Effect of Different Ratios of Red and Blue LED Light on Lettuce Production and Phytochemical Accumulation

Light-emitting diodes (LEDs) emit a short span, monochromatic light, which permits the creation of a custom light spectrum that can more closely resemble the light spectrum needed by plants for photosynthesis. A combination of blue and red wavebands can result in higher photosynthetic activity and better photomorphogenetic characteristics than red or blue alone. Red light is important in the development of the photosystem complex and morphogenesis through the mutation of phytochrome apparatus while blue has been identified as affecting chlorophyll concentrations, photomorphogenesis and stomatal openings. The LEDs can be selected to target the wavelengths absorbed by plants, enabling the growers to customize the wavelengths of light required to maximize production and limit wavelengths that do not significantly impact plant growth. Plant response to light from the red and the blue spectra has been documented extensively. The purpose of this study was to determine which combination of red and blue LED light ratio was optimum for lettuce and pepper fruit production. In this study three red to blue ratio levels (5:1, 10:1 and 19:1) compared to 100% red light for lettuce (Lactuca sativa). Seedlings were cultured in a growth chamber at 25 °C (+/–2.5°C), ambient CO₂, and a 16-hour light, 8-hour dark photoperiod under fluorescent light. Seeds were germinated and two-week-old seedlings were planted in 1-inch rockwool cubes in trays for soilless hydroponic culture with half-strength Hoagland’s solution and transplanted in LED light chamber. Lettuce plants were harvested after 4 weeks of transplant. The fresh biomass (aerial and fruit) was dried according to the ASABE standard (2007), with a temperature of 95 °C for no less than 72 hours and subsequently weighed. Leaf tissue samples were frozen before lyophilization (Gamma 1-16 LSC; Martin Christ, MBI, Kirkland, Quebec,Canada). Pigments were extracted and analyzed from freeze dried tissues. Among the treatments of light, red to blue ratios of LEDs were found superior. The lowest significant growth (fresh mass and dry mass) and phytochemicals accumulation were observed in 100% red LED. This research will facilitate the improved selection of LEDs for control environment production.