Carbon dioxide exchange in lichens: Partition of total CO2 resistances at different thallus water contents into transport and carboxylation components

The total resistances to CO₂ uptake by Sticta latifrons Rich, and Pseudocyphellaria amphisticta Kremp. were separated into transport and carboxylation components by calculation after transformation of net photosynthesis rate against CO₂ concentration curves into a linear form. The use of this technique circumvented the problem of measuring the internal CO₂ concentration of the lichen thalli. Both species exhibited an increase in transport resistance at high thallus water contents and an increase in both transport and carboxylation resistances at low water contents. At low and intermediate water contents internal transport resistances were larger than carboxylation resistances when measured at limiting CO₂ concentrations. However, at ambient CO₂ concentrations carboxylation processes were the dominant factors limiting photosynthesis at all, except the high, water contents.