The annual variation in stomatal ammonia compensation point of rye grass (Lolium perenne L.) leaves in an intensively managed grassland

The stomatal ammonia compensation point for ammonia (NH3) of an intensively managed pasture of rye grass (Lolium perenne L.) was followed from mid January till November 2000. Leaf samples were taken every week. Simultaneously, the ambient NH3 concentration was measured. Meteorological data (temperature, wind speed, rainfall and radiance) were collected from a nearby field station. The vacuum infiltration technique was used to isolate the apoplastic solution of the leaves. From the determined ammonium (NH4+) concentration and pH in the apoplast, the gaseous NH3 concentration inside the leaves was calculated, i.e. the so-called stomatal compensation point (χs). Temperature appeared to have a predominant effect on χs, partly by affecting the equilibrium between gaseous NH3 inside the leaf and NH3 dissolved in the apoplast and partly by affecting physiological processes influencing the NH4+ concentration in the apoplast. Results of the present study suggest that these temperature effects were counteracting. On one hand temperature increase during early spring stimulated NH3 volatilisation from the apoplast, on the other hand it led to a decline in apoplastic NH4+ from 0.9 to 0.2 mM, thereby diminishing the emission potential of the leaf. The low NH4+ concentrations during spring and summer coincided with a low total leaf N content (<3% dw). However, there was no clear relationship between these two variables. The total N content of the leaf tissue is therefore an inadequate parameter for prediction of the potential NH3 emission from rye grass leaves. No annual trend was found for the apoplast pH. With a few exceptions, pH varied between 5.9 and 6.5 throughout the experimental period. The calculated values for χs varied between 0.5 and 4 μg m−3. The gaseous NH3 concentrations inside the grass leaves were, with a few exceptions, always smaller than the measured ambient NH3 concentrations. The present study indicates that under the current ambient NH3 concentrations in the Netherlands, the grass canopy is unlikely to be a major source of NH3 emission.