Uptake and accumulation of perchlorate by lettuce (Lactuca sativa L.): Controlling factors at environmentally relevant concentrations

The health implications of low-level contamination of perchlorate is a topic of ongoing debate, and exposure through produce has remained largely ignored. Evidence of perchlorate uptake by plants exists; however, the mechanistic basis of such uptake is not well understood. The overarching goal of this research was to understand the factors controlling environmentally relevant-concentrations of perchlorate uptake lettuce.;We developed a method to measure perchlorate in lettuce at environmentally relevant-concentrations down to 0.8 microg/kg FW. A series of hydroponic uptake experiments were then conducted using several varieties of lettuce exposed to environmentally-relevant concentrations of perchlorate. We found that perchlorate accumulated in the fresh leaves to varying amounts, ranging from 4 to 192 mug/kg FW and the ranking of perchlorate accumulation was iceberg > butter head > romaine > red leaf > green leaf. Perchlorate is generally found in higher concentrations in the outer leaves of lettuce, and the level of perchlorate uptake depends on the plant's transpiration rate.;A series of competition experiments revealed that increasing nitrate, pH, and bicarbonate led to a decrease in perchlorate uptake in all types of lettuce tested, whereas chloride and sulfate had no effect. Thus, perchlorate is likely taken up due to active transport across root cell membranes and perchlorate and nitrate likely share a common ion transporter in higher plants.;A stable isotope of perchlorate was used to assess the extent of perchlorate metabolism through chlorate and chlorite metabolites to chloride; however, metabolites of perchlorate were not detected in any of our lettuce extracts despite low detection limits of 291 and 13.1 nmol/kg for chlorate and chlorite, respectively. By using treatments of 37C1-labeled perchlorate and then natural (nonlabeled) perchlorate, we were able to determine that perchlorate does not readily move through the phloem in lettuce.;Perchlorate is persistent in fresh produce and may accumulate to potentially harmful levels in lettuce, even when exposed to concentrations below the proposed drinking water standard. Human exposure to perchlorate through consumption of leafy produced can be minimized by choosing varieties/species that accumulate less perchlorate, and by removing outer leaves prior to consumption. Little can be done to reduce exposure via manipulation of soil chemical conditions.