Stimulation of H⁺extrusion and plasma membrane H⁺-ATPase activity of barley roots by ammonium-treatment

H⁺extrusion and the plasma membrane H⁺-ATPase activity were analyzed using barley roots exposed to ammonium. H⁺extrusion from roots was enhanced 4 h after the addition of 5 mol m^-3ammonium to the medium. However, the stimulation of the plasma membrane H⁺-ATPase activity occurred at least 8 h after the addition of ammonium. Furthermore, this stimulation was found to be caused by the increase in the content of H⁺-ATPase proteins based on the results of immunoassay. These results indicate that H⁺was actively extruded to the medium through the increase in the amount of plasma membrane H⁺-ATPase proteins, when barley roots were exposed to ammonium for more than 8 h. Thus, it is assumed that in the long term exposure to ammonium nitrogen, ammonium-induced stimulation of the H⁺-ATPase activity may be important to maintain intracellular pH in root cells under the acidic conditions associated with the uptake of ammonium.