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 asco rbic acid and nonprotein thiols, and activities of ascorbate peroxidas e (AP), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and quinone reductase (QR) were dete rmined in leaves of three barley cultivars inoculated by a Hungarian i solate of Erysiphe graminis f. sp. hordei. Markedly increased malondia ldehyde levels (enhanced lipid peroxidation) were observed in leaves o f the resistant cultivar Amsel after infection but not in two suscepti ble cultivars. In the diseased susceptible cultivars Emir and GK-Omega , however, the ascorbic acid levels substantially decreased. A substan tial increase of AP and a decline of DHAR activities also were observe d in mildewed susceptible plants. A dramatic induction of NADPH-consum ing activity was found in the inoculated leaves of the highly suscepti ble cultivar Emir concomitantly with decreasing 1-electron QR activity . Less-pronounced changes in the parameters were found in the resistan t cultivar Amsel. Thiol levels increased moderately in cultivar Amsel and in susceptible cultivar GK-Omega. No significant change in GR acti vity was found in either cultivar. GST activity was induced in each in oculated cultivar, most substantially in highly susceptible Emir (up t o about 360% of the control). Several antioxidative processes seemed t o be activated in compatible host-parasite relationships, which may di minish 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 ef ficiently activated in the incompatible relationship, which may lead t o an early necrotization in the resistant host.