The growth response of leaf lettuce at different stages to multiple wavelength-band light-emitting diode lighting

• Different light formulas were used on lettuce at different growth stages.
• Giving different light formulas resulted in different nitrate contents.
• The higher the chlorophyll, the lower the nitrate content.
• Light-red plus light-blue light treatment showed the lowest plant quality.

This study investigated the effects of a combination of light-emitting diodes (LEDs) that emit different bands of short wavelengths on the growth response of leaf lettuce (Lactuca sativa var. crispa) at the seedling stage (1–14 d old) and vegetative stage (15–28 d old). The wavebands included red (R; range: 623–673 nm), light red (Lr; range: 599–644 nm), blue (B; range: 427–478 nm), light-blue (Lb; range: 435–489 nm), cyan (C; range: 466–532 nm), green (G; range: 494–564 nm), and ultraviolet-A (UV-A; range: 383–426 nm). In the light treatments, a photon flux of 300 ± 12 (standard deviation [SD]) μmol m−2 s−1 was provided for 18 h d−1 at the seedling and vegetative stages with such LED light formulas (combinations) including LrLb (61.5% and 38.5%, respectively), RCB (42.8%, 30.0%, and 27.2%, respectively), LrLbG (49.0%, 36.1%, and 14.9%), and RBUV-A (52.9%, 37.0%, and 10.1%), with each fraction denoting the percentage of total photon flux. The air temperature, relative humidity, and CO2 levels for all treatments were maintained at 22 ± 2 °C, 64 ± 8%, and 570 ± 75 ppm, respectively. The data obtained revealed the variability of light formula effects on lettuce plant physiological indices (including the number of leaves, leaf length, leaf area, dry mass, fresh mass, chlorophyll-a (chl-a) content, and nitrate content). Provision of RBUV-A and RCB LED irradiation during the vegetative stage could increase the shoot fresh mass of leaf lettuce. In addition, providing different LED light formulas to lettuce plants at the seedling and vegetative stages resulted in tremendous differences in nitrate contents, especially with the use of two light formulas with and without reversal of lighting sequence. Finally, the adoption of LrLbG or LrLb LED irradiation during different growth stages resulted in the lowest physiological indices of lettuce plants.