Differential effects of high light duration on growth, nutritional quality, and oxidative stress of hydroponic lettuce under red and blue LED irradiation.

• This study investigated the responses of purple lettuce grown under part-time high light. • Short-term high light irradiation promoted biomass production, but the effect on secondary metabolites accumulation was not significant. • 4 h high light irradiation (500 μmol·m−2·s−1) in the middle of 16 h light period induced higher secondary metabolites content but caused stress on lettuce. • The responses of lettuce to high light irradiation are more sensitive in the early growth stage. In this study, effects of high light irradiation (HLI, 500 μmol m−2·s-1) duration in the middle of light period (16 h, 150 μmol m−2·s-1) provided by red (R) and blue (B) LEDs (4R:1B) on the growth, nutritional quality, and oxidative stress of hydroponic purple leaf lettuce grown in an environmentally controlled plant factory were examined on the 10th and 20th day after treatment. Set up five treatments with 0 h (CK), 0.5 h (HL0.5), 1 h (HL1), 2 h (HL2), and 4 h (HL4) HLI, respectively. We found that the shoot fresh weight of lettuce increased first and then decreased with the prolongation of HLI duration, whether in the early or late growth stage. On the 10th day after treatment, the root fresh weight and specific leaf weight of lettuce increased linearly with the prolongation of HLI duration, while the shoot/root ratio decreased linearly. On the 20th day after treatment, the lettuce of HL1 had the highest shoot fresh weight, leaf area, and shoot dry weight, followed by HL2. Besides, HL4 promoted the accumulation of soluble sugar, anthocyanin, flavonoid, and total phenolic simultaneously decreased the maximum quantum efficiency compared with others on the 10th day after treatment. On the 20th day after treatment, HL0.5 and HL4 stimulated the accumulation anthocyanin, flavonoid, total phenolic, and ascorbic acid compared with other treatments. The results showed that short-term HLI promotes biomass production and secondary metabolite accumulation, but HL4 caused photo-oxidative damage to lettuce. Therefore, lettuce growth and qualities can be purposely adjusted by adopting different duration of HLI provided by red and blue light in the plant factory, also these responses depended on lettuce growth stage closely. [ABSTRACT FROM AUTHOR]