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III. Improvement of Drought Response through Breeding and Management
J. S. BOYER AND H. G. MCPHERSON
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.
PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS
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,
J. S. BOYER AND H. G. MCPHERSON
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
PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS
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
J. S. BOYER AND H. G. MCPHERSON
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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PHYSIOLOGY OF WATER DEFICITS IN CEREAL CROPS
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BIOLOGICAL SIGNIFICANCE OF
ENZYMES ACCUMULATED IN SOIL
S. Kiss, M. Dr6gan-Bularda, and D. Rddulescu
Babes-Bolyai University, Clui-Napoca, Romania
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. ............................................................
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.
s. KISS, M. DRAGAN-BULARDA,
In sd solution
In adwxbed state
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,