Membrane Potentials in Carrot Root Cells

Abstract

Membrane potentials in carrot root xylem parenchyma cells, aged for at least 5 days after excision from the parent organ, were measured under a variety of external conditions by standard intracellular electrode methods. The respiration blocking agents cyanide and carbon monoxide (in the dark) produced large (up to 90 mV) depolarizations which could be described by single exponential decay curves having rate constants (or half-times) of 0.065 s-1 (t*1/2 = 10.8 s) and 0.094 s-1 (t*1/2 = 7.4 s) respectively. The uncoupler carbonyl cyanide m-chlorophenylhydrazone caused a single exponential membrane depolarization with a rate constant of 0.0054 s-1(t*1/2 = 126 s). These effects are thought to reflect the action of an active electrogenic H+ extrusion pump at the plasmalemma. A second, minor electrogenic transport, causing depolarization of about 10 mV in the restlng state, and due to an obligate electrogenic coupling to Cl- influx has been confirmed in KCl-pretreated tissue. In tissue pretreated in more complete nutrient solution, there is no evidence of a Cl- -coupled electrogenic exchange. This seems to be the first report of two independent electrogenic mechanisms operating in a plant cell and, interestingly, they act in opposite senses, the major H+ extrusion causing membrane hyperpolarization, the minor Cl- -influx-coupled exchange causing depolarization, in the normal resting cell.