Continuous Light from Red, Blue, and Green Light-emitting Diodes Reduces Nitrate Content and Enhances Phytochemical Concentrations and Antioxidant Capacity in Lettuce

Abstract

Light-emitting diodes (LEDs) have shown great potential for plant growth and development, with higher luminous efficiency and more flexible and feasible spectral control compared with other artificial lighting. The combined effects of red and blue (RB) LED with or without green (G) LED light and white LED light on lettuce (Lactuca sativa L.) growth and physiology, including nitrate content, chlorophyll fluorescence, and phytochemical concentration before harvest, were investigated. Continuous light exposure at preharvest can effectively reduce nitrate accumulation and increase phytochemical concentrations in lettuce plants. Nitrate accumulation is dependent on the spectral composition and duration of treatment: lettuce exposed to continuous RB (with or without G) LED light with a photosynthetic photon flux (PPF) of 200 µmol·m−2·s−1 exhibited a remarkable decrease in nitrate content at 24 hour compared with white LED light treatment at the same PPF. In addition, RB LED light (R:B = 4:1) was more effective than white LED light at the same PPF in facilitating lettuce growth. Moreover, continuous LED light for 24 hours significantly enhanced free-radical scavenging activity and increased phenolic compound concentrations. We suggest that 24 hours continuous RB LED with G light exposure can be used to decrease nitrate content and enhance lettuce quality.