A comparison of CO₂ and O₂ exchange patterns and the relationship with chlorophyll fluorescence during photosynthesis in C₃ and CAM plants

Patterns of CO₂ and O₂ exchange coupled to chlorophyll fluorescence were examined in the CAM plants Kalanchoe daigremontiana Hamet et Perr. and Hoya carnosa (L.f.) R.Br., and compared with the C₃ species Sonchus oleraceus L. Patterns of CO₂ assimilation during phase IV of CAM were as predicted for Rubisco-mediated atmospheric CO₂ uptake in both CAM plants. The high energetic demand during decarboxylation (phase III) was reflected in a high, CO₂-insensitive rate of gross O₂ evolution. Uptake of external CO₂ was significant during phase III and was not saturated at 1.5% CO₂. Gross oxygen uptake in the light was CO₂ sensitive in H. carnosa during phase III and IV, which suggests Rubisco oxygenase activity. Oxygen consumption comprised around 33% linear electron transport in K. daigremontiana during phase III (7.5 μmol O₂ m^-2 s^-1 at saturating CO₂), but we cannot yet distinguish whether this involves the Mehler reaction of TCA cycle and oxidative phosphorylation during deacidification. CO₂ assimilation was saturated at 0.2% CO₂ when the epidermis was removed in K. daigremontiana, suggesting a large stomatal and mesophyll resistance to CO₂ diffusion. A linear relationship was obtained between the quantum yield of gross O₂ evolution and the quantum efficiency of PSII.