Melatonin stabilizes photosynthesis and scavenges ROS to enhance salt tolerance in cucumber

Soil salinization is a major constraint on crop productivity worldwide. Salt stress severely inhibits the growth of cucumbers by inducing oxidative damage and impairing photosynthetic efficiency. Melatonin (MT), a pleiotropic signaling molecule, potentiates abiotic stress tolerance across diverse crops; however, the mechanism governing its salt tolerance enhancement in cucumber (Cucumis sativus L.) remains unclear. This study revealed that exogenous melatonin alleviates salt stress in cucumber seedlings through combined physiological and transcriptomic mechanisms. Phenotypically, melatonin increased plant height, leaf area, and biomass under salt stress. At the physiological level, melatonin enhanced the activity of antioxidant enzymes, increased the content of ASA and osmotic adjustment substances, and reduced Na+/K+ ratio. Additionally, the application of exogenous melatonin increased the chlorophyll content in cucumbers. Transcriptomic analysis identified 1355 differentially expressed genes in response to melatonin treatment.. GO and KEGG enrichment analyses showed that photosynthesis-related pathways were significantly enriched, such as Porphyrin and chlorophyll metabolism, Photosynthesis-antenna proteins, and Photosynthesis. Weighted gene co-expression network analysis (WGCNA) identified a brown module strongly associated with melatonin treatment, which was enriched in Photosynthesis-antenna protein genes. The gene expression network results showed that the light-harvesting complex (LHC) genes of PSI and PSII were upregulated after melatonin treatment. Photosynthesis-related indicators also demonstrated that the application of melatonin enhanced the photosynthetic rate of cucumbers and reduced the content of H2O2 and O2.-. In conclusion, exogenous melatonin enhances photosynthetic stability and reduces ROS accumulation in cucumbers, providing a mechanistic foundation for improving stress resilience in horticultural crops.