Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NO x emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope "&N\"%N content (δ"&N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ"&N. Plants collected near motorways or busy urban roads had δ"&N values ranging between j6 and k1=, while in rural areas with hardly any traffic these ranged from k2 to k12=. In a separate survey of mosses, the average δ"&N of shoots from busy roadsides in London was j3.66=, whereas from samples collected from farm buildings near poultry or cattle pens it was k7.8=. This indicates that the two main atmospheric N sources, NO x and NH x , have different δ"&N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ"&N as an aid to distinguish between urban (NO x ) and rural (NH x ) forms of N pollution.

Glutamine synthetase (GS) exists as two main isoforms in plants, a cytasolic form (GSl) and a ehloroplast or plastidic form (GS2). Fifty-five species of legume, representing a phylogenetically diverse group of tropical and temperate species, were screened by western blotting for the presence of GS2 in their roots. A remarkably strong correlation was found between the climatic origin of the species and the presence or absence of a GS2-like polypeptide in the root. Root GS2 was found in all 31 temperate species examined (30 papilionoids, one caesalpinoid), but was not detected in any of the 17 tropical papilionoid species. It was also absent in the roots of four out of seven tropical non-papilionoid species. The ''in vivo' NR activities of roots, stems and leaves of 46 of the legume species were analysed to establish their major site of nitrate reduction, and the ratio of nitrate:reduced N in the xylem sap was determined for some species, but no clear correlation between possession of a root GS2 and a preference for root nitrate assimilation was found. We discuss the possibility that expression of GS2 in the root was part of a more extensive physiological adaptation to root nitrate assimilation that evolved in temperate species to suit the alkaline, nitraterich soils found in the centres of origin in temperate latitudes.

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 m methionine sulphoximine (MSO) to inhibit glutamine synthetase (GS), the enzyme of ammonium assimilation. Leaf tissue NH % + concentration of control plants was 0n03-0n1 µmol g −" f. wt. Although NH % + accumulated in the leaf tissue of MSO-treated plants of all three species to similar concentrations (6-10 µmol g −" f. wt after 4 d), emissions were only detected from the leaves of M. perennis, with potential rates of 2n5 nmol m −# leaf s −" . 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 −" f. wt. Comparisons with Hordeum vulgare grown under the same conditions showed that tissue NH % + concentration reached a plateau of about 40 µmol g −" f. wt after 2 d. Emissions of NH $ during the 5 d of treatment reached a maximum rate of 10 nmol m −# s −" 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 NH $ emission between species. The higher the apoplastic pH, the greater the likelihood of loss of NH $ 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. 5n6, whereas that of M. perennis (emitter) was c. pH 6n3. 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 NO $ − fertilization, H. vulgare had an apoplastic pH of 6n8 but this increased to 7n3, 3 d after feeding with NO $ − . Short-term fumigation (2 h) of shoots of H. vulgare with 60 µg ($ 32 mg NH $ m −$ ) of labelled gaseous "&N-NH $ (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 NH % + 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 NH $ were collected for 3 h, then c. 6n5 µg of N was recovered, of which c. 17% was "&N-labelled. Some of this label could have resulted from desorption of NH $ from leaf surfaces, but it was more likely that the remaining "%N isotope was from sub-stomatal emissions of NH $ . It is argued that non-nitrophilous plants tend to rely on mixed sources of N (NO $ − , NH % + 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 NO $ − mainly in their shoots), and at atmospheric concentrations most wild species are likely to be ne...

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