Pb in Carrots Grown On Pb-Rich Soils Is Mostly within the Xylem.

Our previous research examined Pb uptake by carrots and other crops grown on old orchard soils or urban gardens rich in Pb from historic contamination processes. We found that carrot, but not potato, accumulated significant amounts of Pb in the edible root vegetable. Peeling the carrots showed that the concentration of Pb in the peel layer of washed carrots was lower than the rest of the storage root, indicating that Pb was not present due to soil particles in/on the peel. With these results, we hypothesized that in carrot the Pb could be localized in the xylem which passes thru the storage root, in contrast with potato which is phloem fed with no internal xylem. Carrots were grown in orchard and urban garden soils rich in Pb and examined using synchrotron XRF mapping to determine the localization of Pb across the root, and XAS to attempt to identify the form of Pb present. μ-XRF examination of carrot cross sections for Pb and other elements showed strong localization of Pb in the xylem part of the carrot. Other elements were mostly evenly distributed across the carrot. We conclude that in carrots grown on Pb-rich soils, Pb moving thru the xylem is partially trapped by sorption on xylem surfaces or precipitated within the xylem. It was already known that Pb ingested with food had low bioavailability to humans. The sorbed or precipitated Pb in carrot storage root xylem would be expected to have even lower bioavailability. Thus carrot may be grown on urban garden soils up to at least the soil Pb concentration which requires limitation of children’s exposure to the garden soil.