OPTIMUM IRRIGATION PERIOD FOR GRAIN PRODUCTION IN SPRING WHEAT

The effect of irrigation during various periods of the development of spring wheat (Triticum aestivum L. cv. Haruyutaka) on the grain yield and yield components was studied to determine the optimum period of irrigation for increasing grain yield. The grain yield increased with the increase in the number of effective spikes per unit area which was more affected by irrigation than the one-grain weight and the number of grains per effective spike. The number of effective spikes was considerably influenced by the difference in the irrigation period. Irrigation from the beginning of tillering to the heading stage resulted in the production of a larger number of effective spikes than did irrigation during any other period and induced a higher grain yield. It can be considered that the promotive effect of irrigation during the period described above on the number of effective spikes involved the following sequence: 1) promotion of N absorption by the crop due to irrigation, 2) increase in the number of tillers in that period by the promotion of N absorption, 3) increase in the number of effective spikes derived from the increase in the number of tillers during that period. On the basis of these results, it was concluded that the optimum irrigation period for increasing the grain yield of spring wheat extended from the beginning of tillering to the heading stage.