Carbon isotope discrimination and yield of upland rice as affected by drought at flowering

Field experiments involving upland rice genotypes, sown in various dates in late season, were carried out to assess the relationship of carbon isotope discrimination with grain yield and drought resistance. In each one of the three years, one trial was kept under good water availability while other suffered water shortage for a period of 18-23 days, encompassing panicle em ergence and flowering. Drought stress reduced carbon isotope discrimination measured on soluble sugars (Deltas) extracted from stem uppermost internod e at the end of the imposition period, but had relatively less effect on bu lk dry matter of leaves, sampled at the same period, or that of uppermost i nternodes and grains, sampled at harvest. The drought-induced reduction in Deltas was accompanied of reduced spikelet fertility and grain yield. In th e three trials subjected to drought, genotypes with the highest yield and s pikelet fertility had the lowest Deltas. However, this relationship was wea k and it was concluded that Deltas is not a sufficiently reliable indicator of rice drought resistance to be useful as a screening test in breeding pr ograms. On the other hand, grain yield and spikelet fertility of genotypes which were the soonest to reach 50% flowering within the drought imposition period, were the least adversely affected by drought. Then, timing of drou ght in relation to panicle emergence and to flowering appeared to be a more important cause of yield variation among genotypes than variation in Delta s.