**1. Introduction**

As sessile organisms, plants are constantly exposed to a wide variety of stress factors, such as desiccation, environmental pollution, temperature changes, and UV radiation. Ultraviolet radiation is a part of the nonionizing radiation region of the electromagnetic spectrum and comprises about 9% of the emitted solar

radiation; according to the ISO 21348 standard, it is divided into three types: UV-C (200–280 nm), UV-B (280–315 nm), and UV-A (315–400 nm) [1, 2].

The ozone layer (O3 ) efficiently filters much of the shortwave UV radiation (UV-C). However, this absorption decreases rapidly for radiation with wavelengths greater than 280 nm, reaching a rate of 0% absorption for wavelengths greater than 330 nm. Factors, such as elevation above sea level, cloud cover ground reflectance, geographic latitude, and ozone gradient and can affect the amount of UV-B and UV-A radiation that reaches the Earth's surface [3].

In normal conditions, the ozone layer filters around 80% of UV-B radiation, but human activities have caused a decrease in the stratospheric ozone concentration through the emission of compounds such as chlorofluorocarbons (CFCs), carbon tetrachloride (CCl4) and hydrochlorofluorocarbons (HCFCs). Therefore, UV-C radiation and an increased percentage of UV-B radiation can pass through [3, 4].

Although UV radiation is a minor fraction of solar energy that reaches the earth's surface, it significantly affects plants. UV-B radiation affects important biomolecules directly, including nucleic acids and proteins; these molecules absorb UV radiation easily when presenting π electrons, and this absorption can lead to metabolic, biochemical, and morphological alterations, as well as alterations in the genetic material [5, 6]. UV-A radiation produces similar effects, although they are part of the constitutive regulation of plant metabolic and morphological processes, such as photosynthesis, biomass production, and synthesis of pigments and antioxidant compounds [7].

Since the discovery of the thinning of the ozone layer, the consequent penetration of UV-B radiation into the atmosphere and its undisputable contribution to global warming of the planet, the effects of UV radiation on plants have been closely studied. Plants can use sunlight not only as a source of energy to produce carbon compounds but also as a source of environmental information; that is, they can detect it as a signal and trigger different systemic responses related to photosynthesis, phototropism, photoperiodicity, and photomorphogenesis. These same processes can be affected by the abnormal incidence of UV radiation in the atmosphere; therefore, the impact of its damage has been studied in recent decades [8]. This assessment has led to the creation of initiatives such as the Montreal Protocol, which aims at mitigating the negative effects of climate change-derived increased UV exposure through international policies [9].

In addition, the analysis of the causes of the morphological alterations shown by plants under UV light stress is difficult because they can be affected simultaneously by other environmental factors such as temperature, salinity, or drought, which together can modify development at the cellular level. The objective of this chapter is to describe the effects of UV radiation on different biochemical, morphological, and genetic processes in plants.

### **2. Morphological alterations**

Photomorphogenesis (light-regulated plant development) in the presence of UV light has been extensively studied [10]. Plants of several species modify the development of their organs in the presence of UV light; for example, the length of the stems tends to shorten, although they form a greater number of axillary buds, while the roots tend to be longer and more abundant, akin to the development of plants that grow in conditions of low light radiation [11].

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

One of the stages of plant development most susceptible to the incidence of light is germination, which is also greatly affected by UV-B radiation. In *Arabidopsis thaliana* seedlings irradiated with UV light, the growth of the hypocotyl was slower [12] compared to seedlings germinating under normal conditions; even the growth of the hypocotyl is lower in etiolated plants developed in the shade but irradiated with UV light [13]. On the contrary, in this same species, it has been observed that, under these conditions, the cotyledons tend to expand, even with short periods of UV light exposure [14].

Leaves also modify their structure, tending to decrease their surface area and increase their thickness in many broadleaf plant species that have been tested for their response to UV light. Apparently, this change in morphology depends on the imbalance between cell proliferation and elongation among the different leaf tissues, which can cause a decrease in leaf area, abnormal thickening, or rolling, resulting in slow plant development [15]. While searching for modifications at the cellular level that explain the alterations in the morphology of plants under UV stress, Krasilenko et al. demonstrated in 2013 [16] that UV radiation can cause depolymerization or fragmentation of microtubules in *A. thaliana* cells, causing the reorganization of the cytoskeleton and the cell in general, so that elongation and cell division are reduced, resulting in the formation of shorter leaves, which affects the development and the complete morphology of the plant.

It is currently accepted that some plant species avoid excess light radiation by forming a waxy cuticle on the epidermis. Exposure to UV radiation-induced deposition of wax in plants of species, such as *Coffea arabica*, *Coffea canephora*, *Hordeum vulgare*, *Cucumis sativus,* and *Phaseolus vulgaris*, which results in an increase in the thickness of the cuticle. Additionally, molecules such as phenolic acids and flavonoids can accumulate in the cuticle, functioning as photoprotectors against UV light or as UV light attenuators, respectively [17].

Stomata, the structures where gas exchange occurs, are also affected by the presence of UV light. High UV irradiation causes loss of stomatal opening and closing control in response to environmental stimuli, apparently due to an altered guard cell conductance. Since the stomatic function is vital for CO2 fixation in the lightindependent reactions of photosynthesis, its deregulation can deeply affect plant development and physiology [18].

UV light plays an important role in plant development, but extreme exposure can be detrimental. Unable to relocate, plants must balance the positive and negative effects of UV radiation mostly through intracellular mechanisms, as described in the following sections.

## **3. Photosynthetic alterations**

Photosynthesis is a light-dependent process, so it is almost inevitable that it be affected by the presence of UV radiation. There are several reports about the damage caused by UV radiation in specific sites of the photosynthetic apparatus of green plants (**Figure 1A**) [17]. Much of the damage is caused by the enhanced production of reactive oxygen species (ROS) that are involved in UV-induced responses, both as signaling agents within normal cellular processes and as damaging agents. ROS can cause damage to the proteins that make up the light-harvesting complexes of the photosystems or to those found in the protein complexes where the electron carriers of photosynthesis are concentrated, their accumulation is even known to cause the

#### **Figure 1.**

*Effects of UV-B light on plants and alterations caused by UV-B radiation in photosynthetic metabolism (A) and secondary metabolism (B).*

destruction of ribulose bisphosphate carboxylase/oxygenase [19], and, therefore, a decrease in atmospheric carbon fixation and plant biomass occurs. Another important damage caused by ROS is the oxidation of fatty acids in the membranes, which, in combination with peroxidation and photooxidation because of UV light, breaks the essential integrity of the thylakoid membranes in the chloroplast, generating alterations in the organization of the membrane-embedded photosynthetic complexes, decreasing their photosynthetic capacity [20].

Ultraviolet light also causes damage to plant proteins; in fact, one of the effects on photosynthesis is the damage, it exerts on the enzymes that synthesize pigments such as chlorophylls [21]. In addition, pigments are also degraded by UV light, especially chlorophyll b and carotenoids, so exposure to this type of radiation can cause an imbalance in the proportion of pigments, with the consequent alteration of the photosynthetic apparatus, as has been observed recently in maize. After being exposed to UV radiation for 19 days, fluorescence and chlorophyll concentration decreased in several maize lines, although in different proportions in a line-dependent manner [22].

Several elements at Photosystem II, the site where photosynthesis begins, are sensitive to UV radiation. This complex is formed by the association of pigments and proteins, and many of these proteins are part of electron transport centers; therefore, their alteration or degradation affects the electron transport chain of photosynthesis, reducing their levels under UV light stress. In an elegant work, Ihle [23] reported that proteins D1 and D2, which are found in the reaction center of photosystem II, are

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

especially susceptible even to low intensities of UV radiation (1 μmol m−2 s−1) [24]. The degradation of proteins D1 and D2 adds to the alteration of the manganese (Mn) oxidizing group of water, which together cause the loss of function of the reaction center and, therefore, the inhibition of electron transport [25]. Damage at the beginning of the electron transport chain of photosynthesis makes it difficult to investigate downstream transporters; however, some reports indicate the change in the ratio of photosystems II and I due to the decrease in absorption at 700 nm—absorbed by Photosystem I—observed after prolonged exposure to UV light [26].

Plants are highly susceptible to the presence of ultraviolet light. Through research over the past four decades, it has been possible to discover the mechanisms related to damage in plant morphology, development, and metabolism. However, many questions remain to be investigated until the problem of the penetration of UV radiation into the atmosphere is resolved.
