Combined application of zinc oxide nanoparticles and melatonin improves growth, photosynthesis and redox homeostasis of cotton seedlings under salt stress

Salt stress severely limits the sustainability of cotton production. This study evaluated the synergistic potential of combined application of 100 mg L -1 zinc oxide nanoparticles (ZnO NPs) and 50 mg L -1 melatonin (MT) in mitigating salt induced injury in cotton seedlings, specifically focusing on osmotic adjustment, photosynthetic performance, redox homeostasis, and cell wall dynamics. Results showed that 200 mM NaCl stress significantly impaired plant growth, characterized by biomass reduction, photosynthetic inhibition, and ROS accumulation alongside increased cell wall rigidity. Conversely, combined application of ZnO NPs and MT effectively reversed these effects, promoting biomass recovery and accelerating third true leaf emergence. This synergistic restoration was driven by an integrated physiological response. The co-application enhanced osmotic adjustment by inducing proline and soluble sugars accumulation to maintain cellular turgor. It simultaneously protected the photosynthetic apparatus and alleviated stomatal limitations, restoring carbon assimilation efficiency. Integration of enzymatic and non enzymatic antioxidant pathways significantly lowered ROS levels, enabling strategic reallocation of metabolic resources from defense toward biomass construction. Furthermore, downregulation of pectin content optimized cell wall flexibility, providing physical support for leaf expansion. Collectively, these findings demonstrate that ZnO NPs and MT act synergistically to reconstruct physiological resilience through osmotic adjustment, redox homeostasis, and cell wall remodeling, providing a high efficiency strategy for crop production in salt alkaline environments. • Salt stress strongly impairs growth, photosynthesis, and redox balance in cotton seedlings. • Foliar ZnO NPs and MT co-application markedly enhances growth under salt stress. • Combined treatment maintains chlorophyll and enhances photosynthetic efficiency. • Synergistic effects enhance antioxidant defense and osmotic adjustment. • Modulation of cell wall composition promotes structural stability and stress tolerance.