Abstract The time course of hypertonic and hypotonic turgor regulation was studied in cells of Ventricaria (Valonia) using pressure probe and I/V (current-voltage) analysis. Out of 11 cells, 9 exhibited hypertonic turgor regulation, ranging from 100% regulation in 150 min to 14% regulation (14% recovery of the decrease in turgor) in 314 min. Some cells began regulating immediately, others took up to 90 min to begin. The resting PD (potential difference) became more positive in most cells. The I/V characteristics became more nonlinear with high resistance between -150 and -20 mV and negative conductance region near -70 mV. Prolonged (16 sec) voltage clamps to negative levels (-100 to -150 mV) showed progressively more rapid current turn-off, but subsequent I/V characteristics were not affected. Clamping to +150 mV, however, abolished the high conductance between -50 and +100 mV to yield a uniform high resistance I/V characteristics, similar to that in high [K⁺]_o. Decreasing illumination from 2.02 micromol.sec^-1 .m^-2 to 0.5 micromol.sec^-1.m^-2 had a similar effect. Two out of a total of three cells exhibited hypotonic turgor regulation. Both cells started regulating within minutes and achieved near 50 % regulation within 50 min. The PD became more negative. The I/V curves exhibited high resistance between +50 and +150 mV. The characteristics were similar to those in cells exposed to low [K⁺]_o. Prolonged voltage clamps to both negative and positive levels showed slow current increase. Decreased illumination increased the membrane resistance.

  Key words: Ventricaria (Valonia), Pressure probe, I/V analysis, Hypertonic and hypotonic turgor regulation, electrogenic pump