Studies on the Short Period Cultivation of Direct Sown Rice Plant : 4. Effect of the sowing time on the plant growth and rice yields

Abstract

This experiment was carried out in order to make clear the relation of the sowing time to the plant growth and rice yields. Sample varieties used for this experiment were extremely early-maturing varieties (Yukara and Sasahonami) and early-maturing varieties (Fujiminori and Koshijiwase). Seeds were sown in varied seeding density ranging over four times from the late May to early July. The authors tested the growth and yields affected by variations in sowing time. 1) Grain yields decreased linearly with the delay in sowing time, showing a decrease of 0.3 to 0.45 kg. per are per day. The decrease in yields from the yields in late May sown was at the rate of 0.5 to 0.9 per cent. This was thought that the reduction in yields was caused by decrease in the dry matter accumulation in panicles and by failure in full-ripeness. 2) The decrease in dry matter accumulation in panicles with the delay in sowing time was due to the reduction in the rate of increase in dry matter production during the ripening periods. The lowering of increasing rate in dry matter during the ripening periods was due to the decrease in the net assimilation rate and leaf dry weight, and affected greatly by the decrease in the net assimilation rate in the case of extremely early-maturing varieties and by the decrease in the leaf dry weight in the case of early-maturing ones. 3) The failure in full-ripeness by the delay in sowing time was due to the lowered ratio of "the quantity of dry matter accumulation in panicles per unit area" to "the number of hulls per unit area". namely, it was due to the rate of decrease in the number of hulls per unit area was less than that of decrease in the quantity of dry matter accumulation in panicles per unit area. 4) During the ripening period, the leaf dry weight in extremely early-maturing varieties were less than in early-maturing varieties, but net assimilation rate in extremely early-maturing ones was higher than in the early-maturing ones. As a result, no significant differeces were found in the rate of increase in dry matter weight between both varieties. Consequently, in the variation of sowing date no significant differences were found in yields between extremely early-maturing varieties and early-maturing ones. 5) Yukara showed higher yield than Sasahonami, and Fujiminori than Koshijiwase. This was ascribed to the fact that Sasahonami was very poor in ripeness (excess of hulls) and the dry matter increasing rate for Fujiminori was high throughout the entire plant growth period. There were generally a few differences in yields according to seeding density.