DIFFERENTIAL AMMONIA-ELICITED CHANGES OF CYTOSOLIC PH IN ROOT HAIR-CELLS OF RICE AND MAIZE AS MONITORED BY (2-CARBOXYETHYL)-5-CARBOXYFLUORESCEIN-FLUORESCENCE AND (2-CARBOXYETHYL)-6-CARBOXYFLUORESCEIN-FLUORESCENCE RATIO

Intact hair cells of young rice (Oryza sativa L.) and maize roots (Zea mays L.), grown without external nitrogen, were specifically loaded w ith 2',7'-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein acetoxyme thyl ester to monitor fluorescence ratio cytosolic pH changes in respo nse to external ammonia (NH4+/NH3) application. In neutral media, cyto solic pH of root hairs was 7.15 +/- 0.13 (O. sativa) and 7.08 +/- 0.11 (Z. mays). Application of 2 mM ammonia at external pH 7.0 caused a tr ansient cytosolic alkalization (7.5 +/- 0.15 in rice; 7.23 +/- 0.13 in maize). Alkalization increased with an increase of external pH; no pH changes occurred at external pH 5.0. The influx of N-13-labeled ammon ia in both plant species did not differ between external pH 5.0 and 7. 0 but increased significantly with higher pH. Pretreatment with 1 mM 1 -methionine sulfoximine significantly reduced the ammonia-elicited pH increase in rice but not in maize. Application of 2 mM methylammonia o nly caused a cytosolic pH increase at high external pH; the increase i n both species compared with the ammonia-elicited alkalization in 1-me thionine sulfoximine-treated roots. The differential effects indicate that cytosolic alkalization derived from (a) NH3 protonation after pas sive permeation of the plasma membrane and, particularly in rice, (b) additional proton consumption via the glutamine synthetase/glutamate s ynthase cycle.