Light Stresses on Plants: Comparison
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This review summarizes the last scientific findings concerning the use of UV and visible spectrum LED lighting, as ‘Green, sustainable, and low-cost Technologies’, to improve quality of sprouts, microgreens, and baby leaves, to enhance their health-promoting compounds focusing on their mode of action, while reducing costs and energy. These technologies applied either during growing and/or after harvesting have shown to be able to improve physiological, and morphological development of young plants in their first stages of development, while increasing their bioactive compounds content without compromising safety and other quality attributes. The novelty is to summarize the main findings published in a comprehensive review, including the mode of action, and remarking the possibility of its postharvest application where the literature is still scarce.

This entry summarizes the last scientific findings concerning the use of UV and visible spectrum LED lighting, as ‘Green, sustainable, and low-cost Technologies’, to improve quality of sprouts, microgreens, and baby leaves, to enhance their health-promoting compounds focusing on their mode of action, while reducing costs and energy. These technologies applied either during growing and/or after harvesting have shown to be able to improve physiological, and morphological development of young plants in their first stages of development, while increasing their bioactive compounds content without compromising safety and other quality attributes. The novelty is to summarize the main findings published in a comprehensive review, including the mode of action, and remarking the possibility of its postharvest application where the literature is still scarce.

  • seed germination
  • ultraviolet
  • illumination
  • light-emitting diodes
  • abiotic stress

1. Introduction

Horticultural products are the most important and most-studied foods as a source of nutraceutical compounds. In this sense, due to the richness of the Mediterranean diet in fruits and vegetables, this dietary pattern has been considered one of the healthiest and was declared an Intangible Cultural Heritage of Humanity by UNESCO in 2013 [1]. Furthermore, young plants specifically in their first stages of development as sprouts, microgreens, and baby leaves have been demonstrated having more than 20-fold bioactive compound content compared to adult plants [2,3][2][3]. This makes them an important source of phytochemicals with a great benefit to be included in our daily balanced diet.
In this context, over the past several decades, and according to consumer requests, conventional crops and agri-food industries have widely increased and developed their production chains to satisfy the food demands related to the global growth of the population [4]. Regarding the social concern, the promotion of a healthy diet and regular physical activity have increased the necessity of nutraceutical foods and ingredients, which can provide positive effects to the human body, specifically since the recent pandemic due to the SARS-CoV-2 virus [5].
Lighting is one of the main energy sources during plant development and plays an essential role in the biosynthesis of health-promoting compounds. To adapt food industries to the new era of contemporary climate change, avoiding economic and energy waste is an essential challenge to focus on in research and development. Therefore, reducing the energy consumption during lighting is an essential point to take into consideration.
To make lighting more efficient, light-emitting diodes (LEDs) have been developed, which emit light at different wavelengths when the energy flows throughout the semiconductor and releases it transformed into photons. Technological advances in this technology have resulted in relevant improvements in agriculture, where intensity and spectral properties can be customised to adapt them to optimise the crop yield [6]. Illumination with regions of the light spectrum (from ultraviolet (UV) to infrared) can influence the biosynthesis of phytochemicals in a different way, as well as improve the plant development at different stages of growth, elongation, flowering, or fruit-setting.
Therefore, physical elicitation produced by the light perceived can be controlled to trigger the activation of the plant secondary metabolism as a defence mechanism against light incidence through the use of LEDs, also stimulated by photoreceptors and genes that are activated against abiotic stresses [7].
As a matter of fact, many authors have confirmed the relation between the stress generated by lighting and the increase in the biosynthesis and accumulation of nutraceutical compounds. For instance, low doses of UV-B and UV-C have been demonstrated to be good elicitors of the biosynthesis of carotenoids and flavonoids in bell peppers [8], as well as of glucosinolates and isothiocyanates in broccoli [2[2][9],9], radish sprouts [10], and kale sprouts [11], and of phenolic compounds in carrots [12]. Moreover, other less-energetic regions of the electromagnetic spectrum have also been shown to be good elicitors of such bioactive compounds, without the potential quality affectation in fruit and vegetables due to the high energetic UV radiation, which may induce cellular damage.

2. Elicitor’s Classification: Biotic and Abiotic

Horticultural crops are subjected to a wide range of stresses, which can reduce their yield and quality. Such stressors can be abiotic (UV, light, floods, drought, heavy metals, salinity, extreme temperatures, wounding, etc.) or biotic (bacteria attacks, fungi, nematodes, insects, and little animals, among others). Their uncontrolled application is one of the main causes of worldwide crop losses [13]. Nevertheless, when some of these stressors are applied in a customised way and under controlled conditions, such exogenous agents stimulate and generate signalling pathways in plants, which are translated into desired cellular responses.
Currently, a compound acts as an elicitor when it can stimulate by itself any kind of plant defence. Therefore, plan elicitors can be described as exogenous stressors (abiotic or biotic), which can enhance the production of secondary metabolites under indirect action, such as phenolic acids, flavonoids, and carotenoids, among other pigments, and glucosinolates and/or their derivatives: isothiocyanates. Although pathogenic bacteria were described as the first biotic elicitor [14], nowadays the essential molecule that produces the desired effect is directly added to generate such stress. Specifically, biotic elicitors activate different signal pathways to synthetise secondary metabolites as defensive molecules.
Independently from their origin, stimuli caused by stressors are received from the sensory receptors located on the cellular wall and are transferred throughout the cytoplasm to the nucleus, which triggers transcriptional changes that helps the plant to tolerate the suffered stress. In consequence, the defence against a pathogen attack includes the triggering of physiological barriers, proteins, and enzymes that confer the resistance against such stress.
Nevertheless, the application of biotic elicitors from pathogen microorganisms is not an attractive way of enriching horticultural commodities for human consumption. Therefore, abiotic stressors, which share an inert and mainly technological origin, have been developed in last few decades to be applied during growing and after harvesting. In this sense, to enhance the biosynthesis and accumulation of nutraceuticals, Cisneros-Zevallos [15] in 2003 first proposed the application of controlled postharvest abiotic stresses, which was recently updated in 2020 [16], and divided it into physical and chemical elicitors.
Physical elicitors are external agents applied under control to the plant in the form of physical damage, which can be produced by wounding, gas composition (modified atmospheres), temperature, humidity, salinity, osmolarity, or lighting. For instance, wounding carrot tissues has been shown to be an interesting way to increase the content of phenolic acids, mainly chlorogenic acid, especially combined with temperatures higher than 15 °C, UV-C lighting, and/or the application of methyl jasmonate, which can be a consequence of the activation of transcriptomic genes involved in the biosynthesis of phenolics [13,17][13][17]. In addition, the enzyme phenylalanine ammonia lyase (PAL) is activated by abiotic stresses against reactive oxygen species (ROS) accumulation, which is linked to the production of phenolic compounds [13,17][13][17]. In fact, the wounding of fresh carrot has been demonstrated to be effective also in combination with UV-A, UV-B, and UV-C lighting, whose application has been related to the production of ROS and, consequently, to the production of antioxidant compounds to fight against such oxidative processes [18,19][18][19]. In this sense, a direct correlation has been found between the UV-B application and the biosynthesis of nutraceuticals, which was recently corroborated in red prickly pears [20], broccoli by-products [21], carrots [22], and red bell peppers [8,23][8][23] to enhance the accumulation of glucosinolates and sulforaphane, phenolics, and carotenoids, respectively. These studies showed that the external stress provoked by UV increases the transcription of genes (FMO GS-OX5 -flavin-monooxygenase glucosinolate S-oxygenase 5- and MYB51) involved in the biosynthetic pathway of glucosinolates [2], as well as the genetic code in charge of the secondary metabolism to counteract the ROS action when UV RESISTANCE LOCUS 8 (UVR8) is activated [24].
Furthermore, besides its use as a sanitiser, UV-C in low doses has been shown to be an elicitor of the biosynthesis of carotenoids and flavonoids in red bell peppers on its own or combined with UV-B [8], and in tatsoi baby leaves combined with hyperoxia conditions [25]. Additionally, as a physical elicitor, modified atmospheres containing high oxygen partial pressure act by enhancing the biosynthesis of phenolics in carrot [26]. These findings have been associated with the direct response of the plant against the oxidation processes produced by such elicitors. In fact, these physical elicitors act throughout the ionisation of the water molecules of the plant cells, which increase the ROS accumulation and the production of natural antioxidants to avoid their effect [27].
Regarding chemical elicitors, inorganic salts, ethanol, ethane, benzothiadiazole, acetic acid, and heavy metals in the appropriate dose can also increase phytochemical production, as is the case in the use of AgNO3 or CdCl2 to increase the production of alkaloids in crops [28]. Moreover, several plant hormones, such as jasmonic and salicylic acid, methyl jasmonate, methyl salicylate, ethylene, gibberellin, and cytokinin, are considered abiotic stressors to stimulate the accumulation of health-promoting compounds, because they act as transcription factors and mediators in hormone-signalling pathways [9,29][9][29].

3. UV and Visible Spectrum Light Sources

Illumination is essential for plant growth and development. Artificial light sources have substituted sunlight for many years to optimise crop yields. In this sense, lighting with specific regions of the UV and visible spectrum are nowadays applied in growth chambers, greenhouses, and vertical farming to obtain standardised and healthier fresh fruit and vegetables.
LEDs are diodes that only allow current to flow in one direction, called the forward direction. A solid diode is made by adding several layers of silicon, but a diode does not emit light by itself. LEDs emit light due to their components In, Ga, and N, which are mixed and, thanks to the current, emit bluish light. On the other hand, by mixing Al with In and Ga, we get a reddish light. When we mix several of these components in the layers of solid diodes and give them electricity, we get them to light up [30]. With different mixtures of the above components, different colours are achieved, but not all of them.
Particularly, LEDs have several advantages that makes them attractive for all their applications. For instance, their use supposes a decrease of 80% of the energetic cost compared with halogen lights, besides their long life (>100,000 h) without any maintenance. As a matter of fact, the lighting yield of LEDs, which is the electric energy converted to visible light, is 75% higher than conventional halogen lights with 15%, as well as the chromatic reproduction index (CRI), which increases to 90% and is the quality of the light source to reproduce the colours.
In the case of UV light, a UV LED lamp is generally produced on aluminium nitride (AlN) to reach wavelengths lower than 350 nm [31]. New generations of UV LED are more efficient, have a longer service life, save more energy and, hence, protect the environment, despite the high purity of the UV rays emitted by UV LED lamps. Although traditional UV lamps were produced on high-pressure mercury, they produced ultra-high temperatures that are avoided by UV-LED lamps, which do not raise the temperature during the treatment, acting as a cold light source and as a kind of protection for the equipment and for the product [32]. Moreover, the UV LED tube does not use ozone, as high-pressure mercury lamps do, and there are no heavy metal elements inside, which is why it is environmentally friendly [32]. Furthermore, the specific range of emission of the electromagnetic spectrum (UV-C, UV-B, UV-A, and visible spectrum lights) allows it to optimise plant development and the activation of the plant secondary metabolism (Figure 1), acting as an abiotic elicitor of the production of nutraceuticals, especially during the first growth stages, as is the case for sprouts, microgreens, and baby leaves.
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Figure 1. Main photoreceptors implied in light capture according to the UV and visible light spectrum.

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.
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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][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][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][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).
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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][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][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.

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