End-of-production Ultraviolet A and Blue Light Similarly Increase Lettuce Coloration and Phytochemical Concentrations

Anthocyanins are a group of human-health-promoting phenolic compounds that influence the pigmentation of red-leaf lettuce (Lactuca sativa). Ultraviolet A (UVA; 315-399 nm) and blue (B; 400-499 nm) light can increase the concentrations of phenolic compounds but also suppress cellular expansion, which can limit harvest -able biomass accumulation. It is not known whether UVA or B light is more effective at increasing phenolic compound concentrations when they are each applied at the same photon flux density. Our objective was to evaluate the efficacy of UVA and B light when added during the end of production (EOP) at promoting phenolic com-pound synthesis and red-leaf coloration without limiting biomass accumulation. We grew red-leaf lettuce 'Rouxai' in a controlled indoor environment at an air tempera-ture of 22 degrees C under warm-white and red light-emitting diodes (LEDs). On day 24, 30 or 60 mu mol center dot m-2 center dot s-1 from UVA, B, UVA plus B, or red plus green LEDs was added during the last 6 days of the 30-day production period. UVA and B light, alone or combined, similarly increased leaf redness (by up to 72%), total phenolic concentra-tion (by up to 92%), total anthocyanin concentration (by up to 2.7-fold), and relative chlorophyll concentration (by up to 20%) and did not inhibit growth, compared with lettuce grown without EOP supplemental lighting. Considering B light was as effec-tive as UVA light at increasing leaf color and phytonutrient density and that B LEDs are more electrically effective, economical, and durable, an enriched blue-light spec-trum at the EOP is a comparatively sustainable method to increase crop quality with-out suppressing biomass accumulation.