Air quality thresholds for O$ for the protection of human health and vegetation set by the European Union (EU) have been exceeded in Europe regularly in the 1990s. Because target reductions for oxides of nitrogen (NO x ) set for the year 2000 are unlikely to be achieved, these O $ exceedances are likely to continue into the next millenium. Improvements of plant tolerance towards O $ are being investigated but very little work has been done to explore NO x tolerance and plant acclimation to NO # and NO. However, it is clear that within the populations of some plant species there is wide variation, and some individuals can fix NO x and use the nitrogen directly from the atmosphere, rather than rely upon, for example, root uptake of nitrate. It is possible that individuals capable of fixing NO x could be selected for a range of species, and genotypes with high rates of uptake could be of value as crops or for forestation in polluted areas (e.g. landscaping in the vicinity of motorways) to reduce tropospheric concentrations of NO x significantly and also to decrease the potential for O $ production.