The mechanism of the cessation of cytoplasmic streaming upon membrane excitation in Characeae internodal cells was investigated. Cell fragments containing only cytoplasm were prepared by collecting the endoplasm at one cell end by centrifugation. In such cell fragments lacking the tonoplast, an action potential induced streaming cessation, indicating that an action potential at the plasmalemma alone is enough to stop the streaming. The active rotation of chloroplasts passively flowing together with the endoplasm also stopped simultaneously with the streaming cessation upon excitation. The time lag or interval between the rotation cessation and the electrical stimulation for inducing the action potential increased with the distance of the chloroplasts from the cortex. The time lag was about 1 second/15 μm, suggesting that an agent causing the rotation cessation is diffused throughout the endoplasm. Using internodes whose tonoplast was removed by replacing the cell sap with EGTA-containing solution (tonoplast-free cells, Tazawaet al. 1976), we investigated the streaming rate with respect to the internal Ca2+ concentration. The rate was roughly identical to that of normal cells at a Ca2+ concentration of less than 10-7 M. It decreased with an increase in the internal Ca2+ concentration and was zero at 1 mM Ca2+. The above results, together with the two facts that Ca2+ reversibly inhibits chloroplast rotation (Hayama and Tazawa, unpublished) and the streaming in tonoplast-free cells does not stop upon excitation (Tazawaet al. 1976), lead us to conclude that a transient increase in the Ca2+ concentration in the cytoplasm directly stops the cytoplasmic streaming. Both Ca influxes across the resting and active membranes were roughly proportional to the external Ca2+ concentration, which did not affect the rate of streaming recovery. Based on these results, several possibilities for the increase in Ca2+ concentration in the cytoplasm causing streaming cessation were discussed. © 1979 Springer-Verlag.
