Mode of action of the dichloroacetamide antidote BAS 145-138 in corn

The effect of BAS 145-138 (BAS) on metazachlor injury to corn and on the fate of [14C]metazachlor in corn seedlings was investigated. Corn shoot and root growth were inhibited by metazchlor. The antidote, BAS, increased corn shoot and root tolerance to metazachlor 10.7- and 7.6-fold, respectively. The antidotal activities of BAS and dichlormid were similar. Corn seedlings grown in soil treated with [14C]metazachlor +/- BAS were dissected at two growth stages prior to emergence and one growth stage immediately after emergence. Parent [14C]metazachlor was present as < 6% of the total radioactivity with an estimated tissue concentration of < 1 micromole in all tissues except the pericarp. This suggests that metazachlor was metabolized rapidly in both antidoted and control plants and that a very low concentration of metazachlor is required for phytotoxicity. BAS treatment reduced the concentration of parent metazachlor in the developing leaves by 82-84%. BAS treatment had three effects that contributed to the reduced amount of parent [14C]metazachlor in the developing leaves: (i) shoot absorption of [14C]metazachlor was slightly reduced by antidote treatment; (ii) the mobility of 14C was reduced in antidoted seedlings, as indicated by the 63-86% decrease of total 14C reaching the developing leaves; (iii) metabolism of metazachlor in growing tissues may have been stimulated by BAS, as suggested by the lower percentage of 14C present as parent metazachlor. The coleoptile plays a critical role in corn shoot tolerance to metazachlor, since more metazachlor is absorbed through the coleoptile than through the mesocotyl and corn is more sensitive to metazachlor absorbed through the coleoptile than the mesocotyl. Reduced absorption and movement of metazachlor through the coleoptile apparently contribute to antidote activity. Results are consistent with the hypothesis that BAS protects corn from metazachlor injury by reducing levels of parent metazachlor present in sensitive and rapidly growing tissues such as the developing leaves.