Vergara - 1976 - Physiological and morphological adaptability of ri

WATER DEFICITS IN CEREAL GRAINS 335
Because of the newness of our 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 a certain insight.
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
ivater per unit of 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 plant produced double the yield. Water loss by
evapotranspiration from a crop can also be quite dramatically altered by appropriate
changes in the crop. Ritchie and Burnett (1971). for example, found
that evapotranspiration was substantially btBlOW 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 evapotranspiratiiwn of a
developing maize crop with incomplete ground cover was less than half the
ratc ol‘ an adjacent luecrnc 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 offield
crops has yet to be established. Iloivever. 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 betv/een measured stomatal resistance for maize. paspalum,
and lueemc and the resistance to crop cvapotranspiration based on measurements
of half-hourly evapotranspiration rates.
One of the problems with many drought-avoidance characters is that they
are developed at the expense of photosynthesis. For example, delaying canopy!
closure reduces interception of photosynthetically’ active radiation and may
thereby reduce rates of photosjrnthesis 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 WlllCh 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 tyvti possible ways of improving drought tolerance
in cereal grains. The first would consist of a selection for the capacity’ of