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-degrees-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 enhanc ed detoxification of active oxygen. The results support this hypothesi s since PBZ stimulated a 16, 32 and 21% increase in the activities of superoxide dismutase, ascorbate peroxidase (AP), and glutathione reduc tase (GR), on a fresh weight basis. The increased activities of AP and GR were maintained to a higher degree than their corresponding contro ls after exposure to both heat and paraquat stress. Ascorbate and glut athione pools were 14 and 8% higher respectively in the PBZ-treated wh eat than in the controls. Within the cytoplasm, PBZ increased the acti vities of catalase (45%) and guaiacol peroxidase (29%) on a fresh weig ht basis and these higher activities, over the controls, were conserve d after stress. It is suggested that the damage caused by two differen t 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.