Current-Voltage Curves for Plant Membrane Studies: A Critical Analysis of the Method

Abstract

The evolution of the steady-state current-voltage (I/V) method is followed to its present sophistication of bipolar staircase. When applied to Chara plasmalemma the resultant I/V characteristics vary enormously depending on the physiological state of the membrane (which is, in turn, dictated by the outside conditions). This variety of response makes formulation of ‘correct’, that is artefact-free, I/V technique quite a challenge. The time-dependence of the clamp current in each electrophysiological state is different and this must be taken into consideration when steady-state I/V is investigated. Clamping of the plasmalemma alone is also necessary, as under some conditions the conductance of the two membranes becomes similar and the effects of the tonoplast can no longer be neglected when both membranes are clamped in series. The data obtained by point clamping and space clamping are compared. The use of bipolar staircase introduces more subtle artifacts: the I/V profiles can be severely misinterpreted due to transient conductance changes arising from the excitation or the outward and inward rectifier currents. The excitation can be blocked by temporary exposure to lanthanum ion, but the concentration must be carefully chosen. It is necessary to optimize the staircase parameters to allow the rectifier currents to return to resting level before the next staircase pulse. Finally, brief comparison of the Chara I/Vprofile to that of other cells is included and the relevance to patch clamp studies discussed.