Seedlings of 12 provenances of European beech (Fagus sylvatica) were exposed to ambient, non-filtered air (NF) or NF+50 nl l−1 ozone (NF50) for 8 h d−1 in open-top chambers (OTCs), from 1 June to 4 October 1995. In 1996 exposure was continued from 31 May to 1 October at four levels: charcoal-filtered air (CF), NF, NF50 and NF+100 nl l−1 ozone (NF100). Provenances were grown for both seasons in outside reference plots. All treatments were replicated. Ozone did not affect gas exchange in the provenances until late in the second season. NF100 reduced photosynthesis by 18% in August 1996 compared to CF. In September, photosynthesis was reduced by 22% in NF50 and by 29% in NF100. After two seasons, ozone reduced the root:shoot ratio by 24% when comparing CF and NF100; this was caused by reductions in the root biomass. Ozone did not affect height growth or stem diameter, and there were no ozone×provenance interactions for any growth parameter. There was, however, a significant ozone×provenance interaction for photosynthesis, showing northwest European provenances to be more sensitive to ozone than southeast European provenances when comparing dose–response estimates. This is interpreted in terms of genetic adaptation of the photosynthetic apparatus to regional growing conditions. Seedlings in the chambers grew 45% taller, and had 28% more shoot biomass and 29% smaller root biomass, resulting in a 44% reduction of root:shoot ratios compared to seedlings outside. Increased temperature and decreased PAR inside the chambers relative to the outside were probably the main causes for the differences. The magnitude of the chamber effects in OTCs raises doubts about conclusions drawn from ozone exposures in such chambers. This and previous ozone experiments with OTCs may have reached inaccurate conclusions concerning the size of ozone responses due to chamber effects.

Emissions of reactive oxidized nitrogen (NO and NO2), collectively known as NOx, from human activities are c. 21 Tg N annually, or 70% of global total emissions. They occur predominantly in industrialized regions, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important contribution to global totals, but also play a part in regulating the dry deposition of NO and NO2 (NOx) to plant canopies. Soil microbial production of NO leads to a soil ‘compensation point’ for NO deposition or emission, which depends on soil temperature, N and water status. In warm conditions, the net emission of NOx from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NOx emissions from soil is to reduce net rates of NO2 deposition to terrestrial surfaces over large areas.

Increasing anthropogenic emissions of NOx have led to an approximate doubling in surface O3 concentrations since the last century. NOx acts as a catalyst for the production of O3 from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O3 concentrations in urban areas.

Removal of NO and NO2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO2 is stomatal uptake. Long-term flux measurements over moorland in Scotland show very small deposition rates for NO2 at night and before mid-day of 1–4 ng NO2-N m−2 s−1, and similar emission rates during afternoon. The bi-directional flux gives 24-h average deposition velocities of only 1–2 mm s−1, and implies a long life-time for NOx due to removal by dry deposition.

Rates of removal of O3 at the ground are also influenced by stomatal uptake, but significant non-stomatal uptake occurs at night and in winter. Measurements above moorland showed 40% of total annual flux was stomatal, with 60% non-stomatal, giving nocturnal and winter deposition velocities of 2–3 mm s−1 and daytime summer values of 10 mm s−1. The stomatal uptake is responsible for adverse effects on vegetation. The critical level for O3 exposure (AOT40) is used to derive a threshold O3 stomatal flux for wheat of 0·5 μg m−2 s−1. Use of modelled stomatal fluxes rather than exposure might give more reliable estimates of yield loss; preliminary calculations suggest that the relative grain yield reduction (%) can be estimated as 38 times the stomatal ozone flux (g m−2) above the threshold, summed over the growing season.

The relative ozone resistance of 20 European and two American populations of Plantago major was examined, and relationships with climatic factors at the source of the plant material were explored using data provided by participants in the ICP-Crops initiative (International Co-operative Programme to Investigate the Effects of Air Pollutants and Other Stresses on Agricultural and Semi-Natural Vegetation). Plants grown from seed were exposed to either charcoal/Purafil® filtered air (CF<5 nmol mol−1 O3) or CF+ozone (70 nmol mol−1 O37 h d−1) over a 2-wk period in controlled environment chambers, and effects on mean plant relative growth rate (R) and allometric root/shoot growth (K) determined. Ozone resistance (R%) was calculated from (R03/RCF)×100.

Populations exhibited contrasting sensitivities to ozone, without the development of typical visible symptoms of injury. A positive relationship was found between relative ozone resistance and descriptors of the ozone-climate at the site of seed collection for the year of, and the 2 yr before, seed collection. The best predictors of inherent ozone resistance were shown to be cumulative ozone exposure indices calculated according to current United Nations Economic Commission for Europe (UN-ECE) critical level guidelines for the pollutant (i.e. the accumulated hourly average ozone exposure over a threshold level of 40 nmol mol−1 (AOT40) or 30 nmol mol−1 (AOT30) calculated during daylight hours for the consecutive 3-month period of the year experiencing the highest ozone concentrations). No relationships were found between ozone resistance and climatic factors (temperature, precipitation, sunshine hours, humidity) or the concentrations of other air pollutants (SO2, NO2, NO).

These findings support the view that current ambient levels of ozone in many regions of Europe are high enough to promote evolution of resistance to the pollutant in native plant populations. The significance of these findings to the debate over the establishment of separate critical levels for the protection of natural and semi-natural vegetation is discussed.