**4. Oxidative stress by UV light induction**

Ultraviolet radiation is an important stress in plants that elicits protective mechanisms such as the accumulation of secondary metabolites in the cell (**Figure 1B**) [27–29] and an increase in leaf thickness [30]. Interestingly, UV radiation is a hormetic stimulus, that is severe exposure is harmful, but exposure to lower sub-acute levels can stimulate protective mechanisms [31]. Consequently, plants can become resilient to UV after repeated exposure [32].

The changes in the secondary metabolism of plants from all taxa under exposure to UV radiation have been widely documented. For instance, in the moss *Pohlia nutans*, UV-B radiation enhanced flavone biosynthesis through increasing type I flavone synthase activity [33]. In *Taxus cuspidate*, UV-B radiation (3 W/m2 ) provoked the accumulation of toxoids and flavonoids [34]. Also, the flavonoid contents in *Scutellaria baicalensis* reached the maximum concentration (41.86 mg/g−1) after seven days under UV-A radiation [35]. In *Pisum sativum* leaves, exposure to UV-B radiation increased the nicotinamide and trigonelline content; the nicotinamide induction is an oxidative stress reaction [36]. In an analysis performed on two different ecotypes of the *Paubrasilia echinata* tree, it was shown that UV-B radiation inhibited stem growth, biomass accumulation, CO2 assimilation, and photochemical efficiency in a shade-tolerant ecotype inhibition; in contrast, a sun-tolerant ecotype showed a positive response: UV-B increased flavonoids, lignin, and antioxidant properties, but reduced cell respiration [37]. In *Pinus radiata,* UV radiation provoked an early response reducing photosystem activity and accumulation of photoprotectors; even the primary metabolism was rearranged to minimize ROS production, also the isoprenoids compounds like carotenoids, tocopherols, phytol, and gibberellins were decreased [38]. Under exposure to UV-B radiation followed by dark treatment, the number of flavonoids and coumarins in *Clematis terniflora* increased significantly; while proteins related to photorespiration, the tricarboxylic acid cycle, and mitochondrial permeability showed differential expression profiles, indicating that UV-B radiation induces a reduction in energy consumption and maintains energy balance [39]. *Nymphoides humboldtiana* increased antioxidant activity and production of flavonoids like phloroglucinol, chlorogenic acid, epicatechin, quercetin, and ferulic acid after 13 days of exposition of UV-B radiation [40]. *Colobanthus quitensis* under UV-B radiation increased the biosynthesis of flavonoids, particularly flavone C-glycosides,

metabolites located within the most metabolically active cells [41]. *Melisa officinalis* showed changes in the glycolysis and phenylpropanoid pathway under UV radiation stress with differential recovery times [28].

Studies on algae have shown similar mechanisms, as expected by their phylogenetical relation to plants. For example, UV-B radiation-induced ROS production in peroxisomes and chloroplasts in *Ulva prolifera* provokes irreversible damage under 5 W m−2 [42]. In *Chlamydomonas reinhardtii,* UV-C radiation stress increased ROS levels and production of antioxidant polyphenols, a phenolic including caffeic acid, cinnamic acid, coumaric acid, salicylic acid, and protocatechuic acid, among others [43].

A notable example among plant-derived compounds is the alkaloid mimosine, present in the seedlings of *Leucaena leucocephala* spp. Glabrata is particularly interesting due to its therapeutic uses as anti-cancer, antifungal, and antimicrobial, which increase its economic interest. Acute UV-C exposure of *L. leucocephala* seedlings induced a strong accumulation of mimosine, which could be implicated in general oxidative stress modulation [44].

The effect of UV-radiation stress has also been extensively studied in plants used in traditional medicine. For example, *Morus alba*, used in traditional Chinese medicine, reduces its growth and secondary metabolism after exposure to UV-B [45]. *Gingko biloba* leaves, after long-term exposure to UV-B radiation, increase flavonoids biosynthesis, and these are beneficial as therapeutic active ingredients [29]. Two different species of the Chinese herb Astragalus modified their secondary metabolite production under UV-B radiation; *A. mebranaceus* produced increased hydroxycinnamic acid derivates, while *Astragalus mongholicus* accumulated myricitrin and isoflavones, showing different tolerance to UV-B stress [30]. The flowers of *Lonicera japonica* are used as a medicinal herb in Asian countries. Under UV radiation, *L. japonica* increases the levels of oxidative pentose phosphate and secondary metabolites such as secologanic acid, secoxyloganin, and isochlorogenic acid [46]. In *Adhatoda vasica*, also used in Asiatic traditional medicine, UV-B radiation (7.2 kj m−2 day−1) induces a reduction of superoxide radical production while increasing hydrogen peroxide production [47]. Finally, *Centella asiatica*, used in Asian and African traditional medicine, accumulated saponins and epidermal flavonols under UV-B radiation in younger leaves with high levels of saponins; in contrast, in older leaves, sapogenins were the most abundant metabolites [48].

As shown in these studies, UV light has forced algae, bryophytes, and plants to modify their metabolism—particularly the secondary metabolism—to increase their ecological success rate. However, this also has important consequences for plants of commercial interest, as seen below.

## **5. UV radiation as functional quality of plant foods of commercial interest**

Historically, economically important plants have been exhaustively studied; recent studies have focused particularly on UV light stress, searching for alterations in organoleptic properties and secondary metabolism. In modern horticulture, plants of economic interest have been irradiated with UV light during the flowering/fruiting period, with the purpose of stimulating oxidative stress pathways as well as antioxidant production [49]. In tomato juice production, the stress caused by UV radiation in plants decreased pectolytic enzymes, improving and preserving tomato characteristics for a longer period of time [50]. Also, in a tomato cultivar, UV-A and B radiation produced higher ripening synchronization and smaller fruits.

#### *Ultraviolet Radiation and Its Effects on Plants DOI: http://dx.doi.org/10.5772/intechopen.109474*

Exposure to UV-A radiation-induced accumulation of phenolics and flavonoids, making these fruits more appealing to consumers [51]. Furthermore, in tomato seedlings under UV-B radiation, carotenoid content increased as well as antioxidant enzyme activities [52].

Another plant of great economic importance is soybean (*Glycinine max*), which increases the isoflavone content of the sprouts under UV radiation [53]. In soybean seedlings, nitric oxide is induced as a protection against UV-B stress [54]. Meanwhile, on germinated soybean, UV-B radiation increased the contents of linoleic acid and erucic acid content, as well as isoflavones, phenolic acids, vitamin C, folate, and chlorophyll, improving nutritional and functional qualities [55]. Conversely, excessive UV-B exposure damaged cells and decreased the amount of isoflavones within them [56]. In cultured soybean, UV-C radiation increased the amount of genistein-Oglucoside and genistein-O-glucosyl-malonate, suggesting *in vitro* culture to obtain a high level of metabolites [57]. Moreover, in germinated soybean under UV-B radiation, total protein content and endogenous H2O2 were increased [58].

Cereals and ornate flowers also have responses to UV radiation. Wheat seedlings under UV stress showed an increase of phenylalanine ammonia-lyase only in the roots, indicating that UV-B radiation has a positive or negative impact, depending on the type of secondary abiotic stress factor observable in the production of phenolic compounds [59]. Also, germinated wheat under UV-B radiation increased phenols, ferulic acid, and coumaric acid. Exogenous Ca<sup>2</sup> + positively affected free and bound phenolic accumulations [60]. In amaranth (*Amaranthus cruentus* L.), UV-C radiation improved postharvest quality by increasing levels of quercetin, kaempferol, copene, lutein, β-carotene, and caffeic acid derivates [27]. In lily bulbs, UV-C radiation increased total phenolic content and antioxidant activity, indicating that UV-C radiation is a safe alternative for processing lily bulbs in storage [61].

Likewise, the effects of UV radiation have been studied in economically important herbs. In spinach cultivars, UV-C induced a hormetic effect that increased total phenolic compounds and reduced the presence of the parasite fungi *Alternaria* ssp. in the crops [62]. In barley seedlings, UV-B radiation up-regulated enzymatic activity, resulting in the accumulation of phenolic acids [63]. *Mentha aquatic* responded to UV-B radiation on a morphological level, increasing glandular trichomes, and on a biochemical level, increasing oxidative metabolism and overexpressing genes implicated in terpene biosynthesis, particularly volatile oils as camphene, β-pinene, and germacrene [64]. In wounded carrots under UV-A and C radiation, ROS increased, acting as a signal for ethylene synthesis, which activated the synthesis of jasmonic acid leading to the accumulation of phenolic compounds [65]. In fresh-cut carrots, UV-C doses inhibited ascorbic acid, total carotenoid, respiration, total phenols, lignin, malondialdehyde, and ethylene production; all data collected indicated extended shelf-life and overall quality maintenance [66]. In parsley, UV-C doses resulted in an increase of antioxidants such as phenylpropanoid and phenolic compounds, as well as enzymes involved in the synthesis of phenylpropanoid [49]. The effect of UV radiation induces the production of 6″-0-malonylapiin, which is a flavone glycoside, as well as the 12-oxo-phytodienoic acid [67]. UV-B radiation (1.5 kJ m−2) maintained the color of broccoli florets during storage, and induced glucosinolates and hydroxylcinnamates, raising their antioxidant properties. These findings suggested that UV-B radiation is likely to induce the indole glucosinolate pathway [31], maintaining the quality of broccoli florets in low-temperature storage [68].

Fruits are also of economic interest and respond differentially to UV. Grape berries (Jumeigui variety) decreased sugar content under UV-C, promoting the accumulation of stilbenes and some flavonoids [69]. In contrast, berry clusters (red table emperor) under UV-A and B radiation decreased the amount of quercetin 3-O-glucoside and quercetin 3-O-glucuronide, suggesting that UV radiation induces postharvest changes in phenolic metabolites [70]. In fresh-cut strawberries, UV-C increased phenolic compounds, anthocyanin, cyanidin 3-glucoside, pelargonidin 3-glucoside, and cyanidin 3-glucoside-succinate, activating the phenylpropanoid pathway, thus improving antioxidant capacity without losing fruit quality [71]. In two blueberry cultivars (*Vaccinium corymbosum*), exposure to UV radiation showed that the amount of phenylpropanoid compounds was higher in the Legacy cultivar than in the Bluegold cultivar, which indicates that UV-B acclimation is different between cultivars [72]. Moreover, in highbush blueberry leaves (*V. corymbosum* L. cv. Brigitta and Bluegold), photosynthesis decreased in the Bluegold variety under UV-B radiation; in contrast, the Brigitta variety increased the photosynthesis rate as well as antioxidant activity [73]. In fragrant pear, postharvest UV-C radiation controlled blackhead disease through chitinase, β-1,3-glucanase, peroxidase, superoxide dismutase, catalase, ascorbate peroxidase, and phenylalanine ammonia-lyase [74]. In nectarine, UV-C radiation induced an increase in anthocyanin biosynthesis and promoted the antioxidant system, stimulating the phenyl propane pathway. Together, these compounds exerted antifungal action against *R. stolonifera* [75]. In young leaves of *Vitis vinifera*, low UV-B radiation increase sitosterol, stigmasterol, and lupeol, probably as an acclimation response. In contrast, diterpenes, tocopherol, phytol, E-nerolidol, monoterpenes as careen, α-pinene, and terpinolene were present in high amounts in mature leaves; these results showed that the synthesis of terpenes is an adaptive response to UV-B radiation stress [76]. In postharvest lemon fruits after UV-B radiation, phenolic compounds increased in flavedo, indicating that lemon peel modifies enzymatic activities involved in sucrose metabolism [77].

Furthermore, in *Olea europaea*, UV-B radiation increases secoiridoids and 2″-methoxyoleuropein metabolites, while decreasing oleuropein as an antioxidant defense against UV [78]. The peach (*Prunus persica*) diminishes the synthesis of anthocyanins and phenolic compounds under UV-B exposure, but after 36 h, it increases anthocyanins, cyanidin, and delphinidin compounds [79]. In Luffa seedlings, the oxylipins such as methyl jasmonate and 12-Oxo-phytodienoic acid mitigated the UV-B stress via improved photosynthetic and nitrogen metabolism, respectively [80].

Even economically important algae respond to UV radiation. In several Spirulina species, mild stress by UV-B radiation has been useful in increasing physiological and nutritional competencies in growth, rendering UV radiation useful in producing this functional food [81].

Although most of the above-mentioned economically important species appeared to benefit from UV exposure, it has been detrimental to some species. Rice (*O. sativa*) plants treated with UV-C had less palatability and were easily infested by the weevil *Sitophilus oryzae*, which provoked lower consumer acceptance and purchase intention [82]. Also, sweet cherry fruits under UV-C radiation diminished respiration, but increased rhamnose, mannose, galactarate, threonate, and aspartate contents [83].

This evidence highlights the importance of studying UV stress in plants of economic interest, as it can lead to higher yields and thus higher profits. However, care must be taken before implementing UV irradiation as a production-boosting resource because some species might be impacted negatively, as evidenced by the effects of the increased exposure to UV derived from climate change. Plant litter decomposition,

especially in regions with low annual rainfall and reduction of photosynthetically active radiation (PAR), further strain crop production [84].
