Sterility of Thermo-Sensitive Genie Male Sterile Line, Heterosis for Grain Yield and Related Characters in F_1 Hybrid Rice (Oryza sativa L.)

The thermo-sensitive genie male sterile (TGMS) T29^S line was sterile when exposed to daily mean temperatures of 24.1℃ or above during the critical stage (from 15 to 11 days before heading). It was completely sterile for both pollen and spikelet when the plant headed from June 16 to November 7 in Okinawa, because the temperature exceeded 24.1℃. The heterosis of the F_1 hybrids between the T29^S line and seven indica cultivars was examined. Most of the F_1 hybrids showed positive heterosis over the male parent for grain yield per plant and the number of spikelets per panicle, and one of them manifested heterosis for grain yield over a F_1 hybrid from a cytoplasmic male sterile (CMS) line. Dry matter accumulation per plant at the panicle initiation stage in most F_1 hybrids was higher than that in the respective male parent or mid-parent, and it was correlated with a larger number of tillers and leaf area per plant. All F_1 hybrids produced a larger number of panicles per plant than their respective male parent. Positive heterosis over the male parent for the number of filled grains per panicle, 1000-grain weight and harvest index was obtained in several F_1 hybrids. A positive correlation was found between grain yield and the dry matter accumulation per plant in F_1 hybrids. Both the larger number of panicles per plant and the larger number of spikelets per panicle were more important for the positive heterosis for grain yield in F_1 hybrids rather than the higher 1000-grain weight. Among the yield attributes, a larger number of filled grains per panicle mainly contributed to a higher grain yield of F_1 hybrids