The role of carbon dioxide (CO 2 ) as a signal in biochemical regulation networks of plants is fathomed. Transport mechanisms of CO 2 and HCO 3 − are surveyed, which are the prerequisite for signalling. A CO 2 sensor is not known to date, but any reaction where CO 2 / HCO 3 − is a substrate can be a candidate. Carbon concentrating mechanisms, e.g., in higher plants C 4 -photosynthesis and crassulacean acid metabolism (CAM), generate high internal CO 2 concentrations, important for photosynthesis, but also as a basis for signalling via diffusion of CO 2 . Spatiotemporal dynamics of desynchronization/synchronization of photosynthetic activity over leaves can be followed by chlorophyll fluorescence imaging. One example of desynchronization is based on patchiness of stomatal opening/closing in heterobaric leaves due to anatomic constraints of lateral CO 2 diffusion. During CAM, largely different internal CO 2 concentrations prevail in the leaves, offering opportunities to study the effect of lateral diffusion of CO 2 in synchronizing photosynthetic activity over the entire leaves.