Impact of gaseous nitrogen deposition on plant funcitioning

Dry deposition of NH$and NOx(NO and NO#) can affect plant metabolism at the cellular and whole-plant level.Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH$dissolves to formNH%+, whereas NO#dissolves to form NO$−and NO#−. The latter compound can also be formed after exposureto NO. There is evidence that NH$-N and NOx-N can be reversibly stored in the apoplast. Temporary storagemight affect processes such as absorption rate, assimilation and re-emission. Once formed, NO$−and NO#−canbe reduced, and NH%+can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitritereductase and the glutamine synthetase}glutamate synthase (GS}GOGAT) cycle. Fumigation with lowconcentrations of atmospheric NH$increasesin vitroglutamine synthetase activity, but whether this involves bothor only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation withlow concentrations of NH$andin vitroGDH activity. The contribution of atmospheric NH$and NO#depositionto the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations andrelative growth rates (rgrs). It is concluded that at current ambient atmospheric N concentrations the directimpact 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