The direct carbon dioxide effect on plants

Abstract

Carbon dioxide may affect plants by changing the climate, but it can have another more subtle and quite separate influence, through its direct effects on plant physiology. Since CO₂ is fundamental to photosynthesis, it makes sense that increasing the amount of CO₂ in the atmosphere will tend to allow plants to photosynthesize faster. This then is one-half of the direct CO₂ effect on plants. But there is also another less straightforward direct effect of CO₂ on the water balance of plants. Why should this be?