Vergara - 1976 - Physiological and morphological adaptability of ri

336 cusmnz Ann RICE
cell elongation in seedlings that had been 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 rcductase 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 intennediate stage of development. perhaps with several
leaves. and the imposition of a drought which could be maintained for several
dajrs. The plants would then be rewatered and scored visually for signs of leaf
senescence. Those plants which 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 wa_vs: they should promote growth under moderately
dry conditions and reduce the tendency to 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 increased
production if water were restored, but 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 insure 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-g
however. severe desiccation represents a very small percentage of the total
instances of drought. Furthennore. the economic effects of drought become