Generation of action potentials in Chara corallina by turgor pressure changes

A depolarisation of the membrane potential difference (about-170 mV) of Chara corallina is observed in response to changes in cell turgor pressure using the pressure probe technique. The depolarisation occurs in phase with the pressure pulse (0.2 s duration) and is independent of the direction of the applied pressure gradient. This latter finding is in contradiction to results predicted on the basis of electro-kinetic phenomena. Pressure induced electrical leakages were ruled out by measuring the depolarisation in response to pressure in individual segments of the internode which were electrically isolated from one another. The changes in potential were recorded by external electrodes and an internal electrode which was positioned close to the micropipette of the pressure probe inserted through one of the electrically isolated nodes. The depolarisation in response to increasing positive or negative pressure gradients in the intact node region and in the intact middle segments was comparable to that monitored in the node region containing the pressure probe. Action potentials were initiated when the pressure gradients exceeded at least 2 bar. The action potentials were elicited at random in one of the two compartments adjacent to the node regions, but were never found to be initiated in the node regions themselves. The pressure-induced action potentials are explained in terms of an electro-mechanical compression (or expansion) of a local membrane area and discussed in their relevance to the propagation of pressure signals in response to water and salt stress in higher plants.