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

Abstract

The stomatal ammonia compensation point for ammonia (NH 3) 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 NH 3 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 (NH 4 +) concentration and pH in the apoplast, the gaseous NH 3 concentration inside the leaves was calculated, i.e. the so-called stomatal compensation point ( χ s). Temperature appeared to have a predominant effect on χ s, partly by affecting the equilibrium between gaseous NH 3 inside the leaf and NH 3 dissolved in the apoplast and partly by affecting physiological processes influencing the NH 4 + 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 NH 3 volatilisation from the apoplast, on the other hand it led to a decline in apoplastic NH 4 + from 0.9 to 0.2 mM, thereby diminishing the emission potential of the leaf. The low NH 4 + 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 NH 3 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 χ s varied between 0.5 and 4 μg m −3. The gaseous NH 3 concentrations inside the grass leaves were, with a few exceptions, always smaller than the measured ambient NH 3 concentrations. The present study indicates that under the current ambient NH 3 concentrations in the Netherlands, the grass canopy is unlikely to be a major source of NH 3 emission.