While end-of-production sole-source lighting at a moderate intensity increased nutrient, water-soluble vitamin, and carotenoid content, the anthocyanin concentration of red leaf lettuce decreased.

In controlled environments, light intensity and quality can be manipulated to enhance beneficial phytochemicals, vitamins, and foliage color by increasing anthocyanin levels before harvest. Our objectives were to quantify whether a 50% reduction in end-of-production (EOP) light intensity, enriched with blue (B) light alone or with red (R) light, would yield red leaf lettuce of comparable or higher quality than that grown under high-intensity white light. Seedlings of red leaf lettuce (Lactuca sativa) 'Barlach', 'Rouxai', and 'Thurinus' were grown under light-emitting diodes (LEDs) that provided a photosynthetic photon flux density (PPFD) of 300 µmol·m-2·s-1 with a ratio (%) of 19:39:39:3 B:green:R:far-red light. During the last 6-8 days of production, plants were either left in the same conditions or placed under LEDs providing a ratio (%) of 100:00, 75:25, or 50:50 B:R light and a PPFD of 150 µmol·m-2·s-1. Compared to untreated plants, the shoot fresh mass of 'Rouxai' was 16%, 17%, and 21% lower, and the shoot dry mass was 25%, 27%, and 29% lower under 100:00, 75:25, and 50:50 B:R EOP treatments, respectively. 'Thurinus' exposed to 100:00 B:R EOP lighting contained 22%, 22%, 47%, 35%, 24%, 10%, 40%, 31%, 25%, 50%, and 38% greater N, P, Ca, Mg, S, B, Cu, Fe, Mn, Mo, and Zn concentrations, respectively, compared to the control. Plants exposed to EOP generally developed increased concentrations of water-soluble vitamins. Vitamin C of 'Barlach', 'Rouxai', and 'Thurinus' was between 37% and 42%, 36% and 45%, and 49% and 57% greater, respectively, when exposed to EOP B or B:R sole-source lighting. The carotenoid concentration of 'Rouxai' under 100:00 B:R was 29% greater than that of the control. Each cultivar developed the highest concentration of violaxanthin, neoxanthin, zeaxanthin, and α-carotene when not exposed to EOP lighting. Under EOP lighting, 'Barlach', 'Rouxai', and 'Thurinus' accumulated between 52% and 68%, 55% and 57%, and 33% and 43% lower anthocyanin concentrations, respectively, than the control plants. Our results indicate that maintaining a higher light intensity during the EOP phase was more effective at increasing anthocyanin production in red leaf lettuce than lowering light intensity and increasing the fraction of blue light.