Dry matter production before heading and determination of number of spikelets of rice cultivar "Takanari"

Dry matter production and the determination mechanism of the number of spikelets of the semidwarf indica rice cultivar, Takanari, were examined in comparison with those of the standard japonica cultivar, Kochihibiki. Nine field experiments were conducted from 1992 to 1996 with high nitrogen application of 18 g m(-2). The average above-ground dry weight at heading (DWh) of Takanari was 1108 g m(-2), and the significant differences of DWh between cultivars were observed in three experimental plots out of four early June transplanting plots. The coefficient of variation (CV) of DWh of Takanari was 7%,, which was lower than that of Kochihibiki (11%). The average number of spikelets (Sp) of Takanari was 48400 m(-2) and the significant differences of Sp between cultivars were observed in all the experimental plots. The CV of Sp of Takanari was 5%, which was slightly lower than that of Kochihibiki (6%). Although the increasing rate of leaf area expansion under the warmer temperature caused an increase in dry matter production in the early growth period, dry matter production in the later growth period before heading was reduced because of reduced growth duration under the warmer temperature. The less-reduced growth duration of Takanari under the warmer temperature, compared with that of the photoperiod-sensitive cultivar, Kochihibiki, was thought to be one cause of the stability of DWh of Takanari. The stability of Sp of Takanari was maintained by the large increase of Sp per panicle when the number of panicles decreased. The characteristics that more secondary rachis-branches were differentiated on the primary rachis-branch of Takanari was thought to be related to the larger number of Sp per panicle and Sp of Takanari.