Inhibition of photosynthetic processes in foliose lichens induced by temperature and osmotic stress

Negative effects of osmotically-induced dehydration of two foliose lichen species, Lasallia pustulata and Umbilicaria hirsuta, was studied at physiological (22 degrees C), low (5 degrees C) and freezing temperature (-10 degrees C), using chlorophyll (Ch1) fluorescence. In both species, exposure to increasing sucrose concentrations led to a pronounced decrease in potential (F-V/F-M), and actual (Phi(2)) quantum yields of photochemical processes in photosystem 2. L. pustulata was more sensitive to osmotic stress, because comparable osmotic dehydration inhibited F-V/F-M and Phi(2) more than in U. hirsuta. Critical concentration of sucrose that fully inhibited photochemical processes of photosynthesis was 2.5 M, which represented water potential (Psi(w)) of -18.8 MPa. Decrease in background Chl fluorescence (F-0) and increase in non-photochemical quenching (qN) revealed two phases of osmotic stress in lichens: phase I with no change (Psi(w) 0 to -6.6 MPa) and phase II (Psi(w) -11.3 to -18.8 MPa) typical by substantial change in Chl fluorescence parameters. Effects of thallus anatomy on species-specific response to osmotic dehydration is discussed and attributed to the results obtained by optical microscopy and Chl fluorescence imaging technique.