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Horticulturae 2017, 3, 36 7 of 11 5. Light as a Regulator: Controlling Herb Production with LEDs Herbs grown under broadband sunlight that shifts both in intensity as well as in spectral distribution resulted in varied yield and phytochemical concentrations. Providing sufficient volume of high quality herbs to meet an increasing demand is especially challenging with global climate change and shortage of resources, such as arable land and water. Therefore, application of LEDs in CEA provides an additional method for herb production that has enormous potential for targeted regulation of plant growth and metabolic responses. Based on previous research, future studies of herb cultivation with LEDs should concentrate on the several aspects that follow. Combinations of red and blue light with different BP—Red and blue light are the most efficient spectra for plant photosynthesis. Previous research only examined the effects of monochromatic red or blue light, and few studies investigated the effects of combined red and blue light, and for those that did so, the BP threshold was not determined. A single LED solution will not solve all problems and may achieve different goals. Therefore, combinations of red and blue light with different BP should be investigated to meet the varied requirements of herbs for specific species, growth stages, and multiple cultivation purposes. Addition of photosynthetically less-efficient lights—It has been demonstrated that photosynthetically less-efficient lights such as green, far-red, and UV light could improve plant photosynthesis [47], hasten flowering and reverse the effects of phytochromes [42], and enhance the biosynthesis of pigments and phytochemicals in some plants [49,56], respectively. The other light spectra within the photosynthetically active region, such as cyan, yellow, and orange light, are also absorbed by photosynthetic pigments and utilized in leaves [12]. However, the effects of photosynthetically less-efficient lights on the growth and phytonutrient accumulation of herbs are still unknown. To make the best use of photosynthetically active light, the effects of UV, cyan, green, yellow, orange, and far-red light on herb production need to be investigated. Sideward lighting—When artificial light is given downward to a densely-populated plant community, most light energy, especially red and blue light, is absorbed by the upper leaves while light energy received by lower leaves is nearly equal to or lower than the light compensation point, which significantly decreases photosynthetic productivity [12]. The low surface temperatures of LEDs makes it possible to install them alongside and close to plants or within the plant canopy, reducing the uneven vertical distribution of light energy between upper and lower leaves. Several studies have been conducted with tomato (S. lycopersicum), rose, and micropropagated plantlets [12,67], but this has not been investigated for herbs. Such placement may affect herb morphology, physiology, and improve plant yield. Interactions of light with other environmental factors—Light is one of the most important environmental factors influencing plant growth and secondary metabolism, and use of LEDs would allow growers to induce desired responses in herbs. On the other hand, other factors such as temperature, humidity, carbon dioxide, and fertilization should be adjusted accordingly with light conditions to promote plant growth and development. To achieve high yield with enriched phytonutrient concentrations in herbs, it may be recommended to provide optimal environmental conditions initially to get a fast growth rate and then provide environmental stresses several days before harvest to enhance the production of medicinal components. Acknowledgments: This research is partially supported by USA. Department of Agriculture National Institute of Food and Agriculture Hatch project TEX090450. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agencies. Author Contributions: All authors contributed extensively to the paper. Haijie Dou wrote the paper; Genhua Niu, Mengmeng Gu, and Joseph G. Masabni helped to revise and improve the paper. Conflicts of Interest: The authors declare no conflict of interest.PDF Image | Light Quality on Growth and Phytonutrient Accumulation of Herbs
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