As the CO2 content of the air rises, most plants respond by increasing their photosynthetic rates, which results in greater leaf carbohydrate concentrations, including starch. It has been suggested that starch accumulation may signal photosynthetic acclimation in response to growth in elevated CO2 conditions. However, in the present study, transformed potatoes lacked the ability to accumulate starch, yet they still displayed reduced rates of photosynthesis. This observation suggests that rates of starch synthesis can influence rates of photosynthesis.