Green Light Proportion Modulates Growth, Antioxidant Capacity, and Nutrient Utilization in Hydroponically Grown Lettuce Under Red–Blue Light

To investigate the effects of red–blue–green light ratios on the growth, photosynthesis, nutritional quality, and mineral nutrient accumulation of hydroponic lettuce, a controlled experiment was conducted using the deep flow technique (DFT) system. Four light treatments were applied under a constant photosynthetically active radiation of 350 μmol·m−2·s−1: the red–blue light control (CK, R:B = 60:40) and 10% (T1), 20% (T2), and 40% (T3) green light substitution treatments. Growth, photosynthetic characteristics, quality and nutrient indexes were assessed. The results showed that 40% green light (T3) significantly enhanced biomass production, increasing the fresh and dry weights by 33.16% and 21.40%, respectively, compared with the CK (control). In addition, the soluble sugar and vitamin C contents increased by 61.79% and 13.43%, respectively, while the nitrate content decreased by 16.31%. T1 (10% green light) was the most effective at promoting antioxidant compound accumulation, resulting in 148.61% and 81.24% increases in the flavonoid and polyphenol contents. All green light treatments increased the net photosynthetic rate and transpiration rate while decreasing the intercellular CO2 concentration and stomatal conductance. Green light supplements generally promoted phosphorus and magnesium uptake, and T3 remarkably enhanced the total accumulation of major mineral elements in shoots. In conclusion, 40% green light substitution optimizes both the biomass and nutritional quality of lettuce, whereas 10% substitution significantly promotes antioxidant accumulation. These findings provide valuable insights for optimizing light spectra in controlled environments for hydroponic lettuce production.