Extent of intracellular pH changes during H(+) extrusion by maize root-tip cells

(31)P-Nuclear-magnetic-resonance spectra of maize (Zea mays L.) root tips, that had been induced to extrude large amounts of H(+) in response to fusicoccin (FC) in the presence of potassium salts, indicate that the cytoplasmic pH does not become higher than that of controls. In fact, the cytoplasmic pH may become slightly (approx. 0.1 pH unit) lower in cells extruding H(+). Estimations of the buffer capacity of the cells show that without active intracellular pH regulation, H(+) extrusion caused by FC would cause the intracellular pH to rise by at least 0.6 pH unit h(-1). Our results indicate that intracellular pH is tightly regulated even during extreme rates of acid extrusion, and that a rise in cytoplasmic pH is not the signal linking H(+) extrusion with enhanced organic-acid synthesis or other intracellular responses to H(+) pumping.