Canopy Position Influences Cultivar Differences in Leaf Gas Exchange in Apple Trees

The canopy light environment creates contrasting microclimatic conditions that influence leaf physiological acclimation within fruit tree canopies. However, whether cultivar differences in leaf gas exchange are expressed similarly across canopy positions remains unclear. This study evaluated the influence of canopy position on leaf gas exchange in two commercial apple (Malus x domestica Borkh.) cultivars, ’Golden’ and ‘Braeburn’, under well-irrigated conditions during two growing seasons. Stomatal conductance (gs) and transpiration (E) were monitored on exterior (Ex) and interior (In) leaves of the canopy, while photosynthetic light-response curves characterized their physiological acclimation. The two seasons differed markedly in atmospheric evaporative demand, while soil water availability remained non-limiting. Exterior leaves showed significantly higher maximum photosynthetic rates, light saturation points, gs, and E than interior leaves in both cultivars. Golden exhibited a substantially higher Pmax in exterior leaves (22.89 ± 1.79) than in interior leaves (10.56 ± 1.56), whereas Pmax remained relatively similar between exterior (13.88 ± 1.63) and interior leaves (12.70 ± 0.33) of Braeburn. Braeburn also exhibited higher gs and E than Golden in interior leaves, particularly in 2022. Seasonal differences in cultivar responses were associated with contrasting atmospheric evaporative demand rather than soil water availability. Overall, the results of this study indicate that cultivar physiological performance cannot be fully characterized without considering canopy position and environmental context.