Unexpected Responses of Bean Leaf Size to Elevated CO₂

CO₂ is currently a growth-limiting resource for plants with C₃ metabolism, and elevated CO₂ also often reduces stomatal conductance, reducing plant water stress. Increased photosynthesis and improved water status might be expected to result in increased leaf size. It is therefore unexpected that leaf size is in some cases reduced in plants grown at elevated CO₂, and also unexpected that elevated CO₂ applied only during darkness can increase leaf size. These experiments compared leaf size responses to day and/or night elevated CO₂ in six cultivars of Phaseolus vulgaris grown with either constant or varying temperature in controlled environment chambers. Diverse responses of leaf size to elevated CO₂ were found among the cultivars, including increased leaf size with elevated CO₂ applied only during darkness in some cultivars and temperature regimes. However, leaf size responses to elevated CO₂ and cultivar differences in response were unrelated to differences in leaf water potential or turgor pressure.