Growth and Photosynthetic Efficiency of Two Lettuce Cultivars Under Light–Emitting Diode Monochromatic Light

This study discussed the influence of light–emitting diodes (LEDs) with red, blue, green, and yellow light on the growth and photosynthetic efficiency of Boston lettuce and Ziyan lettuce as a reference for lettuce production in plant factories. The experiments were conducted in a plant factory under a 120 µ mole ・ m –2 ・ s –1 photosynthetic photon flux density, CO 2 concentration of 1000 ppm, and daytime and nighttime temperature of 25°C/18 h and 17°C/6 h, respectively. The experiment results revealed that after 15 days of treatment under different spectral qualities, the appearances of the two lettuce leaves differed slightly. The fresh and dry weights of the Boston lettuce were highest under green light treatment. The fresh and dry weights of the Ziyan lettuce were higher under red and green light, and blue light helped the leaves to change color. In addition, photosynthesis analyzers were used to investigate the photosynthetic efficiency of the two types of lettuce under the four spectral qualities under the six luminous intensity levels of 20, 40, 60, 80, 100, and 120 µ mole ・ m –2 ・ s –1 and six CO 2 concentrations of 400, 600, 800, 1000, 1200, and 1400 ppm. The photosynthetic efficiency of the two types of lettuce generally increased with increasing luminous intensity and CO 2 concentration. When the luminous intensity was 100 and 120 µ mole ・ m –2 ・ s –1 and the CO 2 concentration was 1200 ppm and 1400 ppm, the Boston lettuce had the highest photosynthetic effi- ciency under green light. When luminous intensity was 120 µ mole ・ m –2 ・ s –1 and the CO 2 concentration was over 1000 ppm, the Ziyan lettuce had the highest photosynthetic efficiency under red light. These results revealed that the monochromatic LED light most suitable for growth differed across types of lettuce. In this experiment, the yield and photosynthetic efficiency of Boston lettuce were highest under green light. Those of Ziyan lettuce were highest under red light.