Branches exhibit greater photosynthetic tolerance than leaves in dwarfing apple trees under water stress: a single-season study in Loess Plateau

Branch photosynthesis is vital for plant resilience to soil water stress and sustaining productivity. Understanding its response under different stress levels is key to assessing apple tree drought resistance and yield potential. This study investigated the photosynthetic response and interrelationships of annual branches (P branch-a ), perennial branches (P branch-p ), and leaves (P leaf ) under different water storage pit irrigation treatments. Diurnally, P branch-a exhibited a bimodal pattern with greater fluctuations and higher peaks than P branch-p , which followed a unimodal course. Seasonally, P branch-a and P branch-p declined with intensifying water stress during the sprout leaves and shoot growth stage. Conversely, during fruit expansion, increased Rubisco, xylem vessel diameter, and cortical thickness enhanced branch photosynthetic capacity. Branch photosynthesis demonstrated high temperature sensitivity. In contrast, P leaf was more sensitive to water availability, with water stress-induced reductions in tissue thickness, chlorophyll, and water potential decreasing P leaf . Although the diurnal pattern of P leaf resembled that of P branch-a , the daily mean of P branch-a accounted for only 7.58%–13.79% of P leaf . P branch-a /P leaf and P branch-p /P leaf stabilized at approximately 10% and 7% once soil water content reached 23%, the level maximizing leaf photosynthesis. Comparative analysis further identified that P branch-a /P leaf = 15% and P branch-p /P leaf = 12% represent critical thresholds for the functional trade-off in photosynthetic capacity between branches and leaves. Integrated analysis revealed chlorophyll, soluble sugar, and water potential as core regulators of the branch-leaf photosynthetic system, with Rubisco and starch increases linked to photosynthetic recovery. Moderate water deficit effectively promoted the coordinated development and dynamic functional balance of this system. These findings reveal the trade-offs in apple branch and leaf photosynthesis under water stress, offering insights for irrigation management and for understanding drought adaptation in fruit trees.