EFFECTS OF GASEOUS AMMONIA ON INTRACELLULAR PH VALUES IN LEAVES OF C-3-PLANT AND C-4-PLANT

Responses of cytosolic and vacuolar pH to different concentrations (1. 3-5.4 mu mol NH3 mol(-1) gas or 0.940-3.825 mg NH3 m(-3) gas) of gaseo us NH3 were studied in experiments of 3 h duration by recording change s in fluorescence of pyranine and esculin in leaves of C-3 and C-4 pla nts. After a lag phase of 0.5-4 min, the uptake of NH3 at 50-200 nmol m(-2) leaf area s(-1) increased pyranine fluorescence, indicating cyto solic alkalinization in leaves of Pelargonium zonale L. (C-3) and Amar anthus caudatus L. (C-4). A smaller increase in esculin fluorescence i nduced by NH3 indicated some vacuolar alkalization in a Spinacia olera cea L. leaf. Photosynthesis and transpiration remained unchanged durin g exposure of illuminated leaves to NH3 for up to 30 min (the maximum tested). CO2 concentrations influenced the extent of cytosolic alkalin ization. 500 mu mol CO2 mol(-1) gas suppressed the NH3-induced cytosol ic alkalinization relative to that found in 16 mu mol CO2 mol(-1) gas. The suppressing effect of CO2 on NH3-induced alkalization was larger in illuminated leaves of the C-4 Amaranthus than the C-3 Pelargonium. These results indicate that the alkaline pH shift caused by solution a nd protonation of NH3 in aqueous leaf compartments is affected by assi milation of NH3. (C) 1998 Elsevier Science Ltd. All rights reserved.