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Ecosystem-level UV-B Radiation Effects Involving Higher Plants

Competitive Balance

In forests, grasslands, etc., overall primary plant productivity may not be greatly affected by ozone reduction even if the growth of some plants is diminished. However, since plant species differ greatly in growth responsivity to UV-B, it is anticipated that a productivity reduction of one species will probably lead to increased productivity of another, more UV-tolerant species. This is likely because more resources (e.g., light, moisture and nutrients) will be available to the tolerant species. Thus, the overall productivity of the system may well remain about the same while species composition may change. However, a change in the balance of species could have far-reaching consequences for the character of many ecosystems.

    Another mechanism whereby the competitive balance of plant species can be changed by increased UV-B is through changes in plant form. Even if plant production per se is not affected by increased UV-B, changes in plant form can result in changes in which species can more effectively compete for sunlight. This phenomenon has been demonstrated in several experiments. For example, in a six-year field study using modulated UV-B lamp systems, the competitive balance of two species (wheat and a common weed, wild oat) could be changed even though the increased UV-B radiation had no effect on production and growth of these species if grown by themselves (Barnes et al., 1988). A quantitative analysis of competition for sunlight in the mixed stands with and without supplemental UV-B showed that subtle changes in plant form of the two species were sufficient to change the balance of competition for sunlight that is necessary for photosynthesis (Barnes et al., 1995). Therefore, one species can achieve some advantage over the other because one captures more sunlight for photosynthesis. In these experiments, the wheat benefited from increased UV-B and the weed suffered. However, in other mixtures of crop and weeds, the situation might be reversed. Also, other changes in plant form, such as greater allocation of biomass to roots, might change competitive effectiveness of individual species for soil moisture and nutrients. In grasslands and forests that are not managed intensively, similar changes in species composition may be experienced.

    Ecosystem-level experiments with nonagricultural systems are only beginning. Early reports of one experiment in a subarctic heath ecosystem suggest that species composition changes may result from UV-B supplementation (Johanson et al., 1995.)

Plant Susceptibility to Pathogens and Insects

The extent to which plant tissues are consumed by insects or the degree to which pathogens attack plants is regulated by several properties of the plant host tissues. Experiments in which solar UV-B radiation has been modified by selective filters show that present-day solar UV-B radiation can substantially reduce insect herbivory of agricultural and native plant foliage (Ballaré et al., 1996; Mazza et al., submitted; Rousseaux et al., 1998). Field studies involving supplementation of solar UV-B radiation with lamp systems indicated a substantial reduction in populations of a herbivorous insect on a heathland plant (Salt et al., 1998). The reasons for these changes are not always clear, but they may be mediated through changes in plant secondary chemistry or alterations in plant nitrogen or sugar content. Studies involving UV lamps indicated decreased herbivory by a moth caterpillar under elevated UV-B radiation and this was attributed to increases of host pea plant tissue nitrogen content (Hatcher and Paul, 1994). Mulberry plants previously irradiated with UV from lamps suffered less herbivory by silkworms (Bombyx mori) and the lower consumption was attributed to lower sucrose content of the foliage (Yazawa et al., 1992). McCloud and Berenbaum (1994) have shown in laboratory studies that UV-B radiation can increase furanocoumarin content of plant tissue which, in turn, results in slower development of certain insect larvae during early life stages of the larvae. Although the foregoing would suggest that insect herbivory may always be decreased by UV-B radiation, another study shows that herbivory can be increased three-fold (e.g., Buck and Callaghan, submitted).

    The results of most of these studies indicate that the effects on insect herbivory are all due to changes in the host plant tissues. However, there are some indications that some insects may respond directly to solar UV-B radiation. Thrips on soybeans were found to consume less foliage if the foliage had been previously exposed to ambient solar UV-B. Furthermore, the thrips appeared to directly sense and avoid solar UV-B radiation even though they were mildly attracted to UV-A radiation (Mazza et al., submitted).

    Plant fungal and viral diseases react in several different ways to UV-B radiation in several experiments, conducted primarily in laboratory and greenhouse conditions. In four of ten studies, UV-B was found to counteract disease severity and in the other six studies, it promoted disease development (Manning and Tiedemann, 1995). The direction of the UV-B radiation effect on disease severity can also vary with the variety of the host. In a rust-resistant variety of wheat, additional UV-B radiation had little effect, but it promoted the rust infection in a rust-sensitive wheat variety (Manning and Tiedemann, 1995). It is not clear in many of these experiments whether the changes in disease severity were due simply to changes caused by UV-B radiation in the host plant, or whether direct UV-B radiation effects on the fungal or viral pathogens was involved. Cucumber plants first exposed to UV-B radiation were more susceptible to subsequent infection by two fungal pathogens if the host plants were exposed to UV-B radiation prior to infection; but UV-B irradiation after infection had no effect on disease severity (Orth et al., 1990). Such an experiment suggests the effect of UV-B radiation was mediated through changes in the host plant tissues. There is also evidence from solar UV-B exclusion studies showing increased incidence of fungal disease when UV-B is removed (Gunasekera et al., 1997).

    These changes in insect herbivory and disease severity caused by alterations of solar UV-B can be sizeable; they can operate in different directions and have very important implications for both agricultural and nonagricultural ecosystems. They may be much more important than known influences of UV-B radiation on plant production based on realistic field studies.

    Even roots of plants whose shoots are exposed to elevated UV-B radiation can be affected as indicated by root interactions with microorganisms. For example, the nature of microorganism assemblages that were associated with roots of sugar maple trees (Acer saccharum) was altered by exposure of the tree shoots to elevated UV-B radiation (Klironomos and Allen, 1995). This was obviously a systemic effect of UV-B expressed in the roots of the host plant.

Timing of Life Phases

The timing of life phases of plants is a combination of response to environmental factors and the genetic constitution of the plant. For example, as mentioned earlier, UV-B exposure can alter the timing of flowering. This timing of events such as flowering, entering and breaking of dormancy, and even senescence is important not only to the individual plant, but also in how plants interact with other plants and animals. For example, a shift in the timing of flowering can mean that a plant species might not have sufficient insect pollinators available at the new time of flowering either because the insects are not present or because other plant species are attracting these pollinators. Such changes could also conceivably be important in agricultural systems, but intervention with management options may make these changes less important. As indicated earlier in this chapter, increased UV-B has been shown to advance or delay (depending on species) the time of flowering in plants. There is little work at present on flowering responses and virtually nothing on other potential effects of UV-B on life phase timing of plants or other terrestrial organisms.

Other Effects due to Changes in Higher Plant Tissues

In higher plants, secondary compounds, such as lignin, are important as structural materials. These are related to phenolic compounds and may change in composition with elevated UV-B radiation (e.g., Gehrke et al. 1995). If the ratio of lignin to cellulose in plant tissues changes, it can alter the rate of decomposition. This has very important implications for biogeochemical cycles as discussed fully in Chapter 5.

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