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.