The study of leaf photosynthesis in different canopy positions of fruit tree is very important, which can be applied to solving various theoretical and applied tasks, e.g. the study of canopy physiological dynamic, orchard density, pruning and fruit load control. Coupled model of net photosynthetic rate (P_n) of the apple leaves was presented which was developed on the basis of the biochemical model of C₃ photosynthesis and semi-mechanistic model of stomatal conductance. We used an experiential equation to describe the relationship of A_max between the top, mature, sunlit leaves, and others in the canopy. The sensitivity of P_n to microclimatic factors and leaf water potential (Ψ_l) in different RPAR was assessed systemically. The experiment was carried out in a ‘Fuji’ apple (Malus domestica Borkh. cv. ‘Fuji’) orchard from 2006 to 2008. The orchard is in the farm of The Ming Tombs (latitude 40°13'north, longitude 116°13'east, altitude 79m) in Changping county of Beijing.
The simulation showed that leaves P_n in different RPAR were highly sensitive to variations in CO₂ concentration at the leaf surface, and was a function of PAR. The optimum T_a of P_n was about 20-30℃ in different RPAR and shifted to a higher temperature as PAR or CO₂ increased. The influence of RH and Ψ_l on P_n occurred through the stomata, which closed with the decrease in RH and Ψ_l. Only slight effects of RH and Ψ_l on P_n were found in different RPAR. The results indicated that P_n decreased rapidly with the decrease in RPAR interception by the leaves. When the canopy radiation decreased from 3 m to 1 m, the average RPAR and maximal P_n of leaves decreased by 57.18% to 16.22% and by 16.65 to 4.24 μmol·m^-2·s^-1, respectively. The diurnal variations in P_n depended mostly on PAR, represented as double-peak curves when RPAR excessed 60%. During a whole day (24 h), a unit leaf area fixed 420 mmol of CO₂ on average weather data to upper leaves, and only 40 mmol of CO₂ when leaves RPAR was about 30%. When leaves RPAR decreased to 12% in apple canopy, the net photosynthetic rate in a whole day decreased to 0 with meteorological data. The area of canopy where total P_n is zero in growth season can be called useless radiation area. The main aim of tree pruning is to remove useless shoots and leaves, and identification of these parts can be conveniently determined by our model. In conclusion, the coupled model performed well in predicting P_n for leaves in different RPAR (or canopy position). And the model has simple input and output parameters and can be widely used as a module in the fruit simulation model.