A NEW TURGOR MEMBRANE-POTENTIAL PROBE SIMULTANEOUSLY MEASURES TURGOR AND ELECTRICAL MEMBRANE-POTENTIAL

A new combined turgor/membrane potential probe (T-EP probe) monitored cell turgor and membrane potential simultaneously in single giant cell s. The new probe consisted of a silicone oil-filled micropipette (oil- microelectrode), which conducted electric current. Measurements of tur gor and hydraulic conductivity were performed as with the conventional cell pressure probe besides the membrane potential. In internodal cel ls of Chara corallina, steady state turgor (0.5-0.7 MPa) and resting p otentials (-200 to -220mV) in APW, and hydraulic conductivity (0.07 to 0.21 x 10(-5) m s(-1) MPa(-1)) were measured with the new probe, and cells exhibited healthy cytoplasmic streaming for at least 24 h during measurements. When internodal cells of Chara corallina were treated w ith 30, 20, 10, and 5 mM KCl, turgor responded immediately to all conc entrations, and the osmotic changes in the medium were measured. Actio n potentials, which brought the membrane potential to a steady depolar ization 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, t he repolarization of membrane potential consisted of a quick and a fol lowing slow phase. During the quick phase, which took place immediatel y and lasted 1 to 3 min, the plasma membrane remained activated. The m embrane was gradually deactivated in the slow phase, and entirely deac tivated when the membrane potential recovered to the resting potential in APW. Although the activated plasma membrane was permeable to K+, n o major ion channels were activated on the tonoplast, and therefore, i nternodal cells of Chara corallina did not regulate turgor when osmoti c potential changed in the surrounding medium.