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The SO resistance of 12 lichen species with different growth forms and taken from different sites was investigated. The thalli were either exposed to different concentrations of SO gas (concentration at entry into the cuvette: 0.5; 1.0; 2.0 and 4.0 mg SO/m air) or treated with NaSO solutions of differing concentration and pH. As a viability criterion the CO exchange of the thalli was measured with an infrared gas analyzer before and immediately after SO exposure and subsequently at intervals of several weeks. In some cases the chlorophyll content was also determined. 1. Species-specific differences were clearly apparent in the SO gas-exposure experiments. The influence of SO on net photosynthesis and dark respiration in the most sensitive species was detectable after exposure to 0.5 mg SO/m for 14 h in a fully hydrated state. The photosynthetic intensity of Lobaria pulmonaria was actually irreversibly damaged. In contrast, the most resistant species survived a treatment with 4 mg SO/m for the same length of time with little or no permanent impairment of their CO exchange. The reaction of the lichen species investigated to the above treatment allows us to arrange them in decreasing order of resistance: Xanthoria parietina (most resistant), Parmelia scortea, Parmelia acetabulum, Hypogymnia physodes, Parmelia saxatilis, Platismatia glauca, Labaria pumonaria, Parmelia stenophylla, Evernia prunastri. The most sensitive species, Evernia prunastri, is characteristically a fruticose lichen. Lichens with this growth form are known from field studies to be especially sensitive. 2. Examples of the same species (Parmelia saxatilis, Lobaria pulmonaria) can vary in their SO resistance according to their growing site. Morphological and anatomical characteristics (thallus and cortex thickness) may cause these differences. 3. The sensitivity of the lichens to SO is closely dependent upon their moisture status. When the water potential is lowered the SO uptake is reduced and with it the injury. Dried thalli survive high SO concentrations in their surroundings without damage. 4. The treatment with NaSO solutions also brought out species-specific differences in lichen resistance. However, the sequence of decreasing resistance is not the same as that to SO gas treatment. Irreversible damage of photosynthesis is not necessarily correlated with destruction of chlorophyll. 5. The damage caused to the lichens by the NaSO solutions (of the same concentration) is closely dependent upon the pH of the medium. At a low pH the effect is much more pronounced than at a high pH. This can be interpreted as due to the concentration of damaging ions, which changes according to the degree of dissociation of the solution; this is pH dependent. The results are discussed on the basis of Levitt's resistance concept that the total resistance of lichens to SO in the air is dependent upon two components, "avoidance" and "tolerance" (see Fig. 15). Resistance to a specific SO concentration in the air depends upon how much SO is taken up by the thallus, which is conditioned among other things by thallus organization (life form, surface characteristics) and by the degree of hydration of the poikilohydric organism. The toxicity of the SO taken up by the lichen can also be reduced; the pH of the thallus and its buffering capacity (dependent among other things upon site and substrate) play a dominant role in this process. In addition to these "avoidance" factors the total resistance of lichens is also dependent upon the plasmatic resistance of sensitive systems to SO ("tolerance"). This type of resistance, due to the influence of NaSO solution, is subject to considerable deviation (for example due to the developmental state of the lichen). The differences in the sequence of resistance for the investigated lichen species in terms of total resistance (SO treatment) and plasmatic resistance (NaSO solution treatment) show the significance of the "avoidance" component for the total resistance of the organisms.In ecological terms the investigation supports the view that lichens are highly sensitive to SO, even in concentrations which occur due to real immisions. The study also shows the complexity of an ecological interpretation of experimentally determined resistance phenomena.