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 p lant mainly through the stomata, and once in the apoplast NH3 dissolve s to form NH4+, whereas NO2 dissolves to form NO3- and NO2-. The latte r 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. Tempora ry storage might affect processes such as absorption rate, assimilatio n and re-emission. Once formed, NO3- and NO2- can be reduced, and NH4 can be assimilated via the normal enzymatic pathways, nitrate reducta se (NR), nitrite reductase and the glutamine synthetase/glutamate synt hase (GS/GOGAT) cycle. Fumigation with ion; concentrations of atmosphe ric NH3 increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open que stion. There seems to be no correlation between fumigation with low co ncentrations of NH3 and in vitro GDH activity. The contribution of atm ospheric 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.