Is Perchlorate Metabolized or Re-Translocated within Lettuce Leaves? A Stable-Isotope Approach

Perchlorate is an environmental contaminant that is found in drinking water and a variety of foodstuffs, but many questions regarding its uptake, transport, and persistence in higher plants remain unanswered. In a series of hydroponic experiments, a stable-isotope tracer of perchlorate (95% 37ClO4(-)) was utilized to determine the extent of in vivo metabolism and of phloem re-translocation of perchlorate at low (i.e., nmol/kg of fresh weight (FW)) concentrations in lettuce. Chlorate and chlorite metabolites were not detected in lettuce leaves at detection limits of 13.1 and 291 nmol/kg FW, respectively, and chloride isotopic signatures were not substantially different from natural chloride. Perchlorate exhibited no significant movement from older leaves into new leaves, nor to roots. Stable isotopes proved useful in assessing perchlorate metabolism and re-translocation within lettuce at nmol/kg levels. The absence of any metabolism or re-translocation indicates that perchlorate is relatively persistent within leafy produce, and that the primary mode of transport of perchlorate is through the xylem of higher plants.