4. Light Stresses as Elicitors of the Biosynthesis of Phytochemicals: Mode of Action
Elicitation of the biosynthesis of bioactive compounds as a defence response against light incidence depends on key factors that affect morphology and secondary metabolism activation. In this sense, both light quality and quantity are essential factors to control (Figure 2). As a result, when we talk about light quality, we refer to the wavelength applied as the specific region of the electromagnetic spectrum. From another point of view, the light direction (direct or indirect incidence), the photoperiod (application time), and the intensity are the main factors to study the specific dose, referring to the light quantity applied to the plant (Figure 2). As a result, the physiological effects of LED lighting on plant growth and development can vary according to these parameters. The main physiological effects can lead to the activation of the primary metabolism: respiration, senescence, photosynthesis, sprouting, growth, and blooming.
Figure 2. Key light factors influencing the plant secondary metabolism to produce bioactive compounds.
In this way, when the quantity of UV light (200–400 nm) exceeds the tolerance of the plant, instead of acting as an abiotic elicitor it causes DNA damage, slows photosynthesis, reduces flowering and pollination, and affects seed development. Particularly, UV-A can cause plant elongation, being less energetic than UV-B or UV-C and belonging to the beginning of the photosynthetic active radiation (PAR) spectral range
[33].
Before the UV section, blue light (400–500 nm) makes photosynthesis more efficient, which is necessary for this process because it is responsible for vegetative and leaf growth, especially for young plants such as sprouts, microgreens, or baby leaves. In addition, it is essential for opening the stomata and reducing the stretching of the plant by the suppression of spreading growth
[34]. Therefore, plants grown in blue light tend to be shorter and have smaller, thicker, and darker green leaves compared to plants grown without blue light. In consequence, blue photons drive the photosynthetic reaction, although their high energy is not fully utilised. However, a minimum intensity of blue light is needed for normal plant growth.
In this sense, radiation with shorter wavelengths, such as blue and UV lights, stimulates the production of pigments that absorb light and influence leaf colouring, such as chlorophylls and carotenoids. Furthermore, in some leafy green crops such as lettuce, blue also increases the production of health-promoting compounds such as antioxidants (phenolic acids, flavonoids, carotenoids, anthocyanins, and anthocyanidins) and some vitamins (E and C)
[35][36][37]. Thus, the application of blue and UV radiation prior to marketing can increase crop quality attributes such as leaf colouring and the bioactive content. Likewise, in the absence of blue and UV, some plants from the tomato family develop intumescence, or small blisters, on the leaves, stems, and petioles
[38][39]. This physiological disorder usually decreases as blue and UV radiation increases, which has been also tested to demonstrate antifungal activity
[39].
At the end of the PAR region, red light (600–700 nm) is the other critical point for light absorption in leaves. The phytochrome inside the leaves is more sensitive and responsive to red light, and it is important in regulating flowering and fruit production. It also helps to increase stem diameter and stimulates branching
[40]. Therefore, red light is the second largest contributor to photosynthesis, but similarly to blue it produces unique results in plant physiology. In fact, it can provide high plant growth, but without the limiting effect of blue, which darkens the chloroplast to protect it from the blue midday sun. Therefore, red is very efficient in producing tall, strong, fast-growing plants and, in fact, produces some of the highest growth rates of height and stem width in plants
[41]. The phytochrome pigment mediates plant flowering with a photoperiodic response. In general, plants are very sensitive to low intensities of red light, which can control the moment of the flowering and fruit setting
[42].
Far-red light can be found in the higher wavelengths of the visible spectrum (700–800 nm), which can cause plant elongation and trigger flowering in plants. The addition of far-red radiation to red light, in roughly similar amounts, is effective in stimulating flowering of a wide range of long-day plants
[42][43]. Therefore, lamps designed to regulate plant flowering always emit red and, in some cases, also far-red light.
In addition to physiological and morphological changes, light from different regions of the electromagnetic spectrum is also responsible for mediating genetic responses, which trigger the biosynthesis of chlorophylls, carotenoids, phenolic acids, flavonoids, anthocyanidins, anthocyanins, and glucosinolates (Figure 3).
Figure 3. Effect of different wavelengths on plant secondary metabolism: phytochemical accumulation due to photoreceptors and UV-C induced ROS (reactive oxygen species) production. UVR8: UV RESISTANCE LOCUS 8; COP1: COP1 E3 Ubiquitin Ligase; CRYP: cryptochromes; PAR1: t Protease-activated receptor-1; HY5: elongated hypocotyl-5; PIF1: phytochrome-interacting factor-1; Pfr: phototropins.
Specific photoreceptors (
Figure 1 and
Figure 3), such as UV RESISTANCE LOCUS 8 (UVR8) as the main UV photoreceptor
[44]; cryptochromes, phototropins, and zeitlupe family proteins as the UV-A/blue/green photoreceptors
[45][46]; and phytochromes as red and far-red photoreceptors
[47], are in charge of absorbing light signals that the plant receives. When UV/blue light activates photoreceptors, they activate transcription factors, which link with elongated hypocotyl-5 (HY5) as the main phytoene synthase (PSY) stimulator
[48]. Phytochromes also absorb red and far-red wavelengths and interact with the phytochrome-interacting factor (PIF) to trigger the genetic response to stimulate the plant secondary metabolism
[48][49]. Furthermore, there is a specific region of the UV-C narrow band (200–250 nm) that does not share UV photoreceptors (UVR8) with the UV-B section. Therefore, the phytochemical accumulation induced by low UV-C doses, which acts as electron donors, has been mainly associated with a mitochondrial energy-dissipating system, and with a partial water molecule ionisation due to it being the most energetic UV radiation, which produces reactive oxygen species (ROS) throughout the mitochondrial electron transport chain for the primary and secondary signalling pathways under UV-C ray incidence
[27] (
Figure 3).
Accordingly, these reactions are the focus of the main illumination strategies with relevant interest for their application during crop growing and during the postharvest period.