Abstract
In recent years, consumption of herb products has increased in daily diets, contributing to the prevention of cardiovascular diseases, chronic diseases, and certain types of cancer owing to high concentrations of phytonutrients such as essential oils and phenolic compounds. To meet the increasing demand for high quality herbs, controlled environment agriculture is an alternative and a supplement to field production. Light is one of the most important environmental factors influencing herb quality including phytonutrient content, in addition to effects on growth and development. The recent development and adoption of light-emitting diodes provides opportunities for targeted regulation of growth and phytonutrient accumulation by herbs to optimize productivity and quality under controlled environments. For most herb species, red light supplemented with blue light significantly increased plant yield. However, plant yield decreased when the blue light proportion (BP) reached a threshold, which varied among species. Research has also shown that red, blue, and ultraviolet (UV) light enhanced the concentration of essential oils and phenolic compounds in various herbs and improved antioxidant capacities of herbs compared with white light or sunlight, yet these improvement effects varied among species, compounds, and light treatments. In addition to red and blue light, other light spectra within the photosynthetically active region—such as cyan, green, yellow, orange, and far-red light—are absorbed by photosynthetic pigments and utilized in leaves. However, only a few selected ranges of light spectra have been investigated, and the effects of light quality (spectrum distribution of light sources) on herb production are not fully understood. This paper reviews how light quality affected the growth and phytonutrient accumulation of both culinary and medicinal herbs under controlled environments, and discusses future research opportunities to produce high quantity and quality herbs.
Highlights
With rising awareness of health benefits, people are consuming more fresh herbs or taking dietary herbal supplements, among other self-care options, as medicinal alternatives [1]
Blue light increased the ratio of chlorophyll a/b, affected plant photomorphogenesis, and promoted stomatal opening in plants [33,34,35]
L-menthol, the main component of essential oils in M. arvensis, was 1.4 times higher under red light than blue or green light, both at 150 μmol m−2 s−1 PPF and 16-h photoperiod for 28 days [6], while the glycyrrhizic acid concentration in root tissues of Chinese liquorice was highest under red light, followed by white light, and lowest under blue light at 300 μmol m−2 s−1 PPF and 16-h photoperiod for three months [50]
Summary
With rising awareness of health benefits, people are consuming more fresh herbs or taking dietary herbal supplements, among other self-care options, as medicinal alternatives [1]. 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]. Both phytonutrient content and yield may be influenced by the light quality used for herb production under controlled environments. We summarize the effects of red (620–700 nm), blue (400–490 nm), UV (280–380 nm), and photosynthetically less-efficient lights, 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
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