In lichens, polyols (sugar alcohols, e.g. ribitol, arabitol, mannitol) have several physiological roles. They are effective carbon storage and anti-freezing substances. Natural levels of polyols in lichens are species-specific and vary within 1.4-8.8 mg.g-1 DW (ribitol), 0.4-29.0 mg.g-1 (mannitol). In our study, we tested positive effects of extrenally added ribitol on primary photosynthetic processes at low/freezing temperature and evaluated interspecific differences between 3 fruticose species (Cetraria islandica, Cetraria nivalis, Cetrariella delisei collected in Southern Norway and single foliose lichen collected in Svalbard. Lichen thalli were exposed to ribitol concentrations of 0 (control), 16, 32, 40, 50 mM for 168 h at 0 deg C and two subzero temperatures. The chlorophyll fluorescence parameters (potential yield of photochemical processes in photosystem II (FV/FM), effective quantum yield in PS II (Yield), and non-photochemical quenching (NPQ) were monitored in 24-h intervals using a chlorophyll fluorescence imaging (Handy Fluor Cam, HFC-010, PSI, CZ). Contents of polyols and other non-structural saccharides (NSS) were evaluated before and after the treatment by a HPLC. Contents of chlorophyll a, b, and total carotenoids were measured by a spectrophotometer (Spekord, Germany). The results indicated importance of polyols in antifreezing tolerance of lichens since positive affect of ribitol addition was apparent at freezing temperature. Funded by the GAAV KJB601630808. Topic: Environmental Stresses and Acclimation. P01-096.