Active Extrusion of Protons into Deionized Water by Roots of Intact Maize Plants

Abstract

The investigations were focussed on the question as to whether roots of intact maize plants (Zea mays L. cv Blizzard) release protons into deionized H2O. Plants in the six to seven leaf stage depressed the pH of deionized H2O from 6 to about 4.8 during an experimental period of 4 hours. Only one-third of the protons released could be ascribed to the solvation of CO2 in H2O. The main counter anions released were Cl−, NO3−, and SO42−. At low temperature (2°C), the H+ release was virtually blocked while a relatively high amount of K+ was released. The presence of K+, Na+, Ca2+, and Mg2+ in the external solution increased the H+ secretion significantly. Addition of vanadate to the outer medium inhibited the H+ release while fusicoccin had a stimulating effect. Substituting the nutrient solution of deionized H2O resulted in a substantial increase of the membrane potential difference from −120 to −190 millivolts. The experimental results support the conclusion that the H+ release by roots of intact maize plants is an active process driven by a plasmalemmalocated ATPase. Since the net H+ release was not associated with a net uptake of K+, it is unlikely to originate from a K+/H+ antiport.