Development, Characterization, and Application of a Cadmium-Selective Microelectrode for the Measurement of Cadmium Fluxes in Roots of Thlaspi Species and Wheat

A Cd²⁺-selective vibrating microelectrode was constructed using a neutral carrier-based Cd ionophore to investigate ion-transport processes along the roots of wheat (Triticum aestivum L.) and two species of Thlaspi, one a Zn/Cd hyperaccumulator and the other a related nonaccumulator. 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 μm Cd²⁺. Addition of high concentrations of K⁺ and Mg²⁺ to the calibration solution to mimic the ionic composition of the cytoplasm affected neither the slope nor the sensitivity of the electrode, demonstrating the pH-insensitive electrode's potential for intracellular investigations. The electrode was assayed for selectivity and was shown to be at least 1000 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 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.