Publication details

Activity of primary photosynthetic processes in some antarctic lichens and mosses at decreasing water potential of their thalli



Type Article in Periodical
Magazine / Source Terra Nostra, Schriften der Alfred-Wegener-Stiftung
MU Faculty or unit

Faculty of Science

Field Botany
Keywords quantum yield; photosystem II; dehydration; poikilohydric organisms; critical water potential
Description It is generally accepted, that water availability is the most important factor limiting photosynthetic processes and growth of autotrophic organisms in antarctic terrestrial ecosystems. In spite of this fact, our knowledge of the relationships between photosynthetic activity and internal water availability (expressed as water potential of assimilating tissues) of antarctic lichens and mosses is still quite fragmentary. In this study, we have tried to define the dependence of primary photosynthetic processes on water potential of thalli in several species of lichens and mosses widely distributed in maritime Antarctica. The measurement were done in a field laboratory of Vernadsky (former Faraday) Station at the Galindez Island (Argentine Islands, 65o 15 S, 64o 16 W) in February 2003. Wet samples of four lichen species (Umbilicaria antarctica, Xanthoria elegans, Stereocaulon glabrum, Sphaerophorus globosus) and two mosses (Polytrichum alpestre, Sanionia uncinata) were placed on plastic dishes (diameter 35 mm, height 10 mm) and subjected to slow desiccation at room temperature and air humidity (about 20 C and 40 %, respectively). Thallus water potential (WP) and quantum yield of photochemical reactions in photosystem II (FII) were measured simultaneously during desiccation until any activity of photochemical processes was detectable. Dew point water potential meter WP4-T (Decagon, USA) and FL-100 modulated fluorometer (OptiScience, USA) were used for the measurements. Relation of FII to WP was found curvilinear with more or less constant FII in high WP and decreasing FII towards low WP. Optimum WP for primary photosynthetic processes was found both for lichens and mosses in the range of 0 to 10 MPa, in which the FII reached maximum value of 0.5-0.6. At lower WP (from 10 to 20 MPa), steep decrease of FII was found, more pronounced in mosses than lichens. Critical values of WP for photosynthesis (= zero FII) were higher in mosses (about -25 MPa) than in lichens (from -30 to -40 MPa). Our measurements documented that not only poikilohydric antarctic lichens, but also mosses are capable to maintain some photosynthetic activity at extremely low water potential of their thalli. This gives them an advantage to prolong their positive carbon balance or diminish the respiration losses during dry periods of a day or season.
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