The balance between carbon assimilation (net photosynthetic production) and the throughput of water by transpiration (resource use in terms of water) results in a benefit–cost ratio of interest to eco-physiologists and crop physiologists, known as water use efficiency. The differences in concentration of CO2 and water vapor between the intercellular surfaces of the leaf mesophyll and the atmosphere drive the fluxes of carbon dioxide and water through the plant. Hot dry environments provide conditions of high evaporative demand. CO2 concentrations are low in the atmosphere, and this gas diffuses through the stomata, which need to be open to allow gas exchange. There is a need under most environments to conserve water, and under drought stress stomata close which conserves water. Water use efficiency is an expression of the benefit–cost ratio for a plant and integrates the physiology of photosynthesis and plant water relations over a particular growth period or cropping season.