Effect of different levels of light and drought stress on individual spikelet filling in rice (Oryza sativa L.)

Reduction of natural light during spikelet filling reduced the number of filled spikelets per panicle and also reduced single grain weight in both IR58 and IR25588. Percent sterility increased and harvest index was reduced with decreasing amount of sunlight available. Reduction by 85% of natural light during spikelet filling resulted in drastic reduction in single grain weight as well as total yield of both varieties. Among all, the top, middle and lower primary branches, spikelet number 5 was the heaviest grain and less affected by light stress. Spikelets of secondary branches were lighter than those of primary branches. Light stress reduced the number of heavy grains while number of partially filled and unfilled grains increased in both varieties. Light stress reduced the accumulation of dry matter during spikelet filling but did not reduce the spikelet filling period. The most sensitive period of grain growth that was affected by shading was 7 to 14 days after flowering. Chlorophyll content of leaf decreased with time of spikelet filling but plants under very low light had the highest amount of chlorophyll in the leaf, although spikelet filling was less. Both varieties showed similar type of response to light stress in term of yield parameters, spikelet filling, and chlorophyll content of leaf. Panicle weight and harvest index were reduced by water stress. When the plants were subjected to water stress from 1 to 10 days after flowering the filled spikelet number per panicle was reduced and percent sterility increased. The earlier the water stress was imposed, the fewer heavy grains and higher poor grains per panicle were found. Water stress lowered the individual grain weight and also increased the number of empty spikelet per panicle. Grain number 5 in a primary branch was usually the heaviest and less affected by water stress compared to other grains.