Ecosystem-level UV-B Radiation Effects Involving Higher Plants
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.)
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.