Mode of action of a herbicide : johnsongrass and methanearsonic Acid.

Johnsongrass (Sorghum halepense (L.) Pers.) is sensitive to methanearsonate, foliar application resulting in a topkill. Investigation of the pattern of photosynthesis by radioautography revealed an accumulation of malate in methanearsonate-treated leaves. Accumulation of malate was attributed to an inhibition of NADP(+)-malic enzyme which was found to be sensitive to sulfhydryl group reagents including arsenosomethane, CH(3)AsO. Methanearsonate was found to act as an oxidant in the Hill reaction using spinach chloroplasts, the photoproduct being a sulfhydryl group reagent.These results suggest that methanearsonate inhibits CO(2) release from malate in bundle sheath cells, depriving the plant of its source of carbon for sucrose production. The mechanism of inhibition of enzymes sensitive to sulfhydryl group reagents by arsenosomethane is addressed.