Effects of Size-Controlling Apple Rootstocks on Growth, Abscisic Acid, and Hydraulic Conductivity of Scion of Different Vigor

Size-controlling rootstocks are required to attain trees with reduced stature that are necessary for modern orchard management, such as high density plantings. Apple cultivars can be grafted on commercially-available size-controlling rootstocks but new rootstocks are needed for both size-controlling efficacy and improved tolerance of abiotic and biotic stress. This research was designed to determine the consistency of Geneva and Malling rootstocks for size-control with scions of different growth potentials. Hydraulic conductivity (HC) and abscisic acid (ABA) in xylem exudates were measured in selected scions grafted to Malling (from least-to-most size control M.7, M.9, and M.27) and Geneva (from least-to-most size control, G.935, G.5087, G.41, and G.11) rootstocks. Regardless of scion vigor, trees with G.935 and G.41 rootstocks had the most height and diameter growth and the highest HC, whereas trees with M.27 rootstocks had the least. Lower ABA was measured in xylem exudate of ‘Fuji’ grown on G.935 and G.41 than G.5087 and G.11 rootstocks. Regardless of scion vigor, tree growth was less when grown on G.11 and G.5087, which were associated with less HC and elevated ABA in ‘Fuji’ scion. Tree heights of the scion were similar when grown on G.11, G.5087, and M.7. It is proposed that trees grafted to G.11 and G.5087 may be better adapted to dry environments based upon their size and elevated ABA.