Indoor-vertical farms enable the production of high-value crops like lettuce (Lactuca sativa) year-round in a tightly controlled environment that reduces water and pesticide use. Indoor farms require less land to grow more food compared with field production and can be placed in or near large cities. However, they are entirely reliant on electricity to control temperature and provide electric lighting. Light-emitting diode (LEDs) fixtures are primarily used for their high efficacy and delivery of a specific light spectrum, photon flux density (PFD), and photoperiod. In addition to PFD, the light spectrum can greatly affect biomass accumulation, morphology, and quality traits such as leaf coloration and nutritional content. To further investigate how LEDs can be used to manipulate the light spectrum to regulate plant growth, morphology, and quality, we designed experiments in a temperature-controlled growth room equipped with hydroponic growing racks and light-waveband tunable LED fixtures. During all experiments, lettuce seeds were sown and grown in rockwool cubes under broad-spectrum light until they were transplanted into the hydroponic system after the seedling stage. First, to compare the effects of ultraviolet A (UVA, 315-399 nm) to blue (400-499 nm) light, we grew red-leaf lettuce ‘Rouxai’ under red (600-699 nm) plus white light with end-of-production supplemental UVA or blue light. UVA and blue light were similarly effective at increasing lettuce leaf coloration and total phenolic and anthocyanin concentrations, while neither affected fresh mass. Next, we investigated the persistency of periodic supplemental UVA or blue light on quality attributes and biomass accumulation by enriching the light spectrum with either waveband during the beginning, middle, or last phase of production as well as the entire production cycle. End-of-production UVA or blue light were as effective at improving lettuce quality as continuous enrichment but the continuous blue light treatment inhibited biomass accumulation. Next, to more broadly quantify the effects of enriching a white spectrum with various wavebands, we grew lettuce ‘Rouxai’ and ‘Rex’ under two different PFDs supplemented with equal proportions (~30%) of blue, green (500-599 nm), red, far-red (700-799 nm), or white light. Supplemental far-red light increased leaf expansion, while additional red and warm-white light were the most effective at increasing biomass accumulation. Supplemental blue light was the only waveband that increased total anthocyanin concentrations and leaf coloration. Finally, increasing the PFD increased biomass accumulation and total phenolic concentration and responses were generally similar at the low and high PFDs tested. In the last study, we investigated how the efficacy of far-red light depends on other light wavebands, and specifically if the substitution of red light with green light would influence the efficacy of far-red light on increasing plant growth. Lettuce ‘Rouxai’ and ‘Rex’ leaf area continually increased as the far-red light percentage increased, regardless of the green- and red-light percentages. The increase in leaf expansion did not always lead to an increase in fresh mass and was greatest when far-red light represented approximately one-eighth to one-fourth of the total PFD. At higher or lower far-red PFD fractions, fresh mass was similarly lower. Collectively, these studies show that the light spectrum has vast and impactful effects on lettuce growth, morphology, and quality. These studies also highlight the utility of including far-red light and end-of-production blue light in the vertical farming of lettuce. Finally, while changing the light spectrum can elicit certain plant responses, it also influences fixture efficacy and thus, electricity consumption.
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