Leaf photosynthesis of Honeycrisp apple was characterized by gas-exchange measurements using the LCPro+ system (ADC) in order to establish a fully parameterized mechanistic model of leaf photosynthesis for this cultivar. Carboxylation capacity (Vcmax), electron transport capacity (Jmax), day respiration rate (RDay), stomatal sensitivity to humidity, CO2 and assimilation (gfac) have been determined from light- and CO2-response measurements on 42 leaves. The leaves were chosen from 6 trees with variable crop loads and differing proportions of leaf zonal chlorosis in order to cover the full range of light exposures, specific leaf weights, shoot types, and chlorotic area of leaf on each tree. The parameterized model was validated using independent photosynthesis measurements with a root mean square error (RMSE) of 11% for photosynthesis and an RMSE of 18% for transpiration. The results show that neither crop load nor shoot type had a significant influence on components of the photosynthetic mechanism while leaf zonal chlorosis reduced photosynthesis rates in proportion to the leaf area affected. Canopy structure above the measured leaf (gap fraction) had by far the biggest effect on leaf photosynthesis rates.
