Vergara - 1976 - Physiological and morphological adaptability of ri

“tartan DEFICITS IN CEREAL slums 33?
important long before production is reduced to a few seeds. Therefore. breeding
for increased leaf growth and decreased senescence could have a positive effect
on agricultural production if it reduces the effects of mild or moderate drought.
The approach also xvould have the advantage of fostering high yields when water
availability is high.
It has been suggested (Meclerski and Jeffers, 1973) that rather than select
for drought performance under drought conditions as proposed above, it may
be possible to select under optimum growth conditions. Mcdcrski and Jeffcrs
(1973) found that the yields of existing varieties of soybeans had the same rank
order regardless of whether they were grown under moist or under waterdefieient
conditions. While this may apply under some circumstances, it appears
that to screen for physiological characteristics that are only called into play
during drought. one must select under desiecating conditions.
The use of cell elongation and leaf senescence as characters for selection of
superior drought performance may have particular usefulness in rice. Chang et
al. ( I974) have shown that rice varieties capable of grovsing in uplands are less
subject to leaf stunting. leaf rolling. and leaf senescence than are the droughtsensitive
lowland varieties. There is also less delay of heading and panicle
exsertion, and spikelet fertility is higher in the upland varieties during drought.
Grain yield is generally less susceptible to drought in the upland varieties. This
behavior is suggestive of a role for cell enlargement and leaf senescence in the
drought response of rice, but the nature of their involvement requires further
study. In view of the range in drought susceptibilitfv and its relationship to yjield
in this crop, rice should provide promising material for selecting for drought
tolerance. It must be emphasized. however. that any selection for these characters
must be accompanied by extensive field testing. Selections that are significant
at the seedling level should be continued only if they result in a clear
increase in grain yield.
Our two-pronged approach of selecting for increased leaf growth and decreased
senescence neglects one important factor: the photosynthetic activity’
of the leaves. The degree to which differences in photosynthesis might occur
in desiccated individuals of a breeding line is unknown. and the selection for
less inhibition of photosynthesis would require cumbersome measurements.
Nevertheless, the photosynthetic differences that were cited above for species
and for different stages of growth might extend to breeding lines, and it may
eventually prove worthvt-'hile to explore this area. Attempts to sunnount the
measurement problem have recently‘ been made (Nelson et al. 1974). and
similar approaches may provide advances in the future.