Grafting

Grafting is a method of asexual plant propagation widely used in agriculture and horticulture where the tissues of one plant are encouraged to fuse with those of another. It is most commonly used for the propagation of trees and shrubs grown commercially.

In most cases, one plant is selected for its roots, and this is called the stock or root stock. The other plant is selected for its stems, leaves, flowers, or fruits and is called the scion. The scion contains the desired genes to be duplicated in future production by the stock/scion plant.

In stem grafting, a common grafting method, a shoot of a selected, desired plant cultivar is grafted onto the stock of another type. In another common form called budding, a dormant side bud is grafted on the stem of another stock plant, and when it has fused successfully, it is encouraged to grow by cutting out the stem above the new bud.

For successful grafting to take place, the vascular cambium tissues of the stock and scion plants must be placed in contact with each other. Both tissues must be kept alive until the graft has taken, usually a period of a few weeks. Successful grafting only requires that a vascular connection takes place between the two tissues. A physical weak point often still occurs at the graft, because the structural tissue of the two distinct plants, such as wood may not fuse.

Reasons for grafting

Dwarfing: To induce dwarfing or cold tolerance or other characteristics to the scion. Most apple trees in modern orchards are grafted dwarf or semi-dwarf trees planted at high density. They provide more fruit per unit of land, higher quality fruit, and reduce the danger of accidents by harvest crews working on ladders.

Ease of propagation: Because the scion is difficult to propagate vegetatively by other means, such as by cuttings. In this case, cuttings of an easily rooted plant are used to provide a root stock. In some cases, the scion may be easily propagated, but grafting may still be used because it is commercially the most cost-effective way of raising a particular type of plant.

Hybrid breeding: To speed maturity of hybrids in fruit tree breeding programs. Hybrid seedlings may take ten or more years to flower and fruit on their own roots. Grafting can reduce the time to flowering and shorten the breeding program.

Hardiness: Because the scion has weak roots or the roots of the stock plants have roots tolerant of difficult conditions. e.g. many showy Western Australian plants are sensitive to dieback on heavy soils, common in urban gardens, and are grafted onto hardier eastern Australian relatives. Grevilleas and eucalypts are examples.

Sturdiness: In order to provide a strong, tall trunk for certain ornamental shrubs and trees. In these cases, a graft is made at a desired height on a stock plant with a strong stem. This is used to raise 'standard' roses, which are rose bushes on a high stem, and it is also used for some ornamental trees, such as certain weeping cherries.

Pollen source: To provide pollenizers. For example, in tightly planted or badly planned apple orchards of a single variety, limbs of crab apple may be grafted at regularly spaced intervals onto trees down rows, say every fourth tree. This takes care of pollen needs at blossom time, yet does not confuse pickers who might otherwise mix varieties while harvesting, as the mature crab apples are so distinct from other apple varieties.

Repair: To repair damage to the trunk of a tree which would prohibit nutrient flow, such as the stripping of the bark by rodents which completely girdles the trunk. In this case a bridge graft may be used to connect the tissues receiving flow from the roots to the tissues above the damage which have been severed from the flow. Where a water sprout, sucker or sapling of the same species is growing nearby, any of these can be grafted to the area above the damage by a method called in arch grafting. These alternatives to scions must be of the correct length to span the gap of the wound.

Changing cultivars: To change the cultivar in a fruit orchard to a more profitable cultivar, called top working. It may be faster to graft a new cultivar onto existing limbs of established trees than to replant an entire orchard.

Maintain Consistency: Apples are notorious for their genetic variability, even differing in multiple characteristics, such as, size, color, and flavor, of fruits located on the same tree. In the commercial farming industry, consistency is maintained by grafting a scion with desired fruit traits onto a hardy stock.

Curiosities

A practice sometimes carried out by gardeners is to graft related potatoes and tomatoes so that both are produced on the same plant, one above ground and one underground.

Cacti of widely different forms are sometimes grafted on to each other.

Multiple cultivars of fruits such as apples are sometimes grafted on a single tree. This so-called "family tree" provides more fruit variety for small spaces such as a suburban backyard, and also takes care of the need for pollenizers. The drawback is that the gardener must be sufficiently trained to prune them correctly, or one strong variety will usually "take over". Occasionally, a so-called "graft hybrid" or "chimaera" can occur where the tissues of the stock continue to grow within the scion. Such a plant can produce flowers and foliage typical of both plants as well as shoots intermediate between the two. The best-known example is probably +Laburnocytisus 'Adamii', a graft hybrid between laburnum and broom, which originated in a nursery near Paris, France in 1825. This small tree bears yellow flowers typical of Laburnum anagyroides, purple flowers typical of Chamaecytisus purpureus and curious coppery-pink flowers which show characteristics of both "parents".

Ornamental and functional, tree shaping uses grafting techniques to join separate trees or parts of the same tree to itself. Furniture, hearts, entry archways are examples. Axel Erlandson was a prolific tree shaper growing over 75 mature shaped and grafted trees.

Methods

Approach

Approach grafting or inarching is used to join plants that are otherwise difficult to join. The plants are grown close together, and then joined so that each plant has roots below and growth above the point of union. Both scion and stock retain their respective parents that may or may not be removed after joining. Also used in pleaching. The graft can be successfully accomplished any time of year.

Budding

T budding

Grafting with a single eye or bud. Normally performed at the height of the growing season by inserting a dormant bud into a shallow slice under the rind of the tree. The bud is sealed from drying and bound in place. There are many styles of budding depending on the cutting and fitting methods, the most popular being shield budding.

Other budding styles include the inverted T, patch budding, double shield, flute budding and chip budding.

Cleft

The most common form of grafting is cleft grafting. The stock is simply split and the scion is inserted. It is best if the stock is 2-7 cm in diameter and has 3-5 buds, and the cleft is around 7 cm deep. is cut in a wedge shape and inserted into the tree with the cambium. The bare stock is covered with grafting compound, otherwise the cambium layer quickly dries and the graft fails.

Stub

Stub grafting is a technique that requires less stock than cleft grafting, and retains the shape of a tree. Also scions are generally of 6-8 buds in this process.

An incision is made into the branch one centimetre long, then the scion is wedged and forced into the branch. The scion should be at an angle of at most 35° to the parent tree so that the crotch remains strong. The graft is covered with grafting compound.

Awl

Awl grafting takes the least resources and the least time. It is best done by an experienced grafter, as it is possible to accidentally drive the tool too far into the stock, reducing the scion's chance of survival. Awl grafting can be done by using a screwdriver to make a slit in the bark, not penetrating the cambium layer completely. Then inset the wedged scion into the incision.

Veneer

Veneer grafting, or inlay grafting, is a method used for stocks larger than three centimeters in diameter. The scion is recommended to be about as thick as a pencil. Clefts are made of the same size as the scion on the side of the branch, not on top. The scion end is shaped as a wedge, inserted, and wrapped with tape to the scaffolding branches to give it more strength.

Natural grafting

Tree branches and more often roots of the same species will sometimes naturally graft, this is called inosculation. When roots make physical contact with each other they often grow together. A group of trees can share water and mineral nutrients via root grafts, which may be advantageous to weaker trees, and may also form a larger root mass as an adaptation to promote fire resistance and regeneration as exemplified by the California Black Oak.

A problem with root grafts is that they allow transmission of certain pathogens, such as Dutch elm disease. Inosculation also sometimes occurs where two stems on the same tree, shrub or vine make contact with each other. This is common in plants such as strawberries and potatoes.

Scientific uses

Grafting has been important in flowering research. Leaves or shoots from plants induced to flower can be grafted onto uninduced plants and transmit a floral stimulus that induces them to flower

The transmission of plant viruses has been studied using grafting. Virus indexing involves grafting a symptom-less plant that is suspected of carrying a virus onto an indicator plant that is very susceptible to the virus.

Herbaceous grafting

Grafting is often done for non-woody plants such as a tomato, cucumber, eggplant and watermelon. The main advantage of grafting is for disease-resistant root stocks. In Japan there is an automated process using grafting robots.

History

Grafting with detached scions has been practised for thousands of years. It was in use by the Chinese before 2000 B.C. and by the first known citizens of Mesopotamia. The practice was almost commonplace in ancient Greece

Objectives of Vegetable Grafting

The main objective of grafting is to avoid diseases such as Fusarium wilt in Cucurbitaceae (Cucumber, melon etc.) and bacterial wilt in Solanaceae (tomato, pepper etc.).

Species and Varieties for Grafting

Inter-generic grafting is used in the production of many fruit-bearing vegetables, i.e. cucumber (Cucumis sativus L.) grafted on pumpkin (Cucurbita spp.), watermelon (Citrullus lanatus Matsum. et Nakai), on bottle gourd (Lagenaria siceraria Standl.), melon (Cucumis melo L.) on white gourd (also known as wax gourd) (Benicasa hispida Cogn.). Inter-specific grafting is generally applied to eggplant (Solanum melongena L.). Scarlet eggplant (S. integrifolium Poir.) and S. torvum Swartz are popular root stock for eggplant production. A large number of varieties for root stock have been bred and released for use by growers.

Grafting Methods for Different Types of Fruit-Bearing Vegetable

Tomato plants are mainly grafted by conventional cleft grafting. Tube grafting has recently been developed for vegetable seedlings grown by plug culture.

Cleft Grafting

Figure 1 Time Schedule of Cleft Grafting for Tomato Plants

Figure 2 Schematic Diagram of Cleft Grafting

The seeds of the root stock are sown five to seven days earlier than those of the scion (Fig. 1). The stem of the scion (at the fair-leaf stage), and the root stock (at the four to five-leaf stage) are cut at right angles, each with 2-3 leaves remaining on the stem (Fig. 2). The stem of the scion is cut in a wedge, and the tapered end fitted into a cleft cut in the end of the root stock. The graft is then held firm with a plastic clip.

Tube Grafting

Figure 3 Time Schedule for Tube Grafting of Tomato Plugs (128-Cell Tray).

Tube grafting makes it possible to graft small plants grown in plug trays two or three times faster than the conventional method. The smaller the plants, the more plants can be fitted into healing chambers or acclimation rooms. For this reason, tube grafting is popular among Japanese seedling producers.

The time schedule for tube grafting of tomato plugs is shown in Fig. 3. The optimum growth stage for grafting varies according to the kind of plug tray used. Plants in small cells must be grafted at an earlier growth stage, and require tubes with a smaller inside diameter.

· Figure 4 Schematic Diagram of Tube Grafting for Tomato Plugs on Plug Tray

First, the root stock is cut at a slant. The scion is cut in the same way. Elastic tubes with a side-slit are placed onto the cut end of the root stock. The cut ends of the scions are then inserted into the tube, splicing the cut surfaces of the scions and root stock together (Fig. 4).

Eggplant Cleft Grafting

Eggplant is grafted mainly by cleft or tube grafting. The growth rate differs according to the species of root stock used. The number of days from sowing to grafting varies accordingly.

Figure 5 Time Schedule of Cleft Grafting for Tomato Plants

Cleft grafting of eggplant is a similar process to that done for tomato. The time schedule for cleft grafting of eggplant is shown in

Fig. 5.

Eggplant Tube Grafting

The time schedule and grafting methods for tube grafting of cucumber are similar to those used for tomato plants. However, the seeds of S. torvum must be sown a few days earlier than those of the other root stock species.

Cucumber Tongue Approach Grafting

Figure 6 Time Schedule of Tongue Approach Grafting for Cucumber Plants

Figure 7

Figure 8 Slant-Cut Grafting for Cucumber Plants

The survival ratio of grafted Cucurbitaceae plants is higher if tongue approach grafting is used. This is because the root of the scion remains until the formation of the graft union. In this method, seeds of cucumber are sown 10 - 13 days before grafting, and pumpkin seeds 7 - 10 days before grafting, to ensure uniformity in the diameter of the hypocotyls of the scion and root stock (Fig. 6). The shoot apex of the root stock is removed so that the shoot cannot grow. The hypocotyls of the scion and root stock are cut in such a way that they tongue into each other (Fig. 7), and the graft is secured with a plastic clip. The hypocotyl of the scion is left to heal for 3 - 4 days and then crushed between the fingers. The hypocotyl is cut off with a razor blade three or four days after being crushed (Fig. 7).

Slant-cut grafting is easy to do, and has recently become popular. Stages at which the scions and root stock should be grafted are the same as those shown in Fig. 6. This grafting method was developed for robotic grafting. It is important to remove the 1st leaf and lateral buds when a cotyledon of root stock is cut on a slant (Fig. 8).

Planting Watermelon

Figure 9 Time Schedule of Cut Grafting for Watermelon

Figure 10 Schematic Diagram of Cut Grafting for Watermelon Plants.

Cut grafting is popular for watermelon. The time schedule for cut grafting varies according to the species of root stock used (Fig. 9). A schematic diagram of cut grafting is shown in Fig. 10.

Planting Melon

Figure 11 Time Schedule for Tongue Approach Grafting of Melon.

Melon plants are mainly grafted by tongue approach grafting. The time schedule for tongue approach grafting of melon plants is shown in Fig. 11. Tongue approach grafting for melon is similar to that used for cucumber plants, shown in Fig. 7.

Healing and Acclimatization

Figure 12 Structure of Acclimatization Tunnel and How to Acclimatize Grafted Plants.

Grafting should be carried out in a shady place sheltered from the wind, to avoid wilting of the grafted plants.

Grafted plants are usually healed and acclimated in a plastic tunnel (Fig. 12). The healing and acclimatization are very important for grafted plants to survive. The tunnel is covered with materials which provide shade and maintain inside humidity: silver/white cheese-cloth (outside) and transparent film (inside). During acclimatization, it is recommended to keep light levels at about 3 to 5 klx.

Before grafting:

Expose the scion and root stock to sunshine for two to three days;

Withhold water from the plants to avoid spindly growth, and

Make sure that the scions and root stock have stems of a similar diameter.

All these will improve the survival rate of grafted plants. When grafting is performed, it is important to increase the chances for vascular bundles of the scion and root stock to come into contact , by maximising the area of the cut surfaces that are spliced together, and by pressing the spliced cut surfaces together. The cut surfaces should not be allowed to dry out. After grafting, keeping the grafted plants at about 30°C and with more than 95% relative humidity for three days of healing promotes the survival ratio. Gradually, the relative humidity is then lowered and the light intensity increased. During healing and acclimatization, it is important to keep a constant air temperature in the tunnel, in order to maintain high humidity. If wilting is observed, foliar spraying of grafted plants with water is effective in helping them survive. The shading materials and films should be adjusted according to the daily weather, with more shade on a fine day.

Future Prospects

Grafting is extremely laborious and time-consuming, and growers are trying to reduce the labour input required. Attempts have been made to mechanize grafting operations since 1987. Tube grafting was developed as a manual operation for small plugs by Itagi et al. (1990), and reduced the time required for manual grafting by at least one-half. Morita (1988) and Oda and Nakajima (1992) have applied an adhesive and a hardener to support the graft union in several crops. With the adhesive, five tomato plugs at a two-leaf stage were grafted at the same time, using grafting plates . Grafting robots for plugs have also been developed, by combining the adhesive and grafting plates . This robot makes it possible for eight plugs of tomato, eggplant, or pepper to be grafted simultaneously. Robotic grafting is about ten times faster than conventional hand grafting. Tomato and eggplant grafted by robot produced a yield of fruit similar to that of plants grafted by conventional methods.

Healing has also been mechanized. The survival ratio is consistently high when the newly developed healing chambers are used. Healing chambers in which the environment is artificially controlled are now being used by many nurseries which produce grafted plugs.

As grafting operations and the healing of grafted plants become easier, grafted vegetable crops may become popular all over the world. Since plants gain disease tolerance and vigor by grafting, grafting of vegetables may be useful in the low-input, sustainable horticulture of the future.

Grafting and managing seedlings

This section illustrates how to graft tomato scions onto eggplant root stocks. Similar procedures are used to graft tomato scions onto tomato root stocks.