Elevated carbon dioxide (CO 2) influences photosynthesis ( AN), transpiration (ET), and water use efficiency (WUE) for well-watered potato ( Solanum tuberosum L.). Little is known regarding effects of short-term drought and CO 2. Two experiments, differing in the quantity of solar radiation, were conducted in soil-plant-atmosphere-research chambers. Plants were grown at ambient (aCO 2) or twice-ambient CO 2 (eCO 2) and received one of three irrigation treatments: no water stress (C), short-term (11-16 d) water-withholding during vegetative and post-tuber initiation stages (VR), or post-tuber initiation (R) only. Canopy conductance to CO 2 transfer (tau) and water vapor ( Gv), light use efficiency (alpha), daily AN, and ET decreased at the onset of each drought and were correlated with volumetric water content. The rate of decrease was similar for R and VR. Gv declined more sharply than AN, resulting in higher WUE. Seasonal AN declined with the pattern of C > R > VR and was higher for eCO 2 C and R treatments. Seasonal WUE was higher for eCO 2 at all irrigation treatments. Total dry matter, harvest index, and leaf area were reduced ( p<0.05) for droughted treatments and total dry matter and harvest index were also higher for eCO 2 VR pots. Relative responses to drought and CO 2 were similar among experiments, with greater magnitude of response under high solar radiation. Findings were similar to those reported under longer-term water-withholding studies, suggesting that interactions between CO 2 and drought on carbon assimilation and water use are conserved across production zones with varying radiation and rainfall patterns.
