Effects of gaseous ammonia on intracellular pH values in leaves of C 3- and C 4-plants

Abstract

Responses of cytosolic and vacuolar pH to different concentrations (1.3–5.4 μmol NH 3 mol -1 gas or 0.940–3.825 mg NH 3 m -3 gas) of gaseous NH 3 were studied in experiments of 3 h duration by recording changes in fluorescence of pyranine and esculin in leaves of C 3 and C 4 plants. After a lag phase of 0.5–4 min, the uptake of NH 3 at 50–200 nmol m -2 leaf area s -1 increased pyranine fluorescence, indicating cytosolic alkalinization in leaves of Pelargonium zonale L. (C 3) and Amaranthus caudatus L. (C 4). A smaller increase in esculin fluorescence induced by NH 3 indicated some vacuolar alkalization in a Spinacia oleracea L. leaf. Photosynthesis and transpiration remained unchanged during exposure of illuminated leaves to NH 3 for up to 30 min (the maximum tested). CO 2 concentrations influenced the extent of cytosolic alkalinization. 500 μmol CO 2 mol -1 gas suppressed the NH 3-induced cytosolic alkalinization relative to that found in 16 μmol CO 2 mol -1 gas. The suppressing effect of CO 2 on NH 3-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 and protonation of NH 3 in aqueous leaf compartments is affected by assimilation of NH 3.