General Effects on Organisms
In order for UV radiation to be effective in most organisms, it must effectively penetrate into the tissues and be absorbed. Structural and biochemical changes induced by enhanced levels of UV-B radiation ultimately modify the penetration of UV radiation into plants and other organisms. The UV shielding in most animals is thought to be quite effective in minimizing UV-B damage, but this should be further examined (see Chapter 2). For example, different stages of insect larvae may be less well protected by UV-absorbing pigments. In plants, a certain amount of UV-screening pigments may be constitutive, and additional UV-absorbing compounds (usually phenolic compounds) can be synthesized when plants are exposed to increased levels of UV radiation. This will naturally be important in reducing the penetration of UV-B radiation to underlying tissues. Experimental mutant plants that lack these pigments are very sensitive to natural sunlight UV-B (Li et al., 1993; Reuber et al., 1996). Other adjustments in plant leaves after exposure to increased UV-B radiation may also contribute to a heightened UV defense. At the structural level, increased leaf thickness is often induced by UV-B radiation that reduces UV-B penetration to internal leaf tissues (Bornman and Vogelmann, 1991). Ultraviolet radiation penetration varies among different plant species and this may be reflected in the sensitivity of these species. Penetration of UV-B was found to be greatest in herbaceous dicotyledons (broad-leaved plants) and was progressively less in woody dicotyledons, grasses and conifers (Day et al., 1992). The UV penetration also changes with leaf age; younger leaves attenuate UV-B radiation less than do the more mature leaves, as was shown for some conifers (DeLucia et al., 1991, 1992).
Of the different kinds of molecular damage, radiation damage to DNA is potentially dangerous to cells, because a single photon hit in a single molecule may have dramatic, sometimes even lethal effects. Many different types of DNA damage are known that result from free radicals and reactive oxygen species formed in various photochemical processes. The two most common UV-B induced DNA lesions are the cyclobutane pyrimidine dimers and (6-4) photoproducts which are pyrimidine adducts. These two types of lesions differ from other DNA lesions in that many organisms living in sunlit habitats possess special enzymes (photolyases) that can effectively repair many of these lesions in the presence of visible light and favorable temperatures. Some DNA repair systems can also operate without light (Britt, 1996; Taylor et al., 1996). Much of the research in this area has been conducted under laboratory conditions, but the level of DNA lesions in intact plants has also been measured under field conditions (e.g., Quaite et al., 1992b; Ballaré et al., 1996; Stapleton et al., 1997). While these studies indicate effective repair of DNA damage (Stapleton et al., 1997), the UV component of sunlight is still sufficient to result in some level of persistent damage. Low temperature can slow this enzymatic repair of DNA damage (Britt, 1996; Takeuchi et al., 1996). Therefore, plants, cold-blooded animals and microbes in cold environments may suffer from a less favorable balance between damage and repair than others. Unfortunately, these environments overlap with those exposed to the greatest ozone depletion.
When exposure to increased UV radiation leads to stimulation of UV-absorbing compounds in plant tissues, another protective effect can result from the antioxidant properties that certain of the compounds confer. Enhanced levels of UV-B radiation appear to selectively stimulate those flavonoids (a type of phenolic) with potential antioxidant properties (Cen et al., 1993; Liu et al., 1995; Reuber et al., 1996; Olsson et al., 1998). This selective enhancement can be up to 500% (Reuber et al., 1996). At present, it is not known how extensive this selective induction is within the plant kingdom.
Many genes in plants, animals and microorganisms are regulated by UV-B, and changes in UV-B may have important consequences by altered gene action (Strid et al., 1994; Jordan, 1996; Bender et al., 1997). The mechanisms of how the organism perceives UV-B radiation and how signals are transduced are not yet well understood. Active oxygen can be one trigger for altered gene activity (Mackerness et al. 1998). No matter what the triggering agent, altered gene activity is important, since UV-B radiation is involved in changes of gene expression which are reflected in many aspects of plant function. For example, an increased amount of UV-B radiation results in enhanced synthesis of UV-screening pigments and is due to the expression of particular genes (Jenkins et al., 1997). It appears that the effects of UV-B radiation on photosynthesis, growth and development of plants are caused by altered gene action. This is currently a topic of intensive research.
Decreased elongation also may be due to UV-induced destruction of the plant hormone auxin, that absorbs in the UV-B range and could be photodegraded by high levels of UV-B radiation. Oxidative enzymes, such as the peroxidases, the activity of which is increased by enhanced UV-B radiation, also may be involved in plant hormone-regulated growth responses, as shown in sunflower and rice plants (Ros and Tevini, 1995; Huang et al., 1997). The levels of another plant hormone, ethylene, which causes greater radial growth and less elongation, are increased after UV-B irradiation in sunflower seedlings (Ros and Tevini, 1995) and cultured shoots of pear seedlings (Predieri et al., 1993). Changes in hormone levels ultimately may be due to UV-B-induced gene expression, but this remains to be demonstrated.
|Figure 3.3. (upper panel) Action spectra for "naked" DNA damage (Setlow, 1974) (lines with dashes and dots), DNA dimer formation (a type of DNA damage) in intact alfalfa seedlings (Quaite et al., 1992a)(line with long dashes), a generalized plant action spectrum compiled from various plant spectra (Caldwell, 1971)(continuous line) and a spectrum for putative lipid damage based on a luminescence indicator (Cen and Björn, 1994)(line with short dashes). The lower panel shows solar spectral irradiance at 360 (continuous line) and 180 (dashed line) Dobson Units (DU) of total atmospheric ozone (Dobson Units are an expression used for describing thickness of the ozone layer at standard temperature and pressure (0° and 101.3 Pa); 1 mm ozone layer thickness is equivalent to 100 DU). The solar irradiation is calculated for latitude 49 N at solar noon at the summer solstice (June 21) using the model of Green et al. (1980).|
Also, there are several studies in which the UV component of existing sunlight has been altered by special filters in the field or in special small greenhouses or growth chambers located outdoors. The filters have involved special glass, plastics, or in one series of studies, ozone gas in a UV-transparent plexiglass envelope (Tevini et al., 1990; Mark and Tevini, 1997). Many of these studies involving filtered sunlight have shown that normal ambient solar UV-B can cause somewhat reduced leaf area, smaller seedlings, etc. (Searles et al., 1995; Ballaré et al., 1996; Mark et al., 1996; Saile-Mark and Tevini, 1997).
Plant species vary considerably in their response to UV-B in both controlled-environment and field studies. Also, varieties of the same species can vary in their response. For example, in the field, sizeable differences in response to UV-B were found among varieties of soybean (Teramura et al., 1990) and rice (Dai et al., 1992, 1997; Kulandaivelu et al., 1997). Experiments in greenhouses covered by different materials that transmitted different amounts of UV indicated that varieties of bean (Phaseolus vulgaris) from lower latitudes were less affected than those from higher latitudes under higher UV-B radiation (Saile-Mark and Tevini 1997).