Growth and quality responses of ‘Green Oak Leaf’ lettuce as affected by monochromic or mixed radiation provided by fluorescent lamp (FL) and light-emitting diode (LED)

• ‘Green Oak Leaf’ lettuce in our study were cultured under different light sources till harvest, that is, nursery was also finished under the corresponding light quality.
• Broad-spectrum fluorescent lamps were not only used as control but also mixed with red or blue LEDs as light treatments to meet growth requirements at different stages.
• Growth and quality responses of lettuce to light quality at different growth stages were described respectively.

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.