Plant Grafting in Agriculture and Horticulture
Investigating the Practical Applications of Vegetative Propagation
Unlike other methods of vegetative propagation, the cutting used as a graft does not regenerate new organs but becomes an integral part of the rooted plant. The rooted stem on which the graft is made is called the stock or rootstock. Roots may also be used as the stock.
The cutting that is attached to the stock is the scion. The entire upper portion of the stock, just above the root, may be removed and replaced by the scion, or the scion may be restricted to a branch or several branches and the major part of the plant remains the stock.
Any number of varieties may be grafted upon the same tree, providing the union is successful. Most of our fruit and nut tree, and many ornamentals, are produced by grafting or budding.
Different Methods of Grafting
Grafting is accomplished through budding, cleft grafting, or whip grafting.
Budding is a specialized form of grafting in which a shield-shaped piece of bark containing a single bud is used as the scion. This is inserted in a T-shaped cut in the bark of the stock. The budded site is wrapped tightly to hold scion and stock together.
Cleft grafting involves two small scions grafted onto one larger stock. The scions ends are cut to a wedge shape and forced down into a notch cut in the top of the stock.
Whip grafting uses a stock and scion of about the same sizes. The end of the scion and the top of the stock are cut at an angle and notched. The pieces are then united so their angles and notches match up. The cut surfaces of the grafts are covered with grafting wax to protect the tissues from drying and from fungus infection. Then, like budding, the grafted sites are tightly wrapped to hold scion and stock together.
Proper Positioning is Critical
When making a graft it is important that the cambium region (water and food conducting tubes) of the stock be closely associated with the cambium region of the scion. Under the protection of the wrapping and grafting wax, the cambium and sometimes other thin-walled cells of the inner bark, form a mass called a callus (a lump of tissue with no specific organs). Both stock and scion form callus tissue.
The callus tissues meet and fuse, and a new common cambium arises, uniting the cambium of the stock with that of the scion. The cambium now produces continuous layers of wood and bark, allowing translocation of food, water, and minerals to proceed without interruption between stock and scion.
Reasons for Grafting
- Speed hybrid breeding. In fruit tree breeding programs where hybrid seedlings may take ten years to fruit on their own roots, grafting can shorten the breeding program time.
- Dwarfing. The trees in most apple orchards are grafted dwarf or semi-dwarf trees. Smaller trees allow for more trees per acre, higher quality fruit, and greater ease of harvesting.
- Improve hardiness. Grafting can improve plant tolerance to heavy soils or extreme weather conditions.
- Repair tree trunks. Rodent or lawn mower damage to trees may be repaired by using bridge grafts.
- Maintain consistency of type. Apples can vary greatly genetically on the same tree. Grafting maintains consistent characteristics of each fruit on the entire tree.
- Topworking. To change an orchard to a more profitable cultivar (type) it is far cheaper to cut down all the trees and graft the new cultivar onto the old rootstocks (known as topworking) than it is to plant an entire new orchard.
- Create curiosities. Grafted cacti are popular with gardeners as is a grafted plant that produces tomatoes above ground and potatoes below ground. Strangely shaped trees produced by grafting are in demand as landscape plantings also.
Whatever the reason for the graft or the type of graft utilized, melding plants together through grafting allows us to fashion plants to our specifications.
A Plant Oddity Produced by Grafting
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