Under benign environmental conditions, plant growth is generally stimulated by elevated atmospheric CO concentrations. When environmental conditions become sub- or supra-optimal for growth, changes in the biomass enhancement ratio (BER; total plant biomass at elevated CO divided by plant biomass at the current CO level) may occur. We analysed literature sources that studied CO×environment interactions on the growth of herbaceous species and tree seedlings during the vegetative phase. For each experiment we calculated the difference in BER for plants that were grown under 'optimal' and 'non-optimal' conditions. Assuming that interactions would be most apparent if the environmental stress strongly diminished growth, we scaled the difference in the BER values by the growth reduction due to the stress factor. In our compilation we found a large variability in CO×environment interactions between experiments. To test the impact of experimental design, we simulated a range of analyses with a plant-to-plant variation in size common in experimental plant populations, in combination with a number of replicates generally used in CO×environment studies. A similar variation in results was found as in the compilation of real experiments, showing the strong impact of stochasticity. We therefore caution against strong inferences derived from single experiments and suggest rather a reliance on average interactions across a range of experiments. Averaged over the literature data available, low soil nutrient supply or sub-optimal temperatures were found to reduce the proportional growth stimulation of elevated CO. In contrast, BER increased when plants were grown at low water supply, albeit relatively modestly. Reduced irradiance or high salinity caused BER to increase in some cases and decrease in others, resulting in an average interaction with elevated CO that was not significant. Under high ozone concentrations, the relative growth enhancement by elevated CO was strongly increased, to the extent that high CO even compensated in an absolute way for the harmful effect of ozone on growth. No systematic difference in response was found between herbaceous and woody species for any of the environmental variables considered.

 Dry deposition of NH$ and NO x (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 form NH % + , whereas NO # dissolves to form NO $ − and NO # − . The latter compound can also be formed after exposure to NO. There is evidence that NH $ -N and NO x -N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO $ − and NO # − can be reduced, and NH % + can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase\glutamate synthase (GS\GOGAT) cycle. Fumigation with low concentrations of atmospheric NH $ 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 NH $ and in vitro GDH activity. The contribution of atmospheric NH $ and NO # deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates (s). 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. Key words : Apoplastic storage, gaseous nitrogen deposition, glutamate dehydrogenase, glutamine synthetase, plant nitrogen demand, sulphur dioxide root\shoot interaction, ammonia, nitrogen oxides. Anthropogenic emissions of nitrogen-containing air pollutants far exceed natural emissions in Europe and North America. These emissions cause two types of effect : generation of tropospheric O $ , and excess N deposition, which can result in direct phytotoxic effects, eutrophication, acidification and stimulation of greenhouse gas production. Anthropogenic emissions of N-containing air pollutants have resulted in atmospheric N deposition 5-20 times higher than in natural conditions. Indications of N excess in the field can be seen from : (1) changes in the species composition of indigenous vegetation types, with an increase in nitrophilous species and a decrease in others, and (2) increases in foliar N * To whom correspondence should be addressed. E-mail : g.stulen!biol.rug.nl. Abbreviations : GDH, glutamate dehydrogenase ; GS, glutamine synthetase ; GOGAT, glutamate synthase ; , net nitrate uptake rate ; NR, nitrate reductase ; P95, 95 percentile ; , plant nitrogen content ; , relative growth rate.

Four-year-old saplings of Scots pine (Pinus sylvestris L.) were exposed for 11 weeks in controlled-environment chambers to charcoal-filtered air, or to charcoal-filtered air supplemented with NH3 (40 #g m-3), O3 (110 #g m -3 during day/40 #g m -3 during night) or NH3 + O3. All treatments were carried out at ambient (350 #L L -1) and at elevated CO2 concentration (700 #L L-l). Total tree biomass, mycorrhizal infection, net CO2 assimilation (Pn), stomatal conductance (gs), transpiration of the shoots and NH3 metabolization of the needles were measured. In ambient CO2 (1) gaseous NH3 decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the activity of GS in the needles, while it increased the activity of GDH; (3) exposure to NH3 + 03 lessened the effects of single exposures to NI-I3 and 03 on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to N-Ha + O3 at ambient CO2, were measured in trees exposed to NH3 + 03 at elevated CO2. Exposure to elevated CO2 without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO2 + NH3 strongly decreased root branching and mycorrhizal infection and temporarily stimulated Pa and gs. The exposure to elevated CO2 + NH3 + O3 also transiently stimulated Pn. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO2 clearly did not alleviate the negative effects of NH3 and 03 on mycorrhizal infection. The significant reduction of mycorrhizal infection after exposure to NH3 or 03, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH3 and 03 induced stress.Abbreviations: D W -dry weight, FA-filtered air, FAa -filtered air at ambient CO2, FAe -filtered air at elevated CO2, F W -fresh weight, GDH-glutamate dehydrogenase, GS -glutamine synthetase, gs -stomatal conductance, Pn -net CO2 assimilation, RWR-root weight ratio, SRL -specific root length.

Four‐year‐old seedlings of Scots pine (Pinus sylvestris L.) were exposed to filtered air (FA), and to FA supplemented with NH3 (60 and 240 μg m−3) in controlled‐environment chambers for 14 weeks. Exposure to the higher NH3 concentration resulted in an increased activity of glutamine synthetase (GS, EC 6.3.1.2), and an increase in the concentrations of soluble proteins, total nitrogen, free amino acids and leaf pigments in the needles. The GS activity (μmol g−1 fresh weight h−1) in the needle extract increased to levels 69% higher than in FA and the soluble protein concentration to levels 22% higher. Total nitrogen concentration in the needles was 42% higher than in FA, while the free amino acid concentration was 300% higher, which was caused by an increase in arginine, glutamate, aspartate and glutamine. Chlorophyll a, chlorophyll b and carotenoid concentrations were 29, 38 and 11% higher, respectively. Neither the glutamate dehydrogenase (GDH, EC 1.4.1.2) activity nor the concentrations of free NH4+ and glucose in the needles were affected by exposure to NH3. After NH3 fumigation at 240 μg m−3 the starch concentration decreased by 39% relative to the FA. The results indicate that the metabolism of Scots pine acclimates to concentrations of NH3 which are 3 to 10 times higher than the average concentration in areas with intensive stock farming. The possible mechanisms underlying acclimation to NH3 are discussed.

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