Calcium control of the hydraulic resistance in cells of Chara corallina

The hydraulic resistance (the reciprocal of the hydraulic conductivity Lp) Lp^-1 was measured in cells of Chara corallina by the method of transcellular osmosis. Treatment of cells with 100 mM KCl decreased Lp^-1 significantly. Subsequent treatment of the cells with 70 mM CaCl₂ recovered the decreased Lp^-1 to the original value. To know whether K⁺ or Ca²⁺/Mg²⁺ acts on the cell wall and/or the membrane, the hydraulic resistances of the cell wall (Lp_w^-1) and that of the membrane (Lp_m^-1) were determined in one and the same cell. For this, a pair of cells (twin cells) were made from an internodal cell, one used for measurement of Lp^-1 and the other used for the measurement of Lp_w^-1. From Lp^-1 and Lp_w^-1, Lp_m^-1 was calculated. Both Lp^-1 and Lp_w^-1 were decreased by K⁺, while Lp_m^-1 was not affected by K⁺. The same result was obtained with 5 mM EGTA. Lp_w^-1 was decreased more than it was by KCl but Lp_m^-1 remained constant after EGTA treatment. The recovery of the K⁺-decreased Lp^-1 with Ca²⁺ can be explained exclusively by the recovery of Lp_w^-1 with Ca²⁺. The Ca²⁺ recovery of Lp_w^-1 was observed in the intact cell wall but not in the cell wall tube isolated from an internodal cell. The different response to Ca²⁺ between the intact cell wall and the isolated cell wall was discussed in relation to the tension in the cell wall which may be an important factor for the ionic regulation of hydraulic conductivity.