SENSITIVITY TO PHOTODAMAGE INCREASES DURING SENESCENCE IN EXCISED LEAVES

Primary leaves of rye seedlings (Secale cereale L.) were excised and a llowed to senesce either at a low photosynthetic photon flux (90 mumol m-2s-1) or in darkness. At different stages after excision, leaves we re exposed to a higher photon flux (520 mumol m-2s-1) for 5 h and chan ges in chlorophyll, variable fluorescence (F(v)) and of the enzymes ca talase, guaiacol-dependent peroxidase, glutathione reductase and glyco late oxidase were assayed. Rapid and substantial changes were observed only for catalase and F(v)/F(m) ratios. Up to 1 day after excision ca talase activity was hardly affected by the high light exposure. At lat er stages, particularly in leaves senescing in darkness, increasing de clines of catalase activity were induced. Similarly, sensitivity for p hotoinhibition of photosystem II, indicated by declines of the F(v)/F( m) ratio, increased during senescence. Labeling of leaf segments with L-[S-35]-methionine indicated that the rates of new synthesis of both catalase and the D1-protein of photosystem III declined during senesce nce. By application of L-galactonic acid-gamma-lactone or L-2-oxothiaz olidine-4-carboxylate, the levels of ascorbate or glutathione were mar kedly increased. However, neither increased antioxidant levels nor cyt okinin application were able to prevent apparent photoinactivation of catalase or photosystem II during later stages of senescence. While lo w light intensity retarded the decline of catalase and chlorophyll in senescing leaves, continuous exposure to the high light intensity mark edly enhanced the decline of catalase and slightly increased chlorophy ll degradation, compared with darkness. Because both catalase and the D1-protein of photosystem II have a rapid turnover in light, the decli ning capacity of protein synthesis in senescing leaves appears to be i ncreasingly 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.