Determining the fluxes of ions (Pb2+, Cu2+ and Cd2+) at the root surface of wetland plants using the scanning ion-selective electrode technique

Background and aims Measuring specific ion fluxes from different regions of the root under practical physiological conditions is crucial for understanding metal uptake mechanisms by plants. Methods We developed and tested a neutral carrier-based liquid-membrane Pb2+ and Cu2+ ion selective microelectrode (ISME) to investigate ion-transport processes along the roots of three common wetland plant species. Results The Pb2+ and Cu2+ ISME exhibited a Nernstian response with Pb2+ and Cu2+ activities as low as 1.0 nM and 1.0 mu M in deionized water and simulated soil solution, respectively. Phragmites australis had a region of Cu2+ release for approximately the first 200 mu m, while it exhibited Pb2+ and Cd2+ outward net flux up to the first 500 mu m. Although in older sections of the root of Phragmites australis there were areas of influx of Cu2+, Pb2+ and Cd2+, the overall influx was much smaller than that of Typha latifolia or Canna indica. Such a reduced uptake and/or an increased efflux of metal ions across the root-cell plasma-membrane might explain the higher resistance of Phragmites australis to metals, at least in part. Conclusions The Pb2+ and Cu2+ ISMEs are shown to permit detailed investigation of heavy-metal ion transport in plant roots, especially for plants used for phytoremediation.