Regulation of photosynthesis during the light period in CAM plants - evaluation by a gas-phase O2 electrode and a compensating infrared CO2 analysis system.
The CO2-dependent O2 evolution during the light period, at which the exogenous CO2 uptake was suspended, was evaluated in 12 CAM plants (Ananas comosus, Crassula argentea [C. ovata], Dendrobium ekapol, Hoya carnosa, Kalanchoe blossfeldiana, K. daigremontiana, K. fedtschenkoi, K. gastonisbonnieri, K. pinnata, Peperomia incana, Sedum praealtum and Vanilla fragrans [V. planifolia]) using a gas-phase oxygen electrode. At 5% CO2, the rate of photosynthetic O2 evolution in pineapple (A. comosus) was saturated at a photosynthetic photon flux density (PPFD) of 1500 µmol m-2 s-1 and the maximum rate was 60 µmol m-2 s-1, which was 10 times those obtained at ambient CO2 conditions with the CO2 exchange system and significantly higher than the other CAM plants. At the saturated PPFD, the O2 evolution in pineapple substantially increased with increasing CO2 concentration up to 3% and decreased above 4%. However, in the other CAM plants, such increment was small. By the use of a novel compensating CO2 gas exchange system, the rate of CO2 exchange in pineapple at very high CO2 was measured and found the enhancement of CO2 uptake in both light (phases III and IV) and dark (phase I) periods. Based on the results obtained, possibilities of the further increases in CO2 uptake in CAM plants, especially in pineapple, are discussed in terms of stomatal functions and malate storage capacities.