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Horticulturae 2017, 3, 36 2 of 11 users [3], the annual expenditure on herb products in South Korea increased from $4.4 billion in 2004 to $7.4 billion in 2009 [4], and out-of-pocket spending on natural products in the United States was $14.8 billion in 2008 [5]. Diets with fresh or processed herbs are positively related to prevention of cardiovascular diseases, chronic diseases, and certain types of cancer, due to high concentrations of phytonutrients such as essential oils, phenolic compounds, flavonoids, and carotenoids [6–8]. Phytonutrients, or bioactive secondary metabolites, are organic compounds not directly involved in primary metabolic processes of growth, development, or reproduction of plants. Phytonutrients play an important role in plant defense against insects and pathogens, act as attractants to pollinators and seed dispersers in reproductive processes, and some may create a competitive advantage as poisons for rival species [9]. Phytonutrients have been used in various industries such as medicines, flavorings, dyes, fibers, glues, oils, waxes, and perfumes [2,3,9]. The increasing consumption of herb products has been accompanied by quality and consistency issues associated with field production. Varying climatic conditions among seasons or locations and adverse environmental conditions all lead to fluctuating yield and phytonutrient concentrations of herbs [10,11]. Hence, some growers are turning to controlled environment agriculture (CEA) including high tunnels, greenhouses, and indoor vertical farms for herb production. To provide high-quality and stable herb product supply, CEA technologies are an alternative and complementary to field production, especially in areas with limited daylight or arable land, or those with other unfavorable environmental conditions [12–15]. Most CEA technologies or systems use artificial lights to ensure plant growth, and new lighting technologies such as light-emitting diodes (LEDs) have the capacity to meet the light intensity and wavelength requirements of different plant species [16–18]. Until recently, high intensity discharge (HID) lamps and fluorescent lamps (FLs) have been the most popular light sources in CEA systems [12]. HID lamps, such as metal halide and high pressure sodium lamps, are typically used in greenhouses to provide supplemental light due to relatively high luminous efficacy (max. 200 lumens per watt) and photosynthetically active radiation (PAR) efficiency (max. 40%) [19]. However, the high surface temperatures of HID lamps prohibit close placement to plant canopies, and the unsuitable spectra of some HID lamps also limit their use for plant production. FLs are the most widely used supplemental light source in CEA systems, due to the higher PAR output, lower cost, and lower surface temperatures compared with HID lamps [20,21]. It was reported that 60% of indoor vertical farms in Japan used FLs as a light source in 2013, 13% used HID lamps, and 27% used LEDs [12]. There is great and increasing interest regarding the use of LEDs in horticulture. The global market of LEDs grew robustly by 32% while the overall lighting market declined by 15% in 2009 [22]. The increasing use of LEDs is due to their high PAR efficiency (80–100%) [19], decreased cost, ease of control, and availability of specific wavelengths suitable for horticultural uses [20,23–25]. In terms of economics and sustainability, LED technology is predicted to replace FL and HID lamps in horticultural systems and to revolutionize CEA [19]. Both phytonutrient content and yield may be influenced by the light quality used for herb production under controlled environments. Research has indicated that photosynthetic responses of single leaves under artificial light were generally similar, but the morphological responses were largely species- and cultivar-specific, and both photosynthetic and morphological responses contribute to plant yield [26]. The average quantum efficiency of 22 plant species suggested that plants potentially have higher photosynthetic efficiency under red light than blue and green light [26,27]. Red light supplemented with blue light increased leaf net photosynthetic rate and crop yield more than monochromatic red or blue light, but yield decreased when blue light proportion (BP) reached a threshold, which varied among plant species and cultivars [16,18]. On the other hand, long-wavelength red light could enhance phenolic compound accumulation, and short-wavelength blue light could enhance the biosynthesis of flavonoids and lactones [28,29]. In this review, we summarize the effects of red (620–700 nm), blue (400–490 nm), UV (280–380 nm), and photosynthetically less-efficient lights,PDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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