An Improved Na+-selective Microelectrode for Intracellular Measurements in Plant Cells

The high background K+ concentration in plant cells is a problem for intracellular measurements of Na+ using ion-selective microelectrodes. The discrimination between Na+ and K+ of the microelectrode ionophore molecule limits the usefulness of this technique. A new Na+-selective microelectrode with an ionophore incorporating a tetramethoxyethyl ester derivative of p-t-butyl calix[4]arene has been developed. Microelectrodes made with this new sensor have superior selectivity for Na+ over K+ resulting in a lower limit of detection when compared with microelectrodes made using a commercially available ionophore (ETH227). Both types of microelectrodes were insensitive to changes in ionic strength and physiological ranges of pH, but only the calixarene-based electrodes showed no protein interference. To test the suitability of the calixarene-based microelectrodes for measurements in plants, they were used to measure Na+ in epidermal cells in the zone 10-20 mm from the root apex of barley (Hordeum vulgare L.). Seedlings were grown in a nutrient solution containing 200 mM NaCl for 1-6 d. The range of intracellular Na+ activity (a(Na)) measured varied from < or =0.1 mM (limit of detection) to over 100 mM, and these values increased significantly with time. The membrane potential (E(m)) of these cells was variable, but the values became significantly more negative with time, although there was no significant correlation between E(m) and a(Na). These intracellular measurements could not be separated into distinct populations that might be representative of subcellular compartments.