Ratooning response of rice to preharvest nitrogen application under different availabilities of stem reserves

Plant N nutrition and preharvest stem nonstructural carbohydrates (NSCs) greatly influence ratoon crop yield in a riceratoon-rice system. However, their physiological relationships haven’t been unraveled. We designed this study to test whether greater rice regeneration ability due to preharvest N application is accompanied by increased stem reserves, or whether plant N nutrition and stem reserves independently influence regeneration. First, we evaluated ratooning of crops that receive N after the main crop’s late reproductive stage. Second, we imposed shade to decrease light intensity by 64 %69 % during grain filling of the main crop, and measured the effects of N application on NSCs of the main crop and ratoon crop growths in 2 years. N applied at 5 days after heading of the main crop consistently increased the regeneration ability and ratoon crop yield under non-shaded condition. It did not increase the regeneration ability under heavy shade, when only small amounts of stem NSCs accumulated. Without shade, N application at 5 days after heading increased the concentration of stem NSCs in only one of the two years, whereas the regeneration ability and ratoon crop yield increased in both years. Our results suggest that the increase in ratoon crop yield with preharvest N application requires more than a threshold amount of stem NSCs before the main crop is harvested. However, the preharvest N application can also promote tiller regeneration without further accumulation of stem reserves. N management for ratoon crops therefore depends on light conditions and the main crop’s stem reserves. The relationships between plant N, stem reserves, and regeneration ability revealed here will support improved N management for ratoon rice cultivation.