Introduction

Photosynthesis and photosynthetic productivity in many plant species, although by no means all, can be inhibited by increased exposure to UV-B radiation (Caldwell, Teramura & Tevini, 1989; Tevini & Teramura, 1989; Teramura, Ziska & Sztein, 1991; Tevini, Braun & Fieser, 1991; Middleton & Teramura, 1993; Musil, 1995). To date, there is no consensus for the mechanistic basis of UV-B-induced inhibition of CO2 assimilation in mature leaves. Decreases in Rubisco activity and stomatal conductance have been implicated as factors limiting CO2 assimilation in leaves exposed to elevated levels of UV-B. Prolonged exposure to elevated levels of UV-B has been demonstrated to result in decreases in both Rubisco activity and content (Vu, Allen & Garrard, 1984; Strid, Chow & Anderson, 1990; Jordan et al., 1992; He et al, 1993), and is accompanied by large decreases in the mRNA transcripts of both the large and small subunits of Rubisco (Jordan et al, 1992). Such decreases in Rubisco are consistent with the observed decrease in the leaf carboxylation efficiency, determined from the initial slope of the response of CO2 assimilation to increasing CO2 concentration, when leaves are given supplemental UV-B radiation (Ziska & Teramura, 1992). Exposure to UV-B can also modify the rates of stomatal opening and closing, and reduce the rate of leaf transpiration (Tevini & Teramura, 1989; Middleton & Teramura, 1993; Day & Vogelmann, 1995).