Analysis of photosynthetic capacity of four apple canopy structures in China

Photosynthesis is the most important factor influencing the growth and development of fruit trees. Using field data to model the three-dimensional (3-D) distribution of photosynthesis in hedgerow tree crops in hourly time steps is critical for the accurately estimation of orchard yield and quality. Here in this study, a coupled model was used to simulate the 3-D distribution and diurnal variations of net photosynthetic rate of four canopy structures of apple trees. The four apple tree canopy structures were small and sparse canopy (APS-Ⅰ), dispersed stratified canopy (APS-Ⅱ), spindle canopy (APS-Ⅲ), and open-center canopy (APS-Ⅳ). The experiment was conducted in four "Fuji" apple (Malus domestica Borkh. cv. "Fuji") orchards during the 2010-2012 growth seasons. While the coupled model was based the 3-D distributions of canopy radiation and leaf area from direct field measurements, the leaf photosynthesis model was based on mechanistic knowledge of C3 plants. Experiential equations best described foliage photosynthetic capacity for different canopy positions. The results showed that 3-D distribution of leaf net photosynthetic rate (Pn) was similar to that of relative radiation. A flat curve was observed for canopy top of the 3-D Pn distribution, decreasing rapidly with decreasing photosynthetically active radiation (PAR). However, the 3-D distribution of total photosynthetic rate of a unit cell depended mainly on leaf area density pattern. Average Pn of the four apple tree canopy structures determined by the coupled model were 6.72 μmol·m^-2·s^-1 for APS-Ⅰ, 7.52 μmol·m^-2·s^-1 for APS-Ⅱ, 7.24 μmol·m^-2·s^-1 for APS-Ⅲ and 9.88 μmol·m^-2·s^-1 for APS-Ⅳ, and with canopy top PAR of 1 500 μmol·m ^-2·s^-1. The diurnal variation in Pn was largely driven by PAR, depicted in di-peak curves. Differences in canopy Pn were related to the differences in leaf area index (LAI) of the tree canopy. Under clear day conditions, total photosynthesis per unit ground area was 665.5 mmol·m^-2·d^-1 for APS-I, 791.7 mmol·m^-2·d^-1 for APS-Ⅱ, 752.6 mmol·m^-2·d^-1 for APS-Ⅲ and 601.1 mmol·m^-2·d^-1 for APS-Ⅳ. There was strong agreement between the measured and simulated Pn for the different tree canopy structures. This showed that the coupled model reliably predicted Pn for the different tree canopy structures. The results also showed that the open-center apple tree canopy enhanced fruit quality with bigger Pn while the other canopies enhanced yield with high canopy photosynthetic rate. The main aim of tree pruning was to remove useless shoots and leaves, which parts were easily identified by 3-D plots. Overall, the coupled model performed well in predicting instantaneous photosynthetic rates for different apple tree canopy structures in 3-D space.