CO2 exchange of the endolithic lichen Verrucaria baldensis from karst habitats in northern Italy.

CO2 exchange of the endolithic lichen Verrucaria baldensis was measured in the laboratory under different conditions of water content, temperature, light, and CO2 concentration. The species had low CO2 exchange rates (maximum net photosynthesis: c. 0.45 μmol CO2 m-2 s-1; maximum dark respiration: c. 0.3 μmol CO2 m-2 s-1) and a very low light compensation point (7 μmol photons m-2 s-1 at 8°C). The net photosynthesis/respiration quotient reached a maximum at 9-15°C. Photosynthetic activity was affected only after very severe desiccation, when high resaturation respiratory rates were measured. Microclimatic data were recorded under different weather conditions in an abyss of the Trieste Karst (northeast Italy), where the species was particularly abundant. Low photosynthetically active radiation (normally below 40 μmol photons m-2 s-1), very high humidities (over 80%), and low, constant temperatures were measured. Thallus water contents sufficient for CO2 assimilation were often measured in the absence of condensation phenomena.