Basis for Differential Chemical Selectivity of MG-191 Safener against Acetochlor and EPTC Injury to Maize*

The influence of MG-191 safener on the uptake, translocation and metabolism of [l4C]acetochlor and [14C]EPTC was studied. The amounts of absorbed radioactivity by maize seedlings at 3, 6, 24, and 72 h after applications of [l4C]labeled herbicides and [l4C]MG-191 were different. Plants treated with [l4C]acetochlor took up 30to 50-fold more radiolabel with­ in 72 h than [14C]EPTCor [14C]MG-191-treated plants. Addition of MG-191 caused only mi­ nor changes in the rate of herbicide absorption. EPTC and MG-191 and/or their metabolites moved quickly acropetally and partitioned equally between root and shoot tissues up to 72 h. The amount of acetochlor and/or its labeled metabolites translocated to shoot tissues was less than 10%. MG-191 practically had no influence on herbicide translocation rates. With all chemicals the amounts of water-soluble and unextractable fractions increased while the ratio of hexane-extractable metabolites decreased with time. TLC analyses of both waterand hex­ ane-soluble metabolites confirmed the fast metabolism of acetochlor. The acetochlor metabo­ lism took place via GSH conjugation and more polar, non-conjugated metabolites compared to parent molecule were detected in hexane-soluble fraction. MG-191 enhanced acetochlor metabolism by decreasing the portion of non-metabolized acetochlor. EPTC metabolism re­ sulted in water-soluble metabolites having similar chromatographic properties to those of ace­ tochlor. However, there was no safener effect on non-metabolized EPTC content of plants. It appears that MG-191 protects maize against EPTC by enhancing the early rate of conjugation with GSH after initial oxidative metabolism.