Efficacy of exogenous GA3 and herbicide safeners in protection of Zea mays from metolachlor toxicity

Contents of gibberellic acid (GA3), glutathione (γ-L-glutamyl-L-cysteinyl glycine, GSH) and protein, activities of α-amylase (EC 3.2.1.1) and glutathione S-transferase (GST, EC 2.5.1.18) as well as metolachlor residues, were investigated in shoots of 6-d old maize seedlings during an 8-d period following treatment with metolachlor, either alone or combined with GA3, naphthalic anhydride (NA) or flurazole (FL). Metolachlor provoked a significant decrease in dry weight but had no effect when combined with GA3, NA or FL. Metolachlor induced a significant decrease in GA3 content and in α-amylase activity throughout the entire experiment. Externally applied GA3 relieved this effect while NA and FL relatively raised the enzyme activity but still remained below control levels. Protein content was also reduced by metolachlor, an effect that was counterbalanced by NA or FL but not by GA3. Metolachlor residues progressively accumulated in shoots up to the 4th day, and decreased gradually thereafter. Residues were appreciably lowered by NA or FL but not by GA3. Significant increases of GSH content were induced by metolachlor or its combinations; the magnitude of the increase was more pronounced with FL. GST activity was significantly enhanced by metolachlor; the effect was not influenced by GA3, being augmented by the presence of NA and multiplied by FL. The results indicate that GA3 compensates the loss of the endogenous GA3 content and of α-amylase activity while FL, and to some extent NA, stimulated the detoxification rate of metolachlor by enhancing GSH content and GST activity.