The Effect of Multi-wavelength Light-emitting Diode Lighting on the Growth Response of Leaf Lettuce at Different Stages

This study researches the effect of a combination with light-emitting diode (LED) of different wavelength-bands on the growth response of leaf lettuce (Lactuca sativa var crispa) at different growth stages. The wavelength-bands consist of red (R; 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) LED lighting. The changes in the shoot dry mass, shoot fresh mass, chlorophyll content, and nitrate content of leaf lettuce are analyzed. In the experiment, a photosynthetic photon flux of  300 ±12 standard deviation (SD)  µmol·m^-2·s^-1 is provided for 18 h·d^-1 at seedling stage and vegetative stage with such four LED sets as Lr-Lb (61.5%-38.5%), R-C-B (42.8%-30%-27.1%), Lr-Lb-G (49%-36.1%-14.9%), and R-B-UV-A (52.9%-35%-10.1%). The irradiation of different LED sets is conducted on leaf lettuce at seedling stage and vegetative stage, respectively. The air temperature, relative humidity, and CO₂ levels for all treatments were maintained at 22±2°C , 64±8% , and 570±75 ppm , respectively. In term of total shoot dry mass and shoot fresh mass, the adoption of LrLbG or LrLb with other combinations results in the worst quality of plant. Provision of UV-A and cyan irradiation during the vegetative stage can increase the shoot fresh mass and dry mass of leaf lettuce.