AbstractUsing an open‐system leaf chamber, gas exchange measurements on attached leaves of 3‐4‐year‐old Golden Delicious apple trees, made through two seasons, provided data from which the parameters of a leaf photosynthesis model could be derived. The equation is:where C1is internal CO2concentration and Qpis the incident quantum flux. There was considerable leaf to leaf variation in the values of the parameters but no clear seasonal trends were established. The initial slope (a) had an average value of about 2.5 × 10−3mg μmol−1† (i.e. quantum yield ∼ 0.057); the mesophyll conductance (gm) was about 3.5 mm s−1in extension leaves of trees carrying fruit and 2.5 mm s−1in extension leaves of defruited trees. Differences between the values of gmfor spur leaves with and without subtending fruits were not significant; 2.5 mm s−1may be used. Dark respiration (Rd, mg m−2s−1) increased exponentially with temperature (T°C); Rd∼ 0.006 exp (0.09 T). At saturating photon flux density Pnwas linearly related to Ci, up to Ci∼ 250 mg m−3. Optimum temperatures for Pnwere slightly different in the two y