抄録
Growing rice plants in solution culture under community conditions and stopping nitrogen supply for 21 days at intervals of l0 days at successive growth stages, the authors studied the effects of nitrogen deficiency at different growth stages on the growth, grain yield, plant type and lodging resistance. The results obtained can be summarized as follows: 1. A maximum grain yield (6.84 tons per ha.) was obtained in treatment No.3 which was subjected to nitrogen deficiency for 21 days from 72 to 87 in the leaf-number index (50∼29 days before heading), and the yield was characterized by the highest percentage of ripened grains among the treatments. 2. As the treatment time delayed, the number of panicles per unit area increased, while the number of spikelets per panicle decreased in treatments after the stage of neck-node differentiation. The number of spikelets per unit area ran parallel to the number of panicles per unit area in all the treatments. 3. Since the effectiveness of the nitrogen depletion treatment on the plant growth such as the increase in number of tillers and in dry weight decreased progressively with the delay of treatment time, the treatment at relatively early stages was considered desirable for controlling the plant type in case of an enough number of tillers having been obtained. 4. The most suitable time of nitlogen depletion treatment for improving the plant type as well as increasing the percentage of ripened grains appeared to be the time of Treatment No.3 (72∼87 in the leaf-number index) and No.4 (77∼89 in the leaf-number index) from the viewpoint of the shortening in the length of upper 3 leaves and the controlling of the leaf-area in plants, while it was considered to be the time of Treatment No.2 (61∼78 in the leaf-number index) and No.3 from the view-point of the lodging resistance (which is closely related with the height of plant, the length of lower internodes and the number of culms with 6-th elongated internodes). 5. The high resistance for lodging in Treatment No.2 and No.3 could be characterized by the strong breaking strength at the base part (15 cm from below) of culms in the period of 20-30 days after heading and also by the big bending stress of culms at maturity. 6. On the basis of the facts above mentioned the most reasonable time for improving the plant type and increasing lodging resistance and consequently increasing the percentage of ripened grains as well as yield by means of nitrogen depletion for 21 days was considered to be the period centering around the time of Treatment No.3 (72∼87 in the leaf-number index).
