Volume changes of Commelina communis guard cell protoplasts in response to K⁺, light and CO₂

The effects of K + concentration, light intensity and CO 2 levels on the volume of Commelina communis L. guard cell protoplasts were studied. Two degrees of swelling response were observed, both dependent on an external supply of K + , but not necessarily on the supply of a permeant anion. The presence of K + itself, independent of light or CO 2 level, stimulated swelling at a relatively slow rate. When K + , light and low CO 2 conditions were supplied together, the swelling was relatively rapid and of high magnitude. The rapid swelling was specific for K + and Rb + giving a half maximal effect after 2 h at a KCl concentration of about 18 mmol m −3 . The addition of CaCl 2 at 1 mol m −3 inhibited K + ‐dependent swelling under all conditions tested. The response to light and low CO 2 levels by Commelina guard cell protoplasts is thought to reflect a high degree of physiological integrity.