Light-Dependent Regulation of Drought Tolerance in Cucumber Plants by Melatonin

Water availability, light spectral composition, and phytohormones jointly regulate plant morphogenesis and photosynthesis. Light quality can modify stress responses by altering endogenous regulators such as melatonin. However, it remains unclear whether light spectrum modulates melatonin-mediated protection under osmotic stress. Here, we investigated the effects of 1 μM melatonin on cucumber (Cucumis sativus L.) subjected to polyethylene glycol 6000 (PEG-6000)-induced osmotic stress under three distinct photosynthetically active radiation (PAR) spectral compositions. Our results indicate that PAR spectral composition determines both the magnitude and the primary physiological targets of the melatonin response. Under red-enriched illumination, melatonin predominantly enhanced photosystem II performance, evidenced by increased effective quantum yield of PSII [Y(II)] and electron transport rate (ETR), reduced lipid peroxidation, and altered expression of genes involved in antioxidant defense, melatonin biosynthesis, and auxin signaling. Specifically, expressions of SOD, APX, and ARF were upregulated, while SNAT expression was downregulated. Under blue-enriched illumination, melatonin effects were mainly associated with stomatal regulation: stomata remained more open, stomatal area and density increased, proline accumulated, and guaiacol peroxidase activity rose, collectively supporting water balance and gas exchange. These findings suggest that melatonin functions as a light-dependent regulator of cucumber responses to PEG-induced osmotic stress, with its physiological and molecular effects being modulated by the spectral quality of PAR.