Interaction of UV-B and other Factors
Plants and other organisms in nature seldom are affected by only a single stress factor, such as UV-B radiation. Instead, they typically respond to several factors acting in concert, such as water stress, increased atmospheric CO2, mineral nutrient availability, heavy metals, tropospheric air pollutants and temperature. Therefore, it is important to keep in mind that the effectiveness of UV-B radiation can be greatly increased or decreased by such factors. Visible radiation is an important ameliorating factor and, thus, as natural levels as possible should be applied in laboratory experiments for attaining more realistic results, as discussed earlier.
Among the most common of factors in nature is water stress. In a field study, Sullivan and Teramura (1990) demonstrated that UV-B mediated reductions in photosynthesis and growth were observed only in well-watered soybeans. When soybeans were water stressed, there was no significant effect of the UV-B radiation on either photosynthesis or growth. The interpretation was that water stress resulted in a large reduction in photosynthesis and growth that masked the UV-B effect. Furthermore, water stressed plants resulted in a higher concentration of leaf flavonoids, which in turn, provided greater UV-B protection. Other interactions between UV-B radiation and water status of plants also occur. Elevated UV-B radiation in field experiments tended to alleviate drought symptoms in two Mediterranean pine species (Petropoulou et al., 1995; Manetas et al., 1997). In a moss species, UV-B radiation inhibited growth when the moss was under water stress, but stimulated growth when the moss was well hydrated (Gehrke, 1998).
Increases of atmospheric CO2 are a certain element of global climate change and atmospheric CO2 concentration will likely double by the middle of the next century (Mitchell et al., 1990). Many experiments with elevated CO2 employ a twice-ambient CO2 concentration as a treatment condition. Such a doubling often results in more pronounced plant responses than are evident in many elevated UV-B radiation lamp experiments designed to simulate up to 20% ozone column reduction under field conditions. However, responses to CO2 are small in semi-natural ecosystems where nutrient or water availability may strongly constrain plant growth. For example, Gwynn-Jones et al. (1997) showed that growth responses to elevated CO2 and enhanced UV-B (both alone and in combination) were small during the first three years of experimentation in a sub-arctic heath. Also, most ecosystem-level effects of elevated CO2 are mediated through changes in plant tissues. When studied independently, plant growth responses to changes in UV-B radiation and atmospheric CO2 concentration generally are thought to be in opposite directions. Usually, however, in most experiments employing both elevated CO2 and UV-B radiation, these factors do not yield interactions, with some exceptions (see reviews by Björn et al., 1997; Sullivan, 1997). Elevated CO2 sometimes appears to provide some protection against elevated UV-B radiation for some species; yet, elevated UV-B radiation can limit the ability of some species to take advantage of elevated CO2 in photosynthesis. Allocation of biomass in plants can also change in a complicated fashion with the combination of CO2 and UV-B radiation treatments (reviewed by Sullivan, 1997). Increased temperature is also a predicted element of global climate change. In a study combining two levels of UV-B radiation with two levels of CO2 and two temperatures, the results indicated that either elevated CO2 or somewhat higher temperature had similar effects in reducing the growth-inhibiting effects of elevated UV-B radiation on sunflower and maize seedlings (Mark and Tevini 1997).
Plant uptake and translocation of mineral nutrients within the plant can be affected by elevated UV-B radiation, but the mineral nutrient status of plants also can affect plant responsiveness to UV-B radiation (Murali and Teramura, 1985; Ros, 1995; Musil and Wand, 1994). Nitrogen concentration in plant tissues can increase under elevated UV-B which has been linked with reduced insect herbivory (Hatcher and Paul, 1994; Rousseaux et al., 1998). The uptake of certain nutrients may also be modified by UV-B radiation and cadmium. In oilseed rape (Brassica napus) plants grown under additional enhanced UV-B radiation and simultaneously exposed to different concentrations of cadmium, the manganese content in the shoots decreased in plants exposed to cadmium and UV-B radiation, while significant increases in magnesium, calcium, phosphate, copper and potassium occurred only in those plants exposed to cadmium and UV-B radiation. Cadmium uptake was not affected by UV-B radiation. The UV-B had no additional influence on the nutrient content of the roots (Larsson et al., 1998). An earlier study showed that both cadmium and UV-B radiation negatively influenced photosynthetic efficiency in spruce seedlings (Dubé and Bornman, 1992).
Interaction of UV-B radiation with
tropospheric air pollutants is also of concern although little work thus
far has been conducted in this area. One field study of soybean plants
showed them to be sensitive to ozone in the air, but not sensitive to UV-B
supplements from lamps under the particular test conditions. There were
no significant interactions of supplemental UV-B and ozone (Miller et al.,
1994). However, in pine seedlings grown in a growth cabinet with a simulated
solar UV radiation, increasing the ozone concentration increased the sensitivity
of the pine seedlings to UV-B radiation since the ozone reduced the levels
of UV-B-absorbing pigments in the plant tissues. In another experiment
with tobacco, UV-B radiation increased the level of ozone-induced foliage
lesions (Thalmair et al., 1996).