Potato Gas Exchange Response to Drought Cycles under Elevated Carbon Dioxide

Elevated carbon dioxide (CO2) 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 CO2. Two experiments, differing in the quantity of solar radiation, were conducted in soil‐plant‐atmosphere‐research chambers. Plants were grown at ambient (aCO2) or twice‐ambient CO2 (eCO2) 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 CO2 transfer (τ) and water vapor (Gv), light use efficiency (α), 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 eCO2 C and R treatments. Seasonal WUE was higher for eCO2 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 eCO2 VR pots. Relative responses to drought and CO2 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 CO2 and drought on carbon assimilation and water use are conserved across production zones with varying radiation and rainfall patterns.