Ranges of Ultraviolet Radiation and Biological Activity

The spectrum of solar ultraviolet light is continuous but is commonly divided into three wavelength bands: UV-C (200-290 nm), UV-B (290-320 nm) and UV-A (320-400 nm). UV-C is the most energetic of the three and is known as “germicidal UV” because of its potency against microorganisms (Yin et al. 2016). It is used to disinfect fresh fruit and vegetables to preserve their quality.

However, since it is effectively absorbed by oxygen and ozone in the stratosphere, only a very small fraction reaches the Earth’s surface. UV-A, on the other hand, is not attenuated by atmospheric ozone, and this less damaging type of radiation plays an important role in plant photomorphogenesis. Although a sizeable amount of UV-B is absorbed by atmospheric ozone, its impact on life on our planet is considerable.

The development of the ozone hole—that is, the decrease in stratospheric ozone concentrations due to ozone decomposition by reaction with anthropogenic gases such as halogenated hydrocarbons or nitrogen oxides (IPCC/TEAP Special Report on Ozone and Climate 2005) is therefore observed with much concern. While the seasonally fluctuating ozone hole is particularly pronounced in the polar and subpolar regions, the UV radiation emitted by the sun crosses the atmosphere in regions of high geographical latitude at an angle and thus encounters a significantly “thicker” ozone layer than at the equator, where it takes the shortest path through the atmosphere. Therefore, in spite of the high latitudinal ozone hole, the UV radiation is high in the tropics and relatively low in the polar regions. The well-known altitudinal increase in UV radiation is caused by an attenuation of the tropospheric ozone layer, concomitant with a decrease in the intensity of haze.

UV-B radiation can damage cells and thus is dangerous to organisms (Table 3.3). In particular, plants—as sessile organisms—have developed mechanisms to cope with the ubiquitous and inescapable natural flux density of UV radiation through repair mechanisms for damaged cellular components (such as DNA). Protective measures are accumulation of UV-absorbing pigments in the epidermal layers, thick layers of hairs (Holmes and Keiller 2002; Manetas 2003) and cuticular waxes (Barnes et al. 1996). Generally, plants show adaptation to the UV-B load of their habitat. Plants growing along a latitudinal or an elevation gradient exhibit increased UV-B tolerance (Robberecht et al. 1980, Fig. 3.23).

Table 3.3. Physiological effects of increased ultraviolet (UV)-B radiation. (Modified from Jansen et al. (1998))

Fig. 3.23. Efficiency of ultraviolet (UV)-B screening (percent transmittance of impinging UV-B radiation) through the epidermis of several plant life forms from various provenances. (Modified from Munk (2009), Körner (1999) and Day et al. (1993))

Problems caused by UV may arise for crop cultivars that are not well adapted to the natural UV-B stress occurring at their sites of cultivation, or for mobile organisms such as plankton. Phytoplankton, especially of the cold oceans, appear to be very sensitive to UV-B. In spite of the shallow depth to which UV light penetrates in a body of water (Fig. 3.24), significant reductions in phytoplankton biomass have repeatedly been observed as a consequence of the ozone hole. Likewise, decreases in yields have been reported for UV-sensitive crop cultivars (e.g. of maize and soybeans (Rius et al. 2016)). However, although statistically significant, these reductions were relatively small (usually <10%).

Fig. 3.24. Decrease in ultraviolet (UV) radiation with depth of water a in the northern Adriatic and b in the humin-rich Lake Neusiedler (Austria). Both measurements were conducted on a cloud-free August day. The intensity of the UV radiation at a depth of 5 cm in Lake Neusiedler corresponded approximately to a depth of 5 m in the Adriatic. (Modified from Herndl (1996))

Morphological-anatomical symptoms indicating a still tolerable UV-B stress are swollen and shortened internodes, reduced leaf expansion, curling up of leaf edges and enhanced branching of the shoot through promotion of lateral buds. Leaves tend to show succulence with a particularly thick, usually pigmented epidermis and low density of stomata. Tolerated UV stress often results in accumulation of vitamin C and soluble sugars in fruits as radical scavengers, thereby also increasing the quality of food. However, the transition from UV-B-triggered “normal morphogenetic reactions” to those that have to be considered as stress responses is not clearly apparent, inasmuch as other natural stresses such as drought, nutrient deficiency or low temperature can interact with UV-B effects at the molecular level.

Nevertheless, differentiation between normal development of UV-B tolerance and stress responses is clear when above-ambient levels of UV-B cause damage to DNA, proteins and membrane lipids, and inhibit protein synthesis and photosynthetic reactions. However, since UV-B also generates ROS (mainly the superoxide radical O₂^-) through its impact on photosynthesis, respiration and on enzymes such as peroxidases and oxidases, the origin of UV-B triggered damage is not always obvious.

Investigation of UV-B effects on plants is not trivial, because of the omnipresence of this radiation in nature and, to a lesser extent, in artificial light sources. Many experiments have therefore applied pulses of UV-B overdoses or longer than natural exposure periods. Such treatment can easily overstretch the tolerance—for example, the capacity for repair—of the plants, causing unnatural reactions and even necroses and death. Also, different reactions have been observed, depending on whether the same total overdose of UV-B was applied either in high intensity pulses or as a slightly elevated constant flux over a long period. With the UV stress applied in short pulses, damage and repair are the dominant effects, while continuous but low-intensity stress leads to acclimation and damage avoidance.