Net Photosynthetic CO2 Compensation Concentrations of Some Lichens

CO₂ compensation concentrations (Γ) and rates of net photosynthetic CO₂ uptake and of respiration in light and darkness were determined at 20°C, 300 μmol photons m_-2s_-1 and 21% O₂ in some foliose and fruticose lichens. Large differences were found in the values of F which ranged between 5 and 70 μl CO₂ l_-1 air. Γ values higher than typical for vascular C₃ 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 CO₂-concentrating mechanism. There was no correlation between the values of Γ and the absolute rates of the other CO₂ exchange parameters investigated.