Vergara - 1976 - Physiological and morphological adaptability of ri

WATER DEFICITS IN CEREAL GRAINS 323
cereal grains. and we will restrict our discussion to those cereals in the family
Gramineae. Some experiments. conducted since Salter and Goodefls book appeared,
bear on the problem notably in wheat (from the laboratory of 1.17.
Wardltuv) and in our work with maize. The salient conclusion from the work
of both groups is that drought has a differential effect on physiological processes
known to contribute to grain yield; some processes are highly sensitive to
desiccation of the plant, others are not. Those that are most sensitive warrant
attention because of their possible usefulness for plant breeding.
In what folltnvs. we will adopt the concepts ofDonald (1962) that the economic
yield is some fraction of the total dry matter of the crop, and consequently photosynthesis.
which detemiines dry matter production, is an important determinant
of grain yield. Actually. data will be presented later in the discussion to support
this idea for maize growing under desiccating conditions. For the whole plant,
photosynthesis is determined by the photosynthetic activity of each part of
leaf and the total leaf area of the crop, as well as other factors such as leaf array
and planting density. As a first approximation. then. the essential physiological
factors controlling economic yield in cereal grains become the photosynthetic
activity of the leaves, the total area of the leaves. and the translocation of the
products of photosynthesis to the grain, which determines the fraction of the
dry weight ending in the grain. In an analysis of this sort, of course, we are
ignoring such factors as the effects of desiccation on floral development and
pollination, which deserve detailed attention in themselves. Since the cereal
grains are generally determinate in their flowering habit, this restriction has
the effect of removing only a small portion of the growing season from our
attention. albeit an important one. The effects of desiccation on floral development
and pollination will be dealt with later in this discourse.
LEAF ENLARGEMENI‘
Of all the different aspects of plant response to desiccating conditions, leaf
enlargement is the most sensitive (Boyer, I973; Hsiao, I973). Rates of enlargement
are most rapid when leaf water potentials are —l.5 to —2.5 bars, and
they decline markedly when leaf water potentials fall below these values (Fig. 1).
In maize. sunflower. and soybean, leaf enlargement was 25 percent of the wellwatered
controls or less when leaf water potentials decreased to —4 bars (Boyer.
I968; 1970a). The steepness of the decline indicates that modest changes in the
evaporative conditions or in the soil water supply will have a considerable
effect on leaf growth. In fact, the transition from night to day or a change in the
humidity of the air is frequent enough to bring about significant changes
(Boyer, 1968'. Acevedo et al., 19H),
In view of this high sensitivity, it is curious that at maximum rates of leaf
growth, leaf water potentials are -1.5 to -2.5 bars when the soil may have a
water potential of —O.] bar to —0.3 bar. Cell enlargement requires turgor to
extend the cell wall, and a gradient in water potential to bring ‘water into the