Intracellular measurement of ammonium in Chara corallina using ion-selective microelectrodes

The study of ammonium (NH4+) transport across plant cell membranes requires accurate measurement of NH4+ gradients across subcellular gradients. We have developed an ammonium-selective microelectrode based on the ionophore nonactin. This electrode can detect NH4+ activities (a(NH4)) in vivo in the millimolar range in the presence of cytosolic levels of potassium, the main interfering ion. The electrode was used to measure intracellular a(NH4) in internodal cells of the giant alga Chara corallina. Results from cells incubated in media supplemented with 1 mM NH4+ produced two populations, with means of 7.3 and 30.8 mM, respectively. HPLC analysis of vacuolar sap suggests the higher population represents vacuolar impalements, and the lower population can thus be assumed to be cytosolic. These results suggest a four-fold accumulation of NH4+ in the vacuolar compartment of Chara.