Basis for antagonism of paraquat phytotoxicity to barley by MCPA dimethylamine*

Abstract

Summary: Résumé: ZusammenfassungMCPA dimethylamine in a mixture with paraquat reduced the disruptive effects of paraquat on leaf cell membrane integrity in excised barley (Hordeum vulgare L.) leaf segments. Uptake or translocation of 14C‐paraquat over 24 h was not affected when mixed with either MCPA dimethy‐lamine or MCPA mixed butyl ester. In treatments followed by a period of dark, uptake and translocation of 14C‐paraquat was increased compared to treatments followed by a period of light. A mixture of technical paraquat dichloride and technical MCPA dimethylamine in aqueous solution yielded a dark brown precipitate. All of the paraquat ion was present in the supernatant. The absorption spectrum of the supernatant exhibited a strong maximum of 390 nm and a lesser one at 610 nm. The spectrum was close to that produced by aqueous mixtures of paraquat and dimethylamine. In greenhouse studies the phytotoxicity of the supernatant to barley was less than an equivalent amount of paraquat alone and equal to that of the complete paraquat‐MCPA dimethylamine mixture. The presence of dimethylamine alone resulted in an early enhancement of paraquat activity. Spot application of an aqueous suspension of the precipitate to rapeseed (Bras sica campestris L.) resulted in slight epinastic effects but these were considerably less than when an equivalent amount of MCPA acid was applied. The effects of the precipitate were restricted to the treated leaf whereas the effects of MCPA acid were evident throughout the plant. The results suggest a chemical interaction between the dichloride salt of paraquat and the dimethylamine salt of MCPA with the production of two compounds with comparatively less biological activity.