Grafting
Grafting is a method of vegetative propagation in which a segment
(called scion) of a plant to be propagated is inserted onto another plant
(called stock) in such a way that their vascular tissues combine (Fig.).
Combination of vascular tissues of scion and stock allows growth of the
grafted segment, i.e., scion.
OR
Grafting is the act of placing a portion of one plant (bud or scion) into or
on a stem, root, or branch of another (stock) in such a way that a union
will be formed and the partners will continue to grow. The part of the
combination that provides the root is called the stock; the added piece
is called the scion. When more than two parts are involved, the middle
piece is called the interstock. Grafting is most widely used vegetative
propagation methods.
The principles involved in grafting are based on the matching of scion
and stock cambiums Grafting involves the joining together of plant
parts by means of tissue regeneration.
Uses
In modern horticulture grafting is used for a variety of purposes: to
repair injured trees, to produce dwarf trees and shrubs, to strengthen
plants’ resistance to certain diseases, to retain varietal characteristics,
to adapt varieties to adverse soil or climatic conditions, to ensure
pollination, to produce multifruited or multiflowered plants, and to
propagate certain species (such as hybrid roses) that can be propagated
in no other way.
Compatibility
In theory any two plants that are closely related botanically and that
have a continuous cambium can be grafted. Grafts between species are
often successful, between genera occasionally so, and grafts between
families are nearly always failures. Within the genus the closeness of
botanical relationship is not an infallible guide as to probable success,
but in the absence of recorded experience it is the best available. The
ability of two plants to continue to grow or be compatible when joined
together by the asexual practice of grafting is mediated by many
complex physiological and environmental factors.
The basic technique in grafting consists of placing cambial tissues of
stock and scion in intimate association, so that the resulting callus
tissue produced from stock and scion interlocks to form a living
continuous connection. Stock cambium and scion cambium respond to
being cut by forming masses of cells (callus tissues) that grow over the
injured surfaces of the wounds. The union resulting from interlocking of
the callus tissues is the basis of graftage. In dicots, cambium is usually
arranged in a continuous ring; in woody members new layers of tissue
are produced annually. Monocot stems do not possess a continuous
cambium layer so grafting is seldom possible.
The success or failure of any grafting operation is based upon the
compatibility of each plant part, closeness of fit, and cambial contact.
The union is initially held in place by pressure exerted by the stock, by
grafting tape, or by rubber budding strips applied over the point of
union. Warm temperatures (80–85 °F [27–30 °C]) increase callus
formation and improve “take” in grafting. Thus grafts using dormant
material are often stored in a warm moist place to stimulate callus
formation.
The establishment of union between grafted components is effected
through the formation of a loose growth of cells (callus) contributed by
both elements. These cells fuse into a mass so continuous in compatible
grafts that the precise location of the line of union is frequently
impossible to determine, even microscopically. Just as in wound
healing, union proceeds more rapidly if the wounded areas are
protected against drying out.
