Vergara - 1976 - Physiological and morphological adaptability of ri

322 cLnuArE AND RICE
fied in the seed to produce the polysaccharides. oils, and proteins that finally
accumulate there. The vegetative plant therefore serves as a source and the
developing seed as a modifying factory for the compounds which ultimately
are found in the grain.
Viewed in this setting. the production of grain seems deceptively simple. The
highest yields should be obtained when the physiological processes contributing
to yield match the capacity of the environment to sustain each process. The
environment then controls the yjield. and the role of the plant breeder should
be essentially’ one of maximizing the performance of the physiological process
most limited hy the particular environment in which the crop is to he grown.
Thus. the plant breeder is called upon to manipulate the physiological character
of plants. and he requires a knowledge of the character which controls perfonnance
most closely.
In practice, of course. this goal is seldom achieved. Breeders find themselves
with the need to improve yields, hut they often lack adequate infonnatitin on
the physiological factors limiting yield. Consequently. physiological questions
frequently must be bypassed and the selection of superior plants must be based
primarily on the yield criterion.
This more empirical approach. while sometimes necessary because of urgency.
suffers inevitable disadvantages. As far as drought is concerned, perhaps
the most serious is that plant sensitivity to desiccation varies with the stage of
growth. Furthermore, plant desiccation can occur at different times during the
growing season and can be caused by inadequate soil moisture or by high evaporative
demand. With this kind of variability’. it becomes obvious that the interpretation
of standard yield experiments can be difficult or impossible. It is
necessary to understand the physiological limitations to grain yield during
drought in order to overcome this problem. It should then he possible to incorporate
physiological characters to protect the plant at appropriate stages of
growth. and in this way‘ to provide a degree of independence of crop perfonnanee
from the timing and nature of drought.
It is with this in mind that ive would like to devote our contribution to this
symposium to the identification of the physiological limitations of grain yield in
a specific environment-one with inadequate water. We need not dwell on the
immense significance of drought to agriculture. As this is being written, its
impact is bringing starvation to thousands in Africa. What does need to be
pointed out, hovrcvcr. is that so little is known about the physiological basis of
the problem. In a recent revieyv Yoshida (1972) was unable to find any data
that described the physiological cause of reductions in grain yield during
drought. Salter and (‘mode (I967) devote a book to the effects of drought on
production by‘ various crops. but only 2 papers of the 114 that deal with
cereal grains attempt physiological measurements which shed light on the mechanism
of the drought inhibition of grain yield.
As we are aware of feiv data that deal with the physiological mechanisms that
limit grain yield in rice during drought. this review will be devoted to other