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‘Green Oak Leaf’ lettuce were hydroponically cultured for 35 days under mixed light qualities of white light and a different supplemental light such as blue (B), green (G), yellow (Y), red (R), and far-red (Fr) provided by light-emitting diodes (LEDs). Effects of supplemental light qualities on the growth and nutritional properties of lettuce plants were investigated. The pure white LED (W) was used as control with a photosynthetic photon flux (PPF) value of approximately 135 μmol m −2 s −1 , and the basal white light in each treatment was 105 μmol m −2 s −1 while the photon flux added by supplemental LED was approximately 30 μmol m −2 s −1 . The results indicated that supplemental lights led to obvious morphological changes, plants with WR appeared compact and vigorous while those with WY and WFr looked sparse and twisted, dwarfed plants with large leaves were detected under WB. Compared with those grown in the white light control, the fresh weight of shoots increased by 63.2% and 21.7% with supplemental R and B respectively, while decreased by 35.9% with supplemental Fr. Chlorophyll and carotenoid contents were significantly higher with supplemental R and B than other treatments. Supplemental B and G resulted in decrease of nitrate content. Supplemental G significantly promoted soluble sugar accumulation. Supplemental Fr increased S/R ratio and ascorbic acid accumulation but resulted in lower biomass and pigment contents. However, no positive impact of supplemental Y light was observed in the research.

Seeds of ‘Green Oak Leaf’ lettuce were sown and hydroponically cultured for 50 days (with no transplanting) under different light spectra from the following six lights: fluorescent light plus red LED (FLR), fluorescent light plus blue LED (FLB), monochromic red (R) or blue (B) LED, mixed red and blue LED (RB), and fluorescent light (FL), with equivalent photoperiod (14 h), PPF (133 ± 5 μmol m−2 s−1), and other cultivation conditions. At seedling stage, highest height-growth rate was obtained under FLR and R, plantlets under FLR showed improved morphology that was large and compact while those under monochromic R appeared sparse and fragile. Seedling growth was promoted under FLR and FLB, while inhibited the most under monochromic B as compared to FL. At harvest stage, fresh weight, dry weight and stem diameter were greatest under FLR followed by FLB and lowest under monochromatic B and RB. Chlorophyll and carotenoid contents were also significantly higher under FLR and FLB and lowest under R and FL. The soluble sugar and nitrate contents were significantly higher in plantlets cultured under FL than those under LED or mixture lights of FL and LED. When cultured under FLB, vitamin C content was significantly lower but no significant difference was observed among other treatments. In conclusion, FLR and FLB resulted in improved morphology, greater biomass and pigment contents of lettuce than monochromic R, B, FL or RB. FL mixed with R or B LED could be used as efficient light sources for hydroponic cultivation of ‘Green Oak Leaf’ lettuce.