Enhancing saline-alkali tolerance in cucumber seedlings: The role of exogenous melatonin in redox homeostasis and stomatal function

• Exogenous MT mitigates oxidative damage in cucumber seedlings exposed to saline-alkali conditions. • MT reduces ROS accumulation, including H₂O₂ and O₂·⁻, enhancing cellular protection. • MT modulates the ascorbate-glutathione system, enhancing AsA, GSH levels, and their redox balance. • MT alleviates stress-induced stomatal closure, helping to restore normal stomatal function. • MT improves salt-alkali tolerance, promoting sustainable agriculture in salinized soils. Cucumbers, cultivated globally on 3.7 million hectares, face yield losses due to salinity, highlighting the need for effective mitigation strategies for degraded soils. Melatonin (MT) has gained significant interest for its ability to relieve plant stress. To explore the regulatory role of exogenous MT in maintaining redox homeostasis in cucumber seedlings under saline-alkali stress (SA), this study employed the cucumber cultivar 'Xinchun No. 4′. Simulated saline-alkali conditions were applied, and the effects of exogenous MT on seedling growth, reactive oxygen species (ROS) production, the ascorbate-glutathione (AsA-GSH) cycle, and changes in leaf anatomy were systematically assessed. The findings reveal that exposure to 40 mmol·L -1 saline-alkali stress significantly impaired cucumber seedling growth, reduced biomass, and led to excessive accumulation of hydrogen peroxide (H 2 O 2 ) and superoxide anions (O 2 ·⁻) in the leaves. This, resulted in increased lipid peroxidation (indicated by elevated malondialdehyde (MDA) levels), whichi further compromised the cell membrane. Application of 10 μmol·L -1 MT effectively reduced ROS levels, lowered MDA content, and mitigated electrolyte leakage. MT also enhanced AsA and GSH levels, improved AsA/DHA and GSH/GSSG ratios, and upregulated key AsA-GSH cycle genes ( CsAPX, CsAAO, CsMDAR, CsDHAR, CsGR ), leading to a significant increase in enzymatic activity. In addition, MT alleviated stress-induced stomatal closure, thereby restoring normal stomatal function. These findings suggest that MT enhances saline-alkali tolerance by mitigating oxidative damage, promoting antioxidant defenses, and effectively preserving stomatal function. Thus, our study points to a sustainable strategy to improve crop resilience in salinized environments via MT application.