Exogenous Melatonin Compensates for Moderate Nitrogen Reduction in Upland Rice by Optimizing Leaf Nitrogen Allocation...

Moderate nitrogen (N) reduction can lower production costs in upland rice but may constrain photosynthesis, N assimilation, and yield formation. Melatonin has the potential to regulate photosynthesis and nutrient metabolism; however, whether it improves the effectiveness of reduced N application by altering functional leaf N allocation remains unclear. A 2-year field experiment was conducted in 2024 and 2025 with four N application rates (0, 120, 160, and 200 kg N ha^-1) and foliar applications of either water or 100 μmol L^-1 melatonin. Photosynthetic performance, functional leaf N allocation, N metabolism, and grain yield were evaluated. An additional inhibition experiment using p-chlorophenylalanine (p-CPA) was also conducted. The beneficial effects of melatonin were observed mainly at N application rates of 120 and 160 kg N ha^-1. In 2024 and 2025, grain yield under N2M1 was 7.27% and 8.19% higher, respectively, than under N2M0 and did not differ significantly from that under the conventional N treatment without melatonin (N3M0). Melatonin increased the net photosynthetic rate, maximum Rubisco carboxylation rate (Vcmax), and maximum electron transport rate (Jmax). At 160 kg N ha^-1, the model-estimated proportion of N allocated to photosynthetic functions at the full heading stage increased from 73.55% to 86.79%, whereas the proportion of storage N decreased from 16.66% to 2.86%. Photosynthetic N-use efficiency (PNUE = Pn/leaf N content per unit area) increased by 11.10%, accompanied by improvements in nitrate reductase (NR) and glutamine synthetase (GS) activities and N fertilizer-use efficiency. The p-CPA treatment reduced endogenous melatonin content, photosynthetic performance, and N metabolism, whereas exogenous melatonin supplementation partially restored these responses. In conclusion, the effects of exogenous melatonin on upland rice were strongly dependent on N supply. At 160 kg N ha^-1, melatonin produced marked compensatory effects on photosynthetic performance and grain yield. These responses were associated with improved photosynthetic capacity, model-estimated functional N allocation, and N assimilation, indicating that combining melatonin application with a 20% reduction in N fertilizer has potential for maintaining upland rice yield.