Sensitivity of Photosynthesis, Plant Hydraulic Traits, and Xylem Vessel Anatomy of Two Apple Cultivars Under Long-Term Drought Stress Conditions.

Understanding drought resistance during orchard establishment is increasingly urgent due to the rising frequency and severity of drought events driven by climate change. This study investigates the temporal responses of physiological and anatomical traits and their impact on hydraulic function in apple trees. We conducted a 75-day drought experiment using potted trees of two cultivars-Gamhong (medium maturing) and Fuji (late maturing)-grafted onto M.9 rootstock under control and drought conditions. Drought impacted morphological traits in both cultivars, with Gamhong showing a reduction in leaf length and Fuji in leaf width. An increase in leaf mass per area (LMA) in Gamhong indicated a shift towards structural reinforcement at the expense of photosynthetic efficiency. Temporal declines in net photosynthetic rate (P_n) and stomatal conductance (g_s) were more rapid and severe in Gamhong than in Fuji. The decline in g_s was linked with reductions in leaf water potential (Ψ_Leaf) and xylem sap flow, both more pronounced in Gamhong. A progressive decline in midday leaf water potential (Ψ_MD) was associated with increasing abscisic acid (ABA) levels, with Gamhong exhibiting higher ABA accumulation. Drought-induced changes in xylem vessel traits-vessel diameter, area, and density-were greater in Gamhong, resulting in reduced hydraulic conductivity and sap flow. Stomatal regulation emerged as a key mechanism for preserving xylem integrity, with Fuji maintaining more stable vessel structures and higher sap flow under prolonged drought. These findings highlight cultivar-specific drought strategies, suggesting Fuji is better suited for high-density orchards in drought-prone areas due to its greater hydraulic resilience and stomatal control.