Effects of Combined UV-A/B and Green Light Exposure on Growth Performance, Photosynthesis, and Antioxidant Defense in Grafted Watermelon Seedlings

Green light and UV-A/B radiation are known to influence plant growth, photosynthesis, and antioxidant capacity; however, their combined effects on grafted watermelon seedlings remain unknown. In this study, the supplemental treatment effects from the multispectral lamp (featuring green light + UV-A/B) and the conventional seedling lamp were evaluated. Their light intensities were modulated by adjusting the lamp-seedling distance (LSD) to 60, 80, or 100 cm. The results demonstrated that multispectral supplemental lighting at 80 cm LSD (delivering 1.185 μmol m–2 s–1 photosynthetically active radiation, 0.154 μmol m–2 s–1 green light, 0.7 μW cm–2 UV-A, and 0.2 μW cm–2 UV-B) significantly enhanced scion and rootstock growth while suppressing tendril development. Notably, the disease index decreased by 26.3% under the multispectral lamp at this LSD when compared with the controls, and its net photosynthetic rate was increased by 18.7%. At day 7, the seedling vigor index even exceeded that of the control by 118.87%. The total chlorophyll content and the enzyme activities of superoxide dismutase and catalase were also elevated. Thus, multispectral supplemental lighting at 80 cm LSD enhances seedling vigor and stress tolerance.