EFFECTS OF EXTERNAL PH AND AMMONIUM ON VACUOLAR PH IN MAIZE ROOTHAIR CELLS

Although nitrogen form can have profound effects on pH homeostasis of plant cells, little is known of the effects of solution ammonium on va cuolar pH. The pH of vacuoles of roothair cells of maize (Zea mays L. cv. FRB 73) seedlings was followed by fluorescence microscopy after se lective vacuolar accumulation of the pH-sensitive dye -bis-(2-carboxye thyl)-5(and-6-)-carboxyfluorescein (BCECF). The addition of 10 mM NH4C l to the solution bathing the roots caused a fast change in vacuolar p H with the magnitude being proportional to external pH. Similar effect s were found when NH4Cl was replaced with the non-metabolized ammonium analog methylamine. These similarities between the effects induced by ammonium and methylamine indicated that the effects of NH4Cl on vacuo lar pH were due to ammonium accumulation rather than assimilation. In addition, the use of varying external pH in the presence of NH4Cl prov ided a convenient and reliable way to calibrate ratio data from the fl uorescence of BCECF to pH values. Parallel experiments to assess chang es in cellular metabolism were conducted utilizing in vivo P-31 NMR. I norganic phosphate in an acidic compartment was detected in the root t issue in the presence of 10 mM NH4Cl even at an external pH of 9.0. Th ese results suggested that there was a small but significant proportio n of vacuoles within the root tissue that did not alter their pH in re sponse to external ammonium as observed for vacuoles of roothair cells .