One of the factors to consider when choosing a rootstock is its influence on the ability to induce fruitfulness — defined by horticulturists as precocity. Horticulturists measure this trait in apple rootstocks by observing the length of time from planting to when the cultivar produces flowers. The trend is as the rootstock induces more dwarfing, the scion cultivar will flower earlier in the life of the orchard. For example, if a cultivar is grafted onto a dwarfing rootstock and a rootstock that induces more vigorous growth, the dwarfed tree may flower 5 or even more years earlier in the life of the tree.
How do rootstocks influence this trait? This is poorly understood. One hypothesis published by Rom et al. proposes that scions grafted onto dwarfing rootstocks terminate shoot growth earlier during the growing season, thus allowing for greater partitioning of assimilates to sites of floral primordia.
Emily E. Hoover, University of Minnesota