Yes, we CAM! First evidence of CAM photosynthesis in a carnivorous plant

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation enabling higher CO₂ concentration close to Rubisco and increased water use efficiency. It evolved in at least 38 plant families, none reported to be carnivorous. Here, we investigate CAM activity in the carnivorous genus Pinguicula, so far reported as C₃, with succulent leaves and distributed mainly in Central America. Eight species of Pinguicula, most with seasonal heterophylly, were cultivated under controlled conditions and monitored for changes in diel acidification (ΔH⁺) when grown with abundant or limited water availability. Additionally, leaf anatomy and gas exchange were studied in representative species. In the winter trial, five species had positive and significant ΔH⁺ 1 week after withholding water (P. agnata, P. esseriana, P. gigantea, P. laxifolia and P. moranensis). ΔH⁺ levels were in the range previously reported in weak, facultative CAM plants (15-30 μmol H⁺ g^-1 FW). The summer trial revealed positive ΔH⁺ for most species tested, regardless of water availability. Three of the homophyllous species had an unclear pattern of CAM induction (P. emarginata, P. martinezii) or no CAM induction (P. grandiflora). Gas exchange in P. agnata found no dark CO₂ assimilation, suggesting CAM cycling. We present the first report of CAM in a carnivorous plant, reinforcing the need to search for CAM in other families. Future work should assess reversibility of the C₃-CAM transition, explore the interplay between nutrient and carbon balance, and the contribution of weak CAM to plant fitness.