Melatonin pre-treatment mitigates cadmium toxicity in maize seedlings by regulating hydrological status, physio-biochemical, and hormonal responses

Melatonin (MT) acts as a multifunctional signaling regulator in plant defense against heavy metal toxicity; nevertheless, the coordinated physiological and hormonal networks governing MT-induced cadmium (Cd) tolerance in maize are still poorly understood. Maize seedlings were pre-treated with melatonin and subsequently exposed to 80 µM Cd. This study examined how root-applied MT (40 µM) alleviates Cd-induced damage by regulating water relations, antioxidant defense, and hormonal balance. Maize seedlings treated to 80 µM Cd exhibited severe growth inhibition, decreased relative water content, and disrupted root hydraulic conductivity, accompanied by elevated hydrogen peroxide and malondialdehyde levels. Pre-treatment with 40 µM MT significantly improved shoot and root biomass, increased chlorophyll pigments, and restored leaf water status. MT enhanced activities of antioxidant enzymes (SOD, CAT, APX, and POD), reducing oxidative stress, while also modulating key phytohormones (ABA, IAA, and trans-zeatin) to maintain physiological homeostasis. Principal component analysis further linked MT-induced biochemical improvements to enhanced tolerance. Taken together, MT improves cadmium tolerance in maize by strengthening the antioxidant defense system, stabilizing cellular redox balance, protecting photosynthetic and water-regulating processes, and reprogramming hormonal signaling to maintain physiological homeostasis under Cd stress. In addition, MT alleviates Cd toxicity in maize through joint adaptive mechanisms, highlighting its potential for sustainable crop protection in Cd-contaminated soils.