Exogenous Melatonin Compensates for Moderate Nitrogen Reduction in Upland Rice by Optimizing Leaf Nitrogen Allocation and Photosynthetic Metabolism

Abstract

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.