Characteristics of dry matter and grain production of the early rice in the early season culture in warm southwestern district of Japan

In recent years, the early season culture using the early cultivars has been commonly practiced in early season rice culture on the coastal areas in Kochi Prefecture. In order to clarify the yield-determining factors in this cropping type, we compared the characteristics of dry matter production and the yielding abilities of the early cultivars, 'Tosapika' , 'Natsuhikari' and 'Koshihikari' with those of a medium cultivar, 'Koganenishiki' . The period from the transplanting to the panicle formation stage was the shortest in 'Tosapika' among the early cultivars and followed by 'Natsuhikari' and 'Koshihikari' . The period was shorter when the middle seedlings at the 4.1-4.6 leaf age were transplanted when young seedling at the 3.0-3.3 leaf age were transplanted in all the cultivars. The yield did not differ between the plants transplanted at the middle and young seedling stage in all the early cultivars. The highest brown rice yield was observed in 'Koshihikari' due to a higher percentage of ripened grains and higher number of spikelets per m(2). Significant differences in the brown rice yield were not observed among the other three cultivars. 'Tosapika' was comparable with 'Koshihikari' in the number of spikelets, but had a lower percentage of ripened grains and 1000-grain weight. The top dry weights of early cultivars were lighter than that of 'Koganenishiki' through the whole growth duration. However, early cultivars showed the higher crop growth rate due to the higher net assimilation rates and the higher dry matter partitioning ratios to the panicle during ripening period. On the other hand, the sink/source ratio (number of spikelets per m(2)/leaf area index at full heading stage) was the highest in 'Tosapika' among the cultivars and which might be closely related with the lower degree of grain filling (percentage of ripened grains x 1000-grain weight). It was suggested that the increase of grain filling by controlling source size is necessary for attaining the higher stable yield of 'Tosapika', an extremely early cultivar, in the warm southwestern district of Japan.