Effects of carbon dioxide on the spatially separate electrogenic ion pumps and the growth rate in the hypocotyl ofVigna sesquipedalis.

Carbon dioxide, introduced into the gas phase of the experimental chamber, has distinct effects on two spatially separate membrane potentials and the rate of elongation growth in hypocotyl segments ofVigna sesquipedalis Wight. Both membrane potentials (V ps andV px=the electric potential difference between the parenchyma symplast and the surface of the hypocotyl, and that between the parenchyma symplast and the xylem, respectively) hyperpolarized rapidly but transiently at the introduction of CO2. Prolonged exposure of the hypocotyl to high concentrations of CO2 (above 10%) caused depolarization of membrane potentials above the level before CO2 introduction. When CO2 was replaced with air, the membrane potentials exhibited a distinct depolarization response of transient nature. The growth rate of the hypocotyl segments exhibited similar responses to CO2 as did the membrane potentials (the increase and the decrease of the growth rate were corresponded to the hyperpolarization and the depolarization, respectively), but these responses always followed the changes of the membrane potentials. The CO2-induced maximum hyperpolarization ofV ps and the maximum increase of the growth rate were closely correlated. All these responses were strictly dependent on aerobic metabolism. These results indicate that CO2 may regulate elongation growth in two ways: by affecting the activity of the electrogenic ion pump via intracellular acidification, and also by acting via apoplastic acidification as a wall-loosening acid.