Response of differentiated and degenerated spikelets to top-dressing, shading and day/night temperature treatments in rice cultivars with large panicles

The presence of large panicles enables to reach a sufficient number of surv iving spikelets per unit area for further increase of rice yield. However, the response of differentiated and degenerated spikelets to environmental f actors in terms of number in cultivars with large panicles has not been elu cidated. By applying top-dressing at different rates and frequencies, shadi ng, and day/night temperature treatments in pot experiments, we observed th e response of the main stem in two cultivars with large panicles: Yangdao 4 (Chinese indica) and Akenohoshi (Japanese japonica-indica hybrid). The res ults showed that top-dressing increased the number of differentiated spikel ets per panicle and decreased the percentage of degenerated spikelets. Cons equently, the number of surviving spikelets per panicle increased. Shading increased the percentage of degenerated spikelets, resulting in the decreas e of the number of surviving spikelets per panicle. Higher day/night temper ature treatments led to a higher percentage of degenerated spikelets, The t reatments affected mainly the spikelets on the secondary branches, while th e number of spikelets on the primary branches did not change appreciably, i rrespective of the presence of differentiated, degenerated, and surviving s pikelets. The influence on the number of surviving spikelets per panicle de pended on the number of differentiated spikelets. Shading effect on the per centage of degenerated spikelets also depended on the number of differentia ted spikelets per panicle. The decrease of the percentage of degenerated sp ikelets was associated with the increase of the shoot dry weight (SDW) at h eading and the ratio of SDW-differentiated spikelets, The SDW and N absorpt ion at heading affected more significantly the number of surviving spikelet s compared to the ratio of surviving spikelets-SDW and surviving spikelets- N, respectively. The above results were similar in Akenohoshi and Yangdao 4 . In Akenohoshi, the percentage of degenerated spikelets responded much mor e conspicuously to the change of SDW or the ratio of SDW-differentiated spi kelets than that in Yangdao 4. So did the number of surviving spikelets to SDW and N absorption at heading. Akenohoshi showed a higher ratio of surviv ing spikelets-N than Yangdao 4. These differences may account for the fact that Akenohoshi showed a larger number of surviving spikelets per panicle t han Yangdao 4, even at the same levels of SDW and N absorption.