| Summary: | This study aimed to evaluate the effective photosynthetic quantum yield ( Phi 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 Phi PSII values with decreasing water potential (WP) of thalli. During initial phase of desiccation (WP from 0 to -10 MPa), no decrease of Phi PSII was apparent, further desiccation (WP from -10 to -20 MPa) led to fast Phi 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. Tato publikace je zaměřena na souběžné sledování indexu PRI a efektivního kvantového výtěžku fyotoyntézy během vysychání stélek dvou druhů lišejníků rodu Umbilicaria ze souostroví Špicberky (Svalbard). Byl zjištěno, že PRI s postupujícím vysycháním roste, zatímco u listů vyšších rostlin klesá. This study aimed to evaluate the effective photosynthetic quantum yield (Phi 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 Phi PSII values with decreasing water potential (WP) of thalli. During initial phase of desiccation (WP from 0 to -10 MPa), no decrease of Phi PSII was apparent, further desiccation (WP from ...
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