Comparisons of physiological and anatomical characteristics between two cultivars in bi-leader apple trees (Malus × domestica Borkh.)

The present study examined the physiology and anatomy characteristic of two leading apple cultivars in Korea, Hongro (medium maturing) and Fuji (late maturing). Leaf morphology, physiology, and anatomy of the two cultivars were examined in bi-leader apple trees (Malus × domestica Borkh.) grafted on the M.9 rootstock and grown under well-irrigated field conditions. We found considerable differences in leaf and stem anatomical characteristics between the two cultivars. Compared to Fuji, Hongro was characterized by thicker palisade (PP) and spongy parenchyma (SP) and higher stomatal density (S_d) in leaves and significantly greater xylem vessel area, vessel density, and vessel diameter. Therefore, the sap flow volume was higher and the midday leaf water potential (Ψ_MD) increased significantly in Hongro when compared to that in Fuji. The higher photosynthetic rate (P_n) and stomatal conductance (G_s) in Hongro compared to those in Fuji were attributed to the higher specific leaf weight, higher contents of chlorophyll (a + b) and nitrogen, greater PP, and thicker leaves. In both cultivars, the seasonal P_n increased rapidly after leaf emergence, reaching its maximum level at 91 days after bud break (DABB), remained more or less stable with slight fluctuations until 146 DABB (late August), and decreased thereafter. The magnitude of the P_n difference between the two cultivars was greater (up to 2.72 μmol CO₂ m⁻² s⁻¹) before, but reduced (up to 1.47 μmol CO₂ m⁻² s⁻¹) after the harvest of Hongro. Hongro had significantly higher P_n values compared to Fuji at most of the measured light intensities. The two cultivars in bi-leader trees shared a single rootstock and thus had equal root distribution within the rhizosphere and access to the same soil nutrients, which ensured that the differences observed between the two cultivars reflected the different leaf and stem functional capacities. Hongro exhibited superior physiological traits, as indicated by its anatomical and morphological characteristics. The results presented herein add valuable insights into the physiology and anatomy of the two leading apple cultivars.