Elevated atmospheric carbon dioxide concentration (CO2) is known to enhance growth of many agronomically important crops when nutrients are non-limiting.  Most relevant studies characterized dry matter production, carbohydrate partitioning, and developmental response, with a subset also evaluating nitrogen use and utilization.  Less attention has been directed towards quantifying long- and short-term photosynthesis and transpiration responses, particularly with respect to interactions of CO2 with nutrients other than nitrogen.  Phosphorus (P) is requred in high amounts for potato, especially during early growth stages, but data on P deficiency is largely limited to dry matter production and partitioning.  Information is needed on leaf-level and canopy gas exchange responses to varying CO2 and P factors so that a more complete physiological basis for understanding crop response to climate change and fertility management can be developed.  Two seven-week studies were conducted at USDA-ARS facilities in Beltsville, MD using six soil-plant-atmosphere research (SPAR) chambers.  Three P treatments were applied to four pots within each chamber and chambers were maintained at either ambient (400 ppm) or elevated (800 ppm) atmospheric CO2 concentrations.  Pots were rearranged after five weeks in each study to obtain measurement of whole plant gas exchange rates among uniformly P-treated plants.  Leaf-level gas exchange measurements were also recorded over the course of both studies.  Leaf-level photosynthetic rates and stomatal conductances were consistently lower for low versus medium and high P-treatments across all measurement dates.  Maximum photosynthetic rates were not influenced by CO2 in either study; however, stomatal conductances were reduced at elevated versus ambient CO2, particularly as time progressed.  Canopy gas exchange data showed similar trends as the leaf-level measurements, with a clear reduction in photosynthesis and transpiration from high and low P-treatments, a response presumably due to reduced leaf area production and lower single leaf-level photosynthetic capacity.  Transpiration and water-use were reduced for plants grown at elevated versus ambient CO2 at a given P-treatment.  Results from the experiments are being used to develop and test mathematical crop models in order to develop a systematic approach to predict, adapt, and mitigate response of agricultural systems to climate change impacts.