Dwarfing rootstocks in apple have existed for more than a century, and their use revolutionized apple production worldwide. The most notable dwarfing rootstocks, ‘M9’ and ‘M27’, contain three quantitative trait loci (QTLs), Dw1 Dw2 and Dw3, which are the major genetic regulators for rootstock induced dwarfing in apples. Although genetically controlled, the dwarfing physiological mechanisms are still not fully understood, nor is there one main hypothesis. Several rootstock-scion relationships contribute to the dwarfing effect in apple rootstocks. These include modifications in: flowering genes, scion/rootstock vascular tissue anatomy and morphology, carbohydrate allocation, nutrient/water distribution, and hormonal regulation. Cumulatively, these genetic and physiological factors influence the architecture and mechanisms within the tree to facilitate optimal orchard designs and training systems to promote enhanced fruit quality. The history of dwarfing rootstocks in apple and the main hypotheses for the dwarfing mechanism are discussed herein. Future research will require a lot of coordinated effort to fully understand the dwarfing mechanism in apple and seek to control confounding variables in rootstock studies, such as nutrition and the crop load of the tree.
Keywords: Malus domestica, vigor, tree physiology, genetic control, growth modification