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 (EY A ), and anthocyanin yield per photon (PY A ). 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 (R 80 :B 20 ) and R 20 :B 80 , were selected for an experiment involving five treatments. An optimum light recipe (SR5SB1) including R 80 :B 20 treatment during the early stage of cultivation (weeks 1 through 5 after sowing) followed by R 20 :B 80 treatment during the final stage (week 6) was proposed. The SR5SB1 treatment led to FW, EY A , and PY A of 87.8 g/plant, 1.63 mg/kWh, and 0.57 mg·mol –1 , respectively. This treatment resulted in the highest EY A and PY A , with 159% and 256% more anthocyanin productivity, respectively, compared with cool white treatment (with FW, EY A , and PY A 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.