Plant growth stage and melatonin concentration dependency together drive the metal-nutrient dynamics of rice in paddy soil

Foliar application of melatonin shows promise in alleviating oxidative stress in rice, though its influence on metal-nutrient dynamics remains unclear. This study investigated the optimal dosage, timing, and concentration of melatonin for regulating elemental uptake, maintaining redox homeostasis, and managing nutrient dynamics in rice cultivated in cadmium (Cd) and selenium (Se)-enriched soils. Melatonin (50, 200 µM) was applied at vegetative stages: jointing (J) and tillering (T). At the J stage, melatonin improved biomass and photosynthetic pigments but inadequately regulated metal-nutrient dynamics due to incomplete redox homeostasis. However, applying 200 µM melatonin during the T stage significantly (<i>p</i> < 0.05) enhanced Se and iron (Fe) root uptake by 48% and 11%, respectively, while also improving shoot translocation. Notably, M200 reduced chromium (Cr) translocation to shoots by 82% (<i>p</i> < 0.05), thereby increasing root retention capacity. Additionally, 50 µM melatonin reduced root Cd uptake by 54% and increased its translocation to shoots by 53% (<i>p</i> < 0.05), alleviating root toxicity and enhancing the detoxification response in aerial tissues. Melatonin application reduced oxidative stress markers, increased proline levels, and enhanced antioxidative enzyme activities, with M200 at the T stage showing pronounced effects. This strategy represents a promising technological approach for managing elemental homeostasis in rice cultivation.