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

ABSTRACT The contents of ascorbate and glutathione and the activities of superoxide dismutase and glutathione reductase were increased to levels as high as those in cold‐hardened leaves (CHL) by incubating non‐hardened leaves (NHL) of winter rye (Secale cereale L.) with the precursor substrates L‐galactonic acid‐γ‐lactone and 2‐oxothiazolidine‐4‐carboxylate. Reduced glutathione was rapidly depleted from NHL after application of D, L‐buthionine sulfoximine, an inhibitor of its biosynthesis. In spite of greatly divergent antioxidant 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 degradation in light were reduced in NHL with increased antioxidant levels. Paraquat‐induced photo‐inactivation of PSII was, however, not mitigated. The CHL had a higher capacity to prevent paraquat‐induced oxidation of ascorbate and glutathione than NHL with increased antioxidant contents. Increased antioxidant contents did not establish resistance to low temperature‐induced photo‐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 affected when photorespiration was suppressed by phosphinothricin. Protection of PSII depended also on non‐photochemical quenching of excitation energy.