Stress under the sun: spotlight on ultraviolet-B responses.

The UV-B spectral band (280–315 nm) contributes less than 2% of the short-wave photons received by terrestrial organisms in most natural environments. Concerns about potential impacts of stratospheric ozone depletion contributed to spark interest in studies of plant responses to enhanced UV-B levels during the last two decades (Caldwell et al., 2003). In connection with this research, two related questions have attracted increasing attention in recent years: (a) What are the impacts of ambient, present-day levels of UV-B on plant and ecosystem function? And (b), which are the mechanisms that mediate plant responses to solar UV-B? Regarding impacts of ambient UV-B, two generalizations are beginning to emerge from field experiments carried out in natural and cultivated ecosystems (Fig. 1). First, ambient UV-B at mid latitudes appears to have a measurable (but generally modest) effect reducing plant growth, particularly in the case of herbaceous plants (e.g. Ballaré et al., 1996; Krizek et al., 1998; Mazza et al., 1999a). Second, modification of ambient UV-B levels may have large impacts on the interactions between plants and phytophagous insects (Ballaré et al., 2001; Paul and Gwynn-Jones, 2003). Although there is variation among ecosystems, the most common effect of solar UV-B is increased plant resistance to insects, measured in terms of leaf area consumed and/or insect growth in standard feeding bioassays (Ballaré et al., 1996; Rousseaux et al., 1998; Mazza et al., 1999b; Zavala et al., 2001). Progress in the understanding of the mechanisms that mediate these effects of ambient UV-B has been slow for various reasons. First, no specific UV-B receptors have yet been identified in plants, and no UV-B-perception mutants are available for comparative studies. Second, most of the information on UV-B effects at the molecular level has been obtained in indoor-exposure experiments, with monochromatic or heavily-unbalanced UV-B sources [i.e. sources that produce unnaturally high UV-B to photosynthetically-active radiation (PAR) ratios]. Under monochromatic UV-B, other photoreceptors can be activated (e.g. the phytochromes) and therefore the results cannot be interpreted as specific UV-B responses. Under conditions of unbalanced light treatments (i.e. high UV-B:PAR ratio), the effects of UV-B on plant growth are often greatly exaggerated. Because no major effects of ambient UV-B on growth rate have been detected under field conditions in any system, the physiological significance of the molecular changes observed under conditions of unbalanced UV-B is unclear. Third, UV-B has the potential to damage key macromolecules and cellular structures, particularly when high doses are used in laboratory studies; therefore, specific UV-B responses are difficult to separate from secondary consequences of generalized damage under these conditions. In this issue, three papers shed new light into the mechanisms that mediate plant responses to solar UV-B radiation.