Source–sink relationships in short-duration and hybrid rice cultivars in tropical Asia

Shortening the period of rice cropping enables farmers to reduce irrigation water in tropical lowlands. However, grain yield of tropical short-duration rice is often restricted by poor grain filling, whose causes are yet unknown. This study evaluated the source–sink balance, which is closely associated with grain filling, in tropical rice. We compared the percentage of filled grains and stem nonstructural carbohydrates in a popular short-duration cultivar and a high-yielding hybrid cultivar over a wide range of spikelets m−2 by differentiating the N inputs and planting densities on a lowland farm of the International Rice Research Institute, the Philippines, during four seasons. Grain yield ranged from 2.2 to 8.6 t ha−1, with the hybrid cultivar producing consistently higher yield (by 27% on average). Compared with the hybrid cultivar, the short-duration cultivar had less stem nonstructural carbohydrates at heading per spikelet (by 24% on average) and a lower percentage of filled grains in the wet season (by 13% on average), when the source capacity was lowest. Source capacity per spikelet and stem nonstructural carbohydrates at heading per spikelet were significantly associated with the percentage of filled grains, suggesting that these indicators of the source–sink ratio modulate grain filling in tropical rice. Although the sink capacity of the short-duration cultivar was similar to, or smaller than, that of the hybrid cultivar, its limited source capacity resulted in poorer grain filling. We suggest that the role of stem nonstructural carbohydrates should be emphasized in the breeding of tropical short-duration rice.