Response of rice megavarieties to drought stress

Drought limits rice production in rainfed environments, with the reproductive stage being the most sensitive. Popular varieties are drought-sensitive, but it is not clear whether the same mechanisms are involved. The authors initiated a greenhouse study of ten Asian 'megavarieties' (IR36, IR64, IR66, BR11, Swarna, Mahsuri, Samba Mahsuri, KDML105, RD6 and Theedat Yin) to identify the drought-sensitive events at the flowering stage. Plants were grown in pots to discount rooting differences. Water was withheld when tagged panicles reached the third day before heading; the plants were re-watered 5-6 days later, when the relative water content of the flag leaf reached 40-55 percent. The yield per tagged panicle was greatly reduced for all varieties, with spikelet fertility being the most affected yield component. In no case was sterility due to failure of panicle exsertion but heading delay of 6-7 days was a likely factor. Fertile and sterile spikelets of each genotype were harvested at maturity and grouped according to their final stage of development on an 8-stage scale. Under well-watered conditions, sterility was low (less than 20 percent), except in the longer duration varieties KDML 105, RD6 and BR11; most sterile spikelets had progressed past anther exsertion (stage 4) but not passed ovary enlargement (stage 5). Under drought stress, sterility was greatly increased in all varieties except RD6, and a strong correlation existed between spikelet fertility under drought stress and the fraction of sterile spikelets reaching stage 4 but not stage 5. In BR11, sterile spikelets failed to reach stage 4 or even stage 3 (anther filament elongation), whereas in Swarna, IR36 and Theedat Yin sterile spikelets tended to reached stage 3 but not stage 4. Spikelets of IR64, IR66 and RD6 halted development and the role of heading delay.