Quantitative Evaluation of Electric Light Recipes for Red Leaf Lettuce Cultivation in Plant Factories

Abstract

Red leaf lettuce (Lactuca sativa) has high nutritional value and is frequently used in salads. In a plant factory with full electric lighting, if the spectrum is incorrect, then red leaf lettuce will have incomplete coloration. This study aimed to establish a light recipe for the mass production of red leaf lettuce using electric light sources in a plant factory by using indicators for quantitative assessment, including energy yield (EY) [grams of fresh weight (FW) harvested per kilowatt hour of electricity input for lighting], photon yield (PY) (grams of FW harvested per mole of photons delivered), anthocyanin yield per kilowatt hour (EYA), and anthocyanin yield per photon (PYA). First, the effects of four types of light quality on FW and anthocyanin content were examined. Then, two types of light quality, light-emitting diode with a red-to-blue photon ratio of 80:20 (R80:B20) and R20:B80, were selected for an experiment involving five treatments. An optimum light recipe (SR5SB1) including R80:B20 treatment during the early stage of cultivation (weeks 1 through 5 after sowing) followed by R20:B80 treatment during the final stage (week 6) was proposed. The SR5SB1 treatment led to FW, EYA, and PYA of 87.8 g/plant, 1.63 mg/kWh, and 0.57 mg·mol–1, respectively. This treatment resulted in the highest EYA and PYA, with 159% and 256% more anthocyanin productivity, respectively, compared with cool white treatment (with FW, EYA, and PYA of 65.8 g/plant, 0.63 mg/kWh, and 0.16 mg·mol–1, respectively). The proposed SR5SB1 light recipe enabled cultivation of red leaf lettuce with a balanced yield and anthocyanin production.