Effect of HCO ³_- concentration in the absorption solution on the energetic coupling of H⁺-cotransports in roots of Zea mays L.

Abstract

The effect of HCO ^-₃ on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO₂ partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H⁺ excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NO ^-₃ accumulation were slowed down, while ⁸⁶Rb⁺ uptake and K⁺ accumulation rates were increased by HCO ^-₃ . These effects were similar to those induced by 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-1,3-propanediol (Hepes-Tris). They were more pronounced when the H⁺ excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO₂/H₂CO₃/HCO ^-₃ accelerated the diffusion of equivalent H⁺ inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.