Influence of Ratio of Red and Blue LEDs On Leaf Index, Growth and Phenolic Compounds In Red Leaf Lettuce

Light-emitting diode (LED) with a short wavelength range can be used in closed plant production systems as an artificial light source. Red and blue lights are known as effective wavelength ranges for photosynthesis. In this study, we determined the effect of ratio of red and blue LEDs on leaf index, growth and the accumulation of phenolic compounds in red leaf lettuce (Lactuca sativa L. ‘Sunmang’). Lettuce seedlings grown under normal growing conditions (20ºC, fluorescent lamp + high pressure sodium lamp 175 μmol/m²/s, 12 h photoperiod) for 18 d were transferred to incubators at 20ºC equipped with different combinations of red (655 nm) and blue (465 nm) LEDs (Red:Blue = 10:0, 9:1, 8:2, 7:3, 6:4 or 5:5) under the same light intensity and photoperiod (175 μmol/m²/s, 12 h photoperiod). Leaf width, leaf length, leaf area, fresh and dry weights of shoots and roots, SPAD value, total phenolic concentration, and antioxidant capacity were measured at 2 and 4 weeks after LED treatment. Leaf index of LED treatments containing blue LEDs regardless of the ratio was similar with that of control during whole growth stage. However, 100% red LED treatment induced significantly higher leaf index than the other treatment. Moreover, all the growth characteristics such as fresh and dry weights of shoots and roots and leaf area were the highest in 100% red LED treatment among the treatments. The ratio of blue LED was negatively associated with lettuce growth. Lettuce plants exposed to 10:0 showed around 4 times higher fresh weight of shoot than those under 5:5 at 4 weeks after LED treatment. In contrast, SPAD value, total phenolic concentration, and antioxidant capacity increased in lettuce grown under high ratios of blue LED such as 5:5, 6:4, and 7:3. Absence of blue light, which indicates 10:0, induced a significant decrease in SPAD value, total phenolic concentration and antioxidant capacity. Especially, lettuce plants under 5:5 or 6:4 contained significantly high total phenolics than control plants as well as lettuce plants under 10:0. Thus, this study suggests that the ratio of red and blue LEDs is a crucial factor for morphology, growth, and the concentration of phenolics with antioxidant properties of red leaf lettuce.