Abstract
In 1998, premature heading was observed in the early season culture field of the extremely early rice cultivar ‘Tosapika’ in Kochi Prefecture. To clarify the factors associated with that, the effect of temperatures (constant temperature:25 and 20°C) under natural daylength around the summer solstice (long day:14.15-14.43 hours) and after the autumn equinox (short day :10.32-11.53 hours) on the differentiation and development of young panicle was examined using the seedlings raised in stock-raising pots (each pot size is 1.5 × 1.5 × 2.5cm) at a rate of one seed per pot. The cumulative temperature from sowing to panicle differentiation was lower in ‘Tosapika’ and its parent, Kouiku No.27 than in the cultivars bred in Hokkaido irrespective of the daylength. The effects of daylength and temperature on the panicle differentiation when examined based on the effective cumulative temperature (ECT, base temp. 10°C) were smaller than that when examined based on the cumulative temperature in each cultivar. In the seedling of ‘Tosapika’ the panicle differentiation and development were scarcely influenced by daylength and the panicle formation stage (average length; 1mm) was observed at 301-348°Cday (ECT) from sowing and at 5.3-5.7 plant age in leaf number on the main culm. It was about 20 days bef heading under 25°C condition. Mover, the ECT after sowing to flag leaf expansion, which is closely related with the earliness of cultivars was also lower in ‘Tosapika’ due to the fewer number of final leaves on the main culm in comparison with the cultivars bred in Hokkaido. From these results, it was concluded that the thermo- and photo-sensitivities and basic vegetative growth of ‘Tosapika’ were comparatively low, and these characteristics were similar to those in its parent, Kouiku No.27.
