Nitrogen and melatonin synergistically alleviate drought stress in wheat by enhancing photosynthesis, nitrogen metabolism, and antioxidant defense

• N and MT improved photosynthesis, N metabolism and remobilization, and root architecture. • N and MT enhanced wheat growth and yield by improving physiological characteristics. • N and MT alleviated the drought-induced reduction in yield and water and N use efficiency. Nitrogen (N) and exogenous melatonin (MT) have been demonstrated to effectively mitigate drought stress. However, the regulatory mechanisms underlying their interactive effects on post-anthesis drought responses in wheat remain unclear. This study explored the impacts of N and MT on photosynthesis, plant water relations, antioxidant defense system, N metabolism and remobilization, root architecture, yield, and water and N use efficiency. Compared with drought stress (DS), the co-application of 260 kg N hm -2 (N2) and 100 µmol·L -1 MT (M1) markedly increased the leaf photosynthetic rate root vitality, and root hydraulic conductivity. Under N2 conditions, MT treatment further enhanced the activities of superoxide dismutase, peroxidase, catalase, and glutathione reductase, as well as the contents of proline and betaine, thereby improving antioxidant capacity and cell membrane stability, as evidenced by the lower levels of superoxide anion and malondialdehyde. Additionally, N2 enhanced the regulatory effect of M1 on N metabolism, as evidenced by higher activities of nitrate reductase, glutamine synthetase, glutamate dehydrogenase, and elevated free amino acids content. Compared with DS, N2M1 treatment increased aboveground N accumulation and the contribution of pre-anthesis N translocation to grain N by 30.8% and 11.1%, respectively, resulting in a 20.4% increase in wheat yield. The higher water use efficiency for grain and biomass was mainly attributed to the increased root density and root hydraulic conductivity in the deep soil layer. N use efficiency increased with the rise in MT concentration, particularly under the N1 conditions. Collectively, appropriate N augmentation combined with MT can enhance drought tolerance in wheat, representing a promising cultivation management strategy to mitigate the adverse effects of post-anthesis drought.