Studies on the Short Period Cultivation of Direct Sown Rice Plant : 5. Effect of top dressing of nitrogen at the heading time on the yields

Abstract

This experiment was carried out in order to find out how to keep from decrease in yields caused by the delay in sowing time. The experiment plots were combined the different times of heading (late July, early and late August and early September) with the amounts of top dressing of nitrogen (0, 0.2, 0.5, 0.8 kg. per are) at heading time. Sample varieties used for this experiment were Yukara (extremely early-maturing variety) and Fujiminori (early-maturing variety). 1) The rate of matter increase in both varieties during the ripening stage showed marked increases with the increase in nitrogen supply at heading time, because a higher rate of dry matter increase during the ripening stage was resulted from the increase in leaf dry weight due to the development of leaves of the lately emerged ears in the upper nodes of plants and keeping from death of leaves. 2) The increase in dry matter production during the ripening stage led to the increase in the accumulation of dry matter in panicles, but the relation between them was not so close. Consequently, it was found that the increase in the quantity of accumulation of dry matter in panicles per unit area were not so large as increase in dry matter production per unit area. These results were thought to be ascribed to the accumulation of greater amounts of dry matter in the base of stems and to the lately emerged eares in upper nodes of plants. 3) According to the increasing rate of dry matter production, higher yields in those plots headed in late July were obtained by increase in the number of panicles per unit area due to the lately emerged ears in upper nodes, and in those plots headed on other dates high yields were obtained by a higher rate of ripening. Viewed generally, it was found that the grain yields tended to increase more with the increased supply of nitrogen. 4) The grain yields in both varieties tended to decrease more with the delay in heading time, but the effect of top dressing of nitrogen on grain yields was greater in the case of later heading time than in the case of earlier heading time. Accordingly, by the top dressing of nitrogen at heading time, grain yields were prevented from being reduced due to the delay in heading time (sowing date). 5) The yields increasing rate in the later heading time was high, as compared with that in the earlier heading time. It was due to the fact that in the case of later heading time the yields showed marked increases in comparison with the increase in dry matter accumulation in the panicles. The cause therefore can be ascribed to the fact that in the case of later heading time partially ripened grains (which occur frequently) can be transformed into fully ripened grains on account of increase in dry matter production in panicles by the effect of the additional application of nitrogen at time of heading.