Advances in agronomy volume 27

ADVANCES IN

AGRONOMY
VOLUME 27


CONTRIBUTORS TO THIS VOLUME

DALEE. BAKER
MARVINE. BAUER
J. S. BOYER

WILLARDH. CARMEAN
LEON CHESNIN
M. DRAGAN-BULARDA
0. P. ENGELSTAD

ROBERTL. HEATH
RENU KHANNA
S. KISS

H. G. MCPHERSON
D. RXDULESCU
D. A. RUSSEL
SURESHK. SINHA
IRWINP. TING


ADVANCES IN

AGRONOMY
Prepared under the Auspices of the
AMERICAN
SOCIETY
OF AGRONOMY
VOLUME 2 7

Edited by N. C. BRADY
International Rice Research Institute
Manila, Philippines
ADVISORY BOARD
W. L. COLVILLE,
CHAIRMAN
G . W. KUNZE D. G. BAKER D. E. WEIBEL
G . R. DUTT H. J. GORZ

M. STELLY,EX OFFICIO,
ASA Headquarters
1975

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NUMBER:50-5598

ISBN 0-12-000727-4
PRINTED IN THE UNITED STATES OF AMERICA


CONTENTS

........................................
PREFACE...........................................................
CONTRIBUTORS
TO VOLUME
27

ix
xi

PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

J.

s. BOYER AND H . G . MCPHERSON

I . Introduction ..................................................
I1. Sensitivity of Desiccation .......................................
I11. Improvement of Drought Response through Breeding
and Management ..............................................
References ...................................................

1
2

17

22

BIOLOGICAL SIGNIFICANCE OF ENZYMES ACCUMULATED IN SOIL

S . Kiss. M . DR~CAN.BULARDA.
AND D . RADULESCU
I. Introduction ..................................................
I1. Role of Accumulated Soil Enzymes in the Initial Phases of the
Decomposition of Organic Residues and of the Transformation of
Some Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
111. Enzymatic Activities in Soil under Conditions Unfavorable for the
Proliferation of Microorganisms .................................
IV Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

27
64
76

76

RESPONSES OF PLANTS TO AIR POLLUTANT OXIDANTS

IRWINP. TINCAND ROBERTL. HEATH
I. Introduction

....................................................

I1. Biochemical and Physiological Effects ..............................

111. Development and Predisposition to Oxidant Injury

...................

IV. Environmental Factors Influencing Susceptibility and Sensitivity . . . . . . . .
V . The Role of Stomata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. Conclusions ....................................................
References .....................................................

89
93
105
107
111
117
118

PHYSIOLOGICAL. BIOCHEMICAL. AND GENETIC BASIS OF HETEROSIS

SURESHK . SINHAAND RENU KHANNA

I. Introduction ..................................................
I1. Heterosis in Heterotrophs and Autotrophs .........................
V

123
124


vi

CONTENTS

111.
IV.
V.
VI .
VII .
VIII .

IX.
X.

Occurrence of Heterosis ........................................
Evaluation of Heterosis ........................................
Manifestation of Heterosis ......................................
Present Theories of Heterosis ....................................
Physiological and Genetic Analysis of Heterosis . . . . . . . . . . . . . . . . . .
Synthesis .....................................................
Programming in Heterotic Hybrids ...............................
Future Outlook ...............................................
References ....................................................

125
125
126
127
130
166
168
169

170

FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS

.

.

0. P ENGELSTAD
AND D A . RUSSEL

I.
I1.
111.
IV.
V.

Introduction ....................................................
Brief Description of Tropics ......................................
History of Fertilizer Use in the Tropics .............................
Agronomic Considerations .......................................
Fertilizer Technology Developments ................................
References .....................................................

175
176
182
186
202
204

FOREST SITE QUALITY EVALUATION I N THE UNITED STATES

WILLARDH . CARMEAN
I . Introduction

...................................................

I1. History of Site Quality Estimation in the United States

................
...............................

111. Methods for Estimating Site Quality
IV. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix: Common and Scientific Names of Tree Species . . . . . . . . . . . . . .
References .....................................................

209
211
212
255
257
258

THE ROLE OF REMOTE SENSING I N DETERMINING THE DISTRIBUTION
AND YIELD OF CROPS

MARVINE. BAUER

I.
I1.
111.
IV.
V.

Introduction ....................................................
Remote Sensing Development .....................................
Physical Basis for Remote Sensing ................................
Agricultural Applications of Remote Sensing ........................
Future Role of Agricultural Remote Sensing ........................
References .....................................................

271
272
274
291
300
301


CONTENTS

vii

CHEMICAL MONITORING OF SOILS FOR ENVIRONMENTAL QUALITY
AND ANIMAL AND HUMAN HEALTH

DALEE. BAKERAND LEON CHESNlN
I . Introduction

..................................................

11. Soil Pollution Sources ..........................................
111. Soil and Waste Composition Monitoring ..........................

IV.
V.
VI
VII .
VIII .

.

Methods of Chemical Analysis ..................................
Monitoring of Macroelements ...................................
Monitoring of Microelements ..................................
Toxic Trace Elements. Organometallic Complexes . . . . . . . . . . . . . . . . . .
Recommendations for Continuing Research ........................
References ...................................................

SUBJECT
INDEX ......................................................

306
307
316
323
327
343

358
364
366
375


This Page Intentionally Left Blank


CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

DALEE. BAKER(305), Department of Agronomy, The Pennsylvania State
University, University Park, Pennsylvania
MARVIN
E. BAUER(271 ) ,Laboratory for Applications of Remote Sensing,
Purdue University, West Lafayette, Indiana
J. S. BOYER( l ) , Departments o f Botany and Agronomy, University of
Illinois, Urbana, Illinois
WILLARDH . CARMEAN
(209), USDA Forest Service, North Central Forest
Experiment Station, St. Paul, Minnesota
LEONCHESNIN
( 305 ) , Department of Agronomy, University of Nebraska,
Lincoln, Nebraska
M. DRAGAN-BULARDA
(25), BabepBolyai University, Cluj-Napoca,
Romania
0. P. ENGELSTAD
( 175 ) , Division o f Agricultural Development, National

Fertilizer Development Center, Tennessee Valley Authority, Muscle
Shoals, Alabama
ROBERT
L. HEATH(89), Department of Biology, University of California,
Riverside, California
RENU KHANNA*( 123), Water Technology Centre, Indian Agricultural
Research Institute, New Delhi, India
S. KISS(25), BabepBolyai University, Cluj-Napoca, Romania
H. G. MCPHERSON
( 1 ) , Plant Physiology Division, Department o f Scientific
and Industrial Research]Palmerston North, New Zealand
D. R~DULESCU
(25 ) , BabepBolyai University] Cluj-Napoca, Romania
D. A. RUSSEL( 175), Division of Agricultural Development, National
Fertilizer Development Center, Tennessee Valley Authority, Muscle
Shoals, Alabama
SURESHK. SINHA( 123), Water Technology Centre, Indian Agricultural
Research Institute, New Delhi, India
IRWINP. RNG(89), Department of Biology, University of California,
Riverside, California

* Present address: School of Life Sciences, Jawaharlal Nehru University, New
Delhi, India.
ix


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PREFACE

Soil and crop scientists continue to focus their attention on pressing
human problems, two of the most important of which are food supplies
and environmental contamination. These two topics receive prominent
attention in Volume 27 as has been the case in the two preceding volumes.
Two papers deal with the effects of environmental contamination on
crops and soils. The influence of air pollutant oxidants on plants is reviewed along with the chemical monitoring of soils for pollutants. These
papers emphasize societal concerns for environmental contamination and
attempts by soil and crop scientists to deal with this emerging problem.
Research aimed at obtaining a better understanding of factors affecting
crop production is presented in three papers. Heterosis is the subject of
one, with emphasis being given to the physiological and genetic basis for
this phenomenon. The physiology of drought as it affects cereal crops is
reviewed along with the genetic potential for drought resistance. The third
paper focuses on fertilizer use in the tropics, with emphasis on agronomic
responses peculiar to these areas. Each of these excellent reviews will be
helpful to scientists concerned with food production.
Research on the evaluation of the physical environment in which plants
grow is covered in three papers. First, work on forest site quality evaluation is reviewed and summarized. Emphasis is placed on methods of
evaluating the site quality. Second, research on remote sensing as a means
of determining crop distribution is evaluated. The physical basis for
sensing this distribution and the agricultural applications of remote sensing
receive major attention in this excellent review. The third paper focuses
on enzymes in soils, their role in microbial transformations and their
activities under conditions where microbial activity is minimized.
The authors of the papers presented herein are to be congratulated on
these excellent reviews. I join their colleagues in thanking them for their
contributions.
N. C. BRADY

xi


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PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS
J. S.

Boyer and

H. G. McPherson

Departments of Botany and Agronomy, University of Illinois, Urbana, Illinois, and
Plant Physiology Division, Department of Scientific and Industrial Research,
Palmerrton North, New Zeoland

I. Introduction..........................................................

11. Sensitivity to Desiccation.. .............................................
A. Photosynthesis....................................................

B. Translocation.....................................................
C. Nutritional Quality.. ..............................................
D. Leaf Enlargement..................................................
E. Floral Development and Pollination.. ...............................
111. Improvement of Drought Response through Breeding and Management.. . . . .
References.. ..........................................................

I.

1
2
2
11
12
13

17
17
22

Introduction

A number of central physiological processes contribute to the formation
of grain in crops. Major ones are photosynthesis and the translocation of
photosynthate to the grain, cell division and enlargement, and the accumulation and transport of nutrient elements for storage in the grain and for
the general functioning of cell metabolism. These processes must occur
during the appropriate stages of development, and consequently the timing
of each contribution is important. Superimposed on this set of circumstances is the suitability of the environment for supplying light, water, and
nutrients for the completion of each stage of growth.
This review is devoted to how the availability of a specific environmental
factor, water, affects grain production in crops. Higher plants must encounter desiccation at least once in their life cycle, late in seed development
when the embryo and stored reserves undergo desiccation prior to seed
release. However, in addition to this period of exposure, other episodes
of drought frequently occur, and there is probably no other factor that
limits grain production so extensively and unpredictably. Yield reductions
from drought may be large enough to result in no grain at all, and even
moderate drought can markedly affect grain production.
In spite of the frequency and importance of this problem, little is known

about the physiological reasons for the diminution of grain production dur1


2

J. S. BOYER AND H. G. MCPHERSON

ing dry periods. Salter and Goode (1967), in an extensive review, described numerous experiments that show reduced yield when drought occurred during various stages of crop development. In that portion of their
review devoted to cereal grains, however, only 2 of the total 114 papers
report measurements of physiological parameters that might affect grain
yield under dry conditions. Yoshida (1972), in his description of the physiology of grain production, was unable to find any data to describe the
effects of drought,
In this review we will present some recent work on the physiological
mechanisms that underlie the reductions in yield caused by drought in
cereal crops of the family Gramineae. Because of the growing literature
on the broad metabolic aspects of desiccation in plants, we will emphasize
that which provides insight for grain production. The reader is encouraged
to consult Hsiao (1973) or Kozlowski (1968, 1972) for more general
treatments.
II. Sensitivity to Desiccation

A. PHOTOSYNTHESIS

The photosynthetic capability of plants is determined primarily by the
total leaf area and the activity of each unit of leaf. Since the COz fixed
by photosynthesis represents most of the dry matter accumulated by the
plant, any factor that affects the photosynthetic activity of the leaves is
likely also to affect the total dry matter and, within broad limits, the grain
production by the crop. In most cereals, the growth that occurs after
flowering is characterized by the photosynthetic activity of existing leaves

and the translocation of the photosynthetic products to the grain rather
than by leaf development. During this portion of the life cycle, therefore,
changes in the photosynthetic activity of the leaves are an important means
by which the photosynthetic capacity of the crop is influenced by drought.
Leaf desiccation can cause a marked inhibition in the photosynthetic
activity per unit of leaf (Hsiao, 1973). An example of this can be seen
in Fig. 1, which describes an experiment conducted by the authors at the
Climate Laboratory in Palmerston North, New Zealand. Net photosynthesis
in maize was inhibited in two sets of plants (termed low VP and high
VP pretreatments) when water was supplied to the soil at one-seventh the
rate of the controls, beginning in early grain-fill and continuing for the
rest of the growing season. For the grain-filling period as a whole, photosynthesis in the desiccated plants was only a small percentage of that in
the controls, and there was a considerable reduction in grain yield (see
next page).
Measurements of leaf water potentials in these plants (Fig. 1A) showed


3

PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

A

Control

1201

I

I

I

I

I

I

I

1

1

Time (days since star; of desiccation)

FIG.1. Leaf water potentials (A) and net photosynthesis (B) for maize that was
desiccated throughout most of the grain-fill period by withholding water from the
soil. The pretreatments consisted of growing the plants throughout the vegetative
period at different humidities during the day: low VP = leaf-air vapor pressure
high VP = leaf-air vapor pressure difference of 5 mb ( 0 ) .
difference of 26 mb (0);
The humidities were equalized during pollination and grain-fill.

that the decline in photosynthesis was related to the degree of plant desiccation. At leaf water potentials of -18 to -20 bars, the rate of photosynthesis
was 15% of the controls or less (Fig. 1B). Under these conditions, there
were no symptoms of desiccation other than a slight gray cast to the leaves,
so that the presence of inhibitory desiccation was difficult to detect visually.
In this species as well as in many others, visual symptoms, if they occur

at all, frequently appear after much photosynthetic activity has been lost.
They therefore do not provide a very useful index of plant water deficits,
and quantitative methods of measuring plant water status are to be preferred (Boyer, 1969;Kramer, 1969).


4

J. S. BOYER AND H. G. MCPHERSON

Since net photosynthesis can be affected by either a decrease in gross
photosynthesis or an increase in respiration, the cause of the decrease in
photosynthetic activity need not be associated with a change in photosynthesis itself. With a few exceptions, however, (Schneider and Childers,
1941 ; Upchurch et al., 1955; Brix, 1962), dark-respiration generally decreases, although substantial respiration may still take place after photosynthesis has ceased (Brix, 1962; Boyer, 1970a). In those cases where
dark respiration increased, the increase was observed only initially and was
small (Schneider and Childers, 1941; Brix, 1962). Photorespiration, or
carbon dioxide loss in the light, also was inhibited and had a sensitivity
more like that of photosynthesis (Boyer, 1971b ) . It is clear therefore that
the decline in net photosynthesis cannot be attributed to a rise in respiration but instead must involve a reduction in gross photosynthesis.
At the same time that net photosynthesis decreases, there generally is
a decrease in transpiration which reflects the closure of the stomata in response to leaf desiccation. The decline in transpiration often parallels the
decline in photosynthesis, and this has been interpreted to indicate that
stornatal closure limits both processes (Hsiao, 1973).
There is little doubt that stornatal closure restricts the entry of carbon
dioxide into the leaf, but the supply may or may not control the rate of
photosynthesis, depending on how severe is stornatal closure. An additional
test of the importance of stornatal closure is required in this situation. I n
a recent examination of the response of sunflower leaves to desiccation,
Boyer (1971b) used an increase in the ambient concentration of carbon
dioxide to provide such a test. Despite the increased availability of carbon
dioxide to the cells within the leaf, the rate of photosynthesis did not

change in the desiccated plants, Boyer concluded that photosynthesis could
not be limited by stornatal closure in this particular case and suggested
that changes at the chloroplast level probably account for the changes in
photosynthetic activity. Wardlaw ( 1967) also showed that increased external carbon dioxide did not diminish the inhibition of photosynthesis during drought in wheat.
Since these experiments suggest the possibility of chloroplast changes
during leaf desiccation, several investigators have isolated chloroplasts from
desiccated leaf tissue (Nir and Poljakoff-Mayber, 1967; Fry, 1970, 1972;
Boyer and Bowen, 1970; Potter and Boyer, 1973; Keck and Boyer, 1974).
They showed that electron transport and photophosphorylation are inhibited, and there are reports that carbon dioxide fixation by isolated chloroplasts is also reduced (Plaut, 1971 ; Plaut and Bravdo, 1973). The changes
in electron transport have been demonstrated in vivo (Boyer and Bowen,
1970; Boyer, 1971a,b), and they parallel the inhibition of photosynthesis


PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

5

in sunflower. It seems, then, that in the short term photosynthesis may
be affected by changes at the chloroplast level or by stomatal effects.
Since photosynthesis can be so severely inhibited by desiccation, and
since some of the effects appear to be subcellular, to what extent will photosynthesis recover after supplying water to the soil? When desiccation has
been mild and of short duration, virtually complete photosynthetic recovery
has been observed (Boyer, 1971a). However, when it is more severe, photosynthesis may show aftereffects of desiccation. There appear to be two
types of aftereffects.First, there may be incomplete recovery of leaf water
potential, which causes photosynthesis to remain below the levels of the
controls (Boyer, 1971a). Second, there may be a direct aftereffect of
drought on the photosynthetic process (Boyer, 1971a). Both depend on
the severity of desiccation: the more severe is desiccation, the more severe
are its aftereffects.
The first kind of aftereffect appears to be caused by breaks in water

columns or other modifications of the pathway for water transport in the
plant (Boyer, 1971a). The net result is that the resistance to liquid water
transport increases within the plant. If it increases enough, desiccation of
the leaves may continue despite rewatering of the soil, and leaf death then
ensues. However, if partial rehydration takes place, the resistance to water
transport decreases over a period of days, and the plant gradually returns
to normal hydration levels. During this time, photosynthesis is frequently
inhibited.
The second kind of aftereffect occurs when the leaves return to full hydration after rewatering. In sunflower leaves that were mature during desiccation, photosynthesis continued to be affected by the previous dry period
(Boyer, 1971a) in spite of a return of the leaves to high water potentials.
Chloroplast recovery required 12-1 5 hours, but stomatal apertures remained reduced for several days (Boyer, 1971a). The inhibition was correlated with partial stomatal closure, but other aspects of photosynthesis may
also have played a part. For whole sunflower plants, there was evidence
that older leaves never recovered their former levels of photosynthesis and
that a return to high photosynthetic activities had to await regrowth of
the plant.
The extent of our knowledge of photosynthesis at low leaf water potentials is rather limited and involves only a few species. From these data,
however, it seems that the response differs between species and may change
as the age of the plant varies. For example, photosynthesis in pine, tcimato,
and sunflower seems to behave similarly as leaf water potentials decline
(Brix, 1962; Boyer, 1970a). For young maize, however, photosynthesis
is more sensitive and soybean photosynthesis is less sensitive than in these


6

J. S. BOYER AND H. G. MCPHERSON

species (Boyer, 1970b). In all these cases, stornatal behavior generally
paralleled photosynthetic behavior. The photosynthetic decline was greatest
between leaf water potentials of -10 and -20 bars.

Plant maturity may also influence the response of photosynthetic activity
to desiccation. Limited data suggest that the sensitivity decreases with age.
In vegetative maize about 30 days after planting, photosynthesis declined
to 70% of that in the well watered plants when leaf water potentials decreased to -12 bars (Boyer, 1970a,b). During grain-fill, however, this
degree of inhibition was not observed until leaf water potentials had decreased to about -16 bars (Fig. 2). A similar decrease in sensitivity has
been found for stornatal closure in wheat (Frank et al., 1973).
These differences between species and even between different ages of
the same plants suggest that plants may be capable of adapting to water
availability. Jordan and Ritchie ( 1971) showed that stomata remained
open in field-grown cotton plants having leaf water potentials that caused
closure in laboratory-grown plants (which were presumably less subject
to desiccation beforehand). McCree ( 1974) demonstrated a similar phenomenon in the laboratory with plants having different moisture prehistories. It seems likely that some type of photosynthetic differences should
also have occurred in these plants.
In order to test whether prior exposure to desiccating conditions could
affect the photosynthetic behavior of plants during a subsequent period
of desiccation, we conducted experiments in maize subjected to two differa

0

I

1

-4

I

I

I

-8

-12

-16

b

-20

4

Leaf Woter Potentiol(bars)

FIG.2. Net photosynthesis in maize at various leaf water potentials and two plant
ages. The 65-day plants ( 0 )were those described in Fig. 1 (Dekalb XLAS) for the
were grown under
early portion of the grain-filling period. The 30day plants (0)
similar conditions but are those shown in Fig. 5 (GSC 50 single cross). The younger
plants had not tasseled.


PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

7

ent desiccation pretreatments. The plants were pretreated by growing them
throughout the entire vegetative period in air having two different humidities during the day (low VP pretreatment = leaf-air vapor pressure difference of 26 mb = low humidity; high VP pretreatment = leaf-air vapor
pressure difference of 5 mb = high humidity). Otherwise, the plants were

grown under identical conditions in well watered soil. The net result was
that the two sets of plants were subjected to a different evaporative demand
during the day, which caused leaf water potentials to average 1 bar lower
in the low VP plants than in the high VP plants (although there was considerable variation between leaves because of mutual shading by other
plants in the stand). At tasseling, identical high VP conditions (5 mb)
were imposed on all plants so that pollination occurred under favorable
moisture conditions. After pollination, the soil was desiccated in half the
plants, and the desiccated plants received one-seventh the amount of water
received by the controls for the remainder of the growing season.
Figure 1 shows the results for the two pretreatment conditions and indicates that there were significant differences in leaf water potentials and
net photosynthesis in the two sets of plants during desiccation in the grainfilling period. The plants that previously had been grown at low humidities
exhibited high leaf water potentials and high rates of photosynthesis for
a longer time than their counterparts that had not previously been subjected
to dry conditions. There were no important differences in photosynthesis
between the controls.
Table I shows that the grain yield by the desiccated plants differed according to the pretreatment. Those previously exposed to dry conditions
produced 7970 kg ha-', and those previously exposed to moist conditions
TABLE I
Grain Yield of Maize When Water Was Withheld throughout Most
of the Grain Fill Period
Plantsa

Low VP pretreatmentb

High VP pretreatmentb

Control
Desiccated

11,700 kg.ha-l

7970

10,500 kg.ha-*

4930

Leafwater potentials were -3 to -4 bars and -18 to -20 bars in
control and desiccated plants, respectively, throughout most of the
desiccation period.
b Pretreatments consisted of growing plants in different humidities
during the day (low VP = leaf-air vapor pressure difference of 26 mb;
high VP = leaf-air vapor pressure difference of 5 mb) throughout
vegetative period. Desiccation occurred after humidities had been
equalized (leaf-air vapor pressure difference -- 5 mb).
0


8

J. S. BOYER AND H. G. MCPHERSON

produced 4930 kg ha-', a result that is in a direction predicted from the
photosynthetic measurements. Thus, the saving in grain production was
quite substantial in the desiccated plants that had previously experienced
dry conditions. This amount of grain production (68% of the control for
the low VP plants) is a considerable accomplishment for plants having
so little photosynthesis (37% of the control when integrated) during the
grain-filling period. The grain produced by the controls, however, was relatively unaffected by the pretreatment ( 10,500 and 11,700 kg ha-l) .
The results of this experiment suggest that (1) plants can adapt to desiccation in some way that preserves grain production, and (2) plants can
mobilize photosynthate produced before the grain-filling period and use

it to fill the grain.
The adaptation of the plants could take two forms: avoidance of low
leaf water potentials or tolerance to low leaf water potentials. Figure 3
shows that there was little difference in the tolerance of photosynthesis
to low leaf water potentials in the two sets of plants. For both, net photosynthesis was inhibited initially at leaf water potentials of about -8 bars
and became zero at leaf water potentials of about -1 8 to -20 bars. However,
less water was used under well watered conditions by plants from the dry
pretreatment than by those without the dry pretreatment (Fig. 4 ) . This
resulted in the conservation of soil water, and consequently leaf water potential (Fig. l A ) , transpiration (Fig. 4), and photosynthesis (Fig. l B)
were preserved in the adapted plants for a longer time than in the unadapted plants. In this case, it appears that adaptation to desiccation took
I20

"0

-4

-8

-12

-16

-20

-24

Leaf Water Potentiol ( b a r s )

FIG.3. Net photosynthesis during grain-fill in maize at various leaf water potentials after pretreatment under two humidity conditions. See Fig. 1 for details of the
Low vapor pressure (VP) pretreatment; 0 , high VP pretreatment.

experiment. 0,


PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

9

I

OO

2

4

6

8

10

12

14

Time (days since start a t d e s i c c o t i o n )

FIG.4. Transpiration during grain-fill for whole maize plants that were desiccated
by withholding water from the soil after pretreatment under two humidity conditions.
See Fig. 1 for details of the experiment. 0,

Low vapor pressure (VP) pretreatment;
0 , high VP pretreatment.

the form of avoidance rather than tolerance, and the fundamental ability
of the protoplasm to carry on metabolism at low leaf water potentials was
unaltered, or at the most only slightly altered. While it is true that adaptation took the form of avoidance in this instance, the possibility remains
that other kinds of desiccation pretreatments might cause improved tolerance of the plants to dry conditions. It would seem that further investigation of this possibility may be worthwhile.
The ability of the plants to mobilize reserves for grain-filling when current photosynthate became unavailable is a result quite distinct from the
problem of adaptation. Table I1 shows that plants from the two pretreatments formed grain roughly in proportion to the total dry matter that had
been accumulated during the growing season (Table 11), not according
to the dry matter produced during grain-fill alone (Table 11). Adaptation
had little effect on this trend. Thus, adaptation simply caused more dry
matter to be accumulated by the plants, and this in turn permitted higher
grain yield (Table I ) .
The vegetative portions of the desiccated plants actually lost weight to
the grain as reserves were transported to the developing ears (Table 11).
Thus, as export of photosynthate from the leaf declined, reserves from
other parts of the plant compensated for the reduction in transport to the
grain. Since the proportion of weight lost by the vegetative portions of the
desiccated plants was similar for both pretreatments, there was relatively


J. S. BOYER AND H. G. MCPHERSON

10

TABLE I1
Dry Weights in Maize When Water Was Withheld throughout Most
of the Grain-Fill Period
Low VP pretreatment"

Parameter
Grain
Shoots at end of season
Gain by shoots during grain fill
Gain by nongrain parts of shoot
during grain-fill
Grain :shoot, end of season
Grain :gain by shoots during
grain-fill

Controlb
(g p1-9

Desiccatedh
(g PI-.')

43 rt 18

101 rt 6
195 2C 1 1
68 rt 3
-26 rt 2

0.48
0.80

0.52
I .49

148 rt 24
311 45
184 f 36

High VP pretreatmenta

Controlh
(g p1-9

Desiccated*
(g PI-')

rt 21
2C 24
rt 19
k8

62 4
154 k I
42 f 3
-17 -t 5

133
311
199
69

0.43
0.67

0.40
1.48

a Pretreatment consisted of growing plants in different humidities during the day (low
VP = leaf-air vapor pressure difference of 26 mb; high VP = leaf-air vapor pressure difference of 5 mb) throughout vegetative period. Desiccation occurred after humidities had
been equalized (leaf-air vapor pressure difference = 5 mb).
Leaf water potentials were -3 to - 4 bars and - 18 to -20 bars in control and desiccated plants, respectively, throughout most of desiccation period (see Fig. 1A). Standard
deviations are shown beside means for 9 to 10 plants.

little difference in the ability of the plants to mobilize these reserves (Table
11). This suggests that maize had a fundamental and fairly constant
capacity for using reserves for grain-filling under our conditions.
Table I11 shows that, of the components of yield, the single grain weight
changed by the largest amount with desiccation. This suggests that the size
of the sink represented by ear number and grain number was virtually the
same for all plants, as would be expected since pollination was completed
before the drought occurred. Thus, the differences in grain yield between
the adapted and nonadapted plants can be attributed to differences in the
total amount of photosynthates accumulated by the plants, not to differences in the ability of the plants to mobilize reserves or in the strength
of the sink for photosynthate represented by the grain.
The capability of maize to mobilize reserves for grain-filling indicates
that a considerable amount of potential grain dry weight is present but
never reaches the grain under good moisture conditions. We do not know
whether most crops exhibit the same tendency to accumulate unused photosynthate in favorable environments, but, if so, it is clear that some method
of utilizing these reserves for grain-filling under all conditions could benefit
yield considerably.


11

PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

TABLE 111
Components of Yield in Maize When Water Was Withheld throughout
Most of the Grain-Fill Period
~

~~

Low VP pretreatmentD
Component

Controlb

Ears per plant
Rows per ear
Florets per ear
Filled grains per ear
Single grain weight

I .o 0 . 0
16.7 f 1.4
784 79
471 i-88
0.314 5 0.02

*

+

Desiccatedb

**
*

I .o 0 . 0
16.0 1.7
740 f 75
444 35
0.227 0.02

+

High VP pretreatment"
Controlb

*

1 .o 0.0
16.4 f 1.7
746 f 61
451 f 80
0.295 0.04

+

Desiccatedb
1 .o
16.5
694

371
0.168

f 0.0
f 1.4
f 29
f 32
0.02

*

Pretreatments consisted of growing plants in different humidities during the day (low
VP = leaf-air vapor pressure difference of 26 mb; high VP = leaf-air vapor pressure difference of 5 mb) throughout vegetative period. Desiccation occurred after humidities had
been equalized (leaf-air vapor pressure difference = 5 mb).
* Leaf water potentials were -3 to - 4 bars and - 18 to -20 bars in control and desiccated plants, respectively, throughout most of desiccation period (see Fig. 1A). Standard
deviations are shown beside means for 9 to 10 plants.

B.

TRANSLOCATION

Although photosynthesis is important for grain production in cereal
crops, the transport of photosynthetic products is also essential for the formation of yield. In maize, about half of the dry matter accumulated by
the shoot is ultimately moved into the grain. Thus, the process operates
on a large scale, and any inhibition of it is likely to result in a reduction
in yield.
It is generally agreed that drought results in a diminution of the recent
photosynthate transported to developing grain. Wardlaw ( 1967, 1969,
1971 ) has shown that the rate of translocation of recently fixed 14C was
reduced in wheat growing under desiccating conditions. Translocation in

maize growing in the field showed a similar behavior (Brevedan and
Hodges, 1973).
This reduction in rates of translocation could result either from a reduction in the amount of photosynthate available for transport or from a direct
inhibition of the translocation process. Wardlaw (1969) attempted to distinguish between these possibilities by manipulating the amount of photosynthetic tissue (the source) relative to the amount of utilizing tissue (the
sink) in wheat. When the relative amount of sink in the desiccated plants
was increased, the velocity of transport became the same as in the controls,
although the total quantity of "C transported was less than in the controls.
Wardlaw ( 1969) interpreted these results to indicate that the translocation


12

J. S. BOYER AND H. G. MCPHERSON

mechanism itself was relatively unaffected by desiccation, and that the
effects of desiccation on the source and sink accounted for most of the
changes in translocation. However, Brevedan and Hodges ( 1973) suggest
the reverse, that “C translocation may be more severely affected than photosynthesis during drought in the field.
From the experiments with maize described in the previous section, it
is clear that translocation was less sensitive than photosynthesis to low
leaf water potentials. Leaf photosynthesis virtually ceased (Fig. 1B) while
dry weight from other parts of the desiccated plants continued to accumulate in the grain (Table 11). The proportion of dry weight transported to
the grain was about as large in the desiccated plants as in the controls
(Table 11). Consistent with this finding is the work of Asana and Basu
(1963) with wheat. They found that an inhibition of photosynthesis early
in the grain-filling period was compensated by translocation of stem reserves. Thus, these findings agree with the concept of Wardlaw (1969)
that reductions in the translocation of recent photosynthate do not reflect
an effect on the translocation mechanism itself, but rather on the availability of photosynthate for export from the leaf.

C. NUTRITIONAL

QUALITY
We have so far mainly considered the effects of drought on the quantity
of grain production. Probably just as important from the human standpoint,
however, are its effects on the nutritional quality of the grain. In addition
to the caloric value of the grain, the other major component of nutritional
quality is the protein content and amino acid composition of the grain.
Miller (1938) pointed out that the bread-making quality of wheat (largely
a function of grain protein content) is affected by the dryness of the growing season. For wheat, the percentage of protein increases during a drought,
although total yield decreases. Evidently, the total protein production is
inhibited but total carbohydrate production is inhibited even more.
In the vegetative portions of the plant, this order is reversed and protein
synthesis appears to be reduced before photosynthesis decreases significantly. Recent studies of nitrate reductase synthesis illustrate the point.
In vegetative maize, nitrate reductase is an unstable enzyme that must be
continually synthesized (Beevers and Hageman, 1969). Unfavorable temperature, CO, levels, and water availability reduce the activity of the enzyme (Beevers and Hageman, 1969; Morilla et d., 1973) largely because
of an inhibition of protein synthesis. Desiccation of the leaves resulted in
a marked inhibition of nitrate reductase activity (60-70% ) at leaf water
potentials of -6 to -8 bars (Morilla et al., 1973). Photosynthesis had declined only 10-20% at these water potentials, however (Boyer, 1970b),