Differential Ammonia-Elicited Changes of Cytosolic pH in Root Hair Cells of Rice and Maize as Monitored by 2[prime],7[prime]-bis-(2-Carboxyethyl)-5 (and -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 with 2prime],7prime]-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein acetoxymethyl ester to monitor fluorescence ratio cytosolic pH changes in response to external ammonia (NH4+/NH3) application. In neutral media, cytosolic pH of root hairs was 7.15 plus or minus] 0.13 (O. sativa) and 7.08 plus or minus] 0.11 (Z. mays). Application of 2 mM ammonia at external pH 7.0 caused a transient cytosolic alkalization (7.5 plus or minus] 0.15 in rice; 7.23 plus or minus] 0.13 in maize). Alkalization increased with an increase of external pH; no pH changes occurred at external pH 5.0. The influx of 13N-labeled ammonia 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 only caused a cytosolic pH increase at high external pH; the increase in both species compared with the ammonia-elicited alkalization in 1-methionine sulfoximine-treated roots. The differential effects indicate that cytosolic alkalization derived from (a) NH3 protonation after passive permeation of the plasma membrane and, particularly in rice, (b) additional proton consumption via the glutamine synthetase/glutamate synthase cycle.