Exogenous Melatonin Enhances Salt Tolerance in Radish by Regulating Photosynthesis and Physiological Traits

This study aimed to analyze the attenuating effects of melatonin on the gas exchange, photosynthetic pigments, water relationships, membrane integrity, and nutritional content of radish (Raphanus sativus L.) subjected to salt stress. The plants were exposed to three levels of electrical conductivity of irrigation water (0.50, 2.75, and 5.00 dS m⁻¹) and treated with melatonin at concentrations of 0.0, 0.5, and 1.0 mM. Physiological parameters, chlorophyll contents, relative water content, membrane stability, and nutrient concentrations in leaves and roots were evaluated. Salinity significantly reduced physiological performance, with net CO₂ assimilation dropping by up to 77% under 5.00 dS m⁻¹. However, melatonin application (0.5 mM) effectively mitigated these effects. Compared to untreated stressed plants, melatonin maintained higher stomatal conductance and preserved photosynthetic pigments, while reducing Na⁺ and Cl⁻ accumulation in leaves. These results demonstrate the potential of melatonin as a biostimulant to maintain physiological functionality and nutritional balance under salt stress, making it a promising strategy for the sustainable management of radish crops in salinized environments.