Stomatal compensation points for ammonia in oilseed rape plants under field conditions

Compensation points for gaseous exchange of ammonia (NH3) between stomata and the atmosphere were determined in an oilseed rape (Brassica napus) canopy by analysing the concentrations of NH4+ 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 NH4+ and H+ concentrations differed between leaf heights but values were relatively stable over time, both diurnally and during a 2-week period. Stomatal NH3 compensation points calculated on the basis of apoplastic NH4+ and H+ concentrations and corrected for ambient leaf temperatures were found to correlate positively with the net NH3 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 NH3 solubility and NH4+ dissociation in the apoplast, together with similar effects of temperature on the net NH3 flux. Very high NH4+ concentrations were also found in extracts of fallen litter and resulted in NH3 partial pressures significantly exceeding NH3 levels in the atmosphere close to the ground. By comparison of vertical atmospheric NH3 concentration profiles in the plant canopy with the stomatal NH3 compensation points determined here at three different plant heights, as well as NH3 partial pressures in the litter, it is shown that plant residues on the soil surface would have been the primary NH3 source while attached leaves acted as an NH3 sink. Although it was not possible to measure apoplastic concentrations of siliques (seed cases), bulk tissue NH4+/H+ concentrations and vertical atmospheric NH3 concentration profiles indicate that these may have acted as an NH3 source.