Several carbon concentrating mechanisms (CCM) have evolved to counteract the deficiencies of the classical photosynthetic pathway (C3 photosynthesis). Their multiple origins represent one of the most striking examples of convergent evolution and make them especially suitable for comparative evolutionary studies. In land plants, the two main CCM are CAM and C4 photosynthesis, which confer an advantage in arid and warm environments. These CCM are ecologically and economically very important, but their evolutionary history remains poorly understood. In particular, the number of transitions between the different photosynthetic types and their directions are still unknown. In this project, a multi-scale phylogenetic approach will be used to address the evolvability and reversibility of the different photosynthetic types together with the factors that influence the probability of a given plant group evolving one or the other CCM. Using two plant groups with a high variability in the photosynthetic types (Poaceae and core Caryophyllales), the genetic changes on an enyzme involved in both CAM and C4 pathways will be identified and, in combination with species phylogenies, will serve to retrace past photosynthetic transitions. The effect of different plant characteristics on CCM evolvability will be tested through comparative analyses in the generated phylogenetic framework. This approach will finally be transferred to a grass genus extremely variable in its photosynthetic characteristics, to shed light on CCM microevolutionary processes. Overall, this project will bring important insights about the causes and consequences of one of the greatest ecological and evolutionary successes in plant history.