CAM photosynthesis is a water-saving mode of C-fixation and its implementation into a C3 crop plant is a promising engineering target as all enzymes involved in the CAM cycle are present in C3 plants and some facultative CAM species can switch from C3 to CAM photosynthesis. In this presentation, I ask the question: what are the metabolic and morphological limitations to implementing CAM or CAM-like mechanisms in a C3 leaf in different environments? This question is tackled by employing a time-resolved, large-scale metabolic leaf model which is coupled to a gas-exchange model. The model thereby takes into account the two main determinants of water-loss through the stomata - temperature and relative humidity and is used to investigate emergent flux modes when water-saving constraints are applied in addition to high productivity. I will highlight three main findings, relating to leaf anatomy, enzyme activity and the impact of the environment.