RESPONSE OF SPIKELET NUMBER TO PLANT NITROGEN CONCENTRATION AND DRY-WEIGHT IN PADDY RICE

Spikelet density, defined as number of spikelets per unit area of crop, is a major determinant of paddy rice (Oryza sativa L.) yield. Nitrogen accumulation at the panicle formation stage is closely associated with spikelet density, but regional variation has been observed in this relationship. Elucidating the reason for this interregional variation will improve our understanding of spikelet density response to N nutrition. A number of functions were tested to identify an appropriate relationship between N accumulation at the panicle formation stage and spikelet number in japonica rice. The data were collected from 157 field experiments conducted in widely different regions in Japan. The correlation with total N accumulated at the panicle formation stage was significant, but relatively low (r2 = 0.64). A considerably larger part of variation (r2 = 0.84) could be accounted for by a model based on a linear function of plant N concentration and an asymptotic function of aboveground biomass at the panicle formation stage. The standard error of the estimate was 3309 spikelets m-2 for the set of data that ranged from 10 500 to 46 400 spikelets m-2. This model can serve to give a fair estimate of potential grain number in japonica rice. The two distinct responses indicated that vigorous biomass growth with heavy dilution of plant N concentration up to the panicle formation stage could result in inefficient use of N for spikelet formation.