Characterization of Lead Precipitate Following Uptake by Roots of Brassica juncea

Seedlings of Brassica juncea (L.) Czern. were grown in solution culture for 14 d prior to exposure to Pb₂+ at an activity of 31 microM for 72 h. Electron dense deposits found within the apoplast and symplast were analysed using scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) to determine the chemical identity of the deposits and potential toxicity resistance mechanisms. Irrespective of the cellular compartment in which they were found, the deposits contained Pb, O, P and Cl. For the extracellular deposits, the average Pb: P : O atomic ratio was 1 : 0.54 : 3.0, which together with the hexagonal crystal system suggests that Pb is present as chloropyromorphite (Pb₅(PO₄)₃Cl). A weak Ca signal was also detected in about half of the spectra, possibly indicating the presence of small concentrations of phosphohedyphane (Pb₃Ca₂(PO₄)₃Cl). The evidence suggests that B. juncea resists Pb toxicity by storing precipitated Pb in the vacuole.