Scientists' Contributions  
   

Atmospheric ozone pollution: bioindication tools for calibrated expertise of symptoms in leaves and needles from trees

P. Vollenweider, M. Ottiger and M. S. Guenthardt-Goerg

Swiss Federal Research Institute, Snow and Landscape Research (WSL), Switzwrland
http://www.wsl.ch/forest/wus/bioindic/bioin-en.htm
E-Mail: vollenweider@wsl.ch, goerg@wsl.ch

    Despite important reduction of precursor emission, atmospheric ozone remains the main air pollutant in Switzerland and in central Europe. Injury is consistently found as early as two weeks to two months after leaf formation during an ozone-rich and sunny weather period. Visible ozone symptoms have been reported in leaves and needles of 81 forest species in Central, and 42 in Southern Europe. Similar symptoms have been observed in the field and in experiments carried out in open-top and climate chamber facilities. Ozone injury symptoms on leaves and needles can be diagnosed roughly by eye or with a magnifying glass and documented with slides and herbarium samples. Twig exposition and crown position of symptomatic leaves help to ascertain the cause of the injury independent of the species (Fig 1A). However, symptom morphology varies considerably between the species. Mottling without clear-cut borderlines is a frequent symptom on conifer needles, whereas in broad-leaved species, different dark red to brownish pigmentation on the upper leaf surface is classified (sometimes quantified) as stippling, leaf reddening or bronzing (Fig 1B). This variability demands additional markers of accumulative and oxidative stress for a calibrated expertise on ozone symptoms and to avoid any possibility of confusion with symptoms of other origin. Microscopical and cytochemical methods are useful techniques to ascertain the diagnosis of ozone injury and characterise cell and subcellular reactions in target tissues. By combining different staining and microscopical observation techniques, it became clear that most of the various visible symptoms observed in European broad-leaved shrubs and trees had microscopical characteristics in common. Ozone injury symptoms generally are restricted to the leaf intercostal fields. Here necrotic cells particularly in the palisade parenchyma are neighboured by cells with modified contents, including different markers which indicate oxidative or accelerated senescence processes (Fig 1C-F). The combination of detecting ozone injury in the field and microscopical testing is presently developed and applied in several Swiss intensive monitoring plots to assess the effects of ozone on forest vegetation (Table 1). Preliminary results show that the proportion of sensitive woody species, the intensity of visible symptoms and the severity of microscopical injury correlate with the local ozone concentrations. Additional research aims to differentiate the existing ozone stress markers by systematic comparison with those in samples from controlled experiments with symptoms induced by abiotic (acid rain, heavy metals) or biotic stress. Reliable and possibly quantitative bioindication tools are developed to be offered in the new 'Regional Co-ordination Centre for calibrated expertise of ozone injury in Central Europe', which is to be formed at WSL.

    Table 1: Different microscopical alterations which ascertain ozone injury in the leaves with visible symptoms of 7 broad-leaved and 2 coniferous species from 2 recent expertises of Swiss and French samples (Pinus cembra L. analysed in collaboration with GIEFS, Nizza).

    Fig. 1: Macroscopical and microscopical detection of ozone-induced symptoms in beech leaves (Fagus sylvatica L.) sampled in southern Switzerland. A: Branches with symptomatic leaves are observed in the crown of tree individuals exposed to full light. These leaves exhibit an increasing bronze staining from the branch tip to the base (bronze colours translate into lighter grey shades on black and white pictures, as pointed by arrows). B: Symptom development increases from light bronzing to brown stippling (appearing as a darker and more irregular grey portion on black and white pictures, as pointed by arrow). Microscopical symptoms in the symptomatic (D and F) compared to the asymptomatic (C and E) leaves (bars: 20 micro m). D: In symptomatic leaves, antioxidative phenolic compounds are induced mainly in the palisade parenchyma (ppal) which is darker stained (vanilin-acid stain for proanthocyanidins). F: Section through a stipple (acid fuchsin followed by a toluidine blue metachromatic stain). Necrotic cells begin to collapse (arrows) while the enlarged vacuome (v) accumulates different phenolic compounds (P) in the epidermis (ep), the palisade (ppal) and the spongy parenchyma (psp).

       
  Scientists' Contributions  
   

[To Contents]    [To Next Topic]