Abstract
An increase in rice yield in warmer areas in Japan is limited by the low number of spikelets per unit area, for which a lower efficiency in spikelet production per unit plant nitrogen (N) has been considered to be the main reason. As a basis for breaking through the lower N efficiency, differentiation of spikelets and rachis-branches and their degeneration were investigated in relation to plant N conditions for Nipponbare rice subjected to various N treatments around the reproductive stage. A total of 28 N treatments were applied at different rates and times in 2 cropping seasons in each of 1988 and 1989. The number of differentiated spikelets per square meter ranged form 17800 to 56600 of all the treatments. These differing results were highly correlated with two factors : the amount of plant N at the early stage of spikelet differentiation (ESD) and the change of plant N concentration between the panicle initiation (PI) and ESD. The increase in the N concentration between PI and ESD significantly promoted the number of differentiated spikelets through the promotion of secondary rachis-branch differentiation on a primary rachis-branch. It was suggested that, in warmer areas, luxurious dry matter production after PI rapidly lowers the plant N concentration, which results in lower efficiency in spikelet production per unit N absorbed.
