Tải bản đầy đủ - 0 (trang)
CHAPTER 1. PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

CHAPTER 1. PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

Tải bản đầy đủ - 0trang

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),



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

CHAPTER 1. PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS

Tải bản đầy đủ ngay(0 tr)

×