Stomatal compensation points for ammonia in oilseed rape plants under field conditions
by Husted S., Schjoerring J. K., Nielsen K. H., Nemitz E., Sutton M. A. (2000)
Søren Husted, Plant Nutrition Laboratory, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
Mark A. Sutton, Centre for Ecology and Hydrology, Edinburgh, Bush Estate, Penicuik, Midlothian EH26 OQB, UK
in Agricultural and Forest Meteorology 105(4): 371-383 – ISSN :0168-1923 –
Compensation points for gaseous exchange of ammonia (NH 3 ) between stomata and the atmosphere were determined in an oilseed rape (Brassica napus) canopy by analysing the concentrations of NH 4 + and H + in leaf apoplastic solution.
This bioassay approach was applied for the first time in the field, allowing the first intercomparison with compensation points derived from micrometeorological measurements.
Apoplastic NH 4 + and H + concentrations differed between leaf heights but values were relatively stable over time, both diurnally and during a 2-week period.
Stomatal NH 3 compensation points calculated on the basis of apoplastic NH 4 + and H + concentrations and corrected for ambient leaf temperatures were found to correlate positively with the net NH 3 emission from the canopy estimated by micrometeorological measurements. As there was little diurnal variability in apoplastic concentrations, this correlation was largely due to the effect of temperature on NH 3 solubility and NH 4 + dissociation in the apoplast, together with similar effects of temperature on the net NH 3 flux.
Very high NH 4 + concentrations were also found in extracts of fallen litter and resulted in NH 3 partial pressures significantly exceeding NH 3 levels in the atmosphere close to the ground. By comparison of vertical atmospheric NH 3 concentration profiles in the plant canopy with the stomatal NH 3 compensation points determined here at three different plant heights, as well as NH 3 partial pressures in the litter, it is shown that plant residues on the soil surface would have been the primary NH 3 source while attached leaves acted as an NH 3 sink.
Although it was not possible to measure apoplastic concentrations of siliques (seed cases), bulk tissue NH 4 + /H + concentrations and vertical atmospheric NH 3 concentration profiles indicate that these may have acted as an NH 3 source.