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Red and blue light have significant effects on plant growth; however, most of the current studies have focused on common horticultural crops such as cucumber and tomato, and there are fewer studies on how red and blue light affect the growth of melon seedlings. Therefore, in this study, we used melon (Cucumis melo L.) as the experimental material to investigate the effects of red and blue light ratios on the photomorphogenesis and photosynthesis of melon seedlings. Five red and blue light ratios were set at a fixed light intensity 200 μmol·m−2·s−1, including R:B = 1:9, R:B = 3:7, R:B = 1:1, R:B = 7:3, and R:B = 9:1. The results showed that with the increase in red light ratios, melon seedling height, stem diameter, total leaf area and stomatal pore size of melon seedlings increased, while the upper epidermis, palisade tissue, spongy tissue, leaf thickness, and stomatal density showed a decreasing trend. Under the R:B = 7:3 treatment, melon seedlings were optimized in all morphological indexes and had higher photosynthetic efficiency; these results indicated that the growth of melon seedlings could be regulated by adjusting the ratio of red to blue light, thus promoting the morphogenesis of melon seedlings. [ABSTRACT FROM AUTHOR]