EFFECTS OF POWDERY MILDEW INFECTION OF BARLEY ON THE ASCORBATE-GLUTATHIONE CYCLE AND OTHER ANTIOXIDANTS IN DIFFERENT HOST-PATHOGEN INTERACTIONS

Rate of lipid peroxidation (malondialdehyde formation), levels of ascorbic acid and nonprotein thiols, and activities of ascorbate peroxidase (AP), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and quinone reductase (QR) were determined in leaves of three barley cultivars inoculated by a Hungarian isolate of Erysiphe graminis f. sp. hordei. Markedly increased malondialdehyde levels (enhanced lipid peroxidation) were observed in leaves of the resistant cultivar Amsel after infection but not in two susceptible cultivars. In the diseased susceptible cultivars Emir and GK-Omega, however, the ascorbic acid levels substantially decreased. A substantial increase of AP and a decline of DHAR activities also were observed in mildewed susceptible plants. A dramatic induction of NADPH-consuming activity was found in the inoculated leaves of the highly susceptible cultivar Emir concomitantly with decreasing 1-electron QR activity. Less-pronounced changes in the parameters were found in the resistant cultivar Amsel. Thiol levels increased moderately in cultivar Amsel and in susceptible cultivar GK-Omega. No significant change in GR activity was found in either cultivar. GST activity was induced in each inoculated cultivar, most substantially in highly susceptible Emir (up to about 360% of the control). Several antioxidative processes seemed to be activated in compatible host-parasite relationships, which may diminish the damaging effects of oxidative stress. This supposition was confirmed by infecting one barley cultivar (Amsel) with compatible and incompatible mildew races. These antioxidative processes were less efficiently activated in the incompatible relationship, which may lead to an early necrotization in the resistant host.