Abstract
It has hitherto been considered that even if the number of spikelets per unit field area is excessive, the percentage of ripened grains may be reduced, but the yield will be nearly constant so long as the plants hold the constant amount of carbohydrates. The present paper, however, has clarified that in fact there is an optimum number of spikelets per unit field area and an optimum percentage of ripened grains equivalent to the amount of carbohydrates per unit field area for maximizing the yield. An optimum number of spikelets per unit field area is represented as a linear function of the amount of carbohydrates but an optimum percentage of ripened grains is found to be nearly constant (about 80 to 85 %) for any amount of carbohydrates to obtain the highest yield. So, the degree of the percecentage of ripened grains can be taken as an index of the diagnosis of the defect in rice cultivation. When the rice plants have less number of spikelets than an optimum, their grain yield decreases in proportion to the number of spikelets, however high the percentage of ripened grains may be, and the amount of carbohydrates which remaines in shoot at maturity increases in inverse proportion to the number of spikelets. While, when the rice plants have more number of spikelets than an optimum, their grain yield decreases in inverse proportion to the number of spikelets, because of the number of imperfectly ripened grains being increased. The main reason for this fact can be explained by that as the number of spikelets increases, the given amount of carbohydrates must be shared with the more numerous spikelets, causing a more than proportional increase in number of grains which have an insufficient share of carbohydrates and consequently resulting in proportional decrease in the number of fully ripened grains.
