Diel net CO₂ exchange of carnivorous Pinguicula is consistent with low-level CAM photosynthesis

Crassulacean acid metabolism (CAM) photosynthesis is a water-conserving pathway of photosynthesis that typically occurs in leaf- or stem-succulent plants of warm, water-limited habitats. Key features of CAM photosynthesis are nocturnal CO₂ uptake and nocturnal increases in tissue acidity. CAM photosynthesis was recently reported in water-deficit stressed carnivorous plants of the genus Pinguicula (Lentibulariaceae). The report is remarkable because CAM has neither been documented in the Lentibulariaceae nor in a carnivorous species before. Furthermore, Pinguicula plants are vastly different in appearance from archetypal CAM plants like cacti and agaves. Evidence of CAM activity in Pinguicula was solely based on small nocturnal increases in leaf titratable acidity. Day-night CO₂ gas exchange was measured, but results were inconclusive. To independently verify the presence of CAM in Pinguicula, net-CO₂ exchange during 12 h light/12 h dark cycles was measured in plants exposed to different degrees of water-deficit stress using a custom-designed stationary gas-exchange system which enables long-term CO₂ monitoring and the detection of small CO₂ fluxes. Measurements included eight plants of Pinguicula 'Weser' and encompassed 36 day-night cycles of CO₂ exchange. Net CO₂ uptake occurred largely in the light via C₃ photosynthesis. However, distinct transient reductions in the rate of nocturnal CO₂ release were regularly observed during dark periods, as were transient reductions of net CO₂ uptake during light periods. In the middle of 14 of 36 dark periods, transient reductions in CO₂ release eventually turned into small rates of net dark CO₂ fixation. These results provide unequivocal evidence of CAM activity and confirm previous suggestions of low-level CAM in Pinguicula.