Reaktivierung der Photosynthese trockener Flechten durch Wasserdampfaufnahme aus dem Luftraum: Artspezifisch unterschiedliches Verhalten

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Summary It is known in principle, that dry thalli of lichens are able to take up water vapor from air of high relative humidity and that this can stimulate CO2 exchange so that high rates of net photosynthesis are reached without exposing the lichens to liquid water. The present investigation compars different types of lichens from different climatic regions with respect to their ability to be activated by water vapor uptake. Thalli of dry lichens (kept in a desiccator with silicagel) were enclosed in gas exchange chambers and treated for 50 to 100 h with air of a relative humidity of 96.5% at 15°C (corresponding to a water potential of ca. —45 bar); subsequently they were sprayed with liquid water. During the course of the experiment, their rate of CO2 exchange in the light (150 μE m−2 s−1 photosynthetically active radiation) and dark respiration was monitored. With all of the heteromerous lichen species with green algae, water vapor uptake leads to reactivation of metabolism. Photosynthetic CO2-assimilation as well as mitochondrial respiration, increases during the course of treatment with moist air and reaches an equilibrium value when water potential of the thallus has equilibrated with that of the surrounding air. However, there are essential differences in the length of time required for reactivation of different species. Several fruticose and folious lichen species (e.g., Parmelia hypoleucina, Thamnolia vermicularis) reach 50% of maximum net photosynthesis already after less than 5 h treatment of the initially dry thalli with moist air. Alpine and Antarctic Umbilicaria-species need 15 to 16 h, and the Antarctic Caloplaca regalis requires approximately 34 h. Habitat adaptations are clearly recognizable. This is apparent for instance for species from a Chilean fog oasis, which depend almost entirely on air humidity as a source for water. There are also distinct differences between lichen types in the response to moistening with liquid water. With some species, spraying with water after water vapor uptake results in an increase of net photosynthesis. With other species there is often a decrease. In extreme cases, CO2 exchange decreases below the compensation point in the light, which appears to be due to a strong and reversible stimulation of mitochondrial respiration by liquid water. The gelatinous lichens are of exceptional interest. None of the investigated Collemataceae (2 Collema, 1 Leptogium species) is able to make use of water vapor for attain positive net photosynthesis rates. With these lichens, photosynthetic primary production depends solely on hydration with liquid water. This must strongly limit the distribution of these species. The results indicate that the gelatinous species must represent an independent life form among lichens. Differences in anatomical and morphological structure of their thallus appear also to be associated with differences in function which profoundly alters the way in which photosynthetic metabolism responds to available water in the environment.