Fig. 4. O2 evolution at different CO2 concentrations around sunflower leaf discs in the O2 electrode. A, Intact disc. B, Peeled disc lacking the abaxial epidermis. For exposure to various CO2 concentrations, four discs were removed from the same leaf of a plant with a high or low Ψw. Three were used for O2 evolution, one for each CO2 concentration and one was used to determine Ψw at the sampling time, shown alongside each curve. This is a representative experiment from two repetitions with similar results. O2 evolution was measured after 10 min at an irradiance of 1200 µmol m–2 s–1 at 25 °C in saturated air containing 21 % O2.