Exogenous Melatonin Confers Thermotolerance in Water Spinach by Modulating Redox Metabolism and the Ascorbate-Glutathione Cycle.

Aims High-temperature (HT) stress significantly impairs plant growth by impairing antioxidant enzyme activities and the photosynthetic apparatus. Melatonin (ME) is a novel plant hormone that plays diverse roles in regulating tolerance to abiotic stresses. This study examined the role and potential of ME in mitigating the negative impacts of heat stress on water spinach plants. Methods Different concentrations of ME (50, 100, 150, and 200µM) were foliar-sprayed for 12 days (every 3 days). After ME pretreatment, water spinach plants were exposed to heat stress (40 °C) for 96hours. Results HT stress dramatically decreased biomass production by hindering pigment concentration, lowering gas exchange parameters, impairing root architecture, and altering antioxidant enzyme activities. In contrast, foliar application of ME (100µM) significantly improved biomass production, photosynthetic performance, and root architecture. In addition, ME (50-200µM) effectively reduced the accumulation of H2O2 and O2•-, restored cellular membrane integrity by reducing MDA accumulation, and regulated the ascorbate-glutathione cycle. Furthermore, foliar application of ME considerably increased secondary metabolite concentrations (anthocyanin, flavonoids, and phenols), and these secondary metabolites strongly controlled ROS overproduction. Conclusions Our study found that ME (100µM) may effectively mitigate heat-induced stress in water spinach by enhancing the photosynthetic machinery, regulating redox homeostasis, reducing cellular membrane damage, and modulating antioxidant defense and the AsA-GSH cycle.