Using field plots, three species (Mercurialis perennis L., Rubus fruticosus L., and Trientalis europaea L.) were tested for their potential to emit gaseous ammonia to the atmosphere. Canopies were misted with 5 mM methionine sulphoximine (MSO) to inhibit glutamine synthetase (GS), the enzyme of ammonium assimilation. Leaf tissue NH4+ concentration of control plants was 0·03–0·1 μmol g−1 f. wt. Although NH4+ accumulated in the leaf tissue of MSO-treated plants of all three species to similar concentrations (6-10 μmol g−1 f. wt after 4 d), emissions were only detected from the leaves of M. perennis, with potential rates of 2·5 nmol m−2 leaf s−1. Experiments carried out in a controlled environment confirmed this rate of emission over 9 d, during which time leaf tissue ammonium increased to 66 μmol g−1 f. wt. Comparisons with Hordeum vulgare grown under the same conditions showed that tissue NH4+ concentration reached a plateau of about 40 μmol g −1f. wt after 2 d. Emissions of NH3 during the 5 d of treatment reached a maximum rate of 10 nmol m−2 s−1 by the third day.

Apoplastic pH of the plants was determined, and it is suggested that this is an important factor explaining the differences in NH3 emission between species. The higher the apoplastic pH, the greater the likelihood of loss of NH3 from sub-stomatal spaces to the atmosphere. T. europaea (non-emitter) had an apoplastic pH of 5, R. fruticosus (non-emitter) a pH of c. 5·6, whereas that of M. perennis (emitter) was c. pH 6·3. The apoplastic pH is thought to be dictated in part by the N nutrition of a species, nitrophilous species tending to have high pH. Without NO3− fertilization, H. vulgare had an apoplastic pH of 6·8 but this increased to 7·3, 3 d after feeding with NO3−.

Short-term fumigation (2 h) of shoots of H. vulgare with 60 μg (≈32 mg NH3 m−3) of labelled gaseous 15N-NH3 (in the absence of MSO) showed that a substantial proportion (60%) of the applied label was found in the leaves, as well as in stems and roots (3%). There was also a change in amino acid pools, with an increase in shoot amino acids and a decrease in those in the root, while tissue NH4+ was very low in both shoots and roots. This provided indirect evidence that some of the applied label had been incorporated into an organic form. Following the fumigation treatment, emissions of NH3 were collected for 3 h, then c. 6·5 μg of N was recovered, of which c. 17% was 15N-labelled. Some of this label could have resulted from desorption of NH3 from leaf surfaces, but it was more likely that the remaining 14N isotope was from sub-stomatal emissions of NH3.

It is argued that non-nitrophilous plants tend to rely on mixed sources of N (NO3−, NH4+ or organic-N) and are more likely to favour root rather than shoot assimilation. Under these circumstances, their apoplastic pH is relatively low (compared with that of nitrophiles, which tend to assimilate NO3− mainly in their shoots), and at atmospheric concentrations most wild species are likely to be net assimilators, rather than emitters, of atmospheric ammonia.