A reduction in spikelet number and fertility causes yield vulnerability in high-yielding rice

In Japan, the indica × japonica rice (Oryza sativa L.) cultivars released since 1990 are high-yielding under favorable weather conditions. However, climate change may delay the end of the rainy season to midsummer and decrease irradiance in the dominant rice-growing season. The objectives of this study were to identify the critical yield component factor causing yield instability in these high-yielding cultivars under fluctuations of solar radiation. One standard-yield japonica, three high-yielding japonica-dominant, and two indica-dominant cultivars were grown in western Japan over 3 yr under contrasting radiation levels. Rough grain yield (RY) ranged from 520 to 980 g m^–2 and was positively correlated with spikelet number (SNO). The SNO decreased with a decrease in solar radiation during the panicle-formation stage; therefore, under low radiation (75% of the highest), the benefit of a high SNO disappeared in the high-yielding cultivars. A decrease in cumulative radiation during the grain-filling period decreased both RY and the spikelet-filling percentage (%F) (RY/potential grain yield [PY]) in the cultivars with high PY. %F was highly correlated with spikelet fertility (%S), where %S was lower in the high-yielding cultivars than in japonica and fluctuated. There was a higher correlation between % S and cumulative radiation for 2 wk before heading per PY. Restricting the assimilate-supply by leaf removal near the heading time significantly lowered % S. These results suggest that the limitation of the assimilate-supply due to low irradiance at the panicle-formation and flowering stages reduces SNO and %S, respectively, which restrains the potential of the high-yielding cultivars.