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III. Improvement of Drought Response through Breeding and Management

III. Improvement of Drought Response through Breeding and Management

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the breeding and management of cereal crops themselves. For example,

the effect of desiccation in crops where the economic yield is vegetative

is likely to be greater than in grain producing crops during the grain-filling


Third, the availability of previously accumulated reserves can substantially protect yield during desiccation. They may also represent a potential

resource for increasing yield under favorable conditions.

Fourth, the physiological factors most likely to be limiting during one

part of the season may be unimportant during another part of the season.

For cereal grains, the vegetative phase of growth is probably limited more

by cell enlargement than by other factors unless drought is severe. During

grain development, however, grain production is probably affected most

by the photosynthetic activity of the leaves. The relatively brief flowering

period between these stages is important largely because of the potential

for disruption of floral development, anthesis, fertilization, and the number

of seeds set.

Timing, then, is very important and efforts to find superior performance

of certain physiological types may be frustrated unless this is taken into

account. It does little good to breed for improved photosynthetic activity,

for example, if yield is limited by the effects of early drought on cell enlargement. For an environment in which drought is sporadic, the problem

of timing is most difficult, as results could suggest superior performance

in one season but inferior performance in another. Thus, it would seem

that breeding for improved performance on the basis of field experiments

will have the greatest success in those areas where drought occurs in the

same part of the growing season year after year. Management, like breeding, will be most effective if it is based on a sound understanding of the

relative timing of environmental demands and crop sensitivity. Decisions

of what crop to plant in given environments, and when to irrigate, should

be made against the background of such information.

Unfortunately, the improvement of plant response to drought has been

rare, and the writers are aware of only one instance where selection or

breeding has succeeded in improving the tolerance of crop varieties to

drought (Wright and Jordan, 1970). In this instance, the selection criterion

was somewhat specialized and was based on seedling survival during a

drought following germination. This approach may or may not have an

effect on grain production.

At this time, with our limited and inadequately integrated knowledge

of plant performance under desiccating conditions, any suggestion of how

to aim a plant improvement program must be tenuous at best. However,

it may be helpful to speculate on the problem at this point because such

speculation may provide some insight.



The ability of a crop to produce a high yield of grain during a dry season

probably depends on two fundamentally different phenomena, which may

be thought of as drought avoidance on the one hand, and drought tolerance

on the other.

Drought avoidance permits a crop to grow longer in a given environment, usually because it is able to tap a larger part of the water stored

in the soil (by having a more extensive and well placed root system), or

because it uses less water per unit time. Water use rates can be affected

both by supply and by demand. Thus, the penetration of the soil by roots

and the resistance to water loss by the canopy can have effects on drought

avoidance. In a pot experiment, Passioura (1972) forced wheat plants to

rely entirely on one seminal root early in the season. The treatment resulted

in double the amount of water being available at heading and the plants

produced double the yield. Water loss by evapotranspiration from a crop

can also change dramatically with changes in the canopy. Ritchie and Burnett ( 197 l ), for example, found that evapotranspiration was substantially

below potential rates in cotton and grain sorghum until the canopies had

developed a certain amount of ground cover. Kerr et al. (1973) found

similar effects where the evapotranspiration of a developing maize crop

with incomplete ground cover was less than half the rate of an adjacent

lucerne crop. The characteristics of stomata and associated diffusive resistance to water loss have received considerable attention in recent years

and it is well established that they play a role in regulating water loss.

The importance of these characteristics in regulating water use rates of

field crops has yet to be established. However, recent work indicates that

the substantial differences in diffusive resistances that occur among crops

can correlate highly with measured evapotranspiration rates. Kerr et al.

(1973) found a correlation of 0.89 between measured stomatal resistance

for maize, paspalum, and lucerne and the resistance to crop evapotranspiration based on measurements of half-hourly evapotranspiration rates.

Unfortunately, drought avoidance characters are often developed at the

expense of photosynthesis. For example, delaying canopy closure reduces

interception of photosynthetically active radiation and may thereby reduce

rates of photosynthesis per unit ground area; stomatal closure may inhibit

carbon dioxide uptake as well as water loss; and larger root systems can

only be developed at the expense of top growth. It would be preferable

to identify characters which would not result in a sacrifice of plant growth.

Drought tolerance is potentially more desirable from this point of view,

since it would permit a crop to produce more yield at a given tissue water

potential. It seems to us that there may be two possible ways of improving

drought tolerance in cereal grains. The first would consist of selection for

the capacity of cell elongation in seedlings that were subjected to a steady,



but suboptimal water availability. A vermiculite system similar to that used

by Meyer and Boyer ( 1972) could be employed, and seedling performance

could be judged by eye. Superior seedlings could be removed from the

vermiculite, and planted for seed. Since screening could be based on visual

criteria, large numbers of individuals could be processed rapidly. This procedure would select for increased rates of cell enlargement during desiccation, and superior performance would result from the ability of the plant

to compensate osmotically for drought. Increased growth under desiccating

conditions should also select for improved rates of protein synthesis and

nitrate reductase activity since these are generally positively correlated with

high rates of growth. Additional benefits would be increased seedling emergence in dry soil and continued leaf growth during moderate drought.

There is also a possibility that elongation of stamens, styles, and possibly

germination tubes of pollen grains could be enhanced if the effects of seedling selection carried over to flowering.

The second approach to selecting for superior performance would involve the growth of plants to an intermediate stage of development, perhaps

with several leaves, and the imposition of a drought that could be maintained for several days. The plants would then be rewatered and scored

visually for signs of leaf senescence. Those plants that showed less senescence would be used as the seed source for the next generation. This level

of selection should retain those plants capable of continued production,

or at least those with less death of tissue, under desiccating conditions.

For cereal grains, these two levels of selection for drought tolerance

might improve production in two ways: they should promote growth under

moderately dry conditions and reduce the tendency for senescence (which

is so characteristic of the grasses) in severe conditions. The criteria for

selection are predicated on the assumption that there will be at least sporadic increases in the availability of water and that the crop will be protected by the farmer against the severest droughts. Thus, the production

of leaves and the lack of loss of leaf tissue would keep the photosynthetic

tissue capable of production when rain came. At the same time, of course,

this represents a compromise because selection would be made against the

natural tendency for the grasses to fill a small amount of grain while leaf

surface senesces. The net result would be an increased production if water

were restored, but an increased susceptibility to very severe droughts.

In the native environment, survival in dry conditions may require the

production of a few seeds for the next growing season to ensure the continuation of the species. Since desiccation can rapidly become severe and

metabolic activity may be inhibited or altered at that time, the genetic

mechanisms that control leaf enlargement and senescence must respond

rapidly. For the plant, this means that the production of at least a few



seeds is assured. Agriculturally, however, severe desiccation represents a

very small percentage of the total instances of drought. Furthermore, the

economic effects of drought become important long before production is

reduced to a few seeds. Therefore, breeding for increased leaf growth and

decreased senescence could have a positive effect on agricultural production

if it reduced the effects of mild or moderate drought. Furthermore, the

approach would have the advantage that it would foster high yields when

water availability was high.

It has been suggested (Mederski and Jeffers, 1973) that rather than

selecting for drought performance under drought conditions as proposed

above, it may be possible to select under optimum growth conditions.

Mederski and Jeffers (1973) found that the yields of existing varieties of

soybeans had the same rank order regardless of whether they were grown

under moist or water-deficient conditions. While this may apply under some

circumstances, it appears that to screen for physiological characteristics

that are only called into play during drought one must select under desiccating conditions. It must be emphasized, however, that selections for seedling performance, as suggested above, should be accompanied from the

outset by extensive field testing and that selections that appear significant

at the seedling level should be continued only if they result in a clear increase in grain yield.

The use of cell elongation and leaf senescence as characters for selection

of superior drought performance appear to have particular usefulness in

rice. Chang et al. ( 1974) have shown that rice varieties capable of growing

in uplands were less subject to leaf stunting, leaf rolling, and leaf senescence than were the drought-sensitive lowland varieties. Deep rooting and

the capacity to withstand a dry spell were correlated as well. There was

less delay of heading and panicle exsertion, and spikelet fertility was higher

in the upland varieties during drought. Grain yield was generally less susceptible to drought in the upland varieties. These are suggestive of differences in cell enlargement and leaf senescence, which reflect tolerance, but

performance was also related to differences in avoidance, such as rooting

depth. Thus, rice displays both kinds of response to desiccation and it

should provide promising material for selecting for improved drought performance either in terms of tolerance or avoidance.

It is also well to note that the two-pronged approach of selecting for

increased leaf growth and decreased senescence neglects one important

factor: the photosynthetic activity of the leaves. The degree to which differences in photosynthesis might occur in desiccated individuals of a breeding

line is unknown, and the selection for less inhibition of photosynthesis

would require cumbersome measurements. Nevertheless, the photosynthetic

differences that were cited above for species and for different stages of



growth might extend to breeding lines, and it may eventually be worthwhile

to explore this area. Attempts to surmount the measurement problems have

recently been made (Nelson et al., 1974), and similar approaches may

provide advances in the future.


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S. Kiss, M. Dr6gan-Bularda, and D. Rddulescu

Babes-Bolyai University, Clui-Napoca, Romania

Introduction. .........................................................

Role of Accumulated Soil Enzymes in the Initial Phases of the Decomposition

of Organic Residues and of the Transformation of Some Mineral Compounds

A. Carbon Cycle.. ...................................................

B. Nitrogen Cycle.. ..................................................

C. Phosphorus Cycle..................................................

D. Sulfur Cycle.. ....................................................

111. Enzymatic Activities in Soil under Conditions Unfavorable for the Proliferation

of Microorganisms.. ...................................................

A. Physical Factors ...................................................

B. Chemical Factors..................................................

IV. Summary. ............................................................

References. ...........................................................
















Enzyme activity of soil results from the activity of accumulated enzymes

and from the enzymatic activity of proliferating microorganisms. By definition, accumulated enzymes are regarded as enzymes present and active

in a soil in which no microbial proliferation takes place. Their amount

in terms of weight is very small.

Sources of accumulated enzymes are primarily the microbial cells. Enzymes in soil, however, can also originate from plant and animal residues.

Enzymes accumulated in soil are free enzymes, such as exoenzymes released from living cells, endoenzymes released from disintegrated cells, and

enzymes bound to cell constituents (enzymes present in disintegrating cells,

in cell fragments, and in viable but nonproliferating cells). Proliferating

microorganisms produce enzymes that are released into the soil, while

others remain within the multiplying cells.

Free enzymes in soils are adsorbed on organic and mineral soil particles

and/or complexed with humic substances. The amount of free enzymes

in the soil solution should be much smaller than in the sorbed state. Cells

and cell fragments also may exist in an adsorbed state or in suspension.






In sd solution


In adwxbed state


In suspension

FIG.1. Components of the enzyme activity in soil.

Components of the enzyme activity of soil' can be classified as shown

in Fig. 1 .

Activity of most soil enzymes is assayed in samples in which the proliferation of microorganisms is prevented by the addition of toluene or the

microorganisms are killed by irradiation with 7-rays or an electron beam.

Enzyme activity determined under these conditions is due to the accumulated enzymes. Dehydrogenase activity in soil is assayed without preventing

microbial proliferation. Consequently, the measured activity is due to dehydrogenases primarily of the proliferating microorganisms.

It is well known that perpetuation of life on our planet is conditioned

by the mineralizing action of soil and water microorganisms on the plant

and animal residues. It is also well known that the mineralizing action of

microorganisms is inseparably related to the activity of enzymes. However,

do the enzymes accumulated in soil play a role in decomposition and mineralization processes, or are these processes attributable exclusively to the

proliferating microorganisms? In other words, do the accumulated soil en-

' Presumably, the enzyme activity of water and mud comprises the same components as that of the soil. It is worth noting in this respect that free, dissolved enzymes

(invertase, amylase, cellulase, lipase, protease, phosphatase) have been found in lake

waters (Steiner, 1938;Overbeck and Babenzien, 1963, 1964; Reichardt ef al., 1967;

Berman, 1969, 1970; Jones, 1971, 1972; Berman and Moses, 1972; Reichardt, 1973;

Wunderlich, 1973 ) and in sea waters (Goldschmiedt, 1959;Strickland and Sol6rzan0,


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