A COMPARISON OF CO2 AND O-2 EXCHANGE PATTERNS AND THE RELATIONSHIP WITH CHLOROPHYLL FLUORESCENCE DURING PHOTOSYNTHESIS IN C-3 AND CAM PLANTS

Patterns of CO2 and O-2 exchange coupled to chlorophyll fluorescence w ere examined in the CAM plants Kalanchoe daigremontiana Hamet et Perr. and Hoya carnosa (L.f.) R.Br., and compared with the C-3 Species Sonc hus oleraceus L. Patterns of CO2 assimilation during phase IV of CAM w ere as predicted for Rubisco-mediated atmospheric CO2 uptake in both C AM plants. The high energetic demand during decarboxylation (phase III ) was reflected in a high, CO2-insensitive rate of gross O-2 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 sensit ive in H. carnosa during phases III and IV, which suggests Rubisco oxy genase activity, Oxygen consumption comprised around 33% linear electr on transport in K. daigremontiana during phase III (7.5 mu mol O-2 m(- 2) s(-1) at saturating CO2), but we cannot yet distinguish whether thi s involves the Mehler reaction or TCA cycle and oxidative phosphorylat ion during deacidification. CO2 assimilation was saturated at 0.2% CO2 when the epidermis was removed in K. daigremontiana, suggesting a lar ge stomatal and mesophyll resistance to CO2 diffusion. A linear relati onship was obtained between the quantum yield of gross O-2 evolution a nd the quantum efficiency of PSII.