WATER RELATIONS IN LICHENS AT SUBZERO TEMPERATURES - STRUCTURAL-CHANGES AND CARBON-DIOXIDE EXCHANGE IN THE LICHEN UMBILICARIA-APRINA FROM CONTINENTAL ANTARCTICA

Photosynthetic activity and structural changes at subzero temperatures were monitored in the foliose lichen Umbilicaria aprina Nyl. from con tinental Antarctica. Carbon dioxide gas exchange measurements revealed that net photosynthesis and dark respiration occurred at subzero temp eratures regardless of whether a lichen thallus saturated with liquid water was exposed to subzero temperatures, or if a dry thallus was re- hydrated only from snow at subzero temperatures. When water-saturated thalli of U. aprina were slowly cooled at subzero temperatures ice nuc leation activity could be detected at -5.4 degrees C, indicating extra cellular freezing of water. Using low-temperature scanning electron mi croscopy (LTSEM) it was demonstrated that extracellular ice formation leads to cytorrhysis in the photobiont cells and to cavitation in the mycobiont cells. Both processes were reversible if the lichen thallus was re-warmed. When dry lichen thalli were covered with snow at subzer o temperatures a substantial re-hydration from snow could be observed in LTSEM micrographs and measured gravimetrically. The final thallus w ater content was strongly dependent on the temperature regime and gave water contents between 20% d. wt at -21 degrees C and 56% d. wt at -4 .5 degrees C after 16 h exposure. Carbon dioxide gas exchange measurem ents revealed that metabolic activity was initiated during re-hydratio n from snow at subzero temperatures. It is proposed that water uptake from snow at subzero temperatures occurs in the gaseous phase and depe nds only on the temperature-related differences in water potential bet ween the cell contents and the surrounding atmosphere in equilibrium w ith snow. Photosynthetic activity and re-hydration from snow at subzer o temperatures are of great ecological importance for primary producti on in extreme environments such as Antarctica where metabolic activity is severely limited by water availability and low temperatures.