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

BACKGROUND: Soil salinization severely constrains crop productivity worldwide. In cucumber (Cucumis sativus L.), salt stress induces oxidative damage and impairs photosynthesis. Melatonin (MT) improves abiotic stress tolerance across diverse crops; yet its salt tolerance mechanism in cucumber remains unclear. RESULTS: Under 100 mM NaCl, foliar application of 100 µM MT increased plant height, leaf area, and biomass. Physiologically, MT enhanced antioxidant enzyme activities, increased ascorbate (ASA) and osmolyte levels, and lowered the Na⁺/K⁺ ratio. MT also elevated chlorophyll content (SPAD), indicating protection of the photosynthetic apparatus. Transcriptomics identified 1,355 melatonin-responsive DEGs, with GO/KEGG enrichment in photosynthesis, photosynthesisantenna proteins, and porphyrin-chlorophyll metabolism. WGCNA revealed a brown module strongly associated with MT and enriched in light-harvesting complex (LHC) genes; LHC genes of PSI and PSII were upregulated by MT. Consistently, MT improved photosynthetic rate and reduced H₂O₂ and O₂·⁻ accumulation. CONCLUSION: Melatonin enhances cucumber salt tolerance by stabilizing photosynthesis-via LHC gene regulation-and mitigating ROS stress, providing a mechanistic basis for improving stress resilience in horticultural crops.