Potential phytotoxicity of diquat accumulated by aquatic plants and sediments

The adsorption and desorption of Reglone A^R to freshwater algae, Myriophyllum spicatum and a soil-sand mixture, singly and in combination was examined to investigate the potential for phytotoxicity of diquat residues in aquatic sediments. The diquat adsorption capacity of the aquatic plants ranged from 0.6 to 2.4 mg diquat g dry wt^?1. Forty to seventy percent of the adsorbed diquat could be desorbed with 5 M ammonium chloride. The adsorption capacity of the soil system was about 2.5 mg diquat g dry wt^?1 and approximately 35% of this diquat could be desorbed. Reglone^R inhibited the growth of blue-green algae (Anabaena flos-aquae and Anacystis nidulans) at concentrations greater than 0.03 ppm. Eukaryotic algae were less sensitive, growth of Navicula pelliculosa was inhibited at concentrations exceeding 0.3 ppm and Chlorella vulgaris was unaffected by 3 ppm Reglone. In the presence of soil, growth inhibition by Reglone was eliminated. Reglone was added to a water-soil mixture system at levels up to 334 ppm to simulate chronic usage. Anabaena flos-aquae or Lemna sp. were used to bioassay the availability of the diquat adsorbed to this soil system. Significant growth inhibition of both bioassay plants was observed in soil treated with 33.4 ppm Reglone and the 334 ppm treatment was lethal. These experiments suggest that residual phytotoxicity becomes apparent in this soil system at about 7% of the diquat adsorption capacity. The Reglone adsorption-desorption isotherm of a natural, organic lake sediment was measured to predict the number of Reglone treatments at the recommended application rate before residual phytotoxicity would become apparent.