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Résumé :
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The growth of potted birch cuttings (one clone of Betula pendula) was studied under low O3 concentrations (0, 0.050, 0.075, 0.100 µl l-¹) throughout an entire growing season. With increasing O3 dose, 20 - 50% of all leaves formed were prematurely shed, while 40 - 70% of the remaining foliage displayed advanced discoloration by the end of the season. Ozonation affected the S, P and N concentration of leaves and increased d13C in leaves and stem, while the CO2 assimilation rate declined with increasing CO2 concentration in mesophyll intercellulars. While whole-plant production correlated negatively with the O3 dose, ozone increased the specific leaf weight (i.e. leaf weight/leaf area, SLW) but decreased the ratios of stem weight/stem length and root/shoot biomass. Neither the latter ratio nor SLW changed in experimentally defoliated control plants, whereas in ozonated plants starch accumulated along leaf veins and phloem tissue was deformed in the leaf petioles and the stem. Only in early summer was the relative growth rate higher in the ozonated than in the control plants. The ratio of whole-plant biomass production versus total foliage area formed was lowered under O3 stress. However, when relating biomass to the actual foliage area present due to leaf loss, this ratio did not differ between treatments. Similarly the ratio of actual foliage area versus basal stem area in cross-section did not differ. Overall, whole-plant production was strongly determined by O3-caused changes in crown structure and began to be limited at O3 doses (approximately 180 µl 1-¹ h) similar to those of rural sites in Central Europe.
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