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

Abstract

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 phases 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 or 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.