Light stress effects and antioxidative protection in two desert plants

Abstract

1. The enzyme catalase was investigated as a sensitive marker of light stress in Retama raetam stems and Atriplex halimus leaves. While the activity of catalase was readily photoinactivated in vitro when crude extracts from these tissues were exposed to a photon flux of 500 μmol m-2 s-1 photosynthetic active radiation (PAR), no apparent losses of the extractable catalase activities were observed when tissues were harvested from desert plants at noon when light levels were as high as 2200 μmol m-2 s-1 PAR. 2. When stems of R. raetam or leaves of A. halimus were exposed to natural daylight of 1000 μmol m-2 s-1 PAR in the presence of cycloheximide (CHI), in order to prevent the new synthesis of catalase, or at a temperature of 42-44 °C, only minor losses of catalase activity occurred in stems of R. raetam, but a marked apparent photoinactivation of catalase was observed in leaves of A. halimus. 3. Similarly, stronger declines of the ratio F(v)/F(m), indicating more severe photoinhibition of photosystem II (PSII), were observed in A. halimus, compared with R. raetam. 4. For R. raetam, activities of some antioxidative enzymes and the carotenoid con-tents were higher in plants growing in the desert than in plants collected from non-desert locations. 5. During the daily increase of light and temperature the antioxidants ascorbate and glutathione became more oxidized and violaxanthin was converted to zeaxanthin in R. raetam stems, while these antioxidants became more reduced and zeaxanthin was not notably accumulated in A. halimus leaves. 6. The results suggest that the desert plants R. raetam and A. halimus apply different strategies of protection against light-stress damage. In the stems of R. raetam antioxidative protection appears to play the major role, while A. halimus appears to avoid light damage in the field by the mutual shading of leaves and reflection of light at the leaf surface.