Significance of antioxidants and electron sinks for the cold-hardening-induced resistance of winter rye leaves to photo-oxidative stress

The contents of ascorbate and glutathione and the activities of superoxide dismutase and glutathione reductase were increased to levels as high as tho se in cold-hardened leaves (CHL) by incubating non-hardened leaves (NHL) of winter rye (Secale cereale L.) with the precursor substrates L-galactonic acid-gamma-lactone and 2-oxothiazolidine-4-carboxylate, Reduced glutathione was rapidly depleted from NHL after application of D,L-buthionine sulfoxim ine, an inhibitor of its biosynthesis. In spite of greatly divergent antiox idant contents the rates of photo-inactivation of photosystem II (PSII) and catalase observed in the presence of translation inhibitors did not differ greatly. The paraquat-induced catalase inactivation and chlorophyll degrad ation in light were reduced in NHL with increased antioxidant levels. Paraq uat-induced photo-inactivation of PSII was, however, not mitigated, The CHL had a higher capacity to prevent paraquat-induced oxidation of ascorbate a nd glutathione than NHL with increased antioxidant contents. Increased anti oxidant contents did not establish resistance to low temperature-induced ph oto-inactivation of PSII and catalase in NHL, The resistance of CHL to low temperature-induced photo-inactivation of PSII and catalase required repair at low temperature and active carbon assimilation but was only little affe cted when photorespiration was suppressed by phosphinothricin, Protection o f PSII depended also on non-photochemical quenching of excitation energy.