Photosynthesis and photoinhibition

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 CO 2 assimilation in mature leaves. Decreases in Rubisco activity and stomatal conductance have been implicated as factors limiting CO 2 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 CO 2 assimilation to increasing CO 2 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).