Physiological basis of heterosis for grain yield in rice

Eight CMS based rice hybrids of medium and mid-early duration were evaluated along with their parents and three check varieties for various physiological parameters like shoot and root dry matter accumulation at different growth stages of the plant, spikelet fertility, panicle dry weight at harvest, harvest index and grain yield per plant. Standard heterosis and heterobeltiosis for growth and yield parameters were computed and hybrids were grouped into three categories based on heterosis for grain yield per plant. The results indicate that yield heterosis in hybrids comes through heterosis for total dry matter at different growth stages coupled with heterosis for panicle dry weight without much change in harvest index. Despite negative heterosis for spikelet fertility, due to increased number of spikelets, three hybrids showed standard heterosis for grain yield mainly due to increased number of filled grains. In all the CMS based hybrids, sink is the major limitation rather than the source because of high spikelet sterility, which may be related to restoration ability of the male parent.