Improving the yielding ability of rice.

Modern plant types possess high yield potential in the tropics primarily because they do not lodge and they tiller profusely. These characteristics, in combination with increased grain number, facilitate biomass distribution. Efforts to further improve yielding ability by increasing photosynthetic rate, biomass, harvest index, and yield components have not significantly increased yield. Recent studies on enhancing yielding ability through the process of grain filling are aimed at increasing the production of high density (HD) grain. HD grains show increase volume and weight and higher milling and head rice recovery. Varietal differences in number of HD grain have been identified. HD grains are mostly located on the primary branches; secondary and tertiary branches show lower grain grades. The fourth leaf from the top appears to be a liability; its removal increased the number of HD grains. Higher active solar radiation and moderate temperatures also are favorable for HD grain production. Applied N has not significantly affected HD grain, but has enhanced spikelet number and improved grain from low grade to average and from average to good, with some increases in yield. Grain size is not related to HD grain production, except that about 40 g/1000 grain is not HD. In the same range acceptable to consumers (15-30 g/1000), manipulation for HD appears promising. Varieties with different grain grades as a characteristic have been identified. Under the same growth conditions, some show HD grain, others good and average grades, from 15d after flowering. This suggests using the existing and endogenous system as a measure of potential. Factors limiting grain filling include availability of assimilates, structure of the pedicel and spikelets, and the nature of growth regulators. Physiological yielding ability combined with suitable morphological and anatomical characteristics is the emerging ideal plant type. Higher production of photosynthetic products during postflowering appears to be the major limitation to higher yield potential.