Effects of Combined Blue–Green Light Treatments on Photosynthetic Characteristics and Antioxidant Activity in Pepper Seedlings

Light spectral composition critically influences seedling physiology in protected horticulture systems. This study investigates the impact of light-quality regulation on the growth and physiological traits of pepper seedlings, using the pepper cultivar ‘GS4’. Plants were exposed to four light treatments: white light (W), blue light (B), green light (G), and combined blue–green light (BG). The results showed that BG significantly promoted the growth of pepper seedlings compared with B and G, and the seedlings treated with BG developed visibly healthier darker green leaves by day 4. Moreover, BG treatment mitigated the green-light-induced cell membrane damage, as indicated by reduced relative electrolyte conductivity, and improved the balance between CO2 assimilation and water loss through the regulation of stomatal function. The BG treatment resulted in a significantly higher net photosynthetic rate than the B and G treatments, while showing no significant difference from the W treatment. Additionally, BG treatment mitigated the inhibitory effects of single light treatments on chlorophyll accumulation and maintained stable chlorophyll a/b ratios. It also enhanced antioxidant enzyme activities, thereby reducing oxidative stress and lipid peroxidation. Furthermore, BG treatment stabilized soluble sugar and protein levels while preventing the excessive proline accumulation observed under the G treatment. Overall, under the tested short-term seedling conditions, BG light treatment improved photosynthetic performance, antioxidant activity, and metabolic stability, alleviated physiological stress associated with monochromatic light exposure, and achieved performance comparable to that of white light.