Comparison of three methods for measuring electrical resistances of plant cell membranes

The reliability of two different membrane resistance-measuring methods that use a single intracellular microelectrode was tested against a conventional method that uses two intracellular microelectrodes. The first single-electrode method used a single square current pulse and required a constant microelectrode resistance. This method was unreliable because the electrode resistance changed markedly on cell penetration and changed with time within the cell. The second method used a high frequency square wave for injecting current into the cell and depended upon the membrane having a much longer RC (resistance × capacitance)-time constant than the microelectrode. The resistance values obtained by this latter method were usually different from membrane resistances obtained at the same time on the same cells using two intracellular microelectrodes. Therefore, neither single intracellular microelectrode method was as reliable as the conventional method. All tests were with coleoptile cells of Avena sativa var. Victory.