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Abstract
Three different LED spectra (W: White light; WFR: W + far-red light; WB: W + blue light) with similar photosynthetic photon flux density (PPFD) were designed to explore the effects of supplementary far-red and blue lights on leaf color, biomass and phytochemicals of two cultivars of red-leaf lettuce (“Yanzhi” and “Red Butter”) in an artificial lighting plant factory. Lettuce plants under WB had redder leaf color and significantly higher contents of pigments, such as chlorophyll a, chlorophyll b, chlorophyll (a + b) and anthocyanins. The accumulation of health-promoting compounds, such as vitamin C, vitamin A, total phenolic compounds, total flavonoids and anthocyanins in the two lettuce cultivars were obviously enhanced by WB. Lettuce under WFR showed remarkable increase in fresh weight and dry weight; meanwhile, significant decreases of pigments, total phenolic compounds, total flavonoids and vitamin C were found. Thus, in the plant factory system, the application of WB can improve the coloration and quality of red leaf lettuce while WFR was encouraged for the purpose of elevating the yield of lettuce.
Highlights
Indoor farms, especially plant factories with artificial light (PFALs), can produce highvalue crops and increase crop production in a stable, controllable and safer manner [1].Lettuce (Lactuca sativa L.) is one of the most common vegetables in PFALs because it is suitable for hydroponic growth and has a short growth period
The growth and biomass of lettuce plant were significantly affected by light quality (p < 0.001) (Figure 1 and Table 2)
While the regulation mechanisms require further investigation through molecule methods, these results still suggested that supplementary far-red and blue lights could be used as effective tools to elevate the lettuce quality. The nutritional compounds such as soluble proteins (SP) and SS of two lettuce cultivars seemed to be less sensitive to WB, while these were significantly increased by WFR in both cultivars (Table 7). These results suggested that the effects of WB and WFR on phytochemicals were vegetable species and cultivar dependent
Summary
Lettuce (Lactuca sativa L.) is one of the most common vegetables in PFALs because it is suitable for hydroponic growth and has a short growth period. Lettuce is popular as a salad vegetable, due to its tender taste, sweet and refreshing flavor and ornamental morphology with vivid colors, and the abundant nutritional and antioxidant compounds such as vitamin C, sugars, proteins and phenolics [2]. Blue light strongly impacts the morphology and nutritional qualities in plants. The hypocotyl length and cotyledon area in kale and mustard had negative correlations with the proportion of blue light (5–30%) in red and blue LED lighting [6]. Canopy height, stem length and internode length of petunia, calibrachoa, geranium and marigold were markedly improved by blue light [7], and leaf angle of these four bedding plants were significantly enlarged by red light. The cucumber leaves accumulated more N, K, Ca, Mg and Mn under monochromatic
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