Influence of Artificial Lights on Nitrate and Sugar Contents of Leaf Lettuce

High nitrate levels of leafy vegetables in diet may be linked to higher incidences of gastrointestinal cancer. This experiment was conducted to determine if different artificial lights influence the tissue nitrate and sugar concentration of leaf lettuce grown hydroponically as well as in pots under light emitting diode (LED), light emitting plasma (LEP) and high pressure sodium (HPS) lamps. The artificial lights were installed in individual cubicles (1 m x 1 m x 1.5 m) lined with fabrics that transmitted less than 3% natural light inside a greenhouse. The photosynthetic photon flux densities (μmol·m^-2s^-1) inside the light boxes were: 89, 389, 400, and 142 μmol·m^-2s^-1, respectively, for LED (2:1 mixture of red and blue lights), LEP, HPS, and natural light. Seedlings of 3 different cultivars (Parris Island, Black Seeded Simpson, Rex Rz) of lettuce were anchored on Styrofoam boards floated inside a tub containing 200 ppm N prepared with 20-20-20 commercial analysis fertilizer. In all cultivars, the nitrate level was lowest when plants were grown under LED compared to other lights. Sugar content determined by a refractometer (Brix) was also lowest under LED, while both LEP and HPS produced highest Brix readings. When plants were grown in a peat-light mix (Sunshine #1) in pots, tissue nitrate levels were about half of those obtained from hydroponic culture using the same nutrient solution. The pattern of nitrate and sugar accumulation also varied by cultivars under the same artificial lights.