(Watson et al. 1996). Existing agronomic practices have the potential to ameliorate adverse effects of increased global temperatures; the greatest uncertainty lies in whether economic systems have the flexibility to incorporate the necessary technological innovations in a timely way.

Increased Ultraviolet-B Radiation

As a result of stratospheric ozone depletion, due principally to emissions of chlorofluorocarbons, levels of ultraviolet-B-radiation (UV-B) (280-315 nm) reaching the earth's surface have increased in all temperate latitudes (Madronich 1993). Increased UV-B radiation is an environmental stress factor that can alter the physiology and development of terrestrial plants in several ways (Caldwell et al. 1989, Tevini 1993), ranging from such cellular and subcellular effects as DNA damage and degradation of photosystem II proteins to such whole-plant effects as stunting, shortening of internodes, alterations in root:shoot biomass ratios (Tevini and Teramura 1989), and alterations in plant primary and secondary chemistry (Ros and Tevini 1995,;McCloud and Berenbaum 1994). At the ecological level, these developmental effects can manifest themselves as yield reductions in several crop species (Tevini 1993). Increased UV-B can also affect interspecific competition in plants (Gold and Caldwell 1983), reduce plant resistance to pathogens (Orth et al. 1990, Panagopoulous et al. 1991), affect pollinator-plant relationships (Collins et al. 1997, Feldheim and Connor 1996), and alter host-plant suitability for herbivores (McCloud and Berenbaum 1994).

Plants have the capacity to reduce damaging effects of UV-B in several ways. Among these is the production of compounds that absorb damaging wavelengths of UV radiation. In several plant species, increased UV-B exposure induces production of flavonoids and other phenolics (reviewed in Caldwell et al. 1989.). Plants can also ameliorate the oxidative damage brought about by UV exposure by enhanced synthesis of enzymatic and nonenzymatic antioxidants (Larson 1988). Alpha-tocopherol, ascorbic acid, and glutathione are all effective scavengers of reactive oxygen species and in several temperate crop species increase in response to UV exposure (Foyer 1993, Grace and Logan 1996).

Inasmuch as antioxidant defensive compounds also serve as essential nutrients for insects, UV-B exposure can potentially increase plant suitability for herbivores. Indeed, Hatcher and Paul (1994) found that elevated UV-B increased nitrogen content of Pisum sativum and digestibility of its foliage by Autographa gamma, a caterpillar. This enhanced nutritional suitability, however, appears to be counterbalanced in other plants by increases in phenolics, flavonoids, and other UV-screening compounds, many of which are toxic or deterrent to insects.

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