Dry deposition of NH$ and NO x (NO and NO # ) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH $ dissolves to form NH % + , whereas NO # dissolves to form NO $ − and NO # − . The latter compound can also be formed after exposure to NO. There is evidence that NH $ -N and NO x -N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO $ − and NO # − can be reduced, and NH % + can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase\glutamate synthase (GS\GOGAT) cycle. Fumigation with low concentrations of atmospheric NH $ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH $ and in vitro GDH activity. The contribution of atmospheric NH $ and NO # deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates (s). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small. Key words : Apoplastic storage, gaseous nitrogen deposition, glutamate dehydrogenase, glutamine synthetase, plant nitrogen demand, sulphur dioxide root\shoot interaction, ammonia, nitrogen oxides.
Anthropogenic emissions of nitrogen-containing air pollutants far exceed natural emissions in Europe and North America. These emissions cause two types of effect : generation of tropospheric O $ , and excess N deposition, which can result in direct phytotoxic effects, eutrophication, acidification and stimulation of greenhouse gas production. Anthropogenic emissions of N-containing air pollutants have resulted in atmospheric N deposition 5-20 times higher than in natural conditions. Indications of N excess in the field can be seen from : (1) changes in the species composition of indigenous vegetation types, with an increase in nitrophilous species and a decrease in others, and (2) increases in foliar N * To whom correspondence should be addressed. E-mail : g.stulen!biol.rug.nl. Abbreviations : GDH, glutamate dehydrogenase ; GS, glutamine synthetase ; GOGAT, glutamate synthase ; , net nitrate uptake rate ; NR, nitrate reductase ; P95, 95 percentile ; , plant nitrogen content ; , relative growth rate.
