Takafumi Hoshino

Analysis of Yield-Determining Process and Its Application to Yield Prediction and Culture Improvement of Lowland Rice : LXXXIII. Relation between the plant types after heading and the varieties differing in the maturation period.

In the previous paper (No. 73) the authors pointed out that the plant type after heading has a close relation to the percentage of ripened grains under luxurious growth conditions and, therefore, the study of plant types is quite necessary for maximizing the yield of rice. In the other papers (No. 58 and 66) they also clarified that the top-dressing of nitrogen at a definite growth stage makes the plant elongate a definite leaf-blade, leaf-sheath and internode, while the restriction of nitrogen supply shortens each of them. On the basis of these facts, the authors considered that under the rice cultivation in which only basical fertilizers are applied and not top-dressed at all, if the varieties differing in the maturation period are transplanted on an identical date, the effectiveness of nitrogenous fertilizers will also appear on an identical date, as a result of it, the longest leaf-blade will be found on upper nodes in short-term varieties, while it will be found on lower nodes in long-term varieties. For examining this point, the authors investigated nine varieties differing in the maturation period for three seasons, confirming the following facts. The longest leaf-blade is found on the second node from above in very short-term varieties, on the third node in short-term and medium-term varieties, and on the fourth node or fifth node in long-term varieties. In other words, the longest leaf-blade appears on the lower node as the variety becomes longer in the maturation period. As to the length of leaf-sheaths, the upper most one is always longest in most cases. But the second longest oneis found on upper nodes in short-term varieties and on lower nodes in long-term varieties as in case of leaf-blades. Since the growth of a leaf-sheath is synchronized with that of the immediate upper leaf-blade, the second longestleaf-sheath is found on the third node from above in very shortterm varieties, on the fourth node in short-and medium-term varieties, and on the fifth or sixth node in long-term varieties.

Analysis of Yield-Determining Process and Its Application to Yield Prediction and Culture Improvement of Lowland Rice : LXVII. Studies on the principles for maximizing yield and their demonstration (1).

Along the line previously reported (67, 69), the authors further studied the principles for maxmizing yield of rice and tried to demonstrate them, obtaining the following results. 1) A target number of spikelets per unit area could easily be obtained in most treatments by increasing the number of panicles, which was brought about by making the plant produce as many tillers as possible at the early growth stages. And a pretty high percentage of ripened grains could also be obtained in most treatments by decreasing the number of spikelets per panicle, controlling excessive growth and improving plant form, which were easily achieved by decreasing or stopping nitrogen supply during the critical period in which the plant form after heading is definitely predetermined. As a result, a yield of 91.9kg of brown rice per are was obtained as the maximum in the present experiment, and 8 treatments yielded more than 75kg per are. 2) A strong positive correlation was found between the number of spikelets per unit area and the amount of nitrogen absorbed by the stage of spikelet differentiation or heading time. From the correlation one can easily estimate the necessary amount of nitrogen to be supplied for producing a target number of spikelets. 3) Decreasing the nitrogen supply in the critical period, in which the plant form after heading is predetermined, made the plant proportionally decrease the amount of phosphate, potassium and silica absorbed, resulting in a decrease in the number of spikelets on the other hand, however, made the plant increase the amount of carbohydrate, resulting in favouring ripening and preventing the plant from lodging. The degree of decreasing the nitrogen supply in the critical period, therefore, must be determined by taking the number of spikelets per unit area, eventual percentage of ripened grains and the resistance of lodging of the plant into due consideration. 4) The optimum leaf-area index of the rice plant at heading stage had hitherto been reported to be about 5 to 6 under the light intnsity of near 320 cal. per day per cm2, while in the present experiment it had likely proved to be about 7 to 8 for producing a maximum dry matter after heading. The leaf-area index, therefore, seems not to be constant even under a given light intensity, but to be considerably variable according to the other factors, i. e. the form and healthiness of the plant etc. 5) From the analytical investigation of high yields in the experiment it had been clarified that the percentage of the amount of carbohydrate accumulated in straw before heading to the final grain yield was always less than 20. Accordingly, the importance of the amount of carbohydrate assimilated after heading can not be overestimated for maximizing yield. For increasing the amount of carbohydrate assimilated after heading, the rice plant must be subjected to the sufficient amount of solar radiation for 25 days just after heading. For that purpose one must find out the most sunny period in the ripening season in each place from the meteorological data investigeted for many years, and must select the variety to be used and determine the planting date so that the rice plant may effectively utilize the most sunny period for carbon assimilation after heading.