Changes in antioxidants and kinetics of glutathione-S-transferase of maize in response to isoproturon treatment

Isoproturon at the recommended field dose (RFD) significantly reduced fresh and dry weights of shoots and roots as well as chlorophyll and carotenoid contents of 10-day-old maize seedlings during the following 20 days. The higher the herbicide dose, the greater the reduction. Meanwhile, ascorbate (AsA) and reduced glutathione (GSH) increased in leaves for only the first few days. Similar increases in activities of superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) were detected. Low doses caused general increases while high doses induced diminutions; however, CAT and APX activities were inhibited by all doses. Nevertheless, H2O2 was significantly accumulated throughout the experiment; the magnitude of accumulation increased with time and herbicide dose. On the contrary, there were significant inhibitions in activities of the glutathione S-transferase (GST) isoforms ( GST((CDNB)), GST((ALA)), or GST((MET))) with no variation in GST((ATR)); the inhibition was greater with increasing isoproturon doses. These findings suggest the occurrence of an oxidative stress induced by isoproturon, a state that prolonged with increasing herbicide dose and/or treatment time. Moreover, V-max of GST was lowered by isoproturon, whereas K-m was unchanged, indicating that the herbicide is a competitive inhibitor of GST.