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To determine the potential impact of contaminants on the aquatic vascular plants Lemna sp., toxicity tests are usually conducted for a 4- to 14-day exposure, and the toxicity is usually expressed as EC50. However, the effects of longer exposure and the recovery potential after exposure to chemicals are other important factors which should be considered. We present the relative risks of a variety of exposure scenarios and recovery potentials from damage, using herbicides with different modes of action. Toxicity was assessed on the basis of both EC50 and relative growth rate (RGR) compared with untreated controls in exposure and recovery. The EC50 of atrazine was found to be 89 ppb, and its phytostatic concentrations were 1600 and 800 ppb for exposure periods of 14 and 28 days, respectively, and no phytocidal effects were observed up to 3200 ppb for a 28-day exposure. The RGR in recovery was not affected by the RGR in exposure, and regrowth was possible even after complete inhibition of growth for 28 days at the highest concentration tested. Alachlor, with an EC50 of 31 ppb, was phytostatic at 400 ppb for a 14-day exposure and phytocidal at 200 ppb for 21- and 28-day exposures. Paraquat, with an EC50 of 31 ppb, showed phytocidal rather than phytostatic effects. All phytostatic fronds could not grow in the recovery period, and the phytocidal concentration decreased with exposure period, from 80 ppb for a 7-day exposure to 20 ppb for 21- and 28-day exposures. The RGR of alachlor and paraquat in recovery was dependent on the RGR in exposure. In the case of cyclosulfamuron, phytostatic concentrations were 100 and 50 ppb for 7- and 14-day exposures, respectively. In the case of exposures longer than 21 days, however, it exhibited phytocidal activity at 10 ppb. The results of this study suggest that it is important to examine the effects of chemicals over a longer exposure period as well as the recovery potential from damage for reliable ecological risk assessment.

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Diverse 2,4-D and 2,4,5-T-degrading bacteria were isolated country-wide from ten Vietnamese soils, with or without a history of exposure to Agent Orange. The 353 degraders were phylogenetically grouped into three major categories; Burkholderias spp. (43.3% of all degraders), Sphingomonas spp. (40.2%), and Ralstonia spp. (15.3%) and two minor ones; Bradyrhizobium sp. (0.8%) and Nocardioides sp. (0.3%). The 2,4,5-T degraders, 65% of all degraders, were isolated from all soil samples and their 16S rRNA genes were the most homologous with that of Sphingomonas spp., Burkholderia spp. or Bradyrhizobium sp. The following four degradative genes were found by PCR: tfdA (tfdAα) in the Burkholderia spp., Ralstonia spp., Bradyrhizobium sp., and Nocardioides sp.; tfdB in all degraders; tftA (cadA) in the Sphingomonas spp., Burkholderia sp. and Bradyrhizobium sp.; tftC only in the Burkholderia sp. The degraders among Burkholderia spp. were isolated only from the central and southern sites, while those among Ralstonia spp. were found only at the north sites with one exception. The Sphingomonas spp. were isolated country-wide, but four phylogenetically different groups were found at one site, while only one group was found at the other five sites. At least three different plasmids that carried the tfd genes were found in the Burkholderia spp. and Ralstonia spp. without relation to the sites and the phylogenetic groups. These results suggest that the 2,4-D-and 2,4,5-T-degrading microbial consortia have spread countrywide and are diverse on a genetic as well as geographic basis.