Fig. 3. Rates of gross CO₂ uptake (TPS, closed squares), net CO₂ uptake (P_N, open squares) (A) and gross CO₂ evolution in the light (E_C) (B) by attached leaves of wild‐type tomato during steady state photosynthesis at saturating light intensity (850 µmol photons m^–2 s^–1) in relation to ambient CO₂. C, Ratio of E_C/TPS. Measurements were made in an open gas exchange system using a ¹²CO₂ /¹³CO₂ isotope technique. Leaves (23 cm²) were supplied with air (gas flow 50 l h^–1) containing 210 ml l^–1 O₂ and 350 µl l^{–1 12}CO₂, or 250 µl l^{–1 12}CO₂ or 100 µl l^{–1 12}CO₂ at 70 % relative humidity and 23 °C. When steady state photosynthesis was reached ¹²CO₂ uptake was measured. Then 350 µl l^{–1 13}CO₂ (or 250 µl l^{–1 13}CO₂ or 100 µl l^{–1 13}CO₂) in 21 % O₂ was supplied for 1 min and ¹³CO₂ uptake as well as ¹²CO₂ evolution were determined. From mass spectrometer readings, TPS, P_N, E_C and the ratio of E_C/TPS were calculated. Points are means of six replicates; s.e. ≤ 10 %.