Studies on the Short Period Cultivation of Direct-Sown Rice Plant : 6. Effect of the sowing time on the plant growth and rice yield in the late seasonal cultivation

摘要

This experiment was carried out in order to make clear the relation of the sowing time to plant growth and rice yields in the late seasonal cultivation. Varieties used for this experiment were Yukara (extremely short-term variety) and Fujiminori (short-term variety). Seeds were sown at six times from late June to early August. The authors tested the growth and yields affected by variations in sowing time. The results are summarized as follows: 1) Grain yields decreased with the delay in sowing time. It was explained that the reduction in yields was caused by decrease in the dry matter accumulation in panicles and in the ripening grade (the weight of 1, 000 kernels x the percentage of ripened grains). 2) The decrease in dry matter accumulation in panicles with the delay in sowing time was due to the reduction in the rate of dry matter production during the ripening period. The reduction in the rate of dry matter production during the ripening period was due to the decrease in the net assimilation rate and the amount of leaves, and affected equally by the decrease in the net assimilation rate in the case of Yukara and by the decrease in the net assimilation and the amount of leaves in the case of Fujiminori, respectively. 3) There were no grain yields when the heading time was so late (Fujiminori, sown on 8 August) as ripening was completely disturbed by unfavourable weather conditions. The decrease in grain yields with the delay in sowing time in Fujiminori was greater than in Yukara. 4) The decrease in ripening degree caused by the delay in sowing time was due to the lowered ratio of the quantity of dry matter accumulation in panicles to the number of hulls per unit area. Namely, it was due to the fact that the decrease in the quantity of dry matter accumulation in panicles per unit area was greater than the decrease in the number of hulls per unit area.