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Horticulturae 2017, 3, 36 4 of 11 the total chlorophyll content in leaves to promote photosynthesis, but inhibited the transport of carbohydrates from leaves to sinks, which suppressed photosynthesis due to the high level of carbohydrates in the leaves [41]. Blue light increased the ratio of chl a/b, improved the activities of ribulose-1,5-bisphosphate carboxylase (Rubisco) and phosphoenolpyruvate carboxylase, and promoted stomatal opening, which improved photosynthesis per unit of leaf area [28]. In terms of plant morphogenesis, red and/or blue light influenced plant yield. Red light promoted cell division and expansion which increased leaf area and root elongation, while blue light inhibited cell division and expansion, thus reducing leaf area [18,30]. The reduced photon capture resulting from reduced leaf area may be the reason for a high level of BP reduced plant growth in spite of increased photosynthesis per unit leaf area. 2.2. Photosynthetically Less-Efficient Lights: Far-Red, Green, and Ultraviolet Light Little research has been conducted to explore the effects of photosynthetically less-efficient lights on herbs. In addition to red and blue light, photosynthetically less-efficient far-red, green, and ultraviolet (UV) lights are also important environmental signals to plants. For instance, far-red light reversed the effects of phytochrome leading to changes in gene expression, plant architecture, and reproductive response [42]. Far-red-rich light caused increased stem elongation, strengthening of apical dominance, acceleration of flowering, reduced assimilate storage, reduced seed set, reduced branching, shortened fruit development, and reduction in seed quality [43,44]. Li and Kubota [45] demonstrated that supplemental far-red light increased the FW and DW of baby leaf lettuce (Lactuca sativa ‘Red Cross’) at 300 μmol m−2 s−1 PPF and 16-h photoperiod for 12 days, but decreased the concentrations of anthocyanins, carotenoids, and chlorophylls compared with white light alone. The hypocotyl elongation of tomato plants (Solanum lycopersicum ‘Aloha’ and ’Maxifort’) grown in the greenhouse was increased by end-of-day far-red light radiation at 3 or 4 μmol m−2 s−1 PPF for 14 days [46]. Although the effects of far-red light on plant growth have been demonstrated in various vegetables, there has been little research using herbs. Green light has the lowest quantum yield, but it can penetrate deeper into the plant canopy due to its high transmittance and reflectance than the other wavelengths. Therefore, theoretically, quantum yield of a dense plant canopy should be more equalized under green light. In fact, Paradiso et al. [47] validated that canopy quantum efficiency of green light was not much lower than that of red light in roses (Rosa spp. ‘Akito’). In addition to intercepting red and blue light, phytochromes and cryptochromes are also green light receptors, which explains why green light affects vegetative growth, organ growth, and plant tropism [42], although the effects of green light may be less significant as light intensity increases [48]. UV light is generally considered a stress factor to plants and leads to smaller plants, reduced photosynthesis, and lower biomass accumulation, but appropriate UV light treatment (suitable light intensity and exposure time) was shown to increase plant yield in some species. Sakalauskaite et al. [49] reported that both 1 h and 2 h supplemental UV-B light per day for seven days increased plant height, leaf area, FW, and DW of sweet basil, and 2 h UV-B treatment had a greater effect than 1 h UV-B treatment. In contrast, a long exposure time at a low intensity or a short exposure time at a high intensity of supplemental UV-B light both decreased net photosynthetic rate of Chinese liquorice (Glycyrrhiza uralensis) [50], and reduced the leaf area and leaf DW of sweet basil and perilla [51,52]. Therefore, the effects of light intensity and exposure times of UV light on plant production needs further investigation. 3. Phytonutrient Accumulation Plants produce a vast and diverse assortment of phytonutrients which are widely used as medicines and flavorings, and are being researched for new drugs, antibiotics, insecticides, and herbicides. The most widely researched phytonutrients in herbs are essential oils and phenolic compounds. Essential oils are naturally occurring volatile aromatic compounds which are found inPDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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