NATURAL SENESCENCE OF PEA LEAVES - AN ACTIVATED OXYGEN-MEDIATED FUNCTION FOR PEROXISOMES

We studied the activated oxygen metabolism of peroxisomes in naturally and dark-induced senescent leaves of pea (Pisum sativum L.). Peroxiso mes were purified from three different types of senescent leaves and t he activities of different peroxisomal and glyoxysomal enzymes were me asured. The activities of the O-2(.-)- and H2O2-producing enzymes were enhanced by natural senescence. Senescence also produced an increase in the generation of active oxygen species (O-2(.-) and H2O2) in leaf peroxisomes and in the activities of two glyoxylate-cycle marker enzym es. A new traction of peroxisomes was detected at an advanced stage of dark-induced senescence. Electron microscopy revealed that this new p eroxisomal fraction varied in size and electron density. During senesc ence, the constitutive Mn-superoxide dismutase (SOD) activity of perox isomes increased and two new CuZn-SODs were induced, one of which cros s-reacted with an antibody against glyoxysomal CuZn-SOD. This fact and the presence of glyoxylate-cycle enzymes support the idea that foliar senescence is associated with the transition of peroxisomes into glyo xysomes. Our results indicate that natural senescence causes the same changes in peroxisome-activated oxygen metabolism as dark-induced sene scence, and reinforce the hypothesis of an effective role of peroxisom es and their activated oxygen metabolism in this stage of the life cyc le.