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Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO₂ (A=350 or E=700 µmol mol^–1, respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Ψ_w) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g_s) and thereby transpiration, but increased net CO₂ assimilation rate (P_n, short‐term measurements); therefore, water‐use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P_n to intercellular CO₂ concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in …