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To understand the dynamic changes of hydroponic lettuce growth, ascorbate (AsA) pool and metabolism under two different dark period light intensities (LL, 20 μmol·m−2·s−1; CL, 200 μmol·m−2·s−1) of continuous light and normal light (NL, 0 μmol·m−2·s−1) provided by red (R) and blue (B) LEDs, the chlorophyll fluorescence parameters, ascorbate pool size, AsA metabolism-related enzyme activities, and H2O2 contents of lettuce were measured at 0, 8, 16, 24, 32, 40, 48, 56, 64, and 72 h after light treatment and the lettuce growth parameters were measured on the 9th day after light treatment. The results showed that compared with the NL, CL treatment for 9 days significantly increased the biomass, dry matter content, and specific leaf weight of lettuce, but had no significant effect on the leaf area and root-to-shoot ratio; LL had no significant effect on lettuce biomass, but it would reduce the root-shoot ratio. Compared with the NL, the AsA content of CL increased significantly within 8 h after light treatment (at the end of first dark period), and then maintained at a relatively stable level with a slight increase; there was no significant difference in AsA contents between NL and LL showing the same circadian rhythm characteristics. Overall, the activities of L-galactono-1,4-lactone dehydrogenase (GalLDH), ascorbate peroxidase(APX), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR) under CL were the highest among the three treatments, and the differences with the other two treatments reached significant levels at several time points; there was almost no significant difference in the activities of GalLDH, APX, MDHAR, and GR between NL and LL; there was no significant difference in the activities of dehydroascorbate reductase (DHAR) under different treatments. Compared with the NL, CL caused a sharp decrease of PSⅡ maximal photochemical efficiency (Fv/Fm) in lettuce within 0–8 h after treatment, which then stabilized at a relatively stable level; the Fv/Fm value under the LL was almost the same as the NL. Except for 32 h, the H2O2 content of lettuce under CL was the highest among the three treatments during the entire experimental period, and was significantly higher than that of NL at several time points; the H2O2 content of LL was almost the same as NL. In summary, lettuce biomass, AsA contents, AsA metabolism-related enzyme activities, chlorophyll fluorescence parameters, and H2O2 contents were regulated by the dark period light intensities of continuous light rather than continuous light signals. [ABSTRACT FROM AUTHOR]

LED red (R) and blue (B) continuous light (CL) is a potential efficient way to increase plant productivity of plant factory with artificial light (PFAL), but limited information was explored about their effects on plant mineral nutrition. In an environmentally controlled plant factory with artificial light (PFAL), the effects of CL of different intensities and spectral qualities, emitted by R and B LEDs on growth and nutrient element content and accumulation of lettuce (Lactuca sativa L.), were conducted in three hydroponic experiments. Two treatments, normal light (12 h/12 h) and CL (24 h/0 h) in experiment 1, three CL intensities (100, 200 and 300 μmol·m−2·s−1) in experiment 2, and three CL light qualities (1R:3B, 1R:1B and 3R:1B) in experiment 3 were designed. The results showed that CL significantly increased the fresh and dry lettuce shoot biomass compared with normal light, and shoot fresh and dry biomass increased with the intensity increment of CL. In experiment 3, shoot fresh biomass was great under high R light proportion CL treatment, while dry shoot biomass remained unchanged. Both CL and CL with increased intensities promoted shoot C content and accumulation in lettuce. CL reduced N, P, K, Ca, Mg, Cu and Zn contents in lettuce shoot, while Fe and Mn contents did not change compared to NL. Moreover, CL increased Ca, Fe and Mn accumulation. 100–200 μmol·m−2·s−1 CL facilitated N, P, Ca, Mg, Fe, Mn, Cu and Zn contents in shoot, but K content was not influenced compared with 300 μmol·m−2·s−1. The data showed that high B light ratio (75%) facilitated C content comparison with low B ratios (50% and 25%). However, lettuce grown under 3R1B treatment had the higher C accumulation. Shoot N, P, K, Ca, Mg, Fe, Mn and Zn contents were higher under 1R1B treatment, and Cu content did not affected by light quality. Moreover, accumulation of N, P, K, Ca, Mg, Fe, Mn, Zn and Cu in shoot was higher under 1R1B treatment, while P, Ca, Mg, Mn accumulation under 3R1B treatment was the lowest. In conclusion, CL tends to reduce shoot mineral element contents due to dilution effect as shoot dry weight increases compared to NL. However, long-term (12 days) CL composed of 1R1B, 100–200 μmol·m−2·s−1 tends to obtain relative higher K, Ca, Fe and Zn contents in the greater dry lettuce shoot. [ABSTRACT FROM AUTHOR]