Lead Translocation and Isotopic Fractionation after Uptake by Brassica juncea (Brown Mustard)

Abstract

The behavior of lead (Pb) during plant uptake is poorly understood, particularly as it pertains to plant isotopic fractionation patterns and whether they can accurately reflect unique geogenic or anthropogenic sources of contamination. Here, we characterize concentrations and changes in plant Pb ratios during uptake to determine the feasibility of bioremediation and isotopic fingerprinting for Pb sourcing in Brassica juncea (B. juncea), a known Pb-tolerant species, which was grown in a controlled greenhouse environment. Twenty individuals were cultivated from three trials using substrates amended with Pb from different sources (natural ore, anthropogenic, and mixed) and a control with no Pb amendment. After maturation, plants were harvested, split into individual parts, and analyzed for both Pb concentrations and isotopic composition. Results demonstrate significant translocation of Pb from the roots to the leaves, showing an average leaf translocation factor of 3.6. Other above-ground parts’ translocation factors were consistently below 1 (TF = 0.4, on average). Results also show no preferential uptake of any given Pb isotope into the plants, resulting in similar isotopic compositions throughout. Above-ground plant part, ²⁰⁶Pb/²⁰⁷Pb (1.17–1.19) were within the range of initial substrate values (1.14–1.18), though the roots did show a significant increase in ²⁰⁶Pb/²⁰⁷Pb (1.21–1.22). We were unable to differentiate plants grown from each treated substrate, making them unreliable for isotopic fingerprinting of substrate Pb sources. This confirms the limitations of B. juncea as a potential biomonitoring apparatus but demonstrates a greater than expected ability to translocate Pb to its above-ground parts, which may indicate some useful and significant phytoextraction potential during phytostabilization projects and subsequent health concerns for those consuming the plant when grown in low [Pb] soils.