Chitosan-Loaded Melatonin Carbon Dots–Copper Nanocomposite Improves Rice Seedling Salt Tolerance via Scavenging–Regulation–Repair Trilevel Synergistic Mechanisms

Soil salinity severely limits rice growth, necessitating effective and safe mitigation strategies. Here, a chitosan-encapsulated melatonin carbon dots–copper nanocomposite (MT-CDs@Cs–Cu) was developed to enhance rice salt tolerance via a trilevel “scavenging–regulation–repair” mechanism. The nanocomposite effectively scavenges reactive oxygen and nitrogen species, activates antioxidant enzymes, and reduces oxidative damage. It suppresses ABA accumulation while upregulating aquaporin genes, improving stomatal conductance and water transport. Under salt stress, MT-CDs@Cs–Cu increases the germination rate, fresh weight, and chlorophyll content, while reducing Na + accumulation and elevating the K + /Na + ratio. Furthermore, it reconstructs the rhizosphere microbiome, enriching beneficial taxa such as Sphingomonas and Actinobacteria, and reverses salt-induced metabolic disturbances. This integrated strategy provides an effective framework for developing nanoagricultural technologies to mitigate soil salinization and enhance sustainable crop production.