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.