RESPONSE OF PLANTS TO CO2 UNDER WATER LIMITED CONDITIONS

The influence of increased atmospheric CO2 on the interaction between plant growth and water use is proving to be one of the most profound impacts of the anthropogenic 'Greenhouse Effect'. This paper illustrates the interaction between CO2 and water in plant growth at a range of scales. Most published work has concentrated on water use efficiency, especially at shorter time scales, and has shown large increases of leaf water use efficiency with increased CO2. However, the magnitude of the effect is variable, and does not consistently agree with predictions from simple leaf gas exchange considerations. The longer the time scales considered, the less the information and the more the uncertainty in the response to CO2, because of the additional factors that have to be considered, such as changes in leaf area, respiration of non-photosynthetic tissues and soil evaporation. The need for more detailed studies of the interactions between plant evaporation, water supply, water status and growth is stressed, as increased CO2 can affect all of these either directly, or indirectly through feedbacks with leaf gas exchange, carbon partitioning, leaf growth, canopy development and root growth.