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IV. Water Deficits and Reproduction

IV. Water Deficits and Reproduction

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20s



similar pollen abortion, thus implicating high ABA levels during dehydrating

conditions. However, the high ABA may have acted by closing stomata and

inhibiting photosynthesis. Increasing CO, pressures around wheat plants overcame some of the reproductive losses (Gifford, 1979), which supports an involvement of photosynthesis. In rice, dehydration of the soil caused especially

severe dehydration of reproductive tissues, and death and bleaching of florets

followed probably because of inadequate cuticular wax (O’Toole et al., 1984).

Therefore, in various crops, there is increasing evidence for metabolic .and

growth regulator effects and some direct dehydration effects that might account

for the susceptibility of early reproduction to water limitation. CO, and ABA

seem to be involved, and photosynthesis also may play a role but each could act

in concert or separately, depending on the crop.

Additional insight may be possible from studies of embryo development in

maize. Westgate and Boyer (1985a) found that the block in embryo development

was correlated with low photosynthetic reserves in the maternal plant. Because

photosynthesis was inhibited during the treatment, the lack of reserves could

have caused embryo starvation. Westgate and Thomson Grant ( 1989) observed

that the sugar content of maize embryos was not significantly different in hydrated and dehydrated plants but concluded that the flux of sugar might differ.

Schussler and Westgate (1991a,b) found that the uptake of sugars was less in

maize ovules isolated from dehydrated plants even though the sugar content was

high, which further confirms that the flux of sugars was more important than the

sugar content of the developing grain. Myers et ul. (1990) showed an inhibition

of endosperm cell division in maize when high ABA levels were present 5 to 10

days after fertilization.



A. MECHANISMS

OF LOSSES

Boyle et a / . (1991a,b) took advantage of the finding of Westgate and Boyer

(1985a; 1986a) that a few days of low water potentials can prevent embryo

growth and developed a system to feed stems a complete medium for embryo

growth during this time. This allowed photosynthetic products and other salts

and metabolites to be supplied to the plants at normal levels without rehydrating

the plants. The controls yielded well, but withholding water for a few days

virtually eliminated grain production because of embryo abortion. Production

was almost fully restored when the plants were infused with the complete medium as low water potentials developed. Infusing the same amount of water alone

showed no restorative activity. Therefore, it was possible to maintain reproduction by feeding substances normally supplied by the parent plant during embryo

development, which indicates that sufficient water was available to the embryos



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so that water itself was not the limiting factor, and embryo growth had been

blocked by some other substance(s) that the parent plant failed to supply. Thus,

reproductive loss appears to be a biochemical problem.

Zinselmeier et al. ( I 995a) found that the active ingredient was sucrose, and

ovary sucrose had to be elevated above controls in order for embryo abortion to

be prevented. Because sucrose is the main translocation form of photosynthate in

maize, this finding showed not only that the lack of photosynthate was the basis

of abortion but also that there was a block in the utilization of sucrose by the

ovaries (Zinselmeier et al., 1995b). Abortion was accompanied by a loss of

starch, which is a product of sucrose metabolism in the ovaries, and starch

returned partially toward control levels when sucrose was fed. This suggests that

the block could have been located between the sucrose supply to the ovaries and

starch synthesis in the ovaries, although there is a possibility that starch degradation also was involved.

It is worth noting that little is known about the role of ovary starch. In contrast

to endosperm starch, which is a terminal pool and has received a great deal of

attention, ovary starch is not a terminal pool. It forms before fertilization and,

because it decreases during dehydration and recovers when sucrose is fed to the

stems of dehydrated plants (Zinselmeier et al., 1995a), it appears to be mobilizable. Apparently, under unfavorable conditions for the parent plant, the products

of breakdown are used to support the growth of the ovary tissues. Fader and

Koller (1985) suggested that ovary starch could be important for developing

soybean pods. Important insight may be gained from a fuller understanding of

ovary starch in early reproduction.

Edmeades and his co-workers found that the time between pollen shed and

silking can be changed by genetic means in maize, and they used early silking to

indicate vigorous development of the ear (BolaAos and Edmeades, 1993a,b;

Bolafios ef al., 1993; Edmeades et al., 1992, 1993). Early ear development may

indicate that the plant supplies more of the biochemical requirements for ear

growth and may be a genetic means of accomplishing the same result as feeding

sucrose to the stem (Zinselmeier et al., 1995a). In effect, early ear development

may be a visual signal for enhanced sucrose availability or utilization by the ear.

Fischer et al. (1989) selected a population of tropical maize for several physiological attributes likely to improve drought tolerance (low canopy temperature,

low leaf death, early silking relative to pollen shed, days to anthesis) and found

that most of these traits conferred a yield gain, but early silking relative to pollen

shed generally accounted for more of the yield gain than the other traits. After

three cycles of selection, grain yield increased by 320, 420, and 4 10 kg-ha- in

the mild, medium, and severe dehydration treatments but not at the expense of

yield in hydrated conditions. The authors point out that progress was accelerated

by using physiological characters in addition to grain yield in the selection

program.



ADV4NCES IN DROUGHT TOLERANCE IN PLANTS



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Tollenaar and Mihajlovic (1991) report that the genetic improvement of maize

yields was associated with improved resistance to the herbicide bromoxynil (4hydroxy-3,5-dibromo-benzonitrile),

which inhibits photosynthetic electron transport at photosystem I1 and probably other aspects of energy metabolism. Tollenaar et d.( I 994) suggest that the mechanism may be related to the production

of oxygen-containing free radicals that may be more rapid during drought or

other unfavorable environments. The production of these agents is destructive to

membrane components, particularly photosynthetic membranes; further, chloroplasts contain high levels of antioxidants (glutathione, hydroquinones, ascorbate,

tocopherols, carotenoids) and the cells possess enzymes (peroxidases, catalases,

superoxide dismutases) that probably are protective. Tollenaar et al. ( 1994)

express the view that modern maize hybrids may have been selected for improved levels of the protective components that would be expressed in protection

against bromoxynil. However, it also seems possible that the selection may have

been toward less penetration of the leaf or greater metabolic degradation of

bromoxynil.

These experiments offer the promise of identifying components that may protect against losses in early phases of reproductive development when plants are

subjected to moderate dehydration. Selection for genotypes that store significant

amounts of mobilizable photosynthate during early reproduction is one approach.

Avoidance of early leaf senescence, which decreases photosynthetic capacity,

might be another. Regardless of the approaches taken, it is clear that the reproductive fraction of the plant can vary from zero to nearly normal during a

drought, which implies that successful protection of reproductive development

may be possible by genetic and cultural means under otherwise inhibiting

drought conditions.



V. DESICCATION TOLERANCE

When seeds mature, it is common for them to dehydrate as part of the maturation process. Barlow et al. (1980) found water potentials as low as -5MPa in

maturing wheat grain. Westgate and Boyer (1986~observed

)

water potentials of

-7 to -8 MPa or lower in maize grain late in the growing season. These seeds

are exposed somewhat to the atmosphere and are known to desiccate to a large

extent by evaporation to the air. Seeds surrounded by a fleshy fruit show a similar

but less severe desiccation. Welbaum and Bradford (1988) observed that water

potentials of melon seeds (Cucurnis melo L.) decreased to about -2 MPa during

maturation, and the surrounding fleshy fruit decreased similarly in water potential. Bradford (1994) considers high solute concentrations to be present in the

apoplast surrounding embryos and proposes that structures may exist to keep the



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J. S. BOYER



solutes localized there, The low osmotic potential of the apoplast solution may

explain how the seeds are dehydrated inside fleshy fruits. Regardless of whether

the seeds air-dry or are dehydrated osmotically inside a fruit, it is clear that

embryos become exceptionally tolerant of desiccation late in maturation despite

their susceptibility to the effects of water limitation when they are young.

Plants lower in the evolutionary scale than seed plants sometimes show a

similar tolerance to desiccation. Some fungi, algae from the intertidal zone, and

a few mosses and lycopods can be desiccated to the air-dry state without losing

viability (Bewley, 1979). There also are some specialized seed plants (Craterostigma species, Myrothamnus jiubellifoliu (Welw.), Xerophyta species) that

can tolerate desiccation (Gaff, 197I , 1977; Gaff and Churchill, 1976). However,

desiccation tolerance is virtually nonexistent in most agricultural species except

for the seeds and pollen. It is curious that most seed plants, which are descendants of plants that crossed the intertidal zone, should have lost the ability to

tolerate the desiccation that is so prevalent in that zone. In land plants, desiccation tolerance often evolved as part of the seed habit because an aqueous medium

generally was absent and the pollen and ultimately the embryo were exposed to

drying conditions during dispersal. In agriculture, this property makes it possible

to store seeds and allows uniform planting times. However, after germination, the

plant generally loses its desiccation tolerance and remains sensitive for the rest of

the life cycle until pollen is produced. Pollen can desiccate to a remarkable

degree in species such as maize without losing viability (Westgate and Boyer,

I986a.b).



A. MOLECULAR

FEATURES

OF DESICCATION

TOLERANCE

An important aspect of severe desiccation is that water contents can become so

low in the cells that enzyme activities can be directly inhibited by the lack of

water, as described by Vertucci and Leopold (1987a,b). Enzymes are affected

directly when sufficient water is lost to remove the hydration shells next to the

protein. The activities begin to decrease when the monolayer of water next to the

peptide surface is all that remains. When the monolayer begins to be lost, activity

decreases and disappears when water covers only a few polar sidegroups in the

peptide backbone (Rupley et al., 1983). Substrates probably are unable to reach

the active site of the enzyme because the aqueous medium is no longer continuous (Skujins and McLaren, 1967). Cells and tissues begin to show these effects

when they are desiccated in atmospheric humidities around 60-70% (water

activities of 0.6-0.7) and below (Skujins and McLaren, 1967). Thus, seeds

desiccated to the air-dry state are likely to be affected by these phenomena. Most

can return to activity when they are rehydrated, provided water contents have not



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



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become so low that the tightly bound water required for viability is lost (Vertucci

and Leopold, 1987a,b).

On the other hand, leaves generally are susceptible to desiccation damage at

much higher water activities. When dehydrated to the air-dry state, leaves of

most crop species show a breakdown of compartmentation that releases cell

constituents to the apoplast (Leopold et al., 1981), and the plasmalemma and

tonoplast show breakage followed by a loss of organelle structure starting at

water activities of about 0.98 (Fellows and Boyer, 1978). In leaves of tolerant

species, the membranes and organelles remain intact at low activities although

they often are distorted (Hallam and Gaff, 1978a,b). Therefore, an important

distinction between tolerance and sensitivity to severe desiccation appears to be

the maintenance of membrane structure and an ability of enzyme activity to

return upon rehydration.

It has been proposed from work beginning with desiccation-tolerant animals

that a possible mechanism to account for preservation of enzymes and cell

structure might be an accumulation of specific sugars such as trehalose or sucrose

whose structure resembles water in certain respects (Crowe and Crowe, 1986).

Sugars having the appropriate stereostructure might form hydrogen bonds with

cell membranes where water would ordinarily bind. Because the sugars would

remain as water is removed, the bonding would be stable and membrane structure might be maintained where otherwise it would become disorganized.

Evidence that the sugar replacement hypothesis may have merit is the accumulation of sugars such as sucrose and raffinose in developing seeds (Caffrey et

al., 1988; Koster and Leopold, 1988). Species such as maize have seeds that can

tolerate desiccation to the air-dry state, and their sugar concentration, while not

high for the seed as a whole, becomes high in the remaining residual water of the

drying seed and could have a stabilizing influence at local sites. As germination

proceeds, the stabilizing sugars are metabolized to nonstabilizing ones such as

glucose and fructose, and desiccation tolerance is lost (Koster and Leopold,

1988). A related hypothesis is that certain sugars may be converted to the glassy

state during dehydration (Williams and Leopold, 1989). The glassy state is

common in sugars such as sucrose used to make candy, and evidence for the

existence of glassy sugars is accumulating for embryos of dehydrated seeds

(Williams and Leopold, 1989).

A similar role has been proposed for certain proteins in seeds (Crowe and

Crowe, 1986; Dure el a l . , 1989). The developing seeds of a range of crops

accumulate hydrophilic proteins in the embryo as normal desiccation begins

(Dure et a l . , 1989). The proteins have been variously called dehydrins, embryo

maturation (Em) proteins, or late embryogenesis abundant (LEA) proteins (Dure

et a l . , 1989). Common to all of them is a high content of hydrophilic amino acids

so that the proteins as a whole are highly water soluble. In some of them, an



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J. S. BOYER



alpha-helix is present that could remain structurally stable during desiccation and

it has been proposed that this portion of the protein could act like a membranestabilizing sugar (Crowe and Crowe, 1986).

The mRNAs for these proteins are not readily detected in leaves or roots of

hydrated plants but can be induced by severe desiccation in very young rape

(Brassica napus (L.)) (Harada er a l . , 1989) and maize and barley seedlings

(Close et al., 1989; Close and Chandler, 1990). There was a marked increase in

dehydrin mRNAs when young wheat seedlings were dehydrated soon after germination (Close and Chandler, 1990). The mRNA expression was especially

increased in shoots, which are most exposed to dehydration under natural conditions. This cellular response suggests that the dehydrin-Em-LEA proteins play a

role in the desiccation tolerance of seedlings. Also, the mRNAs can be induced

by treating hydrated seedlings or immature embryos with high abscisic acid

concentrations (Galau et al., 1986; Hong et al., 1988; Mundy and Chua, 1988).

Abscisic acid levels normally increase in plants subjected to dehydration (e.g.,

Beardsell and Cohen, 1975) and they become high in maturing dehydrating seeds

(Ihle and Dure, 1972). The induction of the mRNAs suggests that there is

molecular control that might be manipulated genetically, thus altering the development of desiccation tolerance of young seedlings and embryos.



VI. CONCLUSIONS

Land plants appear not to be optimally adapted to water shortages imposed by

the environment and indeed we likely would see large improvements in dehydration performance if this chapter could be written after a few hundred million

years to give additional time for beneficial adaptations to evolve. Certain metabolic changes have developed during the course of evolution that have improved

the ability of plants to withstand limited water supplies, particularly in photosynthesis. The recent evolutionary development of C, photosynthesis and

Crassulacean acid metabolism are clear examples, and there is increased water

use efficiency in those species possessing these adaptations. Methods of plant

breeding and genetic modification can speed the transition to more efficient water

use and considerable success has already been achieved. Water acquisition has

been improved by deep rooting and strong osmotic adjustment, cuticular characters have been modified to conserve water, and earliness in reproduction has been

used to avoid late season droughts. It also appears increasingly possible to

improve water use efficiency by genetic means using new techniques for screening for this trait.

Water is required for biological activity, and studies show that water use



ADVANCES IN DROUGHT TOLERANCE IN PLANTS



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efficiency is unchanged by the water supply when the water limitation remains

stable for the entire season. This allows acclimation to occur, and the harvest

index often remains unchanged as well. However, when the water supply is not

stable for the growing season but slowly declines or varies with rainfall, dry

matter is partitioned differently to plant parts depending on the time in the life

cycle. This is particularly true for reproductive structures and can lead to a

variable harvest index, which can be relatively independent of the overall water

use efficiency. Attention to the harvest index may provide a means for maintaining the economically valuable parts of a crop even though the total plant dry mass

may decline.

The demonstration that reproductive losses usually associated with drought

may have a biochemical origin raises the possibility that metabolic modifications

may be useful for improving the harvest index with limited water, and genetic

approaches are being applied to this problem. Also, the molecular mechanisms

of desiccation tolerance suggest that changes in expression of specific genes are

correlated with decreased lethality of severe desiccation at least during late seed

development.

From these principles, it is possible to distill certain conclusions that may help

in efforts to improve the efficiency of water use and drought tolerance of plants.

The approaches at first appear diverse and the complexity makes it tempting to

take shortcuts such as selecting seedlings for rapid growth only under favorable

conditions or in osmotica, or by using single biochemical tests for performance.

In general, the temptation should be avoided because the results have not carried

over to field situations. The approaches that have given the most rapid progress

in improving drought performance have been: (1) using realistic soil conditions,

(2) testing with adequate water and with limited water, (3) understanding the

sources of crop failure in the proposed growing area, and (4) targeting a limited

number of traits for improvement.

In most examples of improvement, there was an intimate knowledge of the

soil, climate, and physiology, and biology of the crop. Physiological tests sometimes could be employed to increase the rate of progress, and the problem could

be reduced to a few traits to simplify the selection effort. Great progress was

made under conditions of realistic water limitation in soils because droughtadaptive factors were called into play and had an opportunity to express themselves. This avoided the problem of selecting only genotypes yielding well in

favorable environments that "crashed" in water-limited environments, or using

pots that restricted root development and prevented the expression of this important tolerance character. It is now clear that successful improvement of drought

performance can come at no sacrifice to performance under favorable conditions

but this can be determined only if performance is tested under both favorable and

unfavorable conditions.



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J. S. BOYER



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