Abstract:A new combined turgor/membrane potential probe (T‐EP probe) monitored cell turgor and membrane potential simultaneously in single giant cells. The new probe consisted of a silicone oil‐filled micropipette (oil‐microelectrode), which conducted electric current. Measurements of turgor and hydraulic conductivity were performed as with the conventional cell pressure probe besides the membrane potential. In internodal cells ofChara corallina, steady state turgor (0.5‐0.7 MPa) and resting potentials (‐200 to −220 mV) in APW, and hydraulic conductivity (0.07 to 0.21 × 10∼5m s−1MPa−1) were measured with the new probe, and cells exhibited healthy cytoplasmic streaming for at least 24 h during measurements. When internodal cells ofChara corallinawere treated with 30, 20, 10, and 5 mM KCI, turgor responded immediately to all concentrations, and the osmotic changes in the medium were measured. Action potentials, which brought the membrane potential to a steady depolarization that measured the concentration difference of K+in the medium, were induced in a concentration — dependent delay and occurred only 30, 20, and 10 mM of KCl. When the solution was changed back to APW, the repolarization of membrane potential consisted of a quick and a following slow phase. During the quick phase, which took place immediately and lasted 1 to 3 min, the plasma membrane remained activated. The membrane was gradually deactivated in the slow phase, and entirely deactivated when the membrane potential recovered to the resting potential in APW. Although the activated plasma membrane was permeable to K+, no major ion channels were activated on the tonoplast, and therefore, internodal cells ofChara corallinadid not regulate turgor when osmotic potential changed in th