Melatonin-iron oxide nanoparticles synergy enhances salt tolerance in Mentha × piperita L

Iron is an essential micronutrient involved in chlorophyll biosynthesis, redox metabolism, and antioxidant enzyme function, and its availability becomes particularly important under salinity stress. This study evaluated the individual and combined effects of green-synthesized iron oxide nanoparticles (IONPs) and melatonin on the salt stress response of Mentha × piperita L. A controlled greenhouse experiment was conducted using a three-factor factorial design with melatonin (0 and 0.1 g L^-1), IONPs (0 and 0.5 g L^-1), and NaCl (0 and 100  mM). Treatments were applied as foliar sprays for four weeks, and plant growth traits, photosynthetic pigments, oxidative damage markers, membrane stability, and antioxidant enzyme activities were assessed. Salinity markedly reduced leaf, shoot, and root growth, decreased the chlorophyll content, and increased hydrogen peroxide, malondialdehyde, and electrolyte leakage. In contrast, melatonin and IONPs alleviated these adverse effects, with the combined treatment producing the strongest response. Under saline conditions, the combined application of IONPs and melatonin increased shoot and root growth, improved chlorophyll retention, reduced oxidative damage, and enhanced the activities of superoxide dismutase, peroxidase, and catalase relative to NaCl-treated plants. These results indicate that the combined treatment improved salt tolerance by strengthening antioxidant defense, preserving membrane integrity, and sustaining photosynthetic performance. These findings suggest that biogenic iron oxide nanoparticles, particularly when integrated with melatonin, may serve as effective nano-biostimulant tools for improving peppermint resilience under salinity stress. This study provides a physiological and biochemical basis for the use of iron-based nanomaterials and plant bioregulators in the management of salt-affected cultivation systems.