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Horticulturae 2017, 3, 36 6 of 11 acid content, the second major component, was highest under white light, followed by blue and red light [59,60]. Therefore, it is possible that the application of supplemental red and/or blue light could enhance phenolic compounds accumulation in some herb species, but the enhancement effect might depend on the species and the specific compounds. Similar results were observed with supplemental UV light. Supplemental UV-B light at 2.5 μmol m−2 s−1 for 1 h each day and 2 h each day for seven days significantly increased the content of total phenolic compounds and anthocyanin concentrations in sweet basil, with the short time UV-B treatment being more efficient for anthocyanin accumulation than the long time UV-B treatment [49]. Levels of caffeic acid and rosmarinic acid of perilla grown under 2 h supplemental UV-A light for seven weeks were eight and seven times higher, respectively, than the plants under sunlight [61]. Accordingly, it could be concluded that supplemental UV light could increase the biosynthesis of phenolic compounds in herbs. The increase of phenolic compounds, flavonoids, and anthocyanins in herbs may be caused by multiple responses to light quality including increased activities of key metabolic enzymes leading to enhanced synthesis of pigments [62]. As key enzymes in the synthesis of anthocyanins, the expression of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), and dihydroflavonol 4-reductase (DFR) increased under blue light, resulting in an enhanced accumulation of anthocyanin in lettuce, Salvia miltiorrhiza, and Gerbera hybrida [28,62]. Polyphenol oxidase (PPO), which causes enzymatic browning of tissues, is also assumed to play a role in plant defense responses to biotic stresses through the production of reactive oxygen species (ROS) or cross-linking of quinones with proteins or other phenolics, forming physical barriers [63]. The expression of PPO was also increased by supplemental red and blue light in lettuce and S. miltiorrhiza, which may be related to the enhancement of phenolic compounds [28]. On the other hand, many plants avoid UV light by accumulating UV-filtering flavonoids and other secondary metabolites. For instance, epidermal flavonoids are enhanced in response to increased UV-B light [12]. The mechanisms of actions of the monochromatic or combined light on phytonutrients have not been clarified yet, but the aforesaid examples suggest that red, blue, UV light and the combinations of different light wavelengths could definitely affect the biosynthesis of phytonutrients. 4. Antioxidant Compounds The human body produces ROS, such as superoxide anion radicals, hydroxyl radicals, and hydrogen peroxide by many enzymatic systems through incomplete oxygen reduction [64]. Large amounts of ROS may favor human disease such as cancers, cardiovascular diseases, aging, and neurodegenerative diseases [64]. Therefore, the daily intake of exogenous antioxidants is beneficial for protecting the human body against the destructive effects of free radicals [65,66]. Most of the antioxidant compounds in a typical diet are derived from plant sources and belong to various classes of phytochemicals. Since light quality does affect the biosynthesis of phytonutrients, it is possible to improve antioxidant activities of herbs by altering the light spectrum, thus providing more natural antioxidants in our daily food. For instance, monochromatic red and blue LED light both enhanced the antioxidant capacities, total phenolics, and flavonoids in Chinese foxglove compared with white FL, and blue light was more efficient than red light [53]. Similar to the effects of light quality on growth parameters and some secondary metabolites, the enhancement of combined red and blue light was more efficient than monochromatic red or blue light only. The antioxidant activity was 2.0, 1.6, and 1.5 times higher, respectively, for coriander plants under 5:1, 10:1, and 19:1 red:blue ratios at 120 μmol m−2 s−1 PPF and 16-h photoperiod for 28 days compared with the plants under red light only [39]. Supplemental UV light enhanced the antioxidant activities of the upper leaves of M. arvensis [55], as well as sweet basil [49]. In conclusion, red, blue, and UV light treatments could be used to enhance the biosynthesis of antioxidant compounds and improve antioxidant capacities of herbs, which protect human body from various disorders through daily intake of fresh herbs or herb supplements.PDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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