Short-duration high blue light at night enhances phytochemicals and morphology without yield loss in three lettuce cultivars

Controlled environment agriculture (CEA) enables precise regulation of plant physiological processes through environmental control. Among these, the spectral composition of photosynthetically active radiation (PAR), particularly the proportion, dose, and duration of blue light exposure, has a strong effect in enhancing plant nutritional quality. This study investigated how end-of-production (EoP) light spectrum (PAR vs. PAR with high blue portion) and application mode (daily continuous for 18 h vs. 18 h plus 2-h nighttime exposure) affect the growth, morphology and nutritional quality of three red leaf lettuce cultivars (Lactuca sativa var. crispa 'Xandra', 'Alaine' and 'Haflex'). Plants grown in climate-controlled conditions for 29 days, were exposed to four light treatments (Ctrl, HB, Night, NightB) with a Daily Light Integral (DLI) of 24.3 mol m^-2 d^-1 during the final seven days. Light spectrum and application mode significantly affected the three lettuce cultivars. Plants treated with continuous high-blue light (HB) had the lowest values for shoot height, leaf area and biomass, but greatest values for pigment content, antioxidant activity and leaf nitrate accumulation. Short-duration, nighttime blue light application (NightB) improved anthocyanins, phenolics and antioxidant activity without yield loss, achieving the highest blue light use efficiency for anthocyanins (BLUE_CYA) and biomass (BLUE_FW). Cultivar-specific responses were evident, with 'Haflex' exhibiting greater yield, rapid anthocyanin response and lower nitrate accumulation, highlighting the potential for genotype-tailored light strategies in CEA. These results demonstrate that concentrated blue light, when applied as short duration supplementation during the dark phase, can be a viable strategy for enhancing visual and nutritional quality of salad leaves with contained energy expense in commercial plant factories.