Fig. 7. Rates of steady state gross CO₂ evolution (E_C, closed circles), photorespiration (PR, open circles) and mitochondrial respiration in the light (R_C, triangles) of attached leaves of tomato hp‐1 mutant during steady state photosynthesis at three different light intensities [850 (A), 400 (B) and 90 (C) µmol photons m^–2 s^–1] in relation to leaf water potential. 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 at 70 % relative humidity and 23 °C. The CO₂ concentration was 350 µl l^{–1 12}CO₂ or ¹³CO₂, respectively, for determination of E_C, and 3000 µl l^{–1 12}CO₂ or ¹³CO₂, respectively, for measurement of R_C. PR is calculated from E_C and U_R. Points are means of six replicates; s.e. ≤ 10 %.