The high mountain plant species Ranunculus glacialis has a low antioxidative scavenging capacity and a low activity of thermal dissipation of excess light energy despite its growth under conditions of frequent light and cold stress. In order to examine whether this species is protected from over-reduction by matching photosystem II (PSII) electron transport (ETR) and carbon assimilation, both were analysed simultaneously at various temperatures and light intensities using infrared gas absorption coupled with chlorophyll fluorescence. ETR exceeded electron consumption by carbon assimilation at higher light intensities and at all temperatures tested, necessitating alternative electron sinks. As photorespiration might consume the majority of excess electrons, photorespiration was inhibited by either high internal leaf CO 2 molar ratio (C i ), low oxygen partial pressure (0.5% oxygen), or both. At 0.5% oxygen ETR was significantly lower than at 21% oxygen. At 21% oxygen, however, ETR still exceeded carbon assimilation at high C i , suggesting that excess electrons are transferred to another oxygen consuming reaction when photorespiration is blocked. Nevertheless, photorespiration does contribute to electron consumption. While the activity of the waterwater cycle to electron consumption is not known in leaves of R. glacialis , indirect evidence such as the high sensitivity to oxidative stress and the low initial NADP-malate dehydrogenase (NADP-MDH) activity suggests only a minor contribution as an alternative electron sink. Alternatively, the plastid terminal oxidase (PTOX) may transfer excess electrons to oxygen. This enzyme is highly abundant in R. glacialis leaves and exceeds the PTOX content of every other plant species so far examined, including those of transgenic tomato leaves overexpressing the PTOX protein. Finally, PTOX contents strongly declined during deacclimation of R. glacialis plants, suggesting their important role in photoprotection. Ranunculus glacialis is the first reported plant species with such a high PTOX protein content.

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The enzyme catalase (EC The enzyme catalase from both animal and plant sources is light sensitive (2,4,7,23 we investigated the dynamics of the catalase polypeptide with labeling and chase experiments and immunochemical techniques; for comparison, we investigated the dynamics of the Dl protein in rye leaves exposed to different photon flux densities.The maintenance of proteins depending on rapid tumover will generally be sensitive to all environmental conditions that either overload or hamper the capacity of repair synthesis, as previously described for low temperature (29). Also, other stress conditions known to affect protein synthesis, such as salt (13, 16) or heat shock (28), can be expected to induce specific declines in catalase and possibly in the Dl protein even in only moderate light, simply by preventing repair. Therefore, their influence on catalase and on Dl was investigated. MATERIALS AND METHODS Plant Material and Growing ConditionsExperiments were performed with leaf sections of 6-d-old rye seedlings (Secale cereale L. cv Halo). Seeds were surface sterilized by a 10-min vacuum infiltration and about 30-min soaking in a freshly prepared, filtered solution of 3% (w/v) calcium hypochlorite chloride, thoroughly washed with demineralized H20, and grown at 220C in glass-covered plastic boxes on filter paper (Schleicher & Schull; No. 598

In leaves of three alpine high mountain plants, Homogyne atpina, Ranuncutus gtaciatis and Sotdanetta atpina., both photosystem II (PSIl) and the enzyme catalase appeared to be highly resistant to photoinactivation under natural field conditions. While the Dl protein of PSII and catalase have a rapid turnover in light and require continuous new protein synthesis in non-adapted plants, little apparent photoinactivation of PSII or catalase was induced in the alpine plants by translation inhibitors or at low temperature, suggesting that turnover of the Dl protein and catalase was slow in these leaves. In vitro PSII was rapidly inactivated in light in isolated thylakoids from H. alpina and R. gtaciatis. In isolated intact chloroplasts from R. gtaciatis., photoinactivation of PSII was slower than in thylakoids. Partially purified catalase from R. gtaciatis and S. alpina was as sensitive to photoinactivation in vitro as catalases from other sources. Catalase from H. atpina had, however, a 10-lbld higher stability in light. The levels of xanthophyll cycle earotenoids, of the antioxidants ascorbate and glutathione, and of the activities of catalase, superoxide dismutase and glutathione reductase were very high in S. atpina, intermediate in H. atpina, but very low in R. gtaciatis. However, isolated chloroplasts from all three alpine species contained much higher concentrations of ascorbate and glutathione than chloroplasts from lowland plants.

Ranunculus glacialis leaves were tested for their plastid terminal oxidase (PTOX) content and electron flow to photorespiration and to alternative acceptors. In shade-leaves, the PTOX and NAD(P)H dehydrogenase (NDH) content were markedly lower than in sun-leaves. Carbon assimilation/light and Ci response curves were not different in sun- and shade-leaves, but photosynthetic capacity was the highest in sun-leaves. Based on calculation of the apparent specificity factor of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), the magnitude of alternative electron flow unrelated to carboxylation and oxygenation of Rubisco correlated to the PTOX content in sun-, shade- and growth chamber-leaves. Similarly, fluorescence induction kinetics indicated more complete and more rapid reoxidation of the plastoquinone (PQ) pool in sun- than in shade-leaves. Blocking electron flow to assimilation, photorespiration and the Mehler reaction with appropriate inhibitors showed that sun-leaves were able to maintain higher electron flow and PQ oxidation. The results suggest that PTOX can act as a safety valve in R. glacialis leaves under conditions where incident photon flux density (PFD) exceeds the growth PFD and under conditions where the plastoquinone pool is highly reduced. Such conditions can occur frequently in alpine climates due to rapid light and temperature changes.