Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/1998
Publication Date: N/A
Citation: Interpretive Summary: Heavy metal (Pb, Zn, Cd, Cu, Cr and Ni) contamination of soils poses serious problems to both human health and agriculture in the U.S. One of the primary sites of entry of toxic heavy metals such as cadmium (Cd) into the food chain is via uptake into plants, with deposition in the edible portions. Thus, it is important that we understand and characterize the absorption and accumulation of heavy metals in plants. Also, there is considerable current interest in using the ability of plants to absorb heavy metals from the soil as a way to clean up contaminated soils. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. One limit to the development of this technology is our lack of understanding of heavy metal transport processes in plants. This lack of understanding, in turn, has been limited by a lack of research technologies to study these processes. In this paper, we report on the development of a new microelectrode that is very selective for the toxic metal Cd. It can measure Cd in solution to very low levels. We show how this microelectrode can be used to study and map Cd uptake along plant roots. This microelectrode will allow us to conduct detailed investigations into plant Cd uptake that previously were not possible.
Technical Abstract: A Cd2+-selective vibrating microelectrode was constructed using a neutral carrier-based cadmium ionophore in order to investigate ion transport processes along the roots of wheat and two species of Thlaspi, one a Zn/Cd hyperaccumulator and the other a related non-accumulator. In simple Cd(NO3)2 solutions, the electrode exhibited a Nernstian response in solutions with Cd2+ activities as low as 50 nM. Addition of Ca2+ to the calibration solutions did not influence the slope of the calibration curve, but reduced the detection limit to a solution activity of 1 uM Cd2. Addition of high concentrations of K+ and Mg2+ to the calibration solution, to mimic the ionic composition of the cytoplasm, neither affected the slope nor the sensitivity of the electrode, demonstrating the electrode's potential for intracellular investigations. The electrode was pH insensitive. The electrode was assayed for selectivity and was shown to be at least one thousand times more selective for Cd2+ than for any of those potentially interfering ions tested. Flux measurements along the roots of the two Thlaspi species showed no differences in the pattern or the magnitude of Cd2+ uptake, at least within the time frame considered. The Cd2-selective microelectrode will permit detailed investigations of heavy metal ion transport in plant roots, especially in the area of phytoremediation.