The distribution of photosynthesis within the canopy determines the productivity and quality in a fruit orchard. A model of photosynthesis was developed to simulate the three-dimensional (3-D) distribution of leaf net photosynthetic rate (P_n), stomatal conductance (G_s) and light use efficiency (LUE) in apple canopy. The experiment was conducted in a ‘Fuji’ apple (Malus domestica Borkh. cv. ‘Fuji’) orchard during the 2007-2009 growing seasons. The orchard is on the farm in Guojia Town (35° 11' N, 105° 21' E, altitude 1530 m) of Gansu Province. The planting spacing was 3 m × 4 m, and each tree was trained as an spindle system. The model was developed on the basis of 3-D distribution of canopy radiation and leaf area determined by direct measurement. Foliage photosynthetic capacity at different canopy positions was described by an experiential equation, and the leaf photosynthesis model is based on the mechanistic understanding of C₃ photosynthesis. The parameters of the leaf photosynthesis model, that is, the maximum carboxylation rate, potential rate of electron transport, the rate of triose phosphate utilisation and their CO₂ and temperature dependence, were determined using gas -exchange data of fully expanded, mature leaves.
In the whole canopy, about 64.51% of the leaves were 1.0-2.0 m from the canopy, 35.49% were at other distances from the canopy. No obvious difference in leaf area distribution was seen in the north-south orientation, and a basin foliage distribution was concentrated around the trunk. In the vertical section, the relative radiation decreased with increasing canopy depth. The rapid depression of the relative radiation was concentrated in the middle of the canopy where the leaf area density was highest. And more than 36% of the relative radiation was on the sides compared to near the trunk. The simulation showed that the 3-D distribution of P_n was similar to that of the relative radiation. When canopy height decreased from upper to bottom canopy, the relative radiation decreased by 89% from 71.18% to 8.05% and P_n decreased by 87% from 15.05 μmol·m^-2·s^-1 to 1.92 μmol·m^-2·s^-1 (PAR=1500 μmol·m^-2·s^-1). A flat step in the top canopy of the 3-D P_n distribution was observed, which decreased rapidly with the decreasing radiation. However, the 3-D distribution of the total photosynthetic rate in a unit cell appeared to mainly track the distribution of the leaf area and was partly correlated with the P_n. The 3-D distribution of G_s patterns was similar to that of P_n. And the distribution of LUE was reverse to P_n, which was more in bottom layer and less in upper layer. Satisfactory correspondence was achieved between measured and simulated values of P_n and G_s in different position of apple canopy, which was showed that the coupled model has the ability to predict P_n in a 3-D apple canopy. The main aim of fruit tree pruning is to remove useless shoots and leaves, and identification of these parts can be conveniently determined in a 3-D canopy plot. Overall, the coupled model performed well in predicting P_n for different leaf area distribution patterns.The 3-D distribution of P_n, G_s and LUE were determined by direct measurement of the 3-D distribution of radiation, leaf area and C₃ photosynthesis models in an apple orchard. The parameters of the model were maintained as simply as possible to facilitate their wide use in canopy photosynthesis simulations, especially for studying the effects of shape and pruning in fruit trees.