title: Sensitivity of photosynthetic processes to freezing temperature in extremophilic lichens evaluated by linear cooling and chlorophyll fluorescence.
abstract: Extremophilic lichens and their photosynthesizing photobionts from the cold regions of Earth are adapted to perform photosynthesis at subzero temperatures. To evaluate interspecific differences in the critical temperature for primary photochemical processes of photosynthesis, we exposed lichen thalli of Usnea antarctica, Usnea aurantiaco-atra, and Umbilicaria cylindrica to linear cooling from +20 to -50 °C at a constant rate of 2 °C min-1. Simultaneously, two chlorophyll fluorescence parameters (FV/FM- potential yield of photosynthetic processes in photosystem II, ΦPSII- effective quantum yield of PS II) evaluating a gradual subzero temperature-induced decline in photosynthetic processes were measured by a modulated fluorometer. For the studied species, the response of FV/FMand ΦPSIIto declining temperature showed an S-curve shape. The decline in FV/FMand ΦPSIIat low temperatures started at -5 and +5 °C, respectively in the majority of cases. The decline was, however, species-specific. U. aurantiaco-atra showed a constant-rate decline of ΦPSIIfrom the physiological temperature 20 °C. U. antarctica exhibited the first sign of FV/FMdecline at -12 °C. The critical temperature related to full inhibition of the photosynthetic processes in PSII (FV/FM), was found at -20 °C. However, this occurred at -30 °C for U. cylindrica. In an individual sample, the critical temperature for FV/FMwas typically lower than for ΦPSII. The method of linear cooling combined with simultaneous measurements of chlorophyll fluorescence parameters proved to be an efficient tool in the estimation of extremophilic species sensitivity/resistance to freezing.
authors.affiliation: Laboratory of Photosynthetic Processes, Section of Plant Physiology and Anatomy, Institute of Experimental Biology, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic. Electronic address: jhajek@sci.muni.cz.
authors.forename: Josef
authors.initials: J
authors.lastname: Hájek
authors.affiliation: Laboratory of Photosynthetic Processes, Section of Plant Physiology and Anatomy, Institute of Experimental Biology, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic.
authors.forename: Miloš
authors.initials: M
authors.lastname: Barták
authors.affiliation: Laboratory of Photosynthetic Processes, Section of Plant Physiology and Anatomy, Institute of Experimental Biology, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic.
authors.forename: Jana
authors.initials: J
authors.lastname: Hazdrová
authors.affiliation: Department of Mathematics and Statistics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czechia.
authors.forename: Marie
authors.initials: M
authors.lastname: Forbelská
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D002734:Chlorophyll / Q000032:analysis; D003080:Cold Temperature; D000071825:Extremophiles / Q000502:physiology*; D005615:Freezing; D008011:Lichens / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000502:physiology; D013045:Species Specificity
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll
keywords: Antarctica; Cryoresistance; James ross island; Temperature stress; Umbilicaria cylindrica; Usnea antarctica; Usnea aurantiaco-atra