Phenological development, shoot dry matter production, grain yield and yield components of rice were examined in
relation to drought occurring at various stages of growth. Rice was sown three or four times at three-week intervals in the
field in each of two years, and performance in three stress trials was compared with that in corresponding irrigation trials,
with the aim of quantifying the response of the crop to water stress of 23-34 days' duration developing at different growth
stages. When drought occurred during vegetative stages, it had only a small effect on subsequent development and grain
yield. The reduction in yield of up to 30% was due to reduced panicle number per unit area in one trial, and reduced number
of spikelets per panicle in another. The effect of water stress on yield was most severe when drought occurred during panicle
development. Anthesis was delayed, the number of spikelets per panicle was reduced to 60% of the irrigated control and the
percentage of filled grains decreased in one crop to zero. This decrease in grain yield to less than 20% of the control was
associated with low dry matter production during the drought period as well as during the recovery period following the
drought. When drought occurred during grain filling, the percentage of filled grains decreased to 40% and individual grain
mass decreased by 20%. The effect of stress was also related to its severity during grain filling. Stress at this stage hastened
maturity. The results suggest that variation in yield components due to water availability is related to the variation in dry
matter production at particular growth stages. Results of a supplementary shading experiment show that the relationship
between spikelet number per panicle or single grain mass and crop growth rate was the same, whether growth rate was
varied by availability of soil water or solar radiation. Filled-grain percentage, however, was more sensitive to drought stress
than shading when comparison was made at a similar crop growth rate.