Physiological, Biochemical, and Omics Mechanisms Underlying Melatonin-Mediated Salinity Tolerance and Yield Stability...

Salinity stress is a critical abiotic factor limiting the growth and yield of legume crops by inhibiting photosynthesis, exacerbating oxidative damage, and disrupting the balance of carbon (C) and nitrogen (N) metabolism. This study aimed to elucidate the physiological regulatory mechanisms by which exogenous melatonin (MT) alleviates growth inhibition and yield loss in adzuki bean (Vigna angularis L.) under salinity stress. Field experiments were conducted over three consecutive years (2023-2025), comprising four treatments: control (CK), control + melatonin (CKM), salinity stress (SW), and salinity stress + melatonin (SM). Systematic analyses were performed on yield formation, physiological and biochemical responses, and the integrated transcriptomic-metabolomic regulatory network. The results showed that salinity stress significantly reduced adzuki bean yield by 25.38%, 37.37%, and 36.46% across the 3 years, respectively. Compared with the SW treatment, the SM treatment increased yield by 10.51%, 6.61%, and 11.75%. At the physiological level, SM treatment significantly increased the net photosynthetic rate ( $P_n$ ) by 5.38% and the leaf area index (LAI) by 12.48% compared to SW. Furthermore, melatonin significantly enhanced the antioxidant defense system, with SOD, POD, and CAT activities increasing by 16.50%, 13.19%, and 14.19%, respectively, in the SM treatment. Additionally, melatonin promoted nitrogen assimilation and carbon metabolism, with NR, GS, and GOGAT activities increasing by up to 23.13%, alongside enhanced activities of enzymes related to sucrose metabolism. Integrated transcriptomic and metabolomic analysis revealed that melatonin significantly activated linoleic acid metabolism, arachidonic acid metabolism, and starch and sucrose metabolism pathways. Compared with SW, the SM treatment increased linoleic acid and arachidonic acid contents by 35.2% and 27.6%, respectively. Concurrently, key carbon metabolism genes, including INV, HK, GPI, and scrK, were significantly up-regulated by 31.2%-45.7%, promoting carbon flow redistribution and maintaining C-N metabolic homeostasis. Metabolite supplementation experiments further verified that linoleic acid enhances antioxidant capacity, and its synergistic application with melatonin further promotes plant growth. In conclusion, exogenous melatonin alleviates oxidative damage and metabolic disorders caused by salinity stress by enhancing antioxidant defense, regulating osmotic balance, and promoting nitrogen uptake and transport. Furthermore, melatonin synergistically regulates lipid signaling and energy metabolism with linoleic acid, thereby maintaining C-N metabolism coordination and stabilizing adzuki bean yield.