Definition - What does Root Stock mean?
Rootstock is the term applied to the host plant in a graft. When plants are grafted, a stem is cut from a donor plant, chosen for the fruit it will bear, the flowers it will host, or other reasons. That stem is then grafted onto a host plant, which is called stock, or rootstock.
The term rootstock applies to any host plant that will be used to support a stem graft. Contrary to the word 'root' appearing in the term 'rootstock', the term does not apply to actual plant roots or root zones.
MaximumYield explains Root Stock
The term rootstock originates from the world of grape growing, in which the rootstock of one vine would be paired with another grape varietal to overcome specific conditions that would otherwise prevent that varietal from being grown in that specific area.
Of course, grapes are only one example of rootstock and grafting. The process can be used with any number of plants, including annuals, perennials, trees, and more. It has become particularly popular with fruit tree growers, but is also practiced by gardeners striving to maximize their per-foot garden yield.
Rootstock is chosen for any number of reasons. One might be the age and development of the stock in question. For instance, an old, established grape vine has very deep roots that supply the entire vine with adequate moisture even in dry years when young vines would die. Grafting another vine onto existing rootstock in this situation would allow a younger vine to thrive.
Rootstock can be chosen for its resistance to pests and diseases, as well. For instance, one variety of grape vine might be resistant to particular types of fungi, mold or nematodes in a specific geographic area, but the grower might prefer a different grape varietal. Grafting allows the desired varietal to be grown in that area without worrying about susceptibility to pests or diseases.