Graft

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Graft

A graft is a horticultural term for a bud or shoot of one variety or species of plant that is positioned on the stem of another, compatible plant, in such a way that integrated growth results. The recipient plant is called the stock or rootstock, and the grafted part is referred to as the scion. A simple method for stem grafting involves both stems being cut with a sharp blade at the same acute angle , in order to maximize the area of contact. Then the stems are joined, and the union is bandaged with waterproof tape (or tape plus wax) until the wound has healed. Variations on this method involve complementary notches and the tongue being cut, according to how sturdy the scion is. Budding is the term applied when a bud with supporting tissue is grafted into a slit or notch cut into the stem of the stock.

Compatibility and incompatibility

The process of wound healing is absolutely necessary for successful grafting. Healing involves the cooperative production of new cells, some of which form cambium. From the cambium, new vascular (transport) tissues develop, permitting the transfer of water , nutrients , and hormones (growth regulators) to and from the scion. This interaction at the cellular level requires that the scion not be rejected by the stock. Hence, grafting is most likely to succeed with plants that are very closely related: either varieties of the same species, or members of the same genus. However, not all members of the same genus are compatible with each other. Sometimes the union can only be successful if one member is always the rootstock. For example, within the genus Prunus, peach scions cannot be grafted onto plum rootstocks, but plums can be grafted onto peach. Surprisingly, some pears (Pyrus species) can be grafted onto quince (Cydonia oblonga), despite the generic difference. Whether a particular combination is compatible can only be discovered by testing.

Advantages of grafting

Despite being labor intensive, grafting is commonly undertaken as a means of vegetative propagation of woody plants for any or all of the following reasons: (1) to impart disease resistance or hardiness, contributed by the rootstock; (2) to shorten the time taken to first production of flowers or fruits by the scion, in some cases by many years; (3) to dwarf the scion, making both its height and shape more convenient for harvesting fruit, as with apples; (4) to allow scion cultivars to retain their desirable leaf , floral, or fruit characters, without the risk of these being lost through sexual reproduction ; and (5) to provide the most economic use of scion material, in cases where there is some difficulty with stem cuttings producing roots.

History and important examples of grafting

The origin of grafting is uncertain. The peoples of ancient civilizations who grew fruit trees may have observed natural unions made by twigs and branches of compatible trees growing next to one another, and copied what had occurred through wind and abrasion. Grafting was applied routinely to apples and pears in England by the eighteenth century, and was utilized to great effect by the English plant breeder Thomas Andrew Knight. Thomas Jefferson wrote that he had "inoculated common cherry buds into stocks of large kind" in 1767, in a Garden Journal he kept for his residence Monticello in Virginia. Jefferson's record predates the work of Knight, and indicates that knowledge of grafting techniques was widespread at that time.

Disease resistance

The rescue of the European wine-grape (Vitis vinifera) industry from the ravages of Phylloxera disease depended on grafting European cultivars onto Phylloxera-resistant rootstocks of native American species: the northern fox grape (Vitis labrusca) and the southern muscadine (V. rotundifolia). Since 1960 another American species, V. champini, has been widely utilized to confer additional resistance to V. vinifera to root-knot nematode worms. This new rootstock also confers salt tolerance, and hence is particularly useful for sultana grapes grown under irrigation .

The practice of grafting onto disease-resistant stocks now extends even to annual plants like tomato. Disease-sensitive cultivars producing high quality fruit, such as Grosse Lisse, are grafted onto wilt and nematode resistant stocks of varieties that would themselves produce fruit deficient in flavor and nutrients.

Hardiness of citrus trees

Most cultivated citrus trees are propagated by grafting desirable types onto hardy rootstocks. For example popular lemon such as Eureka, which has few thorns, is grafted as a bud onto a thorny wild or rough lemon (all lemons are Citrus limon). For other types of citrus such as grapefruit and orange (C. sinensis), use of the rough lemon or sour orange (C. aurantium) as rootstock has been discontinued in favor of the wild orange (C. trifoliata). As a rootstock the latter species can tolerate wetter conditions than the other stocks, and its use does not diminish the quality of sweet oranges as rootstocks of rough lemon do.

Hardiness in flowering shrubs

Among cool-temperate ornamental flowering shrubs, the lilac (Syringa vulgaris) is often grafted onto privet (Ligustrum species), another example of rare, cross-generic compatibility. Rhododendrons, many of which have been deliberately bred for variants of flower size and color , are usually grafted onto a rootstock of Rhododendron ponticum. This species has pale purple flowers and is native from Spain and Portugal to Turkey. Rhododendron ponticum was the first rhododendron introduced to England in the mid-eighteenth century, and it is still the hardiest rootstock available, even surviving fires that destroy the above-ground scion.

Resources

books

Hartmann, H.T., et. al. Plant Science: Growth, Development and Utilization of Cultivated Plants. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1988.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

David R. Murray

KEY TERMS

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Cambium: —A layer of actively dividing cells, from which tissues used for conducting water and nutrients (xylem, phloem) are derived.
Graft incompatibility: —The failure of a scion to establish a viable connection with a rootstock, sometimes involving active rejection by release of toxins.
Hardiness: —The ability of a plant to withstand environmental stresses, such as extremes of temperature, low soil fertility, waterlogging, salinity, drought, ultraviolet light, or shade.
Hybrid: —A plant derived by crossing two distinct parents, which may be different species of the same genus, or varieties of the same species.
Phylloxera: —A fatal disease of grape vines caused by an infestation of the aphid Dactylasphaera vitifoliae in the roots.
Rootstock: —The basal component of a grafted plant.
Scion: —The upper or transferred component of a grafted plant.
Vegetative propagation: —A type of asexual reproduction in plants involving production of a new plant from the vegetative structures—stem, leaf, or root—of the parent plant.