0

w4.5
4.-
-4.0


3.51 I I i I


6.0


5.5


"--











Texas Cream 40

..--Pinkeye Purple
SHull
\
\
N\ _


3.5 I I I
B -


I I I I
23/15 28/20 33/25 38/30
DAY/NIGHT


1.20 r


1.04

S0.88

S0.72
0-
0.56


I-


0.401 I I


0 1 1 1 I I
23/15 28/20 33/25 38/30
TEMPERATURE (C)


Fig. 16 Effect of different temperatures during seed development on seed
composition (mg per seed) of Texas Cream 40' and 'Pinkeye Purple
Hull' cowpeas.

A. nitrogen, B. protein, C. phosphorus, D. starch.


(,,
8.0

=6.5

I- 5
S5.0


( 33

z 27
I-


I







(Spiertz, 1977) and in pearl millet (Fussell et al., 1980). The influence

of temperature on seed vigor of the progeny was not reported in these

studies, however.

Ries (1971) reported that larger seedlings and higher bean yields

were obtained from seeds high in protein compared to those low in protein

of the same size. Austin and Longden (1965, 1966) observed that higher

P contents in seeds of carrot, watercress and pea led to better seed

vigor. These relationships were not found in the present experiments with

cowpea after temperature stress during seed development. The greatest

axis growth of 'Texas Cream 40' and 'Pinkeye Purple Hull' was from seeds

which developed at 280/200C and 330/250C, yet neither had excessively

high N and P concentrations. Seed development at 380/300C led to the

highest concentrations of N, protein, and P in the seeds but the poorest

seedling axis growth in the progeny. After five days of germination,

lack of nutrient supply probably accounted for the reduction in axis

weight from seeds which developed at 380/300C. However, this would not

explain why axis growth of seeds which developed at 230/150C was less

than seeds which developed at 280/200C and 330/25C because seeds

produced at 230/150C were the largest in size. Efficiency in the

utilization of storage nutrients during seed germination might be

impaired by low developmental temperature or embryo damage might have

occurred at low temperature.

Embryo culture

Although seed weights of both cultivars increased as develop-

mental temperatures decreased, embryos excised from these seeds

were, for the most part, similar in size (Table 13). Radicle

length and axis weight of both cultivars after five days culture at

250C were related to the embryo size, although only the axis dry weight

was significantly different among the various temperature treatments.








Table 13

Effect of Different Temperatures During Seed Development
on Embryo Size at Seed Maturity and Its Growth in Culture
of 'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpeas


Day/Niqht Axis length Axis wt
Cultivar temperature Ut/embryo hypocotyl radicle fresh dry
C mg -- ------- -mg/embryo-
Texas 23/15 3.4az 0.99a 3.0a 32a 3.2a
Cream 40 28/20 3.3a 0.96a 3.2a 35a 3.0a
33/25 2.9b 0.97a 2.8a 28a 2.5b
38/30 3.lab 0.95a 2.8a 34a 3.0a

Pinkeye 23/15 4.2a l.lla 3.1a 36a 3.5a
Purple 28/20 3.5b 0.94a 2.8a 30a 3.0b
Hull 33/25 4.3a 1.02a 3.0a 36a 3.6a
38/30 3.9ab 1.27a 2.7a 38a 3.4a


ZValues followed by the same letter within cultivar column are not
significantly different at 5% level of probability according to
Duncan's Multiple Range Test.







Chang and Struckmeyer (1976) reported that onion (Allium cepa)

produced many aborted seeds with retarded endosperm growth when seeds

developed at 430C. Similar seed abortion was observed in 'Texas Cream 40'

and 'Pinkeye Purple Hull' cowpeas at 380/300C. These data were not

included in seed yield. Embryos surviving heat stress were able to

develop normally, although the shorter duration of the seed growth

period and more rapid leaf senescence appeared to lead to a reduction

in nutrient accumulation in the cotyledons. Compared axis growth of

embryos in culture (Table 13) with that of whole seeds (Figs. 12, 14),

embryo size and its relative growth obviously did not lead to the

reduction in seed vigor at stress temperatures of 380/300C and 230/150C

during seed development. Therefore, utilization of food reserves

during germination was probably affected by adverse seed developmental

temperatures.

Partitioning of seedling weight and nutrient composition during germination

Generally, changes in seedling growth and utilization of storage

reserves during germination were similar in 'Texas Cream 40' and

'Pinkeye Purple Hull' seeds which developed at different temperatures

(Tables A-5, A-6). Seedling fresh weight increased almost linearly in

both cultivars, but dry weight decreased slightly over the five day

germination period. Protein content of the whole seedling decreased

during germination, accompanied by an increase in amino acid content.

Loss of starch in the whole seedling during germination was not

accompanied by an immediate increase in soluble carbohydrate.

In fact, soluble carbohydrate decreased during the first three

days of germination from seeds which developed at 280/200C or higher.

This loss of carbohydrate was assumed to be transformed to structural

components and/or used for respiration.








The hydrolysis of storage protein and starch, and the loss of

soluble carbohydrate during germination coincided with rapid radicle

growth and hypocotyl elongation (Tables A-5, A-6). From this time on,

axis weights increased dramatically, accompanied by a decrease in

cotyledon dry weight. The protein, amino acid, and soluble carbohydrate

in the axis during the first day of germination was apparently used for

respiration. No starch was found in the axis throughout germination.

A constantly low amino acid content and a decrease of soluble

carbohydrate in the cotyledons during germination indicated that

protein and starch were hydrolyzed and moved to the axis without

significant accumulation of products in the cotyledons.

Although cotyledon weight and nutrient reserves in 'Texas Cream 40'

seeds which developed at 230/150C and 280/200C decreased at similar

rates during germination, axis weight increase was slower at the

third day of germination in seeds which developed at 230/150C than

seeds which developed at 280/200C (Table 14). The decrease in

amino acid and soluble carbohydrate in the cotyledons and the

corresponding influx of these materials in the axis were also slower

in the former compared to the latter at the third day of germination.

Decreases in axis growth and movement of amino acid and soluble

carbohydrate from cotyledons to the axis, as shown in the partitioning

experiment with the standard germination test (Table 14), might

have been further slowed down after aging. This possibly led to the

low seed vigor in seeds which developed at 230/150C (Fig. 12).

After five days of germination axis growth rates were similar in

'Texas Cream 40' seeds which developed at 330/250C or less, regardless

of seed size at maturity (Table 14). Cotyledon weight and nutrient

reserves decreased more rapidly in seeds which developed at 330/250C








Table 14

Effect of Different Temperatures during Seed Development on
the Percentage Changes in Weight and Nutrient Composition
in the Axis and Cotyledons of 'Texas Cream 40' Cowpea
during Germination


Day/Night Axis Cotyledon
Measurement temperature 1 3 b 1 3 b
uC ----% change % change -

Fresh wt 23/15 452az 6780b 22500a 87a 92a 44a
28/20 531a 8250a 26100a 109a 105a 41a
33/25 501a 7850a 25500a 107a 97a 19b
38/30 374a 6600b 18100b 94a 84a 4c

Dry wt 23/15 25a 530b 1750a -8a -20a -49a
28/20 32a 660a 2000a -3a -20a -53a
33/25 31a 630a 1850a -2a -26ab -65b
38/30 14a 540b 1310b -8a -32b -69b

Protein 23/15 16a 243a 760a la -15a -48a
28/20 -2a 225a 680a 3a -16a -53a
33/25 -2a 218a 670a -6ab -28b -70b
38/30 9a 228a 570a -9b -33b -71b

Amino acid 23/15 87a 1660b 6750b 51a 39a -17a
28/20 113a 1990a 7520ab 49a 19b -37b
33/25 86a 1980a 7670a 19b -9c -61d
38/30 61a 1630b 5170c 29b 15b -48c

Soluble 23/15 -37a 600b 2030a -la -35a -76a
carbohydrate 28/20 -35a 750a 2220a -la -66b -84b
33/25 -35a 730a 2050a -17b -77c -89c
38/30 -47a 540b 1160b -4a -73c -90c

Starch 23/15 -11a -22a -50a
28/20 -8a -17a -57ab
33/25 -6a -21a -67bc
38/30 la -28a -71c
ZValues followed by the same letter within measurement column are not
significantly different at 5% level of probability according to Duncan's
Multiple Range Test.







compared with the lower temperatures. This appeared to be related to

the initial seed size (Table A-5), because changes in total quantities

of weight and nutrients over the five days germination period were

less in seeds which developed at 330/250C than at 280/200C (Table 18).

A decrease in axis weight in seeds which developed at 330/250C over

those which developed at 280/200C during germination was probably due

to a smaller embryo size in the former at seed maturity (Table A-5).

Similar seed and embryo sizes were obtained in 'Texas Cream 40'

when seeds developed at 380/300C and 330/250C (Table A-5). However,

axis growth rate decreased in seeds which developed at 380/300C

after three days of germination (Table 14). Loss of protein and starch

from the cotyledons during germination was similar in seeds which

developed at both of the above temperatures (Table 14), but movement

of amino acid and soluble carbohydrate to the axis was slower in seeds

which developed at 380/300C than at 330/250C (Tables 14, 15).

Therefore, the former seemed unable to utilize reserve food as

efficiently as the latter. A lower starch concentration in the

cotyledons, at seed maturity and during germination, was found

in seeds which developed at 380/300C compared with seeds which

developed at 330/250C (Table 15). This might partially explain the

large decrease in soluble carbohydrate accumulation in the axis of

heat stressed seeds after five days of germination.

'Pinkeye Purple Hull' seeds which developed at 230/150C and

280/200C had similar seed weights at maturity (Table A-6). However,

changes in weight and nutrient composition were generally delayed in

seeds which developed at 230/150C compared with seeds which developed

at 280/200C or 330/250C (Table 16). The actual increases in weight.

























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Table 16

Effect of Different Temperatures during Seed Development on
the Percentage Changes in Weight and Nutrient Composition
in the Axis and Cotyledons of 'Pinkeye Purple Hull' Cowpea
during Germination


Day/Niqht Axis Cotyledon
Measurement temperature 1 3 5 1I 3 5


UC

23/15
28/20
33/25
38/30

23/15
28/20
33/25
38/30

23/15
28/20
33/25
38/30


% change--- ---- change--

259bz 5000b 17000bc 10lab 105a 75a
480a 7340a 22700a 85b 83a 35b
510a 6260ab 19000b 82b 84a 22bc
530a 5030b 14500c 118a 94a 9c

2b 400b 1340bc la -lla -37a
14ab 560a 1710a -7a -22b -51b
25a 520a 1520ab -7a -21b -58b
20ab 450ab 1110c 4a -29b -69c


5a 270a
17a 300a
30a 320a
-la 160b


700b -5a -30a -52a
830ab -10a -29a -63b
890a -12a -31a -75c
440c -7a -35a -84d


Amino acid 23/15
28/20
33/25
38/30


Soluble
carbohydrate



Starch


23/15
28/20
33/25
38/30

23/15
28/20
33/25
38/30


40c 960b 3970b
94b 1550a 5870a
139a 1520a 5380a
81b 1020b 3510b


38b 50a 5a
73a 52a -28b
52ab 9b -47c
34b 14b -54c


-53a 410b 1490b 3a -25a -68a
-58a 640a 2100a -13a -60b -82b
-45a 560ab 1680ab -14a -68b -85bc
-45a 450ab 910c -14a -80c -90c

la -25a -46a
la -26a -56b
4a -25a -63b
Oa -48b -79c


Fresh wt


Dry wt




Protein


ZValues followed by the same letter within measurement column are not
significantly different at 5% level of probability according to
Duncan's Multiple Range Test.


M-







and nutrient composition from day three to five in the axis of the

former were close to those changes in the latter (Table A-7).

Therefore, the reduction in axis growth of seeds from the 230/150C

treatment was probably due to a lag in active growth in the first

day of germination (Table A-7).

'Pinkeye Purple Hull' seeds which developed at 280/200C and

330/250C had similar growth rates in the axis during germination

(Table 16), regardless of seed and embryo sizes at maturity (Table A-6).

Hydrolysis of food reserves in the cotyledons and utilization of the

breakdown products in the axis were, for the most part, at similar

rates in both seeds.

Reduction in growth and nutrient accumulation in the axis of

'Pinkeye Purple Hull' seeds which developed at 380/300C, compared

with seeds which developed at 280/200C and 330/250C, occurred after

three days of germination (Table 16). As germination progressed, the

highest rates (Table 16), but the least actual quantities (Table 18),

of weight and storage reserve loss in the cotyledons were observed

in seeds which developed at 380/300C. The same seeds were the smallest

in size (Table A-6) and contained the least concentration of

starch (Table 17). This most likely led to a reduction in the

amount of carbohydrate available for axis growth. After five days

of germination most of the starch was utilized in these heat stressed

seeds (Table A-5), which, in turn, probably led to the low concentration

of soluble carbohydrate in the axis (Table 17). Because changes in

weight and nutrient composition in the axis and cotyledons during

early germination were unaffected by developmental temperatures of

380/300C and 330/250C, low seed vigor at 380/300C was probably due

to an insufficient reserve food at seed maturity.




















u






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Table 18
Effect of Different Temperatures during Seed Development on Changes
in Weight and Nutrient Composition of Axis and Cotyledons of
'Texas Cream 40' and 'Pinkeye Purple Hull' Cowpea
during a 5 Day Germination Period


Day/Niqht Texas Cream 40 Pinkeye Purpule Hull
Measurement temperature Axis Cotyledon Axis Cotyledon
oC mg mg-

Fresh wt 23/15 776 74 709 137
28/20 821 56 815 64
33/25 722 19 788 32
38/30 602 4 597 7

Dry wt 23/15 55.5 -74 52.9 -62
28/20 57.0 -66 58.3 -86
33/25 48.2 -59 58.8 -78
38/30 40.0 -58 41.9 -51

Protein 23/15 4.8 -10.7 5.2 -15.8
28/20 4.8 -10.7 5.5 -17.7
33/25 4.1 -10.8 5.7 -15.7
38/30 3.7 -10.6 4.0 -12.6

Amino acid 23/15 6.4 -0.3 5.2 -0.1
28/20 7.9 -0.7 6.4 -0.3
33/25 6.6 -0.9 6.0 -0.8
38/30 5.5 -0.7 5.6 -0.8

Soluble 23/15 17.0 -11.3 14.6 -10.2
carbohydrate 28/20 15.4 -10.1 17.0 -11.6
33/25 13.5 -10.2 16.3 -11.4
38/30 9.8 -9.4 8.0 -8.0

Starch 23/15 -40.5 -40.9
28/20 -36.9 -48.2
33/25 -29.5 -42.6
38/30 -26.4 -23.8









Lowe and Ries (1973) reported an influence of storage tissue on

seed vigor of wheat. A greater dry weight in wheat seedlings derived

from high protein seeds than from low protein seeds was attributed to the

endosperm, not the embryo. Reduction in seed vigor due to high

temperature during seed development was reported to be related to a

decrease in seed size of pearl millet (Fussell and Pearson, 1980).

However, information regarding which part of the seedcortributed to

the low seed vigor due to temperature stress was not reported.

In the present study, low seed vigor of 'Pinkeye Purple Hull' cowpea

which developed at high temperature was due to the insufficient

food reserve in the cotyledons, not due to an injury to the embryo.

Although cowpea is tropical in origin, high temperature stress

during seed development had a detrimental effect on seed yield and

vigor. Seed vigor was also reduced by low temperature during cowpea

seed development, even though large seeds were produced. The adverse

effect of high or low temperature on seed vigor was probably related

to a reduction in nutrient build up on the seeds or to a reduction

in the ability to utilize food reserves efficiently during germination.

No apparent injury to the embryo was found in the seeds harvested

from the different temperature treatments in this experiment.

In order to produce seeds of high vigor in cowpea, low or high

temperature during seed development should be avoided by altering

the planting date and/or by using short season cultivars.



Summary

'Texas Cream 40' and 'Pinkeye Purple Hull' cowpeas were grown at

day/night temperatures of 380/300C, 330/250C, 280/200C, and 230/150C




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