Effects of gaseous ammonia on intracellular pH values in leaves of C3- and C4-plants

Responses of cytosolic and vacuolar pH to different concentrations (1.3–5.4 μmol NH3 mol-1 gas or 0.940–3.825 mg NH3 m-3 gas) of gaseous NH3 were studied in experiments of 3 h duration by recording changes in fluorescence of pyranine and esculin in leaves of C3 and C4 plants. 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 cytosolic alkalinization in leaves of Pelargonium zonale L. (C3) and Amaranthus caudatus L. (C4). A smaller increase in esculin fluorescence induced by NH3 indicated some vacuolar alkalization in a Spinacia oleracea L. leaf. Photosynthesis and transpiration remained unchanged during exposure of illuminated leaves to NH3 for up to 30 min (the maximum tested). CO2 concentrations influenced the extent of cytosolic alkalinization. 500 μmol CO2 mol-1 gas suppressed the NH3-induced cytosolic alkalinization relative to that found in 16 μmol CO2 mol-1 gas. The suppressing effect of CO2 on NH3-induced alkalization was larger in illuminated leaves of the C4 Amaranthus than the C3 Pelargonium. These results indicate that the alkaline pH shift caused by solution and protonation of NH3 in aqueous leaf compartments is affected by assimilation of NH3.