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Horticulturae 2017, 3, 36 3 of 11 such as cyan (490–520 nm), green (520–570 nm), yellow (570–590 nm), orange (590–620 nm), and far-red (700–800 nm) light, on growth and phytonutrient accumulation in herbs and discuss the potential for regulating herb productivity and quality through manipulation of light spectra. 2. Photosynthesis and Plant Growth 2.1. Red and/or Blue Light Photosynthesis is the foundation of food production, and photosynthetic processes are often modified in plants grown under artificial lights, since lamps do not usually provide the same spectrum, light intensity or photoperiod as sunlight. Red light seems to be more effective in improving photosynthesis compared than blue or green light [26,27]. For instance, supplementary red light in a greenhouse increased the leaf area of grape (Vitis vinifera cv. ‘Jingxiu’) as well as the dry mass distribution ratio in leaves compared with sunlight [30]. Goto et al. [31] assessed the gross photosynthetic rate (Pg) of both green and red perilla (Perilla frutescens), a popular herb in East Asia used in traditional Chinese medicine, under different combinations of LEDs (405, 465, 530, 595, 660, and 735 nm) at 250 μmol m−2 s−1 photosynthetic photon flux (PPF) and 16-h photoperiod for 14−21 days in a growth chamber. The highest increment of Pg was obtained with red light (660 nm) for both green and red perilla, and Pg was lowest at 465 and 530 nm for red perilla [31]. Consistent with the above results, red perilla grown under red-enriched light treatments (red light alone, a combination of red and blue light, and a combination of red and green light) had more leaves, bigger leaves, and greater dry weight (DW) than plants grown under blue light alone, a combination of blue and green light, or green light alone [32]. The high efficiency of red light on plant growth is easy to understand because red light wavelengths perfectly fit the absorption peak of chlorophylls and phytochromes and, therefore, red light would be the most efficient light to supplement existing light conditions. Combining red and blue light is more effective than monochromatic red light for plant growth. Plants grown under monochromatic red light exhibited elongated hypocotyls and cotyledons, a condition known to be phytochrome-dependent [19]. Blue light increased the ratio of chlorophyll a/b (chl a/b), affected plant photomorphogenesis, and promoted stomatal opening in plants [33–35]. The combination of red and blue light provided better excitation of photoreceptors including phytochromes, cryptochromes, and phototropins, and resulted in higher photosynthetic activity than plants under either monochromatic red or blue light [36]. Dong et al. [37] reported that combined red and blue light (1/3 blue light at 450−460 nm + 2/3 red light at 620−630 nm, at 400 lx and 12-h photoperiod for 60 days) increased the DW and bioefficiency of Cordyceps militaris mushroom compared with blue light alone, red light alone, and sunlight. For most species, combined light wavelengths with a large proportion of red light supplemented with blue light significantly increased plant yield [16]. However, plant yield reached a plateau or decreased when the BP reached a threshold, which varied among species [38,39]. For instance, fresh and dry mass of coriander (Coriandrum sativum), one of the most useful essential oil-bearing herbs as well as medicinal plants, increased when BP increased from 5% to 9% at 120 μmol m−2 s−1 PPF and 16-h photoperiod for 28 days, but decreased when BP was 17%, and was lowest under monochromatic red light [39]. The leaf area of Anoectochilus roxburghii was not different when BP was 17% or 25% under combinations of red and blue LEDs with 12-h photoperiod for 40 days, but decreased when BP reached 50% [40]. In contrast, the fresh weight (FW) of sweet basil (Ocimum basilicum), one of the most commonly grown herbs in the United States, increased when BP increased from 15% to 59% at 200 μmol m−2 s−1 PPF and 16-h photoperiod for 31 days, and the total lateral shoot FW was highest under monochromatic blue light compared with red, green, blue+green, or white light at 100 μmol m−2 s−1 PPF and 16-h photoperiod for 70 days [16,38]. Within the photosynthetically active radiation (PAR) spectrum from 400 to 700 nm, red and/or blue light affect both photosynthesis and plant morphogenesis. Red and/or blue light affected chlorophyll content, photosynthetic enzyme activity, stomatal opening, and the distribution of carbohydrates in plants [26,27,33,34,41]. Compared with other PAR wavelengths, red light increasedPDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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