Cultivar differences in the number of differentiated spikelets and percentage of degenerated spikelets as determinants of the spikelet number per panicle in relation to dry matter production and nitrogen absorption

The plant type in high-yielding cultivars (HYC) bred in recent years has tended to change from a higher number of panicles with smaller stems and panicles (panicle number type) to a lower number of panicles with larger stems and panicles (panicle weight type or super-panicle weight type) with a concomitant increase of the contribution ratio of the spikelet number per panicle to the number of spikelets per square meter. Therefore, field experiments were conducted by using twenty-two rice cultivars with different plant types belonging to both japonica and indica ecospecies to analyze cultivar differences in the number of differentiated spikelets per panicle (NDS) and the percentage of degenerated spikelets (PDGS) which determines the number of spikelets per panicle, in relation to the amount of dry matter produced and nitrogen (N) absorbed until and / or during the panicle formation stage. The NDS, number of degenerated spikelets (NDGS) and surviving spikelets per panicle (NSS) were significantly higher in the indica cultivars than in the japonica ones mainly due to the spikelets on the secondary rachis branches. The NDS ranged from 83.9-149.6 and 133.1-229.9 in the japonica and indica cultivars, respectively. The PDGS ranged from 15.3 to 26.8% in the japonica cultivars and 7.2-38.1% in the indica ones with averages of 21.6 and 27.0%, respectively. Correlation showed that the higher NDS increased the number of both NSS and NDGS per panicle and that the latter one significantly increased the PDGS in both indica and japonica cultivar groups. The NSS was not closely related to the PDGS in both ecospecies. The amount of dry matter per productive stem (per stem) at YPS (young panicle stage: young panicle 1-2 mm long on superior stems) or heading and its increase during the period from YPS to heading were correlated with the NDS, and the amount of absorbed N per stem showed similar correlations with the NDS except at YPS. The amount of dry matter or absorbed N per stem at YPS or heading showed a similar range in both ecospecies, but for the same amount of dry matter, the NDS was remarkably higher in the indica than in the japonica cultivars, i.e., SPE-D or SPE-N (production efficiency of the differentiated spikelets per dry matter or N content) was higher in the indica than in the japonica cultivars. SPE-D or SPE-N showed a high and negative correlation with the amount of dry matter or absorbed N per stem at YPS or heading and their increase during the period from YPS to heading in both indica and japonica cultivar groups. These results imply that the increase in the amount of dry matter or absorbed N per stem at YPS and heading also increased the NDS with decreasing SPE-D or SPE-N. We concluded that the changes in the plant type from panicle number to panicle weight type may be brought about by the increase of NDS and NSS with a concomitant decrease in the spikelets production efficiency per amount of dry matter produced and N absorbed per stem irrespective of ecospecies.