Over last decades, several studies have been focused on short-term high light stress in lichens under laboratory conditions. Such studies reported a strong photoinhibition of photosynthesis accompanied by a partial photodestruction of PSII, involvement of photoprotective mechanisms, and resynthetic processes into gradual recovery. In our paper, we applied medium [800 µmol(photon) m -2 s -1 ] light stress to induce negative changes in PSII funcioning as well as pigment and glutathione (GSH) content in two Antarctic fruticose lichen species. Chlorophyll (Chl) fluorescence parameters, such as potential and effective quantum yield of photosynthetic processes and fast transients (OJIP) recorded during high light exposition and recovery, revealed that Usnea antarctica was less susceptible to photoinhibition than U. aurantiaco-atra. This might be supported by a more pronounced high light-induced reduction in Chl a and b contents in U. aurantiaco-atra compared with U. antarctica. In both experimental species, total GSH showed an initial increase during the first 30-40 min of high light treatment followed by a decrease (60 min) and an increase during dark recovery. Full GSH recovery, however, was not finished in U. aurantiaco-atra even after 5 h indicating lower capacity of photoprotective mechanisms in the species. OJIP curves showed high light-induced decrease in both species, however, the recovery of the OJIPs shape to pre-photoinhibitory values was faster and more apparent in U. antarctica than in U. aurantiaco-atra. The results are discussed in terms of sensitivity of the two species to photoinhibition and their photosynthetic performance in natural environment.

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This study aimed to evaluate the effective photosynthetic quantum yield ( PSII ) and the Photochemical Reflectance Index (PRI) for assessment of photosynthetic performance of two Umbilicaria lichens during gradual desiccation of their thalli. U. cylindrica and U. decussata exhibited curvilinear relationship (S-shape curve) of decreasing  PSII values with decreasing water potential (WP) of thalli. During initial phase of desiccation (WP from 0 to -10 MPa), no decrease of  PSII was apparent, further desiccation (WP from -10 to -20 MPa) led to fast  PSII decrease from 0.6 to 0.1 indicating strong inhibition of photosynthetic processes. Critical WP at which photosythetic processes are fully inhibited was found bellow -25 MPa in both lichen species. Photochemical Reflectance Index (PRI) exhibited curvilinear increase with thalli desiccation (decreasing WP). At full thallus hydration, the PRI reached the value of -0.18 in both species. Under strong dehydration (WP from -20 to -30 MPa), however, U. cylindrica showed somewhat lower value (-0.04) than U.decussata (-0.02 MPa). PRI to WP relationship is discussed and compared to existing evidence from higher plants and poikilohydric organisms.

In this study, we investigated the relationship between relative water content (RWC) of N. commune colonies recorded during gradual dehydration and (i) normalized difference vegetation index (NDVI), (ii) photochemical reflectance index (PRI), and (iii) primary photochemical processes of photosynthesis, effective quantum yield of photosynthetic processes ( PSII ) in photosystem II particular. PRI increased from -0.05 to 0.02 with RWC decrease from 100% (full hydration) to 0% (dry state). NDVI showed somewhat curvilinear relationship with desiccation with minimum value of 0.25 found at 10% RWC. Negative effect of suprasaturation of N. commune colony with water on effective quantum yield ( PSII ) was found at RWC range 80-100%. At the RWC range,  PSII reached only 50 % of maximum found at RWC of 30%. In general, desiccation-response curve of showed polyphasic character with three main phases (phase I -constant  PSII values, phase II -an increase with desiccation at RWC 80-30%, and phase IIIsigmoidal decrease with desiccation at RWC 0-30%). Non-photochemical quenching (qN) of absorbed light energy showed triphasic dependence on RWC as well. qN showed constant values in the phase I, an increase (phase II), and constant values at severe dehydration (phase III).

The authors thank the CzechPolar project for research infrastructure and the PASSEB project for funding traveling to coordination meetings of team members.

In many polar and alpine ecosystems, lichens of genus Umbilicaria represent dominant species forming community structure. Photosynthetic and spectral properties of the lichens may change rapidly according to an actual hydration status of their thalli. In this study, we investigated responses of photochemical reflectance index (PRI), normalized difference vegetation index (NDVI), effective quantum yield of photosynthetic efficiency of photosystem (PS) II (Φ PSII ), and several photosynthetic parameters derived from fast induction kinetics of chlorophyll fluorescence (OJIP) to controlled dehydration. We used U. arctica and U. hyperborea collected close to Nuuk, Greenland. In both the species, PRI showed a curvilinear increase with dehydration, i.e., a decreasing water potential ( w ). The increase was apparent within  w range of 0 to -10 MPa. The PRI increase was less pronounced in U. arctica than in U. hyperborea. NDVI decreased with a progressive thallus dehydration in both the species, however, throughout  w range of 0 to -30 MPa, U. hyperborea had lower NDVI values than U. arctica. The relationship between Φ PSII and  w resulted in a typical S curve. A critical  w at which photosynthetic processes were fully inhibited was -30 MPa in both the species, however, species-specific differences in the S curve shape were found. Analyses of photosynthetic parameters derived from OJIPs revealed that the absorption of radiation energy and a trapping rate increased with dehydration in active reaction centres of PS II, the number of which decreased with a more pronounced lichen thallus dehydration. It is concluded that U. arctica and U. hyperborea possess effective physiological mechanisms to maintain an effective photosynthesis when partly dehydrated (the  w range of 0 to -15 MPa). In spite of similar ecological niches that these two lichens occupy in nature, their spectral and photosynthetic properties differred.

Optimum growth temperature of Trebouxia sp. (re-classified as Asterochloris sp. recently), a symbiotic lichenized alga was evaluated using a batch culture cultivated in a bioreactor. The algae were isolated from lichen thalli of Usnea antarctica collected at the James Ross Island, Antarctica in February 2012. The algae were isolated under laboratory conditions and then cultivated on agar medium at 5°C. When sufficiently developed, the algae were suspended in a BBM liquid medium and cultivated in a photobioreactor for 33 days at either 15, or 10°C. During cultivation, optical density (OD) characterizing culture growth, and effective quantum yield of photosystem II ( PSII ) characterizing photosynthetic performance were measured simultaneously. Thanks to higher  PSII values, faster growth was achieved at 10 o C than 15 o C indicating that Trebouxia sp. might be ranked among psychrotolerant species. Such conclusion is supported also by a higher specific growth rate found during exponential phase of culture growth. The results are discussed and compared to available data on temperaturedependent growth of polar microalgae.Key words: Usnea antarctica, chlorophyll fluorescence, lichen, effective quantum yield, James Ross Island, psychrotolerance, Asterochloris Abbreviations:  PSII -effective quantum yield of photosystem II,  -specific growth rate, OD -optical density, PBRr -photobioreactors

Recently, spectral characteristics of lichens are in focus because of increasing number of spectral data applications in remote sensing of treeless polar and alpine regions. Therefore, species-specific spectral reflectance indices are measured in lichen species dominating polar ecosystems. Hydration status of the lichen thalli, as well as the presence of intrathalline secondary metabolites-which are UV-B absorbing compounds-both affects the spectral reflectance curves as well as numeric values of spectral reflectance indices. In the present paper, the reflectance spectra in 380-800 nm was measured in selected lichens to assess the effects of full hydration, and to evaluate the influence of secondary metabolites, they were wash out from lichen thalli with acetone (i.e. acetone rinsing) and then the spectra were also measured. For these experiments, Antarctic (Xanthoria elegans, Leptogium puberulum, Physconia muscigena and Rhizoplaca melanophthalma) and Argentinean lichens from mountain regions (Parmotrema conferendum and Ramalina celastri) were used. Changes in several spectral reflectance indices were evaluated and discussed in relation with hydration status and the absence of secondary metabolites. For the great majority of studied lichens, MCARI (Modified Chlorophyll Absorption in Reflectance Index) was the most effective index to reflect the changes between dry and wet state of thallus.