Dose-dependent effects of supplemental ultraviolet-B and blue light on nutritional quality–yield trade-offs in red lettuce

• End-of-production UVB and blue light supplementation enhanced pigmentation and antioxidant accumulation in red leaf lettuce. • Nutritional quality–yield trade-offs were dose-dependent and more pronounced under UVB than blue light. • UVB was more effective for enhancing anthocyanins and phenolics, while blue light promoted higher vitamin C content. • Short-term spectral treatments near harvest can effectively improve lettuce quality with relatively low yield penalties. End-of-production (EOP) supplemental lighting can effectively enhance crop nutritional and visual quality in controlled environment agriculture (CEA), as beneficial phytochemicals such as phenolic compounds (including anthocyanins) and ascorbic acid can accumulate within hours to days in response to environmental cues. Narrow waveband ultraviolet-B (UVB; 280–320 nm) and blue light (400–500 nm) are especially effective in improving nutritional quality but may reduce yield even when applied over short durations. This study quantified the dose-dependent effects of EOP UVB and blue light on nutritional quality–yield trade-offs in red lettuce ‘Rouxai’ and ‘Red Salad Bowl’. UVB was applied at 0.5, 1.5, or 2.5 µmol m -2 s -1 (UVB 0.5 , UVB 1.5 , or UVB 2.5 ) and blue light was applied at 30, 60, or 90 µmol m -2 s -1 (B 30 , B 60 , or B 90 ) for 16 h day -1 during the final six days of production. Plant biomass and nutritional parameters were measured at 3 and 6 days after treatment (DAT). All six EOP treatments significantly enhanced anthocyanin accumulation in both cultivars, with the strongest effect observed under the highest-intensity UVB treatment applied for 6 days. Cultivar ‘Rouxai’ was generally more responsive; under UVB 2.5 at 6 DAT, extraction-based anthocyanin content increased by 468 % in ‘Rouxai’ and 154 % in ‘Red Salad Bowl’ compared to controls. Total phenolic concentration showed similar response trends to anthocyanins. Total ascorbic acid concentrations were less affected by the EOP treatments, with levels generally remaining similar to controls at 3 DAT but increasing at 6 DAT; the largest increase occurred under B 90 at 6 DAT (27 % increase in ‘Rouxai’ and 21 % increase in ‘Red Salad Bowl’). However, EOP treatments generally reduced leaf expansion and biomass, with greater reductions at higher supplemental light intensities, longer exposure duration, and under UVB compared to blue light. These results highlight clear dose-dependent quality–yield trade-offs under EOP UVB and blue light treatments. Blue light provided a more favorable balance between maintaining marketable yield and enhancing nutritional quality, whereas UVB more strongly enhanced anthocyanin and phenolic accumulation at the cost of moderate yield reductions.