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Horticulturae 2017, 3, 36 5 of 11 seeds, barks, stems, roots, flowers, and other parts of plants [9]. In addition to contributing to the distinctive smells of plants, essential oils have long been used in food preparation and health-care practices such as reducing pain, enhancing perspiration, and anti-allergic effects [6]. Phenolic compounds are notably found in herbs, spices, and nuts, which have been studied extensively from the perspective of health protection and pharmacological utility, due to their anticancer, antiviral, antitoxic, and hepatoprotective properties [7–9]. With increasing consumption of herbs, growers are adopting CEA systems for herb production to provide sufficient quantity and quality of natural products. As LEDs are increasingly being used in controlled environment horticulture, the effects of light quality (spectrum distribution of light sources) on phytonutrient accumulation have been increasingly investigated [16,36,49,53], which will be discussed in the following sections. 3.1. Essential Oils Red, blue, and UV light enhanced the concentration of essential oils in various herbs compared with white light or sunlight, and the level of enhancement varied among species, compounds, and light treatments. For instance, the total essential oil content of Mentha piperita, M. spicata, and M. longifolia were highest under red light at 500 μmol m−2 s−1 PPF and 16-h photoperiod for 60 days, 39% and 86% higher than blue or white light, respectively, and was lowest under sunlight [36]. Similarly, l-menthol, the main component of essential oils in M. arvensis, was 1.4 times higher under red light than blue or green light, both at 150 μmol m−2 s−1 PPF and 16-h photoperiod for 28 days [6], while the glycyrrhizic acid (an oleanane-type triterpenoid saponin and considered to be 50 times sweeter than sugar) concentration in root tissues of Chinese liquorice was highest under red light, followed by white light, and lowest under blue light at 300 μmol m−2 s−1 PPF and 16-h photoperiod for three months [50]. However, for sweet basil, the total essential oils under blue light were 1.2–4.4 times higher than plants grown under white light, and was lowest under red light, all at 50 μmol m−2 s−1 PPF and 16-h photoperiod for 70 days [16]. Although supplemental UV light generally inhibits plant growth, it might increase essential oils, resulting in enhanced plant defense and protection against UV light [54]. UV-B or combined UV-A and UV-B light with white light was more effective than white light alone in increasing l-menthol and limonene concentrations in M. arvensis [55] as well as the content of essential oils in Chinese liquorice, such as glycyrrhizic acid, liquiritin, liquiritigenin, and isoliquiritigenin [56]. Two weeks of supplemental UV-B light for 2.5 h each day enriched the levels of phenylpropanoid and terpenoid concentrations in sweet basil, which were three times higher than the plants under sunlight [52]. Therefore, supplemental red, blue, and UV light treatments could be used as effective processes to enhance plant phytochemical biosynthesis and provide great commercial advantages. In addition to enhancing the concentrations of essential oils, light quality may also alter the compositions of essential oils in herbs. For example, the second and third major components of essential oils in sweet basil were myrcene and linalool under blue light, and α-pinene and β-pinene under green and red light, and those under white light showed an intermediate response [16]. Mexican mint (Plectranthus amboinicus) grown under blue light exhibited greater amounts of sesquiterpene group compounds, while plants under red light showed greater amounts of monoterpene group compounds [57]. Hence, light quality could be manipulated to enhance targeted compound concentrations for various purposes. 3.2. Phenolic Compounds Phenolic compounds, flavonoids, and anthocyanins in herbs could be enriched by red, blue or UV light, to provide more health-beneficial natural products for human beings. Monochromatic red or blue light treatments significantly increased the concentrations of total phenolic compounds, flavonoids, and anthocyanins in Chinese foxglove (Rehmannia glutinosa) and perilla plants compared with white light, and blue light was more efficient than red light [53,58]. However, the amount of rosmarinic acid—the major component of phenolic compounds in sweet basil—under continuous red and white light was double that under blue light, all at 100 μmol m−2 s−1 PPF for 14 days, whereas chicoricPDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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