Photosynthesis plays a central role as an energy source for plant metabolism, and its efficiency may be drastically reduced due to abiotic stresses. During acclimation to adverse environments, plants try to maintain the photosynthetic efficiency at as high a level as possible, avoiding the energy imbalance and the photo-oxidative damage. When plants are exposed to environmental stresses the availability of CO2 within the leaf (Ci) is restricted and/or the synthesis of ATP is impaired. Moreover, under abiotic stress conditions stomatal closure is caused, slowing CO2 assimilation and consequently reducing the photosynthetic rate. The uptake and release of CO2 in photo synthesis and the respiration and release of H2O during transpiration of plants or leaves is measured using gas-exchange systems.