Diel net <scp> CO ₂ </scp> exchange of carnivorous <i>Pinguicula</i> is consistent with low‐level <scp>CAM</scp> photosynthesis

Abstract 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 CO 2 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 CO 2 gas exchange was measured, but results were inconclusive. To independently verify the presence of CAM in Pinguicula , net‐CO 2 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 CO 2 monitoring and the detection of small CO 2 fluxes. Measurements included eight plants of Pinguicula ‘Weser’ and encompassed 36 day–night cycles of CO 2 exchange. Net CO 2 uptake occurred largely in the light via C 3 photosynthesis. However, distinct transient reductions in the rate of nocturnal CO 2 release were regularly observed during dark periods, as were transient reductions of net CO 2 uptake during light periods. In the middle of 14 of 36 dark periods, transient reductions in CO 2 release eventually turned into small rates of net dark CO 2 fixation. These results provide unequivocal evidence of CAM activity and confirm previous suggestions of low‐level CAM in Pinguicula .