Uptake of EDTA-complexed Pb, Cd and Fe by solution- and sand-cultured Brassica juncea

Direct plant uptake of metals bound to chelating agents has important implications for metal uptake and the free-ion activity model. Uptake of hydrophilic solutes such as metal–EDTA complexes is believed to occur via bypass apoplastic flow, but many questions remain about the relative importance and selectivity of this pathway. In this study, Brassica juncea (Indian mustard) plants grown in solution- and sand-culture conditions were exposed to metal–EDTA complexes and to PTS, a hydrophilic fluorescent dye previously used as a tracer of apoplastic flow. The results suggest that there are two general phases of solute uptake. Under normal conditions, xylem sap solute concentrations are relatively low (i.e., <0.5% of concentration in solution) and there is a high degree of selectivity among different solutes, while under conditions of stress, xylem sap concentrations are significantly higher (i.e., >3% of concentration in solution) and the selectivity among solutes is less. In healthy plants, xylem sap metal–EDTA concentrations were generally an order of magnitude higher than those of PTS and differences among complexes were observed, with CdEDTA²⁻ exhibiting slightly higher xylem sap concentrations than PbEDTA²⁻ or FeEDTA⁻. Metal–EDTA complexes were found to dominate xylem sap metal speciation and the fraction of metal in xylem sap present as metal–EDTA was greater for non-nutrient metals (Pb, Cd) than for the nutrient metal Fe. Despite differences in root morphology between plants grown under solution- and sand-culture conditions, uptake of solutes was similar under both sets of growth conditions.