The physiological effects and fate of dichlobenil (2,6-dichlorobenzonitrile) in an aquatic environment Public Deposited

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  • The amounts of residual dichlobenil (2,6-dichlorobenzonitrile) in water and hydrosoil were compared periodically following application of the wettable powder and granular formulations of the herbicide to separate ponds at ten pounds of active ingredient per surface acre {0.6 ppmw (parts per million by weight) }. Dichlobenil was measured by electroncapture gas chromatography. Maximum residual concentrations in the water after treatment with wettable powder or granules were 1.00 ppmw after four days and 0,68 ppmw after five days, respectively. After 15 days, the amounts of residue in the two ponds were similar, and decreased steadily to 0.001 ppmw or less after 126 days (last sampling date). The maximum concentration of dichlobenil in the hydrosoil occurred 6 days after treatment with wettable powder and was 1.47 ppmv (parts per million by volume), while the granules produced a maximum concentration of 3.70 ppmv in one day. Residues in the hydrosoil of the two ponds reached similar concentrations 34 days after treatment. After 126 days the residue levels in the hydrosoil of both ponds had decreased to less than 0.04 ppmv. The persistence of dichlobenil in pond water and hydrosoil was similar whether applied as a granular or a wettable powder formulation. The growth rates of two aquatic vascular plants, Potamogeton pectinatus and Alisma gramineum var. Geyeri, and one aquatic alga, Scenesdesmus obliquus, were measured after exposure to various concentrations of dichlobenil in laboratory studies P. pectinatus and A. gramineum were sensitive to 0.1 ppmw of dichlobenil, whereas S. obliquus was tolerant to concentrations as high as 10.0 ppmw. Several laboratory studies were conducted in efforts to elucidate the mode of action of dichlobenil. Dichlobenil did not affect photosynthesis in either P. pectinatus or S. obliquus as determined by manometric measurements of oxygen evolution. Utilization of oxygen by isolated potato mitochondria, measured by polarographic techniques, appeared to be stimulated slightly by 25 ppmw dichlobenil in the reaction medium during state 4 respiration. However, measurements of mitochondrial ATPase activity following additions of 10-5 or 10-4 M dichlobenil indicated that the action of dichlobenil in apparently stimulating respiration did not involve the uncoupling of oxidative phosphorylation. The basis for the selectivity of dichlobenil was investigated by exposing P. pectinatus and S. obliquus to 10 ppmw radioactive dichlobenil for 72 hours. Two possible explanations were found for the selective action of dichlobenil. First, S. obliquus, the tolerant species, absorbed dichlobenil at a slower rate than the susceptible species, P. pectinatus. Secondly, S. obliquus apparently metabolized a greater percentage of the .absorbed.herbicide to non-phytotoxic product(s) than did P. pectinatus. Proper standards were not available for the identification of the metabolite.
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