The response of lichen photosynthesis to external CO2 concentration and its interaction with thallus water-status

Three lichen species were studied which differed in net photosynthesis (NP) performance at high thallus water contents (WC): Diploschistes muscorum, showing no depression in NP at high WC, D. scruposus, a slight depression and Fulgensia fulgens, showing severe depression. NP responses to CO2 concentration were constructed by interpolation from a large number of NP response to WC curves prepared at fixed CO2 concentrations from near zero to above ambient levels that alleviated all depression in NP at high WC. Optimised CO2 response curves were extracted which proved to be practically identical for all three species. The major differences in NP at high WC and ambient CO2 concentration were due not just to the appearance of diffusion resistances at high WC, since resistances appeared in all three species, but to how high these additional resistances were. There was a major contrast between the three species in how they managed the interaction between CO2 diffusion pathways and water storage at high WC. Since underlying carboxylation processes were nearly identical our studies confirmed that the difference in NP performances at high WC must have a morphological basis and be a product of the symbiosis rather than just of the photobiont. Methods of calculating various photosynthetic parameters: carboxylation efficiency (CE) and overall affinity of the carboxylation system to CO2 [K-0.5(CO2)] were compared. It is recommended that K-0.5(CO2) be applied when comparing photosynthetic abilities since it is independent of the basis used to calculate NP but both K-0.5(CO2) and CE can be affected by WC and should be measured at optimal WC for NP. All photosynthetic parameters suggest that the three species studied have CO2 concentrating mechanisms (CCM).