Abstract
In 1998, the occurrence of premature heading was observed in the earlyseason culture field of cultivar ‘Tosapika’ in Kochi Prefecture. The effect of growing temperature after sowing on the heading of the extremely early rice cultivar ‘Tosapika’, bred in Kochi Prefecture was studied comparatively with 49 cultivars and lines bred in various regions from Hokkaido to Kyushu.
Pregerminated seeds were sown at a rate of one seed per pot in a stock-raising (kabumaki) pot which had a small hole at the bottom (2.5 cm depth) filled with nursery bed soil on March 10, 2000, which is the prevailing seedling time for the earlyseason culture in Kochi Prefecture. After seedlings emerged, kabumaki, pots were placed on containers, containing paddy soil with nitrogen at the rate of 1.5 g m-2 or 3.0 g m-2 and placed in the phytotron under natural light conditions with about 12 hours of day and night. Day (D) and night (N) temperatures were set at D/N of 30°C/20°C, 25°C/25°C or 25°C/15°C. Total accumulative temperature for the culture period was 1300°C days after sowing.
The percentage of headed cultivars by at the end of the culture period was highest in the plot exposed to 25°C/25°C with 1.5 g m-2 nitrogen. Heading of the cultivars bred in Hokkaido were accelerated in the plot under 25°C/25°C and with 1.5 g m-2N. ‘Tosapika’ headed when the accumulated daily temperature was beyond 1000°C days. The heading was scarcely influenced by temperature and nitrogen level. Moreover, the accumulated temperature after sowing to flag leaf expansion was also lowest in ‘Tosapika’ among the headed cultivars, because of a fewer number of final leaves on the main stem in this cultivar. These two types accumulated temperature in ‘Tosapika’ were insensitive to temperature and nitrogen level, so it was suggested that the heading day of the main stem in this cultivar could be estimated by the accumulated daily temperature after sowing.
