Vergara - 1976 - Physiological and morphological adaptability of ri

334 CIIXIATE AND RICE
IMPROVEMENT OF DROUGHT RESPONSE
THROUGH BREEDING AND MANAGEMENT
Tvt-‘o points of importance emerge from the foregoing. First. the various physiological
processes contributing to grain yield var_v markedly in their susceptibility
to drought. For example. cell elongation is affected by quite normal diurnal
fluctuations in plant water status, while net photosynthesis requires considerably
greater desiccation. and translocation is even less sensitive. Second. 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 effoiis to find superior perfonnance of
certain physiological types may be frustrated unless this is taken into BCCIJLIHI.
It docs 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,
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 based on a sound
understanding of the relative timing of environmental demands and crop
sensitivity. Decisions as to what crop to plant in given environments. and when
to irrigate, should be made against the background of such information.
Ilnfortunately. 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 gennination.
This approach may or may not have an effect on grain production. Developments
that have occurred during the past decade or tw-"o do indicate. however,
that we can expect more effective progress in the future. The recognition that
the soil-plant-atmosphere continuum (Philip. 1966; Kramer. 1969) must be
considered as a complex system has emphasized the advantage of determining
plant- rather than stiil-tvater status and has shtiwn that plant-water status is a
function of the evaporative demand of the atmosphere as well as the ability of
the soil to supply water. Effective methods of measuring plant-water status
have recently become available (Boyer, 1969), and the physiological significance
of a particular plant-water status has begun to emerge.