The oxidising agent is photosynthetically reduced oxygen, the oxygen anion radical. The reaction occurs via the hydrogen sulfite radical:
After its activation with ATP (to adenosine phosphosulfate, APS) sulfate is reduced to S2~ by the well-known assimilatory sulfate reduction pathway. The sulfide ion (S2~) is incorporated into the amino acid cysteine via serine. With protons it may alternatively produce H2S which is released in considerable amounts into the atmosphere. However, this does not generally exceed 10% of the S02 taken up (Rennenberg 1984). Whereas reduction of S02 consumes protons, oxidation to sulfate produces sulfuric acid as well as sulfurous acid. If the strain remains below the threshold of damage, these acids may be buffered intracellularly (Lange et al. 1989). Whether proton transport from S02-stressed leaves via sulfate into the roots and finally into the soil can take place needs further clarification. Stress by sulfate to assimilatory organs is not only caused by uptake and conversion of S02. Research on the sulfur relations of spruce (Kôstner et al. 1998) showed that uptake of sulfur from the soil may exceed gaseous sulfur uptake by a factor of 4 to 5.
Beside the different capacity of plants to assimilate S02 there is no specific resistance against this pollutant, as it has many effects. Known differences in the sensitivity of various plants and their developmental stages are usually not based on S02-specific interactions or reactions, but on general characteristics of individual plant types, e.g. the higher resistance of C4 plants is based on the fact that S02 inhibits PEP-carboxylase less than Rubisco. Other favourable characteristics are frequent renewal of leaves and a greater capacity for buffering or adsorption.
Lichens are an example for a very broad spectrum of S02 sensitivity (Box 1.9.5), because they do not possess stomata and are, therefore, more directly and evenly subjected to pollutants in air than higher plants. Because of the close symbiotic interactions between algae and fungi, lichens are particularly delicate and easily disturbed, and they are not able to drop stressed tissues like shedding leaves.
Figure 1.9.10 summarises possible mechanisms for avoidance and tolerance which are used by lichens to cope with air pollution, particularly by S02. However, the role of the typical compounds of lichens, the lichenous acids, in this process is not quite clear. Possibly these acids adsorb some of the gaseous noxes (Box 1.9.6).
Biomonitoring of air pollutants using lichens
| Table 1. Lichen occupation and communities on trees (bark as an organic substrate) and walls, and the influence of S02 stress (after Larcher 1994)
Average S02 concentration (ng m"3)
Non-eutrophic substrate Alkaline substrate
> 125 Lecanora conizaeoides
Lecanora expallens ca. 70 Buellia canescens
Physcia adscendens ca. 60 Buellia canescens
Xanthoria parietina Physcia orbicularis Ramalina farinacea ca. 50 Pertusaria albescens
Physiconia pulverulenta Xanthoria polycarpa Lecania cyrtella ca. 40 Physcia aipolia
Ramalina fastigiata Candelaria concolor <30 Ramalina calicaris
Lecanora conizaeoides Lepraria incana Hypogymnia physodes Lecidea scalaris Hypogymnia physodes Evernia prunastri
Parmelia caperata Graphis elegans Pseudovenia furfuracea
Parmelia carperata Usnea subfloridana Pertusaria hemisphaerica Lobaria pulmonaria Usnea florida Teloschistes flavicans
Lecanoraion dispersae Conizaedion
Conizaeoidion Acarospora fuscata
Xanthorion (increasing Cladonia spp. diversity)
Up to 20 species of Xanthoria
Increasing diversity, no Lecanora conizaeoides
The high sensitivity of many species of lichens to atmospheric pollutants is often used to monitor long-term air pollution, particularly in towns. The presence of lichens can be mapped on areas to be studied, but several precautions must be observed if there is to be value in the results for prediction. For example, only lichen thalli which occupy bark should be used (because basic rocks provide different strengths of buffering capacity); then only those that grow at a particular height and on the side with optimal growth should be counted and their appearance should be rated. In addition, in any one area a number of trees (10-50) should be examined in order to obtain statistically significant data. Of course, only known species of lichens can be used and it is often recommended that only lichen communities should be analysed (Table 1). Even if these include species which are particularly sensitive to air pollutants (e.g. Xanthoria parietina, Grimmia pulvinata and Parmelia saxatilis for S02), other pollutants must be taken into consideration. Frequently, in urban areas, dusts containing heavy metals attack lichens more strongly than gaseous pollutants. If these data, particularly the cover abundance, have been recorded, area classification into lichen deserts, struggling zone and clean air zone can be accomplished using Table 2. For many cities in Europe, such lichen maps already exist. Nevertheless, for long-term biomonitoring, they must be revised from time to time. Short-term biomonitoring can be done with standardised lichen exposure panels. For this the widely distributed and fast growing foliose lichen Hypogymnia physodes is used. It is picked with sufficient bark as substrate and is exposed on the panel in ten replicates. These panels are then exposed in the study areas and the state of the thalli is recorded from time to time.
| Table 2. Lichen growth in situ as a parameter of lichen zonation (after Seitz 1972)
No growth or crustose lichens, Lichen desert maximum 0.5% cover
Growth only of crustose lichens, Inner struggle foliose lichens absent zone
Foliose and fruticose lichens 1-25% Middle struggle cover abundance (if >25% cover zone then partially dying)
Foliose and fruticose lichens 25- Outer struggle
50% cover abundance (if 50% then zone partially clearly dying)
Foliose and fruticose lichens >50% Clean air zone cover abundance, no damage
Plant condairy metabolism induced by ozone
Erythrose-4-phosphate + Phosphoenolpyruvate
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