Melatonin alleviates acid-induced stress in peanut (Arachis hypogaea L.) through physiological, antioxidative regulation and molecular defense mechanisms

Acidic stress severely restricts crop growth by disrupting nutrient uptake, redox homeostasis, and membrane stability, yet mitigation strategies remain limited. Here, we investigated the role of melatonin (MT) in regulating growth, photosynthesis, oxidative stress, antioxidant defense and proton transport in peanut seedlings under controlled hydroponics acidic (pH 4.0) and near-optimal (pH 6.5) conditions, and validated these findings in naturally acidic field soil (pH 4.3-4.5). Acid stress markedly reduced biomass accumulation, chlorophyll content, and redox balance, while enhancing ROS (H2O2) and lipid peroxidation (MDA). Exogenous MT application, particularly at 50-100 {micro}M, significantly improved shoot and root biomass, restored chlorophyll pigments and reduced H2O2 and MDA accumulation, with more pronounced effects under pH 4.0 than pH 6.5. MT strongly activated antioxidant enzymes (SOD, CAT, APX), while POD activity declined, reflecting melatonins dual role as both a direct ROS scavenger and a regulator of enzymatic redox networks. Notably, MT induced strong, dose-dependent upregulation of HL-ATPase genes (AH1 and AH2) in both leaves and roots under acidic conditions, suggesting enhanced proton extrusion, intracellular pH homeostasis, and stress adaptation. The soil validation experiment confirmed the agronomic relevance of these findings, where MT dose-dependent concentrations improved germination, vegetative growth, chlorophyll fluorescence (Fv/Fm), and yield-related traits under natural acidic conditions. Although MT also conferred benefits at pH 6.5, responses were generally moderate compared with acid stress. Collectively, these results demonstrate that MT enhances peanut tolerance to acid stress across both controlled and natural field-relevant environments, highlighting its potential application for sustainable crop production on low-pH soils.