Abstract
Summary CO 2 compensation concentrations (Γ) and rates of net photosynthetic CO 2 uptake and of respiration in light and darkness were determined at 20°C, 300 μmol photons m -2 s -1 and 21% O 2 in some foliose and fruticose lichens. Large differences were found in the values of F which ranged between 5 and 70 μl CO 2 l -1 air. Γ values higher than typical for vascular C 3 plants (30 to 35 μll -1 under the same conditions) may be due to fungal respiration but were found in foliose and fruticose lichens irrespective of the mass proportion of the phycobiont to mycobiont. Low Γ levels were detected in a lichen with a cyanobacterium as phycobiont ( Peltigera polydactyla ) but also in lichens with green algae ( Cladonia rangiferina , C stellaris , Evernia prunastri ). In low Γ species with green algae a dense plectenchyma of fungal hyphae forms the thallus surface and this high diffusive resistance may assist a CO 2 -concentrating mechanism. There was no correlation between the values of Γ and the absolute rates of the other CO 2 exchange parameters investigated.
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