End-of-production supplemental blue light intensity and duration co-regulate growth, anthocyanin, and ascorbic acid production in red leaf lettuce

Red leaf lettuce (Lactuca sativa) typically requires high light intensities for adequate production of anthocyanins, which are phytopigments with antioxidant properties and potential health-promoting benefits. Plants grown under relatively low light conditions, e.g., in an indoor farm, often have low anthocyanins accumulation and may be perceived as lower in quality. Blue light has been shown to promote anthocyanin production, however, highintensity blue light often causes reductions in leaf expansion and biomass accumulation. We investigated whether the same daily cumulative amount of supplemental blue light applied at varying intensities and durations (i.e., lower intensity over a longer duration or higher intensity over a shorter duration) at the end of production (EOP) may differentially affect plant growth and the accumulation of beneficial phytochemicals. Two red lettuce cultivars, 'Rouxai' and 'Red Salad Bowl', were grown under six light treatments: 1). white light applied at 180 mu mol m- 2 s- 1 for 16 h/day throughout production (W180; control), 2-5). EOP supplemental blue light applied during the last seven days of production at 45 mu mol m-2 s- 1 for 24 h/day (EOP B45), 67.5 mu mol m- 2 s- 1 for 16 h/day (EOP B67.5), 135 mu mol m- 2 s- 1 for 8-h/day (EOP B135), or 270 mu mol m-2 s- 1 for 4 h/day (EOP B270), and 6). higher-intensity white light applied at 247.5 mu mol m- 2 s- 1 for 16-h/day throughout production (W247.5). The EOP supplemental blue light treatments significantly enhanced anthocyanins production in both lettuce cultivars compared to both the control and the W247.5 treatments, with the highest anthocyanins content under intermediate supplemental blue light intensities and application durations (i.e., EOP B67.5 and B135). The extraction-based anthocyanin index in the EOP B67.5 and B135 treatments increased by 161-168 % in 'Rouxai' and by 180-181 % in 'Red Salad Bowl', compared to the control. However, a significant increase in total vitamin C level (by 47.2 %) was only found in 'Rouxai' receiving the B45 treatment compared to the control. Plant yield also differed among the EOP blue light treatments, with B45 resulting in the highest leaf area and fresh/dry biomass in both lettuce cultivars, comparable to those in W247.5. Interestingly, EOP B67.5 and B135 treatments led to the lowest yield among the four blue light treatments in both cultivars, likely due to reduced leaf expansion and possibly lower photosynthetic light use efficiency because of high anthocyanins accumulation. Our results indicate that, given the same amount of EOP supplemental blue light, plant growth, and quality could be more effectively promoted by controlling the blue light intensity and application duration. Adequate intensity and application duration of supplemental blue light are both required for optimal anthocyanin production in plants.