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General Effects on Organisms

Basic Effects of UV-B Radiation on Organisms and their Protective Responses

Enhanced UV-B radiation can have many direct and indirect effects on organisms. However, organisms have developed mechanisms of protection and mitigation of UV-B radiation damage. General deleterious effects include production of active oxygen species and free radicals, DNA damage and, for plants, partial inhibition of photosynthesis. Protective responses include radiation shielding due to structural or pigment changes and specific damage repair systems. Although photochemical lesions of DNA and proteins and damage as a result of active oxygen species and free radicals may occur, many of the effects of UV-B radiation may be expressed through increased regulation rather than sustained damage,

    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.

The Biological Effectiveness of Changes in Sunlight

As explained in Chapter 1, the biological effectiveness of solar UV-B radiation needs to be taken into account in assessing what ozone reduction, and the resulting changes in solar radiation, may mean for biological systems and processes. The biological weighting functions used for this purpose often come from action spectra. Action spectra assumed to be relevant for organisms, especially plants (Fig. 3.3) all indicate that the shorter UV-B wavelengths are the most important. However, the relative importance of shorter vs longer UV-B wavelengths (the slopes in Fig. 3.3) varies considerably. Depending on these slopes and the tails of the spectra extending into UV-A, the Radiation Amplification Factors (discussed in Chapter 1) vary enormously. Only the weighting functions with steep slopes result in RAF values suggesting that ozone reduction is potentially important. Thus, the evaluation of weighting functions (and therefore action spectra) is critical. Although there is evidence that action spectra for some plant functions are steep, indicating that ozone reduction translates into large increases in effective solar UV-B (Caldwell, 1971; Setlow, 1974), some more recent spectra developed specifically for evaluating the ozone reduction problem show somewhat flatter slopes (and therefore somewhat lower RAF values) than the earlier work (Caldwell et al., 1986; Steinmüller, 1986; Quaite et al., 1992a; Cen and Björn, 1994). Still, many of these spectra are sufficiently steep so that ozone reduction must be taken seriously (Flint and Caldwell, 1996; Caldwell and Flint, 1997). Biological weighting functions also are needed to relate solar UV to UV from lamps used in many experiments.
 
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).

Plant Growth Responses

In many plant species reduced leaf area and/or stem growth have been found in studies carried out in growth chambers, greenhouses and in the field (Tevini and Teramura, 1989; Johanson et al., 1995; Mepsted et al., 1996; Tevini, 1996; Rozema et al., 1997a). These studies have traditionally been conducted with specially filtered UV lamps. It is important in such experiments to maintain a realistic balance between different spectral regions since both UV-A (315-400 nm) and visible (400-700 nm) radiation can have strong ameliorating effects on responses of plants to UV-B (Caldwell et al., 1994). In growth chambers and greenhouses, the radiation conditions are usually quite different from those in nature. For example, the visible radiation which is used in photosynthesis (400 to 700 nm, photosynthetically active radiation, PAR) and the UV-B/UV-A/PAR ratios are different from those in the field. If UV-A and PAR are low, the effects of UV-B may be much more severe. Thus, even if realistic levels of UV-B are used in simulating ozone reduction, the plant response may be exaggerated relative to that in the field. In addition, other factors, such as temperature, water and nutrients differ from conditions in the field and this can alter response to UV-B radiation. It is, however, important that these studies conducted under controlled conditions be verified as much as possible under field conditions. Even under field conditions, if applied UV-B is not adjusted downward during cloudy periods, the UV-B sensitivity may be unduly pronounced (Fiscus and Booker, 1995). Unfortunately, the most expensive and difficult experiments, i.e., those conducted in the field with UV-B supplements adjusted for cloudiness and other atmospheric conditions, are seldom undertaken. In the last few years more field experiments have been conducted and many of these employ lamp systems with controls to make continual adjustments according to prevailing sunlight conditions.

    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).

Plant Reproductive Processes

Ultraviolet-B radiation can alter both the timing of flowering (Caldwell, 1968; Ziska et al., 1992; Staxén and Bornman, 1994; Mark et al., 1996; Tevini, 1996) as well as the number of flowers in certain species (Musil, 1995; Klaper et al., 1996; Saile-Mark and Tevini, 1997). Differences in timing of flowering may have important consequences for the availability of pollinators. Such effects may be due to regulatory alterations in the plant rather than damage per se. Poorly protected reproductive organs might, however, be susceptible to damaging effects. Most of the reproductive parts of plants, such as pollen and ovules, are rather well shielded from solar UV-B radiation. For example, anther walls can absorb more than 98% of incident UV-B radiation (Flint and Caldwell, 1983). In addition, the pollen wall contains UV-B absorbing compounds affording protection during pollination as do the other flower parts such as sepals, petals and walls of the ovaries (Day and Demchik, 1996). Only after transfer to the stigma might pollen be susceptible to solar UV-B radiation. In vitro experiments have shown that germinating pollen can be sensitive at this time to UV-B radiation in some cases (Flint and Caldwell, 1984). However, often pollen germination itself is not affected, but pollen tube growth of many species can be retarded as shown in a survey of 34 plant species or varieties (Torabinejad et al., 1998).

Carry-over Effects of UV-B Irradiation in Subsequent Generations

In sexually reproducing populations of an annual desert plant, effects of UV-B irradiation on growth and allocation of biomass appeared to accumulate as subsequent generations were exposed to UV-B irradiation (Musil, 1996). Furthermore, after four generations of UV-B irradiation, the effects persisted in a fifth generation that was not exposed to UV-B treatment (Musil et al., 1998). If this phenomenon is common, it could amplify the effects of UV-B radiation changes. This is somewhat analogous to apparent accumulated effects of UV-B irradiation over several growing seasons in long-lived woody plants discussed later.

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