Alternating Red/Blue Light Increases Leaf Thickness and Mesophyll Cell Density in the Early Growth Stage, Improving Photosynthesis and Plant Growth in Lettuce

Abstract

Numerous studies in plant factories show that alternating irradiation of red and blue (R/B) light promotes growth of leafy lettuce compared with simultaneous R+B (RB) light or white (W) light, even under the same daily light integrals. Here we investigate why alternating R/B light enhances lettuce growth in terms of the anatomical and photosynthetic changes in leaves. At 16 days after sowing (DAS), plants under alternating R/B light for 12/12 hours (R12/B12) had higher leaf thickness, mesophyll cell density, and leaf mass per area than did plants under RB light for 24 hours (RB24) or W light for 24 hours (W24). At 35 DAS, these anatomical values of leaves and the daily carbon gain of leaves evaluated by gas exchange analysis were higher under R12/B12 than under W24, and were similar to RB24. The projected leaf area and maximum leaf length were highest under R12/B12 across all growth stages, which would promote light capture. Marketable fresh weight under R12/B12 at 35 DAS was highest among the treatments. These results indicate that leaf morphological changes affecting the light absorption efficiency and photosynthetic rate could be the primary factors of enhanced lettuce production under the alternating R/B light condition.