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Chapter 5. Advances in Drought Tolerance in Plants

Chapter 5. Advances in Drought Tolerance in Plants

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188



J. S. B O E R



United States have become scarce as municipalities and environmental needs

compete for the same water. Salt-laden water usually is not an alternative because evaporation removes the water and not the salt, degrading the soil. As a

consequence, new irrigation is becoming less possible than in the past and there

is increasing interest in improving the efficiency of water use in irrigation and

determining whether plants can yield well under water deficient conditions (Boyer, 1982).

A number of methods exist for improving the efficiency of water use and have

been summarized by Taylor et al. (1983) and by Stewart and Nielsen (1990). The

methods can be classified in three broad categories: (1) increasing the efficiency

of water delivery and the timing of water application, (2) increasing the efficiency of water use by the plants, and (3) increasing the drought tolerance of the

plants. The first method is practiced most because it depends on engineering and

minimally on the crop. Transporting water with low evaporative loss, preventing

runoff, storing water in catchments, delivering water only to the root zone, and

timing irrigation to the needs of the plant have been successful in improving

productivity per unit of water delivered to the farm. There are estimates that just

by improving irrigation timing, the amount of applied water can be decreased by

half in some crops while maintaining high levels of production (e.g., Bordovsky

et al., 1974). The second and third methods depend on understanding the biology

of the crop and whether it can be manipulated to achieve the same productivity

with less water. The state of knowledge in this area is the focus of this review.



11. WATER USE EFFICIENCY

Water use efficiency (WUE) usually is defined as the total dry matter produced

by plants per unit of water used,

WUE



D

W ’



= -



where D is the mass of dry matter produced (usually aboveground) and W is the

mass of water used (usually including direct evaporation from the soil). For a

field experiment, D and W would be expressed on the basis of land area. For a

single plant, D and W would be measured in the same plant and expressed on the

basis of the whole plant. Sometimes, the D is the economically valuable part of

the crop (for example the grain, tuber, or fruit) and WUE refers to yield. One

may also consider the water use efficiency of a single leaf, and so on. The higher

the production per unit of water use, the higher the efficiency.

There is extensive evidence that WUE varies among species in the same

environment and among climates for the same crop (Briggs and Shantz, 1914; de



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



189



Wit, 1958; Hanks in Taylor er al., 1983; Tanner and Sinclair in Taylor et al.,

1983). Taking advantage of the species and climate effects can help manage

limited water supplies in agriculture. For example, alfalfa (Medicago sativa L.)

has a lower water use efficiency than maize (Zea mays L.) when grown in nearby

sites in the same year (Hanks in Taylor et al., 1983; Miller, 1938). Thus, simply

by changing crops, water consumption can be reduced with little sacrifice in dry

matter production. Relocating production to a new climate with lower evapotranspiration is another possible approach. For economic reasons, however,

these options are not often employed and probably will not be until the cost of

water rises to a level that forces change. What then are the prospects for improving water use efficiency within a species, or protecting against yield loss in a

particular climate when irrigation is not possible?



A. PHYSIOLOGY

OF WATER

USEEFFICIENCY

The dry mass of plants consists mostly of the C and 0 atoms fixed photosynthetically from CO,. These elements are much heavier than the H atoms that

also are prevalent in the dry mass and that originate in the water photolyzed

during the photosynthetic process. As a consequence, D of Eq. ( I ) represents

mostly the net C0,-fixing activity of the plant. Before fixation, the CO, must

diffuse into the leaf and dissolve in the wet surface of the cells where it becomes

available to be fixed. The wet surfaces are exposed to the atmosphere inside the

leaf and transpiration is inevitable. As a result, the photosynthetic cells dehydrate

to varying degrees. Water absorbed from the soil replenishes the water lost by the

cells, but the water potential of the cells must be low enough to maintain absorption. The stomata and waxy cuticle of the epidermis control the transpiration rate

and thus the amount of water needing to be absorbed, and lower water potentials

cause stomata1 closure. This regulation of transpiration and absorption affects the

balance between net CO, gain and water loss and in turn the WUE. Depending on

the leaf anatomy and physiology, the dry matter produced per unit of water used

can vary widely.

In addition to these factors, water use also is affected by physical factors. CO,

enters by diffusing down a concentration gradient to the leaf interior, and the

water vapor in the intercellular spaces inside the leaf likewise diffuses in the

opposite direction. The lower the external humidity, the faster transpiration will

be when all other factors are constant. Leaf temperature plays an important role

by affecting the vapor pressure of water in the leaf. The higher the leaf temperature, the higher the vapor pressure and the more rapid the transpiration. Water

use will differ among sites and seasons for these reasons and the water use

efficiency in Eq. ( I ) thus reflects a complex of plant and environmental factors.

Briggs and Shantz (1914) conducted an extensive survey of the water use



190



J. S. BOER



efficiency of crops, and they expressed it as the water requirement, that is, the

amount of water transpired to produce a unit Df aboveground dry matter, which is

the reciprocal of the water use efficiency. They grew the plants in large containers

of soil and made measurements for the entire growing season. This had the

advantage that a large number of crops could be compared in a uniform climate

during a single season. In their experiments, the transpiration ratio of maize,

sorghum (Sorghum bicolor L.), and millet (Panicurn rniliaceum L. and Sefaria

italica (L.) Pal. = Chaetochloa) was less than for the other crops and, although

Briggs and his co-workers could not have known at the time, the three crops are

C , species possessing a special anatomy and biochemistry that allows CO, to be

concentrated around the site of fixation. This resulted in more photosynthesis per

unit of water transpired and accounted for the lower water requirement.

After the experiments of Briggs and his co-workers, various investigators

measured water use efficiency under field conditions where all the adaptations of

the crop could express themselves (de Wit, 1958; Hanks in Taylor et a l . , 1983).

Typically, the experiments involved season-long exposure to differing amounts of

irrigation. Figure 1 shows examples for Logan, Utah, where wheat (Triticum

aestivum L.) and maize were grown with varying amounts of irrigation in 1975



Wafer Use Efficiency



EVAPOTRANSPIRATION (xi O6 Kg H2O.ha -’)



Figure 1 Production of aboveground shoot dry matter at various levels of water use in several

crops near Logan, Utah. The years in which the crops were grown are given in the symbol key. Water

use was controlled by irrigation that held conditions essentially constant for the growing season.

Water use is the combined evaporation from the soil and transpiration from the plants. A positive

evapotranspiration intercept indicates the amount of water obtained from soil stores. The slope of the

linear relation is the water use eficiency, which was 4.49 g of dry mass per kilogram of H,O for

maize, 2.50 for wheat, 2.36 for alfalfa, and 2.1 I for barley. Maize is a C, plant and the others are C,.

Adapted from Hanks (in Taylor et al., 1983).



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



191



and alfalfa and barley were grown in other years (Hanks in Taylor et a / ., 1983).

There is a linear relationship between water use and dry matter production. The

linearity is mostly caused by the diffusion link between photosynthesis and

transpiration because the visible radiation input is almost completely absorbed by

crops after the canopy closes and, in a given climate, the input tends to be

partitioned in a constant proportion between energy for transpiration and energy

for photosynthesis. The slope of the linear relationship is the water use efficiency

[Eq. ( I ) ] , and the straight line indicates that the water use efficiency does not

vary as the availability of water varies. However, it differs among species, as is

apparent in Fig. 1 for the C, maize and C, wheat, alfalfa, and barley. These

experiments confirm the differences noted by Briggs and his co-workers and

further indicate that water use efficiency does not differ under varying availabilities of soil water in this type of experiment. However, it differs among species,

climates, and from year to year (Briggs and Shantz, 1914; Brown and Simmons,

1979; Garrity et al., 1982; Hanks in Taylor et al., 1983; Kawamitsu el al., 1987;

Pandey et a / . , 1984a,b; Robichaux and Pearcy, 1984; Tanner and Sinclair in

Taylor et al., 1983), and there is a possibility that it will vary with different

mineral nutrient availabilities, plant spacing, and other cropping practices.

In this respect, it is important to note that while differences between C, and C,

species are apparent, similar tests have not been made in species exhibiting

Crassulacean acid metabolism. Pineapple (Ananas cornoms (L.) Merr.) has this

form of metabolism and it concentrates CO, by temporarily fixing the gas in

organic acids at night and releasing it the next day for photosynthesis. During

release, the stomata are closed and water is conserved. This allows CAM plants

to achieve even higher water saving than C, plants, and limited estimates of

water use efficiency are about 20 g of aboveground dry mass per kilogram of

water for pineapple (Joshi et a/., 1965; Neales er a / ., I968), compared to 3 to 5

for C4 plants, and 2 to 3 for C3 plants.

An alternate approach to the usual description of water use efficiency is to

normalize water use for evaporative demand (de Wit, 1958; Tanner and Sinclair

in Taylor el al., 1983) and dry mass for the potential productivity of the crop

(Hanks in Taylor et a / . , 1983). Thus, modified expressions of WUE have been

used, such as



where the fractional dry mass is DID,,,, and is expressed relative to the maximum dry mass produced with optimum water D,,.

The fractional water use

WI W,,,,, is likewise expressed relative to the maximum evapotranspiration W,,,

that would occur with optimum water. This normalization procedure has the

advantage that for a water use of, say, half the maximum evapotranspiration, half



192



J. S. BOYER



1



0



1



Sorghum



2



4



6



EVAPOTRANSPIRATION(x106 Kg H2O.ha -’)

Figure 2 Seed yield at various levels of water use by three sorghum genotypes in West Central

Nebraska. The experiment was conducted under conditions similar to those of Fig. 1. The water use

efficiency for seed yield is the slope of the line and was 1.8 g of dry mass per kilogram of H,O in

RS626. 1.9 in NC+55X, and 1.2 in NB505. The water use efficiency for total shoot dry mass was

3.3 in RS626, 3.2 in NC+55X, and 2.0 in NB505. Adapted from Garrity er al. (1982).



the maximum dry mass would be predicted. This can simplify the job of predicting the impact of water shortages but it requires a knowledge of the maximum

dry matter yield and evapotranspiration of the crop for the year, which will vary.

For practical purposes, the maximum yield and water use usually are not

known and normalization may not be done easily, so the absolute expression in

Eq. (1) is preferable. Moreover, farm income for an irrigated crop is generally

based on the absolute dry mass or economic yield rather than normalized yields,

and expense is based on the absolute amounts of water used. There needs to be a

high absolute production of dry mass to justify pumping large amounts of water

and there should be a high production of marketable yield. For example, Fig. 2

shows that the water use efficiency differed for production of grain dry mass in

sorghum genotypes RS626 and NB505 (Garrity el al., 1982). Normalizing according to Eq. (2) would not distinguish which genotype gives the highest grain

production, but Eq. (1) would detect the difference.



B. IMPORTANCE

OF HARVEST INDEX

The fraction of the crop that is economically valuable, termed the harvest

index, is part of the total dry mass and thus part of WUE. There has been a



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



193



general increase in yields of modern crops with little change in the total aboveground biomass, and according to Gifford (1986) the increase is attributable to an

increase in the harvest index. The increase has come without much change in the

amount of water used, and the result has been a natural improvement in WUE for

yield (Richards et al., 1993). There is a maximum to which the harvest index can

be increased and the maximum probably is being approached in many modern

crops. Therefore, the maintenance of the harvest index is of critical importance

when water is in short supply.

In Fig. 2, the harvest index was nearly constant among the various treatments.

Instead, the differences in WUE were attributable mostly to differences in the

total aboveground dry matter (Garrity et a l . , 1982). The experiments involved

season-long steady exposure to limited water, and the crops acclimated by growing smaller shoots, with flowering and grain fill adjusting in proportion. In this

steady environment, the acclimation allowed the harvest index to be maintained.

Under many field conditions, however, plants encounter variable water deficits

that do not allow the acclimation possible in long-term experiments, and the

harvest index can decrease. This effect can be extreme, and water deficits can

give a harvest index as low as zero (see Boyle et a l . , 1991b, for an example).

Therefore, considerable opportunity exists for maintaining the harvest index in

the face of variable environments, which we will explore more fully later.



C. MEASURING

WATERUSEEFFICIENCY

The most accurate means of measuring water use efficiency is to monitor the

evapotranspiration and harvest the crop for biomass measurements at the end of

the season. The WUE can be determined for the total biomass or any part of the

biomass. However, these are labor intensive and costly measurements. Less

expensive methods have been sought, and one has been to measure directly the

COz and H,O exchange of individual leaves (Bierhuizen and Slatyer, 1965;

Brown and Simmons, 1979; Robichaux and Pearcy, 1984). Because the CO,

molecule contributes most of the dry mass, the gas exchange efficiency can be

defined as the ratio of the mass of CO, gained to the mass of H,O lost. Martin

and Thorstenson (1988) compared the gas exchange efficiency with the actual

water use efficiency for the whole growing season in tomato (Lycopersicon

esculentum Mill. ), its wild relative Lycopersicon pentzeflii (Cor.) D’Arcy, and

hybrids between them. The relationship was poor because of additional factors

affecting dry mass accumulation but not gas exchange. For example, the mass of

the plant is determined not only by photosynthesis but also by respiratory losses

at night and partitioning to nonphotosynthetic organs such as roots. It is altered

by temperature and the molecular composition of the dry mass. Gas exchange for

short times during the day does not detect these additional factors. Therefore,



194



J. S. BOYER



while the gas exchange efficiency gives valuable insight into the physiologic and

metabolic controls that might operate during photosynthesis and transpiration,

the method is being used less frequently than in the past.

Another method is based on the relative abundance of natural isotopes in plant

tissue. Although most of the CO, in the atmosphere is T O , , a small amount is

T O 2 . Because the I2CO2 is lighter, it diffuses more rapidly than 13C02. Also,

ribulose- 1,5-bisphosphate carboxylase fixes the ' T O 2more rapidly than I3CO2.

Consequently, the cells accumulate relatively more 12C than 13C, and the unused

C diffuses out according to the extent of stomata1 opening. This outward diffusion is correlated with transpiration. Because the inward diffusion and use of

I2CO2correlates with photosynthesis and dry mass but the outward diffusion of

13C0, correlates with transpiration, the relative uptake of I2C and I3C correlates

with the water use efficiency. Generally, higher water use efficiency correlates

with lower tissue 12C relative to 13C for wheat, peanut (Arachis hypogaea L.),

barley (Hordeum vulgare L.), and other crops as shown in Fig. 3. Therefore, the

measurements detect differences in WUE among individuals within a species and

they only require the ratio of the isotopes in tissue samples to be compared to a

standard (Bowman et al., 1989; Brugnoli et al. 1988; Condon et al., 1987, 1990;

Farquhar and Richards, 1984; Hubick and Farquhar, 1989; Hubick et al., 1986).

The ratio technique makes it possible to survey a large number of plants at

moderate cost. Differences integrate the conditions over which the plants were



Discrimination (Woo)



Figure 3 Water use efficiency and carbon isotope discrimination compared in various genotypes

of (A) wheat, (B) barley, (C) peanut, and (D) wheatgrass. Adapted from Farquhar and Richards

(1984), Hubick ef al. (1988). Johnson et at. (1990). Hubick and Farquhar (19891, and 'hrner (1993).



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



195



grown. Analyzing the entire shoot indicates the water use efficiency for the time

required to grow the shoot whereas analyzing only leaf starch indicates the water

use efficiency during the time necessary to accumulate the starch. One may

integrate over long or short times which avoids one of the problems of the gas

exchange technique.

Martin and Thorstenson (1988) used this technique to show that differences in

water use efficiency were present between the domestic tomato species and L .

pennellii and their hybrids. Differences in water use efficiency were detectable in

isotope ratio data between the parents and the hybrids particularly when water

was optimally available. The domesticated parent had the lowest efficiency, the

wild parent the highest efficiency, and the hybrids showed intermediate behavior.

Because the species could be crossed, it was possible to correlate the differences

in water use efficiency with restriction fragment maps of the tomato DNA (Martin et af.,1989). Three loci were found to be predictors of the variation in water

use efficiency in field grown tomato. This landmark effort indicates that water

use efficiencies are determined by relatively few genetic loci and implies not only

that agriculturally relevant differences exist but that they might be genetically

manipulated in a simple fashion.

The success of the method suggests that differences in water use efficiency

exist in individual species and might be usefully incorporated into breeding

programs, although this is still in its infancy (Bowman et al., 1989; Brugnoli et

al., 1988; Condon et a l . , 1987, 1990; Hubick and Farquhar, 1989; Hubick e t a l . ,

1986). Genetic variation clearly exists but in crop canopies the variation becomes

less clear. Substantial water limitation usually gives a negative relationship between discrimination and WUE as shown in Fig. 3, but under relatively favorable

conditions, the relationship tends to become less negative or even positive. In the

field, this can obscure relationships developed from pot experiments. For example, Condon and Richards (1993) showed that two wheat genotypes differing by

40-50% in leaf diffusive conductance only differed by 15% in canopy transpiration efficiency. In the field, this difference was not reflected in improved WUE for

the crop because soil evaporation differed in opposition, canceling the 15%

effect. There was a slower development of the canopy in one genotype than in the

other, and this was responsible for the canceling effect (Condon and Richards,

1993). Therefore, differences in WUE may need to be combined with other crop

traits to be realized as water savings.



D. MOLECULAR

BIOLOGYAND WATERUSEEFFICIENCY

Opportunities to improve WUE generally involve many genes and many interactions. Perhaps this is not surprising in view of the massive changes in plant

form and anatomy that have occurred as plants colonized the land. The develop-



196



J. S. BOYER



ment of roots, cuticle, stomata, vascular systems, and seeds all occurred after

plants left their original water environment and, in evolutionary time, these

developments are relatively recent. As a consequence, single genes that substantially change the WUE of plants are difficult to find. With the increased ability to

transform plants genetically in recent years, it would be desirable to apply the

tools of molecular biology to the improvement of WUE. However, the tools work

best when single genes are manipulated. The problem can be seen in C, species

where many genes code for enzymes and leaf anatomy that differ from those in

C, species. While C, photosynthesis confers a clear improvement in WUE, the

genetic complexity of the trait makes it difficult to incorporate in C, plants.

However, there are traits of more limited scope that probably have less complex

genetics and might benefit from the application of molecular biology. Some of

these are considered below as ways to improve the drought tolerance of the plant.



111. DROUGHT TOLERANCE

Plants showing improved growth with limited water are considered to tolerate

drought regardless of how the improvement occurs or whether the water use

efficiency is affected. Some species can avoid drought by maturing rapidly before

the onset of dry conditions or by reproducing only after rain. Examples of these

drought avoiders are ephemerals such as California poppy (Eschscholtzia californica (Cham.)) that can complete their life cycle in a few weeks, or tree crops

such as coffee (Cofea arabica (L.)) and cacao (Theobroma cacao (L.)) that

flower and fruit after drought followed by rain (Alvim, 1960, 1985). Others can

postpone dehydration by growing deep roots or sealing themselves tightly against

transpiration or accumulating large stores of water in fleshy tissues. Examples of

dehydration postponers are upland rice (Oryza sariva L.) with deep roots compared to paddy rice (Chang et al., 1974), or agave (Agave deserti (Engelm.)) or

saguaro cactus (Carnegiea gigantea (Engelm) Britt. and Rose) with thick cuticle

or fleshy tissue. Still other species allow dehydration of the tissues and simply

tolerate it by continuing to grow when dehydrated or by surviving severe desiccation. Certain intertidal algae such as Fucus vesiculosus (L.) or lower vascular

plants such as Selaginella lepidophylla (Hook. & Grev.) can carry out photosynthesis at very low water contents and tolerate desiccation to the air dry state

without losing viability. The seeds of most angiosperms also can tolerate severe

desiccation.

These effects are generally distinct from the factors controlling water use

efficiency. Drought avoiders depend on the timing of development which is under

internal control. They tend to reproduce themselves after a minimal accumulation of dry matter and their success ensures that they are represented in the next



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



197



generation. Dehydration postponers having deep roots may have a water use

efficiency identical to that of other species but will accumulate more dry weight

because of their ability to gain access to a larger amount of water than shallow

rooted types. In effect the slope of the water use efficiency relation in Fig. 4 may

be the same but the deep rooted species work farther out on the curve. Their

adaptations are mostly structural and take time to build, requiring the expenditure

of photosynthetic products. Finally, dehydration tolerators may have the same

water use efficiency as dehydration-sensitive species when water is available but

the tolerators can grow at tissue hydration levels that the other species cannot.

Of the three forms of drought tolerance, dehydration tolerance is most intriguing because it often requires only slight repartitioning of dry mass. An example is

osmotic adjustment (Morgan, 1984; Munns, 1988) which occurs because dry

mass normally used to synthesize new cells instead accumulates in the cells as

solute (Meyer and Boyer, 1972; 1981) or is deposited in fewer or smaller cells

(Fraser et al., 1990; Sharp et a l . , 1990). Only a brief decrease in biosynthesis of

tissue is necessary to accomplish this (Meyer and Boyer, 1981), but the increased

concentration of solutes can markedly increase the ability of the cells to extract

water from the soil. The increased solute is present only under dry conditions. In

other words, there is little cost to the plant when water is scarce and no cost when

water is plentiful.



A. APPROACHESTO IMPROVING

DROUGHT

TOLERANCE

From these examples it can be seen that water use efficiency is important, but

crop improvement under conditions of limited water involves more than water



-



EVAPOTRANSPIRATION (x106 Kg HpO.ha -')



Figure 4 Effect of increasing the amount of water available to a crop without changing the water

use efticiency. Production moves from A to B. An example might be increasing rooting depth.



198



J. S. BOYER



use efficiency. Basing improvement solely on water use efficiency is tempting

because breeding programs could select solely for high productivity when water

is plentiful. The idea is that, for a given climate, water use efficiency will be

highest when dry matter production is highest and the linear relationship of dry

matter to water use (Figs. 1 and 2) would allow the high productivity to carry

over to drought conditions. However, it is clear that many opportunities will be

missed if superior selections are based only on this concept of water use efficiency. Characters such as osmotic adjustment are called into play only during a

water deficit. Roots may penetrate deeper soil layers or leaves may persist better

during a water deficit in some genotypes than in others, and so on. Without plant

selection under water deficient conditions, these beneficial traits will be missed.



800C

Morrow Plots



Urbana, Illinois

Continuous Maize



0



600C



m



A



5n



5



-



-0

Q



First



E

al



n 4000



E



e

U

*9

0



.i?



LL



2000



0



1



I



I



I



1890



1900



1910



1920



I



1930

Year



I



I



I



1940



1950



1960



l! '0



Figure 5 Yields of continuous maize crops at Morrow Plots at University of Illinois. The first

crop was planted in native prairie. Soil nutrients and organic matter were added to some of the plots

beginning around 1905. Hybrids were introduced to all plots in 1937. Yields are 5-year averages.

Genotypes are those popular at the time.



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