Uptake and Exudation Behavior of Perchlorate in Smartweed

Perchlorate uptake has been reported in a variety of vascular plants, but the mechanisms affecting the uptake, translocation, and exudation of ClO4(-) in plants are not well known. The uptake and transport behavior of ClO4(-) was studied in smartweed (Polygonum spp.) grown in aerated hydroponic ClO4(-) solutions with or without other common anions. Uptake followed first-order kinetics (0.017 +/- 0.003 day(-1)) and there was no apparent competitive effect on ClO4(-) uptake for up to 500 mg L(-1) of NO3(-) -N, SO4(2-), PO4(3-), or Cl(-). The net ClO4(-) mass uptake was proportional to transpiration but with some exclusion of ClO4(-) from the transpiration stream. Up to 4% of the ClO4(-) in plants after a 30-d exposure to a 10-mg/L solution of ClO4(-) was exuded from the roots within 15 d after the exposure was ended. Perchlorate exudation followed first-order kinetics with a rate coefficient of 0.0018 +/- 0.0006 (day(-1) Results obtained in this study indicate that evapotranspiration may be the critical variable to predict accumulation potential in plants and that phytoremediation strategies should take into account the re-release potential of ClO4(-) via exudation.