Blue Light Sonata: Dynamic variation of red:blue ratio during the photoperiod differentially affects leaf photosynthesis, pigments, and growth in lettuce

Vertical farming (VF) has unparalleled capacity to highly customize plant growth environments. In VF, red and blue LED lights are predominantly used as the two main wavelengths for photosynthesis. For many plants, red light increases biomass, and blue light can increase nutritional content. Because red light is more cost- and energy-efficient to produce than blue light, refined growth recipes are imperative to mutualistically improve efficiency with crop yield and quality. This study's aim was to balance lighting energy-use with growth and nutritional quality by using “dynamic lighting” recipes to reduce durations of high-intensity blue light. Lettuce (Lactuca sativa L.) was grown for 21 days at 220 μmol m⁻² s⁻¹, receiving one of five R:B ratios (R:B_100:0, R:B_95:5, R:B_89:11, R:B_50:50, and R:B_0:100) for either the whole 18-h photoperiod (Whole Day), the first 6 h of the photoperiod (Morning), or the last 6 h of the photoperiod (Evening). Morning and Evening treatments received low blue (R:B_89:11) for the remaining 12 h of the day. The Morning and Evening high blue treatments had greater fresh weight and leaf area than their respective Whole Day treatments, attributed to reduced instantaneous leaf photosynthesis under high blue. High blue reduced photosynthesis during only the 6 h of Morning and Evening treatments, compared to the full impact of static high blue for 18-h Whole Day treatments. Intriguingly, with only 6 h of R:B_0:100, Morning and Evening treatments had the same high anthocyanin content as lettuce grown for 18 h under R:B_0:100. Therefore, daily blue light fraction can be reduced by using dynamic treatments to more efficiently promote growth and nutritional quality.