Fig. 4. Gross oxygen evolution (E_O) of attached leaves of wild‐type tomato (closed circles) and the hp‐1 mutant (open circles) 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 a closed gas exchange system using a mass spectrometric ¹⁶O₂/¹⁸O₂ isotope technique. Leaves (23 cm²) were supplied with air (gas flow 50 l h^–1) containing 350 µl l^–1 CO₂ and 210 ml l^{–1 18}O₂ in N₂ at 70 % relative humidity and 23 °C. Points are means of six replicates; s.e. ≤ 10 %.