Uptake and localisation of lead in the root system of Brassica juncea

The uptake and distribution of Pb sequestered by hydroponically grown (14 days growth) Brassica juncea (3 days exposure; Pb activities 3.2, 32 and 217 μM) was investigated. Lead uptake was restricted largely to root tissue. Examination using scanning transmission electron microscopy–energy dispersive spectroscopy revealed substantial and predominantly intracellular uptake at the root tip. Endocytosis of Pb at the plasma membrane was not observed. A membrane transport protein may therefore be involved. In contrast, endocytosis of Pb into a subset of vacuoles was observed, resulting in the formation of dense Pb aggregates. Sparse and predominantly extracellular uptake occurred at some distance from the root tip. X-ray photoelectron spectroscopy confirmed that the Pb concentration was greater in root tips. Heavy metal rhizofiltration using B. juncea might therefore be improved by breeding plants with profusely branching roots. Uptake enhancement using genetic engineering techniques would benefit from investigation of plasma membrane transport mechanisms.