Sensitivity to photodamage increases during senescence in excised leaves

Abstract

Summary Primary leaves of rye seedlings ( Secale cereale L.) were excised and allowed to senesce either at a low photosynthetic photon flux (90 µmol m −2 s −1 ) or in darkness. At different stages after excision, leaves were exposed to a higher photon flux (520 µmol m −2 s −1 ) for 5 h and changes in chlorophyll, variable fluorescence (F v ) and of the enzymes catalase, guaiacol-dependent peroxidase, glutathione reductase and glycolate oxidase were assayed. Rapid and substantial changes were observed only for catalase and F v /F m ratios. Up to 1 day after excision catalase activity was hardly affected by the high light exposure. At later stages, particularly in leaves senescing in darkness, increasing declines of catalase activity were induced. Similarly, sensitivity for photoinhibition of photosystem II, indicated by declines of the F v /F m ratio, increased during senescence. Labeling of leaf segments with L-[ 35 S]-methionine indicated that the rates of new synthesis of both catalase and the D1-protein of photosystem II declined during senescence. By application of L-galactonic acid-γ-lactone or L-2-oxothiazolidine-4-carboxylate, the levels of ascorbate or glutathione were markedly increased. However, neither increased antioxidant levels nor cytokinin application were able to prevent apparent photoinactivation of catalase or photosystem II during later stages of senescence. While low light intensity retarded the decline of catalase and chlorophyll in senescing leaves, continuous exposure to the high light intensity markedly enhanced the decline of catalase and slightly increased chlorophyll degradation, compared with darkness. Because both catalase and the D1-protein of photosystem II have a rapid turnover in light, the declining capacity of protein synthesis in senescing leaves appears to be increasingly unable to replace losses by photodegradation, and apparent sensitivity to photodamage is increasing. In darkness, an additional subunit form of catalase of higher apparent molecular mass appeared in excised rye leaves.