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(-3) ammonium 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 Hf 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.