Spikelet Sterility is Associated with a Lack of Assimilate in High-Spikelet-Number Rice

New rice (Oryza sativa L.) genotypes bearing a high spikelet number (HS) occasionally cannot show their high yield potentials due to the lower grain-filling rate. Our objectives were to evaluate the effect of spikelet sterility on grain filling in HS cultivars and to determine whether the assimilation factor around anthesis is related to spikelet sterility. Two japonica x indica and one japonica x javanica HS cultivars were grown and compared with single-standard indica and japonica rice cultivars. Lower spikelet fertility (higher sterility) was accompanied by a lower filling percentage of spikelets (observed/potential grain yield) across cultivars and within the HS cultivars bearing different spikelet numbers as a function of N fertilizer treatments. To increase assimilate supply into the whole or lower part of the panicle, plant density was halved at heading or the upper half of the panicle was removed during the middle-heading period. The halved density, and the removal of the upper panicle in particular, significantly increased spikelet fertility by 1.3 to 1.5 times in the lower part of the panicle in the HS cultivars. The spikelet fertility in HS cultivars could easily be decreased under adverse conditions of assimilation around flowering. The removal of the upper panicle in the HS cultivars significantly increased by 1.3 times the pollen count on the pistil in the low panicle position. These results suggest that spikelet sterility in the lower part of the panicle lowers grain-filling in the HS cultivars and that inadequate assimilate supply around flowering results in spikelet sterility through a fault in the pollination process.