Comparisons of anatomical structure, component and photosynthetic capacity of leaves at different apple canopy positions

The work studied the differences in anatomical structure, component and photosynthetic capacity of leaves at different canopy positions of apple (Malus domestica Borkh. cv. ‘Fuji’) trees. Response of photosynthetic rate (P_n) of apple leaves to microclimatic factors was simulated by C₃ photosynthetic mechanism models. And the stomatal conductance was developed by using semi-mechanistic stomatal models. The results indicate that leaf thickness in the middle and upper canopy increases by 31.8% and 37.0% respectively compared to that in the lower canopy. Palisade thickness also increases by 44.8% and 62.7% respectively. Chlorophyll content in the middle and upper canopy increases by 18.0% and 20.6%, soluble sugar increases by 25.2% and 38.8% and proline increases by 11.7% and 29.0% respectively. Difference in photosynthetic capacity of leaves in different canopy position is similar to that in anatomical structure, component of leaves. Fluctuation in P_n of apple leaves is mainly caused by changes of photosynthetically active radiation, and highly sensitive to variations in carbon dioxide (CO₂) concentration at the leaf surface. From upper to lower canopy, daily P_n decreases by 67% (from 400 mmol·m^-2·d^-1 to 130 mmol·m^-2·d^-1 ) on clear-sky days and by 87% (from 170 mmol·m^-2·d^-1 to 22 mmol·m^-2·d^-1) on cloudy days. Maximal photosynthetic rate also decreases by an average of 67%.