Impact of gaseous nitrogen deposition on plant functioning

Dry deposition of NH3 and NOx (NO and NO2) 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 NH3 dissolves to form NH4+, whereas NO2 dissolves to form NO3- and NO2-. The latter compound can also be formed after exposure to NO. There is evidence that NH3-N and NOx-N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO3- and NO2- can be reduced, and NH4+ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with ion; concentrations of atmospheric NH3 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 NH3 and in vitro GDH activity. The contribution of atmospheric NH3 and NO2 deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates (RGRs). 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