‘Grafting is a process by which two living parts are joined together in such a manner that they would unite together and subsequently grow into a composite plant.’ Usually, the graft has two parts, the scion and rootstock.
Scion refers to that part of a graft combination that becomes the top of the plant. Scion is the short piece of detached shoot containing 3 or more dormant buds, which when united with the rootstock or inter-stock, comprises the upper portion of the graft and from which will grow the stem or branches or both. It should be of the desired cultivar and free from diseases.
The rootstock is the lower portion of the graft, which develops into the root systems of the grafted plant. It may be a seedling, a rooted cutting, or a layered or micro-propagated plant.
Inter-stock is a piece of stem inserted by means of two graft unions between the scion and rootstock. Inter-stocks are used to avoid incompatibility between the rootstock and scion, to produce special tree forms, to control diseases, or to take advantage of its growth-controlling properties.
The vascular cambium is a thin tissue located between the bark and the wood. Its cells are meristematic i.e., they are capable of dividing and forming new cells. For successful graft union, the cambium of the scion is placed in close contact with the cambium of the rootstock.
A callus is a term applied to the mass of parenchymatic cells that develop from and around wounded plant tissues. It occurs at the junction of a graft union, arising from the living cells of both the scion and rootstock. The production and interlocking of these parenchymatic cells (callus) constitute one of the important steps in callus bridge formation between the scion and rootstock in a successful graft.
Reasons for grafting and budding
- Perpetuating clones that cannot be readily propagated by cutting, layers, division or other asexual methods
- Obtaining the benefits of certain rootstocks
- Obtaining the benefits of certain plants (double working)
- Changing cultivars of established plants (top working)
- Precocity in bearing
- Reduction in juvenility
- Obtaining special forms of plant growth
- Repairing the damaged parts of trees
- Study and elimination of viral diseases
- Studying the developmental and physiological processes
Elements for successful grafting
There are five important elements to any successful grafting operation. Which is as follows:
- The rootstock and scion should be compatible.
- The vascular cambium of the scion should be matched with the cambium of the rootstock.
- The grafting operation should be done at a time when the rootstock and scion are in the proper physiological stage.
- Immediately after completion of grafting, all cut surfaces should be protected from desiccation.
- Proper care should be given to the grafts for some period of time after grafting.
Types of Grafting
- Veneer grafting
- This method is used for grafting small potted plants such as seedlings of deciduous trees, shrubs, and fruit crops.
- A shallow downward and inward cut from 2.5-3.5 cm long is made.
- At the base of this cut, a second short inward and downward cut is made, inserting the first cut, so as to remove the piece of wood and bark.
- The scion is prepared with a long cut along one side and a very short one at the base of the scion on the opposite side.
- These scion cuts should be of the same length and width as those made in the rootstock so that the vascular cambium layers can be matched as closely as possible.
- After inserting the scion, the graft is tightly wrapped with polythene strips.
- After the union has healed, the rootstock can be cut back above the scion in gradual steps.
- This method is commercially used for raising mango and walnut plants.
- Splice/whip grafting
- In this method, it is essential that both the stock and scion should be of equal diameter.
- For this, about one-year-old rootstock is headed back at a height of 20-25 cm from the soil and a splice (diagonal) cut of 2.5 to 6 cm long is made at the distal end of the rootstock with the help of a sharp knife.
- A similar slanting cut is made on the proximal end of the scion. The cuts should be smooth and the cut surface of both the rootstock and scion are bound together and tied firmly with a polythene strip.
- After the union has taken place, the polythene tape is cut otherwise it restricts the growth at the point of union and such plants break due to wind. Ex. Walnut, apple, and pear.
- Tongue grafting
- It is a modified form of whip grafting. It differs from whip grafting that a reverse cut is made downward at a point about 1/3rd of the distance from the tip and should be about ½ the length of the first cut. To obtain a smooth-fitting graft, the cut should not split.
- The rootstock and scion are then inserted into each other, with the tongues interlocking. It is important that the vascular cambium layer match along at least one side, preferably along both sides.
- The lower tip of the scion should not overhang the stock as there is a likely hood of the formation of large callus knots.
- The use of scions larger than rootstock should be avoided. After the scion and rootstock are fitted together, they should be securely held by tying them with polythene tape.
- This method gives better success than whip grafting because of better cambial contact between stock and scion due to the formation of the tongue.
- Regular de-shooting of sprouts on stock is required to obtain better growth of the scion.
- Cleft grafting
- It is one of the oldest methods of field grafting. It is used to top work trees, either in the trunk of a small tree or in the scaffold branches of a bigger tree.
- In making the cleft graft, a heavy knife is used to make a vertical split for a distance of 5 to 8 cm down the center of the stub to be grafted. This split is made by pounding the knife in with a hammer.
- The branch is sawed off in such a way that the end of the stub is smooth and free from knots for at least 15 cm.
- In this method, two scions are inserted, one at each side of the stock where the vascular cambium layer is located. The scions should be 8 to 10 cm long and 10-15 mm in thickness and should have two or three buds.
- The side of the wedge, which is to go to the outer side of the rootstock should be slightly wider than the inside edge. When the knife is removed, a hard wooden wedge is inserted to keep it open for the subsequent insertion of the scion.
- The graft should be wrapped with a polyethylene strip properly to prevent wilting of the scion. The scion starts growing after 2 to 3 months of grafting. The right time for cleft grafting is the later part of the dormant season or just before the start of active growth.
- Pears, walnut, hazelnut, pecan nut, and grapes are propagated by this method.
- Wedge grafting (saw-kerf grafting)
- It is performed in late winter or early spring before the bark begins to slip.
- A sharp, heavy, short-bladed knife is used for making a V-wedge in the side of the stub or stock about 5cm long.
- Two cuts are made, coming together at the bottom and as far apart at the top as the width of the scion.
- These cuts extend about 2 cm deep into the side of the stub.
- The base of the scion is trimmed and a wedge shape cut exactly the same size and shape as the opening.
- With the two vascular cambium layers matching the scion is tapped downward firmly into place and slanting outward slightly at the top so that the vascular cambium layers cross.
- After all scions are firmly tapped into place, all cut surfaces including the tips of the scion, should be waxed thoroughly.
- It is called saw-kerf grafting because the cuts in the side of the rootstock can be made with a saw rather than a sharp knife.
- Bridge grafting:
- Bridge grafting is basically not a method of propagation but a form of repair grafting only in plants, which have been damaged either by frost, rodents or insects.
- It may, however, be kept in mind that bridge grafting is only helpful if the trunk is damaged but the root system of the plant is healthy.
- In this method, first, the damaged portion of the stock is cleaned and the irregular edges of the girdled area are cut evenly.
- In the healthy portion of rootstock, an incision is made on the top and bottom portions of the stock.
- The scion of desired variety is inserted in such a way that it is attached at both the upper and lower ends into the living bark. Similarly, it is important that scions should be inserted right side up to ensure the polarity.
- The exposed injured wood must also be covered otherwise; it may serve as an entry channel for decaying organisms.
- Softwood grafting
- This technique of grafting is commercially used for raising mango, sapota, tamarind, and cashew nut in western India. This is also known as In-situ Grafting.
- In this technique, grafting is done with mature, procured scion on the emerging soft, coppery-red shoot of the rootstock, which is 60-70 days old.
- The basic technique involves the beheading of the rootstock with a sharp knife.
- Then, a slit (2.5 – 4.0 cm) is made on the beheaded stock to insert the procured scion.
- The lower portion of a 10-15cm long scion is made in a wedge shape so that both the faces of the scion fit with the stock.
- Both stock and scion are tied with the help of a polyethylene strip.
- In about 3-4 weeks, sprouting starts, and grafts start growing.
- Sometimes the scion is then covered with a polythene bag (100 gauge) and tied with thread to keep the scion fresh till the complete union is formed.
- After sprouting, the bag is removed. To have better success, the leaves on the stock must be retained.
- Epicotyl grafting
- It is also called stone grafting. In this method, germinating seeds of less than two weeks old are wedge or splice grafted with mature scion.
- Moderate temperature and high relative humidity are major factors related to the success of epicotyl grafting.
- For epicotyl grafting, germinating seeds less than 2 weeks old are used as rootstock.
- The seedling is deheaded at a height of 10 cm from ground level.
- A 2-3 cm long slanting cut is made in the deheaded epicotyl at a height of 10 cm from ground level.
- Scion shoots 2 to 3 months age having pencil thickness is used. The leaves of the scion are defoliated 10 days before grafting to facilitate sprouting.
- A wedge-shaped cut, slanting from both sides is made on the lower side of the scion stick.
- The scion stick is then inserted into the saddle-like structure made on the rootstock and pressed properly so that the cambium tissues of the rootstock and scion stick firmly overlap each other.
- The union is then tied with the help of a 150 gauge-polyethylene strip.
- This technique has been commercialized for the rapid multiplication of mango in the Konkan area of Maharashtra.
- Inarching / Approach Grafting
- This is called an attached method of grafting.
- The characteristic features of approach grafting are that two independent, self-sustaining plants are grafted together.
- After the formation of a union, the top of the rootstock plant is removed above the graft union and the base of the scion plant is removed below the graft union.
- Generally, it is performed with one or both of the plants to be grafted, which are growing in a container. Rootstock plants in containers may also be placed adjoining an established plant, which is to furnish the scion part of the new grafted plant.
- The seeding of compatible species is planted around the tree during the dormant season and grafted when active growth commences in them during early spring.
- A thin slice of bark (6-10 cm in length) about 20 cm above the ground level is removed from the stock with a sharp knife. A similar cut is made in the scion. Thus, the cambium layers of both stock and scion are exposed. These cuts are brought together and tied firmly with the help of a polythene strip.
- After the successful union, stock above and scion below the graft union are lopped off gradually. In low rainfall areas, it should be done with the onset of rains, while in regions of heavy rainfall; it should be done soon after the rainy season is over, provided the temperature does not fall below 150
- Double Working
- In some cases, the scion cultivar fails to grow if grafted directly over the rootstock.
- To avert this situation, an interstock is used in between rootstock and scion.
- Care is taken that interstock is compatible with both rootstock and scion. The process of double working is accomplished in two successive years.
- During the first year, grafting of interstock is made on rootstock and during the second year grafting of scion cultivar. Is made on interstock.
- Double working is in practice chiefly in pear.
- Top Working
- It is the process of converting an undesirable plant into a desirable type.
- Usually, seedling stands of fruit plants are considered suitable for top working.
- To begin with the process, the plants are headed back within one-meter height from ground level during spring.
- Young plants with a trunk diameter of 2.5-20 cm are considered ideal for top working.
- The new shoot appears in response to heading back are selected and the vegetative propagation (Budding/grafting) method is resorted during June-July.
- While selecting a shoot, it should be taken into account that pencil-thick shoots fit well to undertake the propagation method.
- In sub-tropical regions, high solar radiation causes sunburn injury to the deheaded main stem. It is counter-checked by whitewashing the stem.
- Frame Working
- Grafting many small secondary scaffold branches high on the tree constitutes frame working.
- It requires the insertion of grafts throughout the main frame of the tree.
- A large number of scions is required to replace the small laterals and scaffold branches.
- Growth comes out of the previous frame of the tree and is removed from time to time to favour the establishment of the scions.
- Frame working is resorted using the budding/ grafting technique. The frame worked plants reward good fruiting. But, the technique being cumbersome and expensive, it is not practiced commercially in the orchard.