IN-VITRO CONJUGATION OF CHLOROACETANILIDE HERBICIDES AND ATRAZINE WITH THIOLS AND CONTRIBUTION OF NONENZYMATIC CONJUGATION TO THEIR GLUTATHIONE-MEDIATED METABOLISM IN CORN

The contribution of nonenzymatic conjugation of acetochlor, alachlor, metolachlor, pretilachlor, and atrazine with glutathione (GSH) in the metabolism of these herbicides by roots and shoots of the Cargill 7567 and Northrup-King 9283 hybrids of corn (Zea mays L.) was investigated . Treatment with 50 muM of each herbicide revealed the following order of chloroacetanilide phytotoxicity: acetochlor > alachlor > metolachl or > pretilachlor. Atrazine did not injure any of the corn hybrids. Ca rgill 7567 was more tolerant than Northrup-King 9283 to metolachlor an d pretilachlor, but the two corn lines did not differ in their respons e to acetochlor and alachlor. Total nonprotein thiol, GSH, and cystein e contents were similar in the two corn hybrids. Glutathione S-transfe rase (GST) activities were comparable in the two corn hybrids. Chloroa cetanilide-specific GST activities were 2-4-fold greater in roots than in shoots of either hybrid, and enzymatic conjugation with GSH was mo re efficient with acetochlor and alachlor than with metolachlor and pr etilachlor as substrates. Assays on the in vitro chemical reactivities of acetochlor, alachlor, metolachlor, pretilachlor, and atrazine with GSH and cysteine revealed that the higher the ratio of thiol to herbi cidal substrate concentrations, the higher the amount of conjugate for med. Nonenzymatic conjugation with GSH may contribute significantly to the overall glutathione-mediated metabolism of chloroacetanilide herb icides and atrazine in corn and seems to be dependent on the concentra tion and molecular structure of the herbicidal substrate as well as on the GSH content of plant tissues (root vs shoot).