| Popis: |
Poplar shoots were exposed for 3–4 weeks to filtered air, ambient (maximum values 50–60 nl l-1) or two times ambient O3-concentrations under controlled environmental conditions in fumigation chambers. A sensitive (Populus nigra ‘Brandaris’) and a tolerant (P. euramericana ‘Robusta’) cultivar were used. At regular intervals the uptake of O3, transpiration and CO2 assimilation rate (Pn) of full-grown leaves were measured with leaf cuvettes. For unaffected leaves, the measured flux of O3 into the leaves appeared to be larger than can be calculated using the stomatal conductance for O3 (gs,o) estimated from the transpiration rates of the same leaves. Resistance analysis revealed that the difference was partly a result of a reaction with the external leaf surface. However, when the O3 flux into the leaf was corrected for this reaction, it was still larger than can be estimated using gs,o. As a consequence, negative residual or internal resistances (ri) for O3 transport into the leaves were assessed. It is postulated that O3 molecules moving into the leaf follow a shorter pathway than effluxing H2O-molecules. P. ‘Brandaris’ leaves showed a reduction in Pn after 12 days of exposure to ambient O3-concentrations, whereas for P. ‘Robusta’ a reduction in Pn was only observed at two times ambient concentrations. A simultaneous decline in the O3-flux was found in both cases. The decline occurred before a decrease in gs,o was observed suggesting that the O3 flux into the affected leaves was first hindered by internal factors. The measured flux of the affected leaves became smaller than the flux estimated using gs,o and, consequently, positive ri-values were estimated. The change in ri suggests that O3 molecules not only penetrated deeper into the leaf, but also were accumulating at a prolonged exposure. Our results indicate that ri may be a potentially important component of the overall resistance for O3-uptake, which may have important consequences for estimating O3 uptake from water vapour flux data. |
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