AN ACTIVATED-OXYGEN-MEDIATED ROLE FOR PEROXISOMES IN THE MECHANISM OFSENESCENCE OF PISUM-SATIVUM L LEAVES

The possible involvement of peroxisomes and their activated-oxygen met abolism in the mechanism of leaf senescence was investigated in detach ed pea (Pisum sativum L.) leaves which were induced to senesce by incu bation in complete darkness for up to 11 d. At days 0, 3, 8, and 11 of senescence, peroxisomes were purified from leaves and the activities of different peroxisomal and glyoxysomal enzymes were measured. Xanthi ne-oxidoreductase activity increased with senescence, especially the O 2.-producing xanthine oxidase (EC 1.1.3.22). The activities of H2O2-ge nerating Mn-superoxide dismutase (EC 1.15.1.1) and urate oxidase (EC 1 .7.3.3) were also enhanced by senescence, whereas catalase (EC 1.11.1. 6) activity was severely depressed. Hydrogen peroxide concentrations i ncreased significantly in senescent leaf peroxisomes, During the progr ess of senescence, glycollate oxidase (EC 1.1.3.1) and hydroxypyruvate reductase (EC 1.1.1.81), two marker enzymes of photorespiratory metab olism, gradually decreased in activity and disappeared. At the same ti me, the activities of malate synthase (EC 4.1.3.2) and isocitrate lyas e (EC 4.1.3.1), key enzymes of the glyoxylate cycle, which were undete ctable in presenescent leaves, increased dramatically upon induction o f senescence. Ultrastructural studies of intact leaves showed that the population of peroxisomes and mitochondria increased with senescence. Results indicate that peroxisomes could play a role, mediated by acti vated oxygen species, in the oxidative mechanism of leaf senescence, a nd further support the idea, proposed by other authors, that foliar se nescence is associated with the transition of leaf peroxisomes into gl yoxysomes.