Divergent strategies of photoprotection in high-mountain plants

Leaves of high-mountain plants were highly resistant to photoinhibitory damage at low temperature. The roles of different photoprotective mechanisms were compared. Mainly, the alpine species Ranunculus glacialis (L.) and Soldanella alpina were investigated because they appeared to apply greatly divergent strategies of adaptation. The ratio of electron transport rates of photosystem II/photosystem I measured in thylakoids from R. glacialis did not indicate a specific acclimation to high irradiance. Low rates of a chloroplast-mediated inactivation of catalase (EC 1.11.1.6) in red light indicated, however, that less reactive oxygen was released by isolated chloroplasts from R. glacialis than by chloroplasts from lowland plants. Leaves of S. alpina and of Homogyne alpina (L.) Cass, but not those of R. glacialis, had a very high capacity for antioxidative protection, relative to lowland plants, as indicated by a much higher tolerance against paraquat-mediated photooxidative damage and a higher cc-tocopherol content. Accordingly, ascorbate and glutathione were strongly oxidized and already largely destroyed at low paraquat concentrations in leaves of R. glacialis, but were much less affected in leaves of S. alpina. Nonradiative dissipation of excitation energy was essential for photoprotection of leaves of S. alpina and depended on the operation of the xanthophyll cycle. Strong nonphotochemical quenching of chlorophyll fluorescence occurred also in R. glacialis leaves at high irradiance, but was largely independent of the presence of zeaxanthin or antheraxanthin. For R. glacialis, photorespiration appeared to provide a strong electron sink and a most essential means of photoprotection, even at low temperature. Application of phosphinothricin, which interferes with photorespiration by inhibition of glutamine synthetase, caused a striking reduction of electron transport through photosystem II and induced marked photoinhibition at both ambient and low temperature in leaves of R. glacialis, while S. alpina was less affected.