Will2358, "rootstock" is a term used in the process of grafting. In grafting 2 or more plant parts in the creation of a grafted plant, rootstock refers to the lowest part of such a plant. It constitutes of the roots and the lower stem up to the point of meeting the scion, or interstem in a 3-part graft. In the process of grafting, the upper parts of the plant serving as the rootstock are sacrificed and removed. The scion portion of the completed graft determines most of the fruit characteristics, although the rootstock has a impact on hardiness, brix content, ripening, and other qualities influencing the fruit.
A plant technically becomes a rootstock only when used for grafting purposes.