Localization of hydrogen peroxide (H2O2) and the roles of peroxidases, malondialdehyde, and reduced glutathione in three apple cultivars were compared in healthy trees, trees infected with apple proliferation phytoplasma (APP), and trees that had recovered from the infection. In recovered apple trees, symptoms of the disease and the pathogen had disappeared from the canopy, but phytoplasmas remained in the roots. H2O2 was detected cytochemically by its reaction with cerium chloride to produce electron-dense deposits of cerium perhydroxides.H2O2 occurred in the plasmalemma of the phloem of leaves of recovered apple trees, but not in healthy or APP-infected leaves. In all cultivars, the peroxidase activity detected in tissue from APP-diseased trees was greater than or equal to that of tissue from recovered trees, which equaled or exceeded that of tissue from healthy trees, at two sampling times (May and September). In contrast, the glutathione content of leaves decreased in the reverse order. More malondialdehyde was observed in leaves from recovered trees than in leaves from healthy or APP-infected trees in three of six cultivar-date combinations; in the other three combinations, the malondialdehyde contents of leaves from healthy, infected, and recovered trees were not significantly different from one another. The results suggest that some components of the oxidant-scavenging system in recovered leaves are not very active, leading to an overproduction of H2O2 and, possibly, to a membrane lipid peroxidation.The production of H2O2 appears to be involved in counteracting pathogen virulence.
