Hygrophilous old-growth forest lichens are highly capable of instantaneous photosynthesis activation after short-term desiccation stress

The vitality of lichens and their growth depend on the physiological status of both the fungal and algal partner. Many epiphytic lichens demonstrate high specificity to a habitat type and hygrophilous species are, as a rule, confined to close-to-natural forest complexes. Tolerance to desiccation stress and the rate of photosynthesis activation upon thallus hydration vary between species. Analyzes of chlorophyll fluorescence and photosynthesis efficiency have been widely applied to determine the viability of lichens. The aim of this study was to determine the activation photosynthesis rate upon hydration in epiphytic lichens exposed to short-term desiccation stress and to find potential links between their activation pattern and ecological properties. The results proved that even highly sensitive hygrophilous lichens, i.e., Cetrelia cetrarioides , Lobaria pulmonaria , Menegazzia terebrata , do not exhibit any delay in the restart of the photosynthesis process, compared to mesophytic or xerophytic ones. All examined lichens achieved nearly 100% of their maximum photosynthetic efficiency just one hour after they had been supplied with a relatively small quantity of water. Moreover, the increase in photosynthesis efficiency, measured at 20-minute intervals upon hydration, started from a relatively high level. In addition, the differences in the content of photosynthetic pigments and water holding capacity between species did not affect the general pattern of activation, which is comparable across various lichens. It can be concluded that healthy hygrophilous lichens do not require long hydration time to regain a high level of photosynthesis efficiency after a short rainless period. This fact supports the idea of applying chlorophyll fluorescence analysis in the field to assess vitality of lichens and the condition of their natural habitat.