Gas exchange parameters were measured using a portablephotosynthesis system (model LCA-4, ADC Bioscientific Ltd., Hod-desdon, UK) and a PLC-4N leaf chamber (11.35 cm2), configuredto an open system. The third fully-expanded leaf was chosen forthe analysis, after 15 days of treatment. Leaves were first equili-brated at a photon density flux of 400 mol m−2s−1for at least20 min. The net photosynthetic rate per unit leaf area (A), tran-spiration rate (E) and stomatal conductance (Gs) were determinedduring the necessary seconds to stabilise the reading, once sta-bilised measurements were reordered every 10 s during 1 min. Themeasurements were made in the middle of the photoperiod inorder to obtain the highest values. During all the measurementsthe cuvette temperature was 25 ± 2◦C, the CO2concentration ofthe cuvette was 380 ± 5 l l−1or 800 ± 10 l l−1for control andelevated CO2conditions, respectively, and the leaf-to-air vapourpressure difference was 2.4 ± 0.4 kPa within the cuvette. Water useefficiency (WUE) was calculated as mol of CO2fixed per mmol ofH2O transpired.