Melatonin Mediates Trade–Off in Antioxidant Responses and Nitrogen Metabolism and Benefits the Growth of Fragrant Rice Seedlings Under Light–Nitrogen Levels

This study aimed to explore the hypothesis that melatonin can benefit morphogenesis under different light − nitrogen levels by regulating the internal physiological metabolism of fragrant rice plants. A hydroponic experiment was carried out with two fragrant rice varieties (Xiangyaxiangzhan and Yuxiangyouzhan) grown under two light conditions (NL: normal light and LL: low light), two nitrogen levels (N1: reduced nitrogen and N2: normal nitrogen), and two melatonin concentrations (M1: 0 µmol·L^− 1 and M2: 100 µmol·L^− 1). The seedling growth, antioxidant response, and nitrogen metabolism parameters were investigated. The results suggested that melatonin application significantly increased the average total dry weight in Xiangyaxiangzhan (8.49%) and Yuxiangyouzhan (14.29%), whose increasement was related to the changes in distribution of dry weight in different plant tissues at different varieties. The antioxidant responses to melatonin varied between the two varieties under light–nitrogen levels. Melatonin also benefited higher activity of nitrogen metabolism enzymes leading to higher total nitrogen accumulation. The correlation analyses indicated that the dry weight was positively correlated with SPAD value, glutamate synthase activity in the stem and nitrogen accumulation. The principal component analysis suggested the fragrant rice growth and physiological responses were mediated by melatonin. Further, the growth parameters, antioxidant response parameters, and nitrogen metabolism parameters were observed to contribute to the two principal components. Collectively, exogenous melatonin could regulate the morphological and physiological performance of fragrant rice seedlings under different light–nitrogen levels, even under dual coercion, which provides a scientific reference for the development of optimized agronomic management practices.