Effects of Substituting B with FR and UVA at Different Growth Stages on the Growth and Quality of Lettuce

This study investigated the effects of substituting B with FR and UVA (50 μmol·m−2·s−1) at two growth stages on the growth and quality of loose-leaf lettuce (cv. Fangni). The basal light was red and blue LEDs at 250 μmol·m−2·s−1. At stage I (the first 10 days of 20-day pre-harvest), there were three treatments: B substituted by FR (FR); B substituted by UVA (UVA); and no substituting (CKI). At stage II (next ten days after stage I), there were 9 treatments: FF (FR + FR), UF (UVA + FR), BF (B + FR), FU (FR + UVA), UU (UVA + UVA), BU (B + UVA), FB (FR + B), UB (UVA + B), and B + B (CKII). At stage I, compare with UV-A and CKI, the shoot fresh weight, leaf area, leaf width, leaf length, and vitamin C content highly increased under far-red light (FR), whereas specific leaf weight and the chlorophyll contents significantly decreased by FR. In CKI, nitrate contents and the antioxidant capacity (FRAP, DPPH) were significantly higher than in FR and UVA. At stage II, higher leaf width, leaf length, leaf area, and shoot fresh and dry weight were observed in FF, UF, and BF. The lowest biomass was shown in CKII. Higher chlorophyll contents were found in FU and FB. The soluble sugar contents significantly increased by all treatments. In addition to UB and BU, soluble protein contents increased by other treatments. There were higher vitamin C contents in UU, UB, and CKII. Large amounts of nitrates accumulated under CKII. The higher antioxidant capacity (DPPH, FRAP) was found in FB and CKII. The highest flavonoid content was found in UB, and higher polyphenols contents were found in UB and BU. In this study, substituting B with FR at 2 stages were the best way to increase lettuce biomass. The optimal measure to both increased lettuce nutrition quality and biomass was FB at stage II.