Improving plant growth, anthocyanin production and oxidative status of red lettuce (Lactuca sativa cv. Lolla Rossa) by optimizing red to blue light ratio with a constant green light fraction in a plant factory

Abstract

Previous studies have reported contrasting effects of light quality on plant growth and anthocyanin content in lettuce leaves. This study aimed to optimize the red-to-blue light ratio with a constant green light fraction to improve biomass and anthocyanin production in red lettuce (Lactuca sativa cv. ‘Lolla Rossa’) in a plant Factory. Six treatments with varying proportions of red (R, 660 nm) and blue (B, 450 nm), along with a constant fraction of green (G, 540 nm) light, recorded as R125:B25:G50, R120:B30:G50, R112:B37:G50, R100:B50:G50, R75:B75:G50, and R50:B100:G50 were set up in this study. The total photosynthetic photon flux density for all treatments was 200 μmol m−2s−1 with a green light intensity of 50 μmol m−2s−1. The results showed that, compared with treatment R50:B100:G50, the fresh and dry weights, leaf length, leaf area and stem width of lettuce plants under treatment R125:B25:G50 showed an increase of 73.4 %, 47.8 %, 35.5 %, 54.9 %, and 58.3 %, respectively. Additionally, red light was found to promote increased weight of lettuce plant roots. Further, the anthocyanin content, nitrate content, and stomata density under treatment R50:B100:G50 were 6.75, 1.5, and 1.42 times higher than those under treatment R125:B25:G50. Enzymatic activities such as hydrogen peroxide (H2O2), superoxide dismutase (SOD), ascorbic acid (AsA), and anthocyanin concentration per fresh weight of plant were higher under treatment R100:B50:G50, comparing with those under treatment R125:B25:G50. The above results indicate that a higher red-to-blue light ratio enhances plant biomass, photosynthesis, and certain nutrients, while increasing the blue ratio promotes anthocyanin accumulation and antioxidants, protecting cells from oxidative stress. Based on this analysis, treatment R100:B50:G50 can be considered as the suitable light spectrum for optimal growth, anthocyanin accumulation and antioxidant activity of lettuce plants. Further researches are needed to explain these effects and refine lighting methods for optimal lettuce production and nutritional quality.