Paclobutrazol Protects Wheat Seedlings from Heat and Paraquat Injury. Is Detoxification of Active Oxygen Involved?

Paclobutrazol (PBZ), a member of the triazole family, protected wheat (Triticum aestivum L.) seedlings from injury due to heat (50°C for 2.5 h) and paraquat (2 mM). Both stresses induced photoinhibition, loss of fresh weight and membrane integrity, suggesting the possible involvement of toxic oxygen species. This study tested the hypothesis that PBZ-induced protection from stress is in part mediated by enhanced detoxification of active oxygen. The results support this hypothesis since PBZ stimulated a 16, 32 and 21% increase in the activities of superoxide dismutase, ascorbate peroxidase (AP), and glutathione reductase (GR), on a fresh weight basis. The increased activities of AP and GR were maintained to a higher degree than their corresponding controls after exposure to both heat and paraquat stress. Ascorbate and glutathione pools were l4 and 8% higher respectively in the PBZ-treated wheat than in the controls. Within the cytoplasm, PBZ increased the activities of catalase (45%) and guaiacol peroxidase (29%) on a fresh weight basis and these higher activities, over the controls, were conserved after stress. It is suggested that the damage caused by two different stresses, heat and paraquat, is in part due to increased generation of active oxygen and that PBZ protects plants by maintaining increased antioxidant enzyme activity.