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A rootstock is a plant, sometimes just a stump, which already has an established, healthy root system, onto which a cutting or a bud from another plant is grafted. In some cases, such as vines of grapes and other berries, cuttings may be used for rootstocks, the roots being established in nursery conditions before planting them out. The plant part grafted onto the rootstock is usually called the scion. The scion is the plant that has the properties that propagator desires above ground, including the photosynthetic activity and the fruit or decorative properties. The rootstock is selected for its interaction with the soil, providing the roots and the stem to support the new plant, obtaining the necessary soil water and minerals, and resisting the relevant pests and diseases. After a few weeks the tissues of the two parts will have grown together, eventually forming a single plant. After some years it may be difficult to detect the site of the graft although the product always contains the components of two genetically different plants.
The use of rootstocks is most commonly associated with fruiting plants and trees, but is the only way to mass propagate many other types of plants that do not breed true from seed, or are particularly susceptible to disease when grown on their own roots.
A variety of rootstocks may be used for a single species or cultivar of scion because different rootstocks impart different properties, such as vigour, fruit size and precocity. Rootstocks also may be selected for traits such as resistance to drought, root pests, and diseases. Grapevines for commercial planting are most often grafted onto rootstocks to avoid damage by phylloxera, though vines available for sale to back garden viticulturists may not be.
The rootstock may be a different species from the scion, but as a rule it should be closely related, For example, many commercial pears are grown on quince rootstock. Grafting can also be done in stages; a closely related scion is grafted to the rootstock, and a less closely related scion is grafted to the first scion. Serial grafting of several scions may also be used to produce a tree that bears several different fruit cultivars, with the same rootstock taking up and distributes water and minerals to the whole system. Those with more than three varieties are known as 'family trees'.
When it is difficult to match a plant to the soil in a certain field or orchard, growers may graft a scion onto a rootstock that is compatible with the soil. It may then be convenient to plant a range of ungrafted rootstocks to see which suit the growing conditions best; the fruiting characteristics of the scion may be considered later, once the most successful rootstock has been identified. Rootstocks are studied extensively and often are sold with a complete guide to their ideal soil and climate. Growers determine the pH, mineral content, nematode population, salinity, water availability, pathogen load and sandiness of their particular soil, and select a rootstock which is matched to it. Genetic testing is increasingly common, and new cultivars of rootstock are always being developed.
AxR1 is a grape rootstock once widely used in California viticulture. Its name is an abbreviation for "Aramon Rupestris Ganzin No. 1", which in turn is based on its parentage: a cross (made by a French grape hybridizer named Ganzin) between Aramon, a Vitis vinifera cultivar, and Rupestris, an American grape species, Vitis rupestris - also used on its own as rootstock, "Rupestris St. George" or "St. George," referring to a town in the South of France, Saint Georges d'Orques, where it was popular.
It achieved a degree of notoriety in California when, after decades of recommendation as a preferred rootstock - despite repeated warnings from France and South Africa about its susceptibility (it had failed in Europe in the early 1900s), it ultimately succumbed to phylloxera in the 1980s, requiring the replanting of most of Napa and Sonoma, with disastrous financial consequences.
Most current day grape rootstocks were and are originally imported from Texas. These were taken from the native wild mustang grapes that grow across Texas. This rootstock also saved France's grape industry in the early 1900s when phylloxera decimated the wine and vineyards of Europe.
See also 
- Mudge K, Janick J, Scofield S, Goldschmidt EE (2009) A history of grafting. In: Janick J, editor. In Horticultural reviews. 35. Hoboken, NJ: John Wiley & Sons. pp. 437–493. www.hort.purdue.edu/newcrop/janick-papers/c09.pdf