CHARACTERIZATION OF LEAD PRECIPITATE FOLLOWING UPTAKE BY ROOTS OF BRASSICA JUNCEA

摘要

Seedlings of Brassica juncea (L.) Czern. were grown in solution culture for 14 d prior to exposure to Pb~(2+) at an activity of 31 μM for 72 h. Electron-dense deposits found within the apoplast and symplast were analyzed using scanning transmission electron microscopy/energy dispersive spectroscopy 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 (Pb_5(PO_4)_3Cl). A weak Ca signal also was detected in approximately half of the spectra, possibly indicating the presence of small concentrations of phosphohedyphane (Pb_3Ca_2(PO_4)_3Cl). The evidence suggests that B. juncea resists Pb toxicity by storing precipitated Pb in the vacuole.