Elevated temperature accelerates leaf senescence and promotes leaf nitrogen transport to improve rice grain protein synthesis

Elevated temperatures during grain filling severely constrain rice yield and quality. Although additional nitrogen can mitigate the adverse effects of elevated temperature, both factors increase grain protein content, and the underlying physiological mechanisms remain poorly understood. Here, we conducted actual field warming (2.28°C day/4.33°C night) during the grain-filling period and applied an additional 60 kg N ha-1. The grain weight of superior spikelets (SS) was 2.1% lower under elevated temperature (ET), while ET increased the grain weight by 4.23% and protein content by 2.95% in inferior spikelets (IS). ET promoted the level of free amino acids, improved the activities of glutamine synthetase and glutamate synthase, and up-regulated the expression of an amino acid transporter gene (OsLHT1), mainly 9-15 days after flowering (DAF), in the leaf, ultimately accelerating leaf senescence post 20 DAF. Regardless of the temperature, the effect of nitrogen on leaves was similar to that of ET, while delaying leaf senescence and further increasing protein content in SS and IS. Metabolomic analysis further confirmed that ET accelerated leaf senescence and amino acid depletion in the leaves. Furthermore, the higher levels of L-histidine, along with increased levels of stress-responsive metabolites (D-raffinose and gentisic acid), collectively contributed to the improved protein content in IS under ET. Overall, the study provides insight into grain protein accumulation under warming.