THE EFFECT OF PLANT NITROGEN CONDITION DU RING REPRODUCTIVE STAGE ON THE DIFFERENTIATION OF SPIKELETS AND RACHIS-BRANCHES IN RICE

An increase in rice yield in warmer areas in Japan is limited by the l ow 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 effic iency, differentiation of spikelets and rachis-branches and their dege neration were investigated in relation to plant N conditions for Nippo nbare rice subjected to various N treatments around the reproductive s tage. A total of 28 N treatments were applied at different rates and t imes in 2 cropping seasons in each of 1988 and 1989. The number of dif ferentiated spikelets per square meter ranged form 17800 to 56600 of a ll the treatments. These differing results were highly correlated with two factors: the amount of plant N at the early stage of spikelet dif ferentiation (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 differenti ation on a primary rachis-branch. It was suggested that, in warmer are as, luxurious dry matter production after PI rapidly lowers the plant N concentration, which results in lower efficiency in spikelet product ion per unit N absorbed.