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Notes: The above-ground parts of two years old seedlings of Douglas fir (Pseudotsuga menziesii) were exposed to filtered air, NH3, NO2+, SO2 (66, 96 and 95 μg m−3, respectively), to a mixture of NO2+NH3 (55 + 82 μg m−3) or SO2+NO2 (128 + 129 μg m−3), for 8 months in fumigation chambers. Both chlorophyll fluorescence and gas exchange measurements were carried out on shoots which had sprouted at the beginning of the exposure period. The chlorophyll fluorescence measurements were performed after 3 and 5 months of exposure (average shoot age 70 and 140 days, respectively). Light response curves of electron transport rate (J) were determined, in which J was deduced from chlorophyll fluorescence. In addition, light response curves of net CO2 assimilation were determined after 5 months of exposure. After 3 months of exposure (average shoot age 70 days) all exposure treatments showed a lower maximum electron transport rate (Jmax) as compared to the control shoots (filtered air). A large reduction (45%) was observed for shoots exposed to SO2+NO2. During the exposure period between 3 and 5 months (average shoot age 70 and 140 days, respectively) a decrease of Jmax was observed for all treatments. Jmax had further declined some time after termination of the exposure, when average shoot age was 310 days.Shoots exposed to SO2 and SO2+NO2 also showed a reduction in maximum net CO2 assimilation (Pmax) as compared to the control shoots. However, shoots exposed to NO2 showed no reduction and even a higher Pmax was observed for shoots exposed to NH3 or NO2+NH3. Needles of these treatments also showed a higher chlorophyll content which might explain the contradictory results obtained for these treatments: the increased amount of photosynthetic units counteracts the reduction in Jmax and consequently no reduction in Pmax is measured. Shoots exposed to SO2 and SO2+NO2 also showed a reduction in maximum stomatal conductance (gs). However, the stomatal opening was larger than could be expected on basis of their (maximum) CO2 assimilation rate. Consequently, water use efficiency of these shoots was lower than that of the control shoots. Also shoots exposed to NO2 had a lower water use efficiency due to a significantly higher maximum gs. Shoots exposed to NH3 showed a high transpiration rate in the dark, indicating imperfect stomatal closure.

Notes: Shoots of poplar (Populus euramericana L. cv. Flevo) were exposed to filtered air, SO2, NH3 or a mixture of SO2 and NH3 for 7 weeks in fumigation chambers. After this exposure gas exchange measurements were carried out using a leaf chamber. As compared to leaves exposed to filtered air, leaves pretreated with 112 μg m−3 SO2 showed a small reduction in maximum CO2 assimilation rate (Pmax) and stomatal conductance (gs). They also showed a slightly higher quantum yield and dark respiration. In addition, the fluorescence measurements indicated that the Calvin cycle of the leaves pretreated with 112 μg m−3 SO2 was more rapidly activated after transition from dark to light. An exposure to 64 μg m−3 NH3 had a positive effect on Pmax, stomatal conductance and NH3 uptake of the leaves. This positive effect was counteracted by an SO2 concentration of 45 μg m−3. The exposure treatments appeared to have no effect on the relationship between net CO2-assimilation and gs. Also, no injury of the leaf cuticle or of epidermal cells was observed.Resistance analysis showed that NH3 transfer into the leaf can be estimated from data on the boundary layer and stomatal resistance for H2O transfer and NH3 concentration at the leaf surface, irrespective of whether the leaves are exposed for a short or long time to NH3 or to a mixture of NH3 and SO2. In contrast SO2 uptake into the leaves was only partly correlated to the stomatal resistance. The results suggest a large additional uptake of this gas by the leaves. The possibility of a difference in path length between SO2 and H2O molecules is proposed.

Notes: Three years old seedlings of Douglas fir (Pseudotsuga menziesii) were exposed lo filtered air, O3 (day and night concentrations of 78 and 30 μgm−3: respectively). NH3 (54 μg m−3) and to a mixture of NH3+O3 (day and night concentrations of 49 + 83 and 49 + 44 μg m−3 respectively), for 5 months in fumigation chambers. Both gas exchange and chlorophyll fluorescence were measured on shoots which had sprouted at the beginning of the exposure period. After 4. 8, 10 and 20 weeks of exposure, light response curves of electron transport rate (J) were determined, in which J was deduced from chlorophyll fluorescence. Net CO2 assimiialion was measured at maximum light intensity of 560) μmol m−2 S−1 (Pn.560). After 8 and 10 weeks of exposure also light response curves of CO2 assimilation were assessed.Shoots exposed to O3 showed a reduction in net CO2 assimilation as compared to the control shoots during the entire exposure period. The reduction was related lo a lower chlorophyll content and a lower electron transport rate, whereas no effect on quantum yield efficiency (qy) was observed. In contrast, shoots exposed to NH3 showed a positive effect on photosynthesis. Shoots exposed to NH3. + O3 showed a rapid increase in Pn.560, in the period between 4 and 8 weeks to a level equal of that of the NH3-treatment. After this period a decline in Pn.560 was observed.After 10 weeks of exposure shoots exposed to O3 showed an increased transpiration rate in the dark as compared to the control shoots. In addition, water use efficiency (WUE) declined as a result of an increase in leaf conductance. Both observations indicate that the stomatal apparatus was affected by O3. A high transpiration rate in the dark was also found for shoots esposed to NHX. However, shoots exposed to NH3+ O3 showed neither an effect on WUE, nor an effect on transpiration rate in the dark. The possibility that NH3 delayed the O3 induced effects on photosynthesis and stomatal conductance is discussed.