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Cherry tree, consolidated "V" graft
Tape has been used to bind the rootstock and scion at the graft, and tar to protect the scion from desiccation.
A grafted tree showing two differently coloured blossoms

Grafting or graftage[1] is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion (/ˈsaɪən/) while the lower part is called the rootstock. The success of this joining requires that the vascular tissues grow together. The natural equivalent of this process is inosculation. The technique is most commonly used in asexual propagation of commercially grown plants for the horticultural and agricultural trades. The scion is typically joined to the rootstock at the soil line; however, top work grafting may occur far above this line, leaving an understock consisting of the lower part of the trunk and the root system.

In most cases, the stock or rootstock is selected for its roots and the scion is selected for its stems, leaves, flowers, or fruits.[1] The scion contains the desired genes to be duplicated in future production by the grafted 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 bud grafting, a dormant side bud is grafted onto the stem of another stock plant, and when it has inosculated successfully, it is encouraged to grow by pruning off the stem of the stock plant just above the newly grafted 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 take place between the grafted tissues. Research conducted in Arabidopsis thaliana hypocotyls has shown that the connection of phloem takes place after three days of initial grafting, whereas the connection of xylem can take up to seven days.[2] Joints formed by grafting are not as strong as naturally formed joints, so a physical weak point often still occurs at the graft because only the newly formed tissues inosculate with each other. The existing structural tissue (or wood) of the stock plant does not fuse.

Advantages

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Graft particular to plum cherry. The scion is the largest in the plant, due to the imperfect union of the two. It can be seen on the enlarged trunk: this accumulation of starch is an indication of imperfection.

Precocity: The ability to induce fruitfulness without the need for completing the juvenile phase. Juvenility is the natural state through which a seedling plant must pass before it can become reproductive. In most fruiting trees, juvenility may last between 5 and 9 years, but in some tropical fruits, e.g., mangosteen, juvenility may be prolonged for up to 15 years. Grafting of mature scions onto rootstocks can result in fruiting in as little as two years.

Dwarfing: To induce dwarfing or cold tolerance or other characteristics to the scion. Most apple trees in modern orchards are grafted on to dwarf or semi-dwarf trees planted at high density. They provide more fruit per unit of land, of higher quality, and reduce the danger of accidents by harvest crews working on ladders. Care must be taken when planting dwarf or semi-dwarf trees. If such a tree is planted with the graft below the soil, then the scion portion can also grow roots and the tree will still grow to its standard size.

Ease of propagation:[3] 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 rootstock. 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 are tolerant of difficult conditions. e.g. many 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: 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.

Disease/pest resistance: In areas where soil-borne pests or pathogens would prevent the successful planting of the desired cultivar, the use of pest/disease tolerant rootstocks allow the production from the cultivar that would be otherwise unsuccessful. A major example is the use of rootstocks in combating Phylloxera.

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. This takes care of pollen needs at blossom time.

Repair:[3] To repair damage to the trunk of a tree that would prohibit nutrient flow, such as stripping of the bark by rodents that completely girdles the trunk. In this case a bridge graft may be used to connect tissues receiving flow from the roots to tissues above the damage that have been severed from the flow. Where a water sprout, basal shoot 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 inarch 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.

Genetic 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.

An example of approach grafting by Axel Erlandson.

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, creating a pomato.
  • 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." Multiple cultivars of different "stone fruits" (Prunus species) can be grafted on a single tree. This is called a fruit salad tree.
  • 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 who grew over 75 mature specimens.

Factors for successful graft

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Compatibility of scion and stock: Because grafting involves the joining of vascular tissues between the scion and rootstock, plants lacking vascular cambium, such as monocots, cannot normally be grafted. As a general rule, the closer two plants are genetically, the more likely the graft union will form. Genetically identical clones and intra-species plants have a high success rate for grafting. Grafting between species of the same genus is sometimes successful. Grafting has a low success rate when performed with plants in the same family but in different genera, and grafting between different families is rare.[4] Possibly the only bifamilial graft is that of Alluaudiopsis marnierana (Didiereaceae) onto Pereskia aculeata (Cactaceae).[5]

Cambium alignment and pressure: The vascular cambium of the scion and stock should be tightly pressed together and oriented in the direction of normal growth. Proper alignment and pressure encourages the tissues to join quickly, allowing nutrients and water to transfer from the stockroot to the scion.[6]: 466 

Completed during appropriate stage of plant: The grafting is completed at a time when the scion and stock are capable of producing callus and other wound-response tissues. Generally, grafting is performed when the scion is dormant, as premature budding can drain the grafting site of moisture before the grafting union is properly established. Temperature greatly affects the physiological stage of plants. If the temperature is too warm, premature budding may result. Elsewise, high temperatures can slow or halt callus formation.[4]

Proper care of graft site: After grafting, it is important to nurse the grafted plant back to health for a period of time. Various grafting tapes and waxes are used to protect the scion and stock from excessive water loss. Furthermore, depending on the type of graft, twine or string is used to add structural support to the grafting site. Sometimes it is necessary to prune the site, as the rootstock may produce shoots that inhibit the growth of the scion.[4]

Tools

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Depiction of general-purpose grafting knives

Cutting tools: It is a good procedure to keep the cutting tool sharp to minimize tissue damage and clean from dirt and other substances to avoid the spread of disease. A good knife for general grafting should have a blade and handle length of about 3 inches and 4 inches respectively. Specialized knives for grafting include bud-grafting knives, surgical knives, and pruning knives. Cleavers, chisels, and saws are utilized when the stock is too large to be cut otherwise.

Disinfecting tools: Treating the cutting tools with disinfectants ensures the grafting site is clear of pathogens. The most commonly used sterilizing agent is alcohol.

Graft seals: Keeps the grafting site hydrated. Good seals should be tight enough to retain moisture while, at the same time, loose enough to accommodate plant growth. Includes specialized types of clay, wax, petroleum jelly, and adhesive tape.

Tying and support materials: Adds support and pressure to the grafting site to hold the stock and scion together before the tissues join, which is especially important in herbaceous grafting. The employed material is often dampened before use to help protect the site from desiccation. Support equipment includes strips made from various substances, twine, nails, and splints.[7]

Grafting machines: Because grafting can take a lot of time and skill, grafting machines have been created. Automation is particularly popular for seedling grafting in countries such as Japan and Korea where farming land is both limited and used intensively. Certain machines can graft 800 seedlings /hr.[6]: 496 

Techniques

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Grafting process (Uzbekistan)

Approach

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Approach grafting or inarching is used to join together 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.[8] 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.[9]

Bud

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T budding

Bud grafting (also called chip budding or shield budding) uses a bud instead of a twig.[10] Grafting roses is the most common example of bud grafting. In this method a bud is removed from the parent plant, and the base of the bud is inserted beneath the bark of the stem of the stock plant from which the rest of the shoot has been cut. Any extra bud that starts growing from the stem of the stock plant is removed. Examples: roses and fruit trees like peaches.

Budwood is a stick with several buds on it that can be cut out and used for bud grafting. It is a common method of propagation for citrus trees.[11][12][13]

Cleft

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Successful cleft graft after 2 years' growth
Same graft after 4 years' growth

In cleft grafting a small cut is made in the stock and then the pointed end of the scion is inserted in the stock. This is best done in the early spring and is useful for joining a thin scion about 1 cm (3⁄8 in) diameter to a thicker branch or stock. It is best if the former has 3–5 buds and the latter is 2–7 cm (3⁄4–2+3⁄4 in) in diameter. The branch or stock should be split carefully down the middle to form a cleft about 3 cm (1+1⁄8 in) deep. If it is a branch that is not vertical then the cleft should be cut horizontally. The end of the scion should be cut cleanly to a long shallow wedge, preferably with a single cut for each wedge surface, and not whittled. A third cut may be made across the end of the wedge to make it straight across.

Slide the wedge into the cleft so that it is at the edge of the stock and the centre of the wedge faces are against the cambium layer between the bark and the wood. It is preferable if a second scion is inserted in a similar way into the other side of the cleft. This helps to seal off the cleft. Tape around the top of the stock to hold the scion in place and cover with grafting wax or sealing compound. This stops the cambium layers from drying out and also prevents the ingress of water into the cleft.

Whip

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Graft needing attention the following season
Successful whip graft

In whip grafting the scion and the stock are cut slanting and then joined. The grafted point is then bound with tape and covered with a soft sealant to prevent dehydration and infection by germs. The common variation is a whip and tongue graft, which is considered the most difficult to master but has the highest rate of success as it offers the most cambium contact between the scion and the stock. It is the most common graft used in preparing commercial fruit trees. It is generally used with stock less than 1.25 cm (1⁄2 in) diameter, with the ideal diameter closer to 1 cm (3⁄8 in) and the scion should be of roughly the same diameter as the stock.

The stock is cut through on one side only at a shallow angle with a sharp knife. (If the stock is a branch and not the main trunk of the rootstock then the cut surface should face outward from the centre of the tree.) The scion is similarly sliced through at an equal angle starting just below a bud, so that the bud is at the top of the cut and on the other side than the cut face.

In the whip and tongue variation, a notch is cut downwards into the sliced face of the stock and a similar cut upwards into the face of the scion cut. These act as the tongues and it requires some skill to make the cuts so that the scion and the stock marry up neatly. The elongated "Z" shape adds strength, removing the need for a companion rod in the first season (see illustration).

The joint is then taped around and treated with tree-sealing compound or grafting wax. A whip graft without a tongue is less stable and may need added support.

Stub

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Successful stub graft, healed

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 1 cm (3⁄8 in) 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.

After the graft has taken, the branch is removed and treated a few centimeters above the graft, to be fully removed when the graft is strong.

Four-flap

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The four-flap graft (also called banana graft) is commonly used for pecans, and first became popular with this species in Oklahoma in 1975.[14] It is heralded for maximum cambium overlap, but is a complex graft. It requires similarly sized diameters for the rootstock and scion. The bark of the rootstock is sliced and peeled back in four flaps, and the hardwood is removed, looking somewhat like a peeled banana. It is a difficult graft to learn.

Awl

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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

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Veneer grafting, or inlay grafting, is a method used for stock larger than 2.5 centimetres (1 in) in diameter.[15] 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.

Rind (also called bark)

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Rind grafting involves grafting a small scion onto the end of a thick stock. The thick stock is sawn off, and a ~4 cm long bark-deep cut is made parallel to the stock, from the sawn-off end down, and the bark is separated from the wood on one or both sides. The scion is shaped as a wedge, exposing cambium on both sides, and is pushed in under the back of the stock, with a flat side against the wood.

Natural grafting

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A "husband and wife tree" blackthorn, (Prunus spinosa), the result of natural grafting

Tree branches and more often roots of the same species will sometimes naturally graft; this is called inosculation. The bark of the tree may be stripped away when the roots make physical contact with each other, exposing the vascular cambium and allowing the roots to graft 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 rootmass as an adaptation to promote fire resistance and regeneration as exemplified by the California black oak (Quercus kelloggii).[16] Additionally, grafting may protect the group from wind damages as a result of the increased mechanical stability provided by the grafting.[17] Albino redwoods use root grafting as a form of plant parasitism of normal redwoods.

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 potato.

Natural grafting is rarely seen in herbaceous plants as those types of plants generally have short-lived roots with little to no secondary growth in the vascular cambium.[17]

Graft chimera

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Occasionally, a so-called "graft hybrid" or more accurately graft chimera 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 this is probably +Laburnocytisus 'Adamii', a graft hybrid between Laburnum and Cytisus, which originated in a nursery near Paris, France, in 1825. This small tree bears yellow flowers typical of Laburnum anagyroides, purple flowers typical of Cytisus purpureus and curious coppery-pink flowers that show characteristics of both "parents". Many species of cactus can also produce graft chimeras under the right conditions although they are often created unintentionally and such results are often hard to replicate.

Communication between parts

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For a graft to be successful in the first place, the rootstock and the scion must coordinate the growth of new tissue at the graft interface. Cells from the two parts form plasmodesmata to link with each other.[18]

Leaves or shoots from plants induced to flower can be grafted onto uninduced plants and transmit a floral stimulus that induces them to flower.[19]

The flowering induction is one of the examples of long-distance communication in vascular plants. This kind of communication still exists in grafts across two genetically different parts. The signals crossing the graft interface include simple molecules like jamonate, small peptides, and RNA molecules like mRNA and miRNA.[20][21]

Grafting can even transfer chloroplasts (plant organelles that can conduct photosynthesis), mitochondrial DNA and the entire cell nucleus containing the genome, making grafting a possible form of natural genetic engineering.[22] Sometimes the exchanged material result in heritable changes, even a new fertile allopolyploid species: this is called a graft hybrid.[23]

Scientific uses

[edit]

Grafting has been important in flowering research using the aforementioned property of flowering stimulation.[19]

Grafting is also used to understand the long-distance communication system in vascular plants, as the genetically differences between the rootstock and the scion allow researchers to identify the origin of information-carrying molecules.[20]

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

Examples

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White Spruce

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White spruce can be grafted with consistent success by using 8–10 cm (3–4 in) scions of current growth on thrifty 4- to 5-year-old rootstock (Nienstaedt and Teich 1972).[24] Before greenhouse grafting, rootstocks should be potted in late spring, allowed to make seasonal growth, then subjected to a period of chilling outdoors, or for about 8 weeks in a cool room at 2 °C (Nienstaedt 1966).[25]

A method of grafting white spruce of seed-bearing age during the time of seed harvest in the fall was developed by Nienstaedt et al. (1958).[26] Scions of white spruce of 2 ages of wood from 30- to 60-year-old trees were collected in the fall and grafted by 3 methods on potted stock to which different day-length treatments had been applied prior to grafting. The grafted stock were given long-day and natural-day treatments. Survival was 70% to 100% and showed effects of rootstock and post-grafting treatments in only a few cases. Photoperiod and temperature treatments after grafting, however, had considerable effect on scion activity and total growth. The best post-grafting treatment was 4 weeks of long-day treatment followed by 2 weeks of short-day treatment, then 8 weeks of chilling, and finally long-day treatment.

Since grafts of white spruce put on relatively little growth in the 2 years after grafting, techniques for accelerating the early growth were studied by Greenwood (1988)[27] and others. The cultural regimes used to promote one additional growth cycle in one year involve manipulation of day length and the use of cold storage to satisfy chilling requirements. Greenwood took dormant potted grafts into the greenhouse in early January then gradually raised the temperature during the course of a week until the minimum temperature rose to 15 °C. Photoperiod was increased to 18 hours using incandescent lighting. In this technique, grafts are grown until elongation has been completed, normally by mid-March. Soluble 10-52-10 fertilizer is applied at both ends of the growth cycle and 20-20-20 during the cycle, with irrigation as needed. When growth elongation is complete, day length is reduced to 8 hours using a blackout curtain. Budset follows, and the grafts are held in the greenhouse until mid-May. Grafts are then moved into a cooler at 4 °C for 1000 hours, after which they are moved to a shade frame where they grow normally, with applications of fertilizer and irrigation as in the first cycle. Grafts are moved into cold frames or unheated greenhouse in September until January. Flower induction treatments are begun on grafts that have reached a minimum length of 1.0 m. Repotting from an initial pot size of 4.5 litre to 16 litre containers with a 2:1:1 soil mix of peat moss, loam, and aggregate.

In one of the first accelerated growth experiments, white spruce grafts made in January and February that would normally elongate shortly after grafting, set bud, and remain in that condition until the following spring, were refrigerated for 500, 1000, or 1500 hours beginning in mid-July, and a non-refrigerated control was held in the nursery.[27] After completion of the cold treatment, the grafts were moved into the greenhouse with an 18-hour photoperiod until late October. Height increment was significantly (P 0.01) influenced by cold treatment. Best results were given by the 1000-hour treatment.[27]

The refrigeration (cold treatment) phase was subsequently shown to be effective when applied 2 months earlier with proper handling and use of blackout curtains, which allows the second growth cycle to be completed in time to satisfy dormancy requirements before January (Greenwood et al. 1988).[27]

Herbaceous grafting

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Grafting is often done for non-woody and vegetable plants (tomato, cucumber, eggplant and watermelon).[28] Tomato grafting is very popular in Asia and Europe, and is gaining popularity in the United States. The main advantage of grafting is for disease-resistant rootstocks. Researchers in Japan developed automated processes using grafting robots as early as 1987.[29][30][31] Plastic tubing can be used to prevent desiccation and support the healing at the graft/scion interface.[32]

History, society and culture

[edit]

Fertile Crescent

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As humans began to domesticate plants and animals, horticultural techniques that could reliably propagate the desired qualities of long-lived woody plants needed to be developed. Although grafting is not specifically mentioned in the Hebrew Bible, it is claimed that ancient Biblical text hints at the practice of grafting. For example, Leviticus 19:19 states "[the Hebrew people] shalt not sow their field with mingled seed" (King James Bible). Some scholars believe the phrase mingled seeds includes grafting,[citation needed] although this interpretation remains contentious among scholars.

Grafting is also mentioned in the New Testament. In Romans 11, starting at verse 17, there is a discussion about the grafting of wild olive trees concerning the relationship between Jews and Gentiles.[33][34]

By 500 BCE grafting was well established and practiced in the region as the Mishna describes grafting as a commonplace technique used to grow grapevines.[35]

China

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Evidence for grafting in China is found in Jia Sixie's 6th century CE agricultural treatise Qimin Yaoshu (Essential Skills for the Common People).[36] It discusses grafting pear twigs onto crab apple, jujube and pomegranate stock (domesticated apples had not yet arrived in China), as well as grafting persimmons. The Qimin yaoshu refers to older texts that referred to grafting, but those works are missing.[citation needed]

Greece, Rome, and the Middle East

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In Greece, a medical record written in 424 BCE contains the first direct reference to grafting. The title of the work is On the Nature of the Child and is thought to be written by a follower of Hippocrates. The language of the author suggests that grafting appeared centuries before this period.

In Rome, Cato the Elder wrote the oldest surviving Latin text in 160 BCE: the De Agri Cultura, and provided extensive instruction about several grafting methods.[37] Other authors in the region would write about grafting in the following years, however, the publications often featured fallacious scion-stock combinations.[citation needed]

Creating lavishly flourished gardens would be a common form of competition among medieval Islamic leaders at the time. Because the region would receive an influx of foreign ornamentals to decorate these gardens, grafting was used much during this period.[35]

Europe and the United States

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Possible deliberate grafts on a sessile oak in Ayrshire, Scotland

After the fall of the Roman Empire, grafting kept being practiced in Christian monasteries and regained popular appeal among lay people during the Renaissance. The invention of the printing press inspired a number of authors to publish books on gardening that included information on grafting. One example, A New Orchard and Garden: Or, the Best Way for Planting, Graffing, and to Make Any Ground Good for a Rich Orchard, Particularly in the North, was written by William Lawson in 1618.

While grafting continued to grow in Europe during the eighteenth century, it was considered unnecessary in the United States as the produce from fruit trees was largely used either to make cider or feed hogs.[35]

French Wine Pandemic

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Beginning in 1864, and without warning, grapevines across France began to sharply decline. Thanks to the efforts of scientists such as C. V. Riley and J. E. Planchon, the culprit was identified to be phylloxera, an insect that infests the roots of vines and causes fungal infections. Initially, farmers unsuccessfully attempted to contain the pest by removing and burning affected vines. When it was discovered that phylloxera was an invasive species introduced from North America, some suggested importing rootstock from the region as the North American vines were resistant to the pest. Others, opposed to the idea, argued that American rootstocks would imbue the French grapes with an undesirable taste; they instead preferred to inject the soil with expensive pesticides. Ultimately, grafting French vines onto American rootstocks became prevalent throughout the region, creating new grafting techniques and machines. American rootstocks had trouble adapting to the high soil pH value of some regions in France so the final solution to the pandemic was to hybridize the American and French variants.[35]

Cultivated plants propagated by grafting

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Apple – grafting
Avocado – grafting
Conifer - stem cuttings, grafting
Citrus (lemon, orange, grapefruit, Tangerine, dayap) – grafting
Grapes – stem cuttings, grafting, aerial layering
Kumquat – stem cutting, grafting
Mango- grafting, budding
Maple – stem cuttings, grafting
Nut crops (walnut, pecan) – grafting
Peach – grafting
Pear – grafting
Rubber Plant - bud grafting
Rose - grafting

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Grafting

View on Grokipedia
from Grokipedia
Grafting is a horticultural technique in which the tissues of one plant, known as the scion, are joined to the root system or stem of another plant, called the rootstock, allowing them to unite and grow as a single organism with distinct genetic contributions from each part.[1] This process relies on the cambium layers of both components aligning to form a vascular connection, enabling nutrient and water transport across the union.[2] Originating over 2,000 years ago, grafting has been documented in ancient practices, such as the cultivation of olives in the Mediterranean region, and has since become a cornerstone of plant propagation worldwide.[1] It is employed to perpetuate cultivars that root poorly from cuttings, combine beneficial traits like disease resistance or stress tolerance from the rootstock with desirable fruit or flower qualities from the scion, and produce dwarfed trees for efficient orchard management.[3] In modern agriculture, grafting enhances economic outcomes by improving nutrient uptake efficiency, yield, and resilience to environmental challenges, particularly in fruit trees and, more recently, vegetable crops.[4] Common grafting methods include whip-and-tongue grafting for dormant stems of similar diameter, cleft grafting for attaching multiple scions to a larger rootstock, and budding techniques like T-budding, which insert a single bud during active growth.[1] These approaches are selected based on plant species, season, and goals, with success depending on compatibility between scion and rootstock to prevent rejection or poor union formation.[3] Beyond traditional uses in woody perennials, grafting has expanded to annual vegetables to combat soil-borne pathogens and optimize resource use in intensive farming systems.[4]

Fundamentals

Definition and Principles

Grafting is a horticultural technique that involves the artificial fusion of tissues from two genetically distinct plants to create a single, functional organism. In this process, a scion—typically a cutting from the upper portion of a desired plant that contributes traits such as fruit quality, flower characteristics, or growth habit—is joined to a rootstock, which is the lower portion providing the root system and often imparting benefits like enhanced vigor, adaptation to specific soils, or resistance to diseases and pests.[5][1][6] The success of grafting relies on several fundamental principles, beginning with the precise alignment of the vascular cambium layers—the thin, meristematic tissue responsible for secondary growth in woody and herbaceous plants—from both the scion and rootstock. This alignment ensures that the conducting tissues can eventually interconnect, allowing for the transport of water, nutrients, and photosynthates between the joined parts. Following alignment, a wound-healing response is triggered, leading to the proliferation of callus tissue, an undifferentiated mass of parenchyma cells that forms a bridge across the graft interface and stabilizes the union.[7][5][8] Ultimately, vascular reconnection occurs as the callus bridge facilitates the differentiation of new xylem and phloem tissues, restoring continuity in the plant's vascular system. A key biological mechanism in this reconnection is the dedifferentiation of parenchyma cells within the callus, which revert to a totipotent state capable of cell division and subsequent redifferentiation into specialized vascular elements, including new cambium. This process, driven by hormonal signals and cellular plasticity, enables the grafted plant to function as a unified entity rather than two separate organisms.[7][1][9]

Compatibility and Physiology

Graft compatibility in plants is primarily determined by phylogenetic relatedness, with success rates decreasing as genetic distance increases. Intraspecific grafts, involving scions and rootstocks of the same species, are almost always compatible due to shared genetic and physiological traits that facilitate union formation.[10] Interspecific grafts, between different species within the same genus, often succeed when the species are closely related; for instance, tomato (Solanum lycopersicum) grafted onto potato (Solanum tuberosum) forms a functional union, enabling nutrient exchange despite minor physiological differences.[11] Intergeneric grafts, across different genera, are less common and typically limited to families with high compatibility, such as Rosaceae, where examples include successful unions between genera like Malus and Pyrus, though these require precise alignment of vascular tissues.[12] Beyond the family level, grafts between plants from different botanical families are generally incompatible and rarely form long-term, functional unions. The phylogenetic distance leads to physiological mismatches, disrupted hormonal signaling, and failure in vascular reconnection, often resulting in graft rejection, necrosis at the union, stunted growth, or complete failure months after initial healing attempts. A notable example is attempts to graft cannabis (Cannabis sativa, family Cannabaceae, related to hops) as scion or rootstock with tomato (Solanum lycopersicum, family Solanaceae, nightshades including potato and pepper). Despite occasional short-term nutrient sharing in experimental settings, these inter-family grafts almost always fail long-term due to genetic and physiological incompatibility. Online myths and forum discussions (e.g., claims of "camouflage" plants or THC-infused tomatoes) are unfounded; no reliable transfer of specialized metabolites like cannabinoids occurs across the graft union in incompatible combinations, as documented in grower reports and limited studies. This contrasts with successful interspecific grafts within the same genus, such as tomato on potato (pomato), where close relatedness enables stable unions and dual cropping. Recent research has explored intra-specific grafting within cannabis itself, using vigorous chemotypes as rootstocks to enhance scion vigor, root biomass, and yield without altering cannabinoid profiles significantly, showing potential for cannabis propagation similar to established vegetable grafting practices. Physiologically, successful grafting relies on coordinated wound healing and vascular reconnection at the graft interface. Upon wounding, auxin signaling, primarily from indole-3-acetic acid (IAA), promotes cell division and callus proliferation, bridging the scion and rootstock to initiate union formation.[13] Enzyme activities, including peroxidases and polyphenol oxidases, facilitate lignification and cell wall remodeling during this phase, strengthening the graft site against mechanical stress.[14] Over time, symplastic continuity is established through the formation of plasmodesmata, specialized channels that allow intercellular transport of solutes, hormones, and signaling molecules, ensuring long-term integration of the vascular systems.[15] Genetically, compatibility involves interactions at loci that regulate recognition and fusion, with epigenetic modifications playing a key role in stabilizing unions. Epigenetic changes, such as alterations in DNA methylation patterns and histone modifications, occur at the graft junction and can propagate systemically, influencing gene expression for vascular development and stress response without altering the underlying DNA sequence.[16] Genes analogous to histocompatibility factors in animals, including those in self-incompatibility pathways, contribute by preventing rejection-like responses.[17] Barriers to graft success manifest as incompatibility, often resulting in failed unions due to disrupted physiological or genetic harmony. Common symptoms include localized necrosis at the graft site, where tissue death occurs from oxidative stress and poor nutrient flow, and inadequate vascular connections that lead to stunted growth or scion wilting.[11] These issues stem from mechanisms such as mismatched self-incompatibility loci, which trigger defensive responses like reactive oxygen species accumulation, blocking callus fusion and symplastic links.[18] In severe cases, incompatibility may not appear immediately but emerge years later as declining vigor from incomplete phloem or xylem integration.

Purposes and Benefits

Advantages Over Other Propagation Methods

Grafting offers distinct advantages over other propagation methods such as seed sowing, cuttings, and layering, particularly in horticultural crops where maintaining specific traits and enhancing performance are critical. Unlike seed propagation, which introduces genetic variability and lengthy juvenile periods, or cuttings and layering, which rely on the inherent qualities of a single plant part, grafting combines the desirable attributes of a scion (the upper portion) with a rootstock (the lower portion), enabling targeted improvements in resilience and productivity. This method is especially valuable for perennial crops, where compatibility between scion and rootstock ensures successful union and trait expression. One primary benefit is superior disease resistance, achieved by grafting susceptible scions onto resistant rootstocks, thereby avoiding soil-borne pathogens that affect seed-grown or cutting-propagated plants. For instance, rootstocks can provide barriers against fungal diseases like Fusarium wilt and Verticillium wilt, which are challenging to manage through other means without chemical interventions. This approach reduces infection risks without altering the scion's genetic makeup, offering a sustainable alternative to replanting entire fields.[19][20][21] Grafting also accelerates fruiting and ensures uniform traits in perennial crops, significantly shortening the time from propagation to harvest compared to seeds, which can take 5–10 years to mature. By using mature scion wood on established rootstocks, plants reach reproductive maturity in 2–4 years, promoting consistent quality and yield across plantings, unlike the variable outcomes from cuttings or layering. This uniformity is crucial for commercial consistency, minimizing variability in size, flavor, and timing that plagues seed-based methods.[22][23] Additionally, grafting facilitates size control and vigor enhancement through specialized rootstocks, such as dwarfing varieties that support high-density orchards while maintaining productivity. Dwarf rootstocks reduce tree height to 30–50% of standard sizes, improving manageability and enabling closer spacing for higher land-use efficiency, advantages not readily achievable with other propagation techniques. This vigor modulation can enhance nutrient uptake and stress tolerance, further outperforming standalone cuttings or layered plants.[23] Grafting preserves sterile or non-seed-producing varieties, including many hybrids, by enabling asexual propagation where seeds are inviable or do not breed true. This allows the multiplication of elite cultivars that cannot be reliably reproduced via seeds, cuttings, or layering due to sterility or genetic instability, ensuring the continued availability of high-value traits.[24] Economically, grafting boosts yields and fruit quality in commercial agriculture, often increasing marketable output by 20–40% through combined disease management and physiological enhancements. These improvements lead to higher returns by extending harvest seasons and reducing losses, making it a cost-effective strategy over time despite initial labor, unlike the lower reliability of alternative methods.[25][26][27]

Common Applications

In agriculture, grafting is extensively applied in fruit tree production to propagate desirable cultivars onto dwarfing rootstocks, enabling more manageable tree sizes and facilitating easier harvesting in commercial orchards. For instance, apple varieties such as Honeycrisp or Gala are commonly grafted onto rootstocks like M.9 or MM.106, which restrict mature tree height to 2-4 meters, allowing for higher-density planting and reduced labor costs compared to standard-sized trees.[28][29] This approach not only accelerates the time to fruiting—often within 2-3 years—but also improves overall orchard efficiency by concentrating yields in accessible heights.[30] In viticulture, grafting serves as a critical tool for vineyard management, particularly to combat soil-borne pests like phylloxera by attaching susceptible European grapevine scions (Vitis vinifera) to resistant American rootstocks such as 3309 or Riparia. This practice, standard since the late 19th century, has preserved iconic varieties like Cabernet Sauvignon and Chardonnay, preventing widespread vine devastation and ensuring sustained production in regions like California and Bordeaux.[31][32] By leveraging the rootstock's disease resistance, growers maintain vine vigor and fruit quality without resorting to chemical treatments, supporting long-term vineyard sustainability.[33] Horticulturists employ grafting for ornamental plants to produce aesthetically unique forms, such as weeping or columnar shapes, and multi-variety specimens that enhance landscape diversity on a single plant. For example, multiple scions from different rose or plumeria cultivars can be grafted onto a single rootstock to create a "fruit salad tree" or floral bouquet effect, offering novelty in gardens and reducing space requirements while showcasing varied colors and forms.[34] This method is particularly valued in urban settings for its visual appeal and ability to propagate non-rooting ornamentals efficiently.[35] In forestry, grafting facilitates the propagation of elite scions from superior timber species to boost growth rates and wood quality in plantations. Species like Pinus radiata or longleaf pine (Pinus palustris) benefit from grafting selected genotypes onto seedling rootstocks, yielding improvements in volume growth of up to 25-30% through enhanced stem straightness and disease tolerance.[36][37][38] This clonal deployment in seed orchards ensures rapid scaling of high-performing trees for timber production, optimizing resource use in managed forests.[39] Restoration ecology utilizes grafting for the clonal propagation of endangered trees, preserving genetic diversity and accelerating reintroduction into native habitats. For critically imperiled species like Florida torreya (Torreya taxifolia), scions from remnant individuals are propagated via cuttings or seedlings on suitable rootstocks for ex situ conservation, addressing seed dormancy and disease challenges before outplanting.[40] Similarly, in efforts to restore American chestnut (Castanea dentata), grafting integrates resistant hybrids, supporting population recovery amid threats like chestnut blight.[41] This technique proves essential for species with low seed viability, ensuring faithful replication of rare genotypes for ecological rehabilitation.[42]

Preparation and Requirements

Factors for Successful Grafts

Successful grafting requires careful attention to timing, which varies by plant type and regional climate. For most woody plants, such as fruit trees, the optimal period is during dormancy in late winter or early spring, before bud swell, when both scion and rootstock are inactive to reduce transpiration and promote healing as growth resumes.[43] In contrast, herbaceous plants like tomatoes are grafted during active growth in spring or summer, aligning with their vegetative phase for faster union formation.[44] Climate influences this; in temperate zones, colder winters necessitate earlier timing to avoid frost damage, while in subtropical areas, mild winters allow flexibility into late fall.[45] Environmental conditions post-grafting are critical for callus development and initial tissue bridging. Temperatures between 20-30°C facilitate rapid callus formation, as rates increase progressively from about 4°C but halt below 0°C and risk tissue death above 32°C.[45] High relative humidity, typically 80-95%, prevents desiccation of the exposed cambium, while moderate shading limits excessive light and heat that could accelerate moisture loss.[46] These conditions mimic a protected microenvironment, supporting the early stages of vascular reconnection without delving into physiological details. Aftercare practices significantly enhance graft viability by safeguarding the union site. Wrapping with moisture-retentive materials like parafilm or grafting tape secures the scion and rootstock, excluding air and maintaining humidity until callus seals the wound.[47] Shading with cloth or bags for 2-4 weeks reduces transpiration stress, and applying sealants or wax over cuts protects against evaporation and environmental pathogens.[48] Consistent monitoring for moisture ensures these measures prevent drying, which accounts for many early failures. Sterility and hygiene protocols are paramount to mitigate infection risks from bacteria, fungi, or viruses entering through fresh cuts. Disinfecting tools with alcohol or bleach solutions before each use, along with sanitizing work surfaces and hands, minimizes contaminant transfer during the procedure.[49] In greenhouse settings, isolating grafted plants in clean, ventilated areas further reduces exposure to airborne pathogens.[50] The operator's skill in executing precise cuts directly impacts success by ensuring maximal cambial alignment and limiting air exposure at the interface. Clean, angled incisions made swiftly with sharp tools promote immediate tissue apposition, reducing oxidation and drying that could disrupt union formation.[51] Inexperienced grafters often see lower take rates due to misalignment, underscoring the need for practice to achieve tight, bubble-free contacts.

Tools and Materials

Grafting procedures necessitate precise cutting tools to prepare scion and rootstock tissues without damaging vascular cambium layers. Pruning shears, also known as hand pruners, are essential for making initial cuts on branches up to about 2 cm in diameter, allowing for clean removal of excess growth prior to grafting.[52] Grafting knives, typically featuring a sharp, single-bevel blade made from high-carbon steel to maintain a keen edge, enable precise, angled incisions that facilitate tight cambial contact between graft components.[5] For larger rootstocks, such as those exceeding 5 cm in diameter used in cleft or bark grafting, fine-tooth saws or chisels provide the necessary leverage to create splits without splintering the wood.[53] Securing materials play a critical role in stabilizing the graft union and retaining moisture during the initial healing phase. Grafting tape, often composed of stretchable parafilm or polyethylene, is wrapped around the joined scion and rootstock to promote close alignment and prevent desiccation while allowing gas exchange.[54] Grafting wax or sealant compounds are applied to exposed cut surfaces to form a protective barrier against pathogens and moisture loss, particularly in field applications where environmental exposure is high.[5] Alternatively, plastic clips or ties can be used to mechanically hold components in place, especially in herbaceous grafting, where they provide adjustable tension without adhesive residue.[55] Support aids enhance post-grafting care by maintaining optimal conditions for union formation. Grafting benches or stable work surfaces offer a dedicated area for precise assembly, often equipped with vises or holders to immobilize rootstocks during cutting. Clamps and stakes provide mechanical support to newly grafted plants, preventing wind-induced movement that could disrupt healing. Humidity domes or chambers, typically transparent plastic enclosures, create a high-humidity microenvironment (around 85-95%) essential for preventing dehydration of the graft site during the first 7-10 days after joining.[56] Specialized items address technique-specific needs in advanced grafting. Budding tubes, small protective cylinders often made of waxed paper or plastic, encase shield buds to shield them from desiccation and physical damage during establishment in methods like T-budding. Electric heat mats deliver bottom warmth (typically 24-27°C) to propagate grafted materials under controlled conditions, accelerating callus formation in bench grafting setups.[57] Safety gear is vital to minimize injury and disease transmission during handling. Cut-resistant gloves protect against blade slips while permitting dexterity for fine work. Disinfectants, such as 10% bleach solutions or alcohol wipes, are applied to tools between cuts to prevent the spread of pathogens like viruses via contaminated surfaces.[58]

Techniques

Approach and Bud Grafting

Approach grafting, also known as inarching, involves joining two independently rooted plants that are grown in close proximity, allowing the cambium layers to align gradually without severing either plant from its root system until a union forms.[59] This method is particularly ideal for species that are difficult to root from cuttings, such as certain tropical fruits or ornamentals, as it minimizes stress and ensures both parts remain viable during the process.[60] The technique relies on the principle of cambium alignment, where the vascular tissues of the scion and rootstock are positioned to touch and heal together over time.[61] To perform approach grafting, select healthy shoots of similar diameter (at least 3/8 inch) from the scion and rootstock plants positioned side by side. Peel back 1-2 inch sections of bark to expose the cambium on both, ensuring the exposed surfaces match in size and shape. Bind the peeled areas tightly together using grafting tape or twine, wrapping securely to maintain contact, and remove excess foliage from the upper plant to reduce transpiration. Over 4-6 weeks during the active growing season, the tissues fuse; once union is confirmed by strong adhesion, sever the scion from its original roots and the rootstock from its top above the graft site.[59] This approach is best suited for field or container settings where plants can be manipulated without disturbance, such as propagating citrus or lychee trees on disease-resistant rootstocks.[62] Bud grafting techniques, including T-budding and chip budding, use a single bud as the scion inserted into the rootstock with minimal tissue removal, making them suitable for beginners and young plants with smooth bark. These methods are performed when the rootstock bark slips easily, typically in spring or summer for soft tissues, and achieve success rates up to 80% in compatible pairings like fruit trees on clonal rootstocks.[63][64] In T-budding, also called shield budding, prepare the dormant scion by cutting a shield-shaped piece (about 1 inch long) that includes the bud, bark, and a thin sliver of wood, removing excess wood if needed for species like maples. On the active rootstock, make a shallow T-shaped incision (1-1.5 inches vertical and 0.5 inches horizontal) in the bark, gently peeling back the flaps to form a pocket. Insert the scion shield downward into the pocket until the cambium layers align, then secure with a rubber band or grafting tape, leaving the bud exposed.[65][66] This variant is commonly applied to woody ornamentals and nut trees in late summer, with the rootstock top removed the following spring to force growth from the bud.[67] Chip budding, suitable for dormant rootstocks or late-season work, involves removing a small chip of bark and wood from both scion and rootstock to create matching wedge shapes. Cut a single downward-sloping chip (about 1 inch long) from the scion below a dormant bud, including a thin layer of cambium. On the rootstock, make two angled cuts to excise a similar chip, forming a pocket. Fit the scion chip into the rootstock pocket with cambium contact, and wrap firmly with tape or parafilm, again exposing the bud.[68] This method yields straighter trunks and higher success in field operations for apples and stone fruits, often performed by teams for efficiency.[69] Diagrams of these techniques typically illustrate the incisions: for approach grafting, side-by-side stems with aligned peeled cambium wrapped in tape; for T-budding, a rootstock stem with the T-cut flaps open and the shield bud inserted; for chip budding, angled cuts forming interlocking wedges before wrapping. These visuals emphasize precise cambium matching and secure binding to promote healing.[65][68]

Cleft and Whip Grafting

Cleft grafting involves vertically splitting the rootstock to create a cleft, into which one or more wedge-shaped scions are inserted to ensure cambial contact for union formation. This technique is particularly suited for topworking larger diameter rootstocks (typically 5 cm or greater) with smaller scions (5 to 10 mm), allowing multiple scions to be placed on opposite sides of the split for balanced growth. It is commonly performed on fruit trees such as apples during late winter while plants are dormant, minimizing desiccation and promoting healing.[70][30][71] Whip grafting, also known as splice grafting, requires making matching diagonal cuts on both the scion and rootstock to form interlocking surfaces, with an optional tongue cut for added mechanical strength and alignment. The bevel angles must be precisely matched to ensure maximum cambial overlap, ideally when the scion and stock diameters are equal for optimal contact. This method is favored for bench grafting of fruit trees like apples and pears in early spring or late winter, providing a strong, rapid-healing union due to the extensive cut surface area.[72][73][5] Both techniques achieve high success rates of 70-90% when performed during the dormant season, provided diameters are closely matched to facilitate vascular integration and reduce stress on the graft union. Emphasis on equal sizing prevents misalignment, which can lower viability, while proper aftercare such as wrapping helps maintain moisture and stability.[74][75][76] Modifications like double-working extend these methods by incorporating an interstock between the scion and rootstock, enabling complex trait combinations such as disease resistance from the rootstock and desired fruit quality from the scion via sequential whip or cleft unions. This approach is used in fruit tree propagation where direct scion-rootstock compatibility is poor, but the interstock bridges the genetic gap effectively.[77][2]

Whip-and-Tongue Graft

The whip-and-tongue graft is ideal for joining scion and rootstock of similar diameter, commonly used for pear trees in late winter/early spring when dormant. Steps:
  1. Select a smooth branch on the rootstock (e.g., young Kieffer pear) about ½–¾ inch thick.
  2. Prepare the scion (e.g., Warren pear): Cut a 4–6 inch piece with 3–4 buds. Make a long diagonal cut (1–1½ inches) at the base.
  3. Create the tongue: About ⅓ from the top of the cut, make a downward slit ¼–½ inch deep.
  4. Prepare the rootstock: Make a matching diagonal cut, then an upward tongue slit.
  5. Join: Interlock the tongues, aligning cambium layers on at least one side.
  6. Secure: Wrap tightly with grafting tape or parafilm, seal top cut with wax.
  7. Aftercare: Provide shade and moisture; check for growth in 4–6 weeks.

Cleft Graft

Suitable for slightly thicker rootstock branches on young trees. Steps:
  1. Cut branch straight across.
  2. Split center 1–2 inches deep.
  3. Trim scion base to wedge, insert 1–2 so cambium contacts.
  4. Wrap and seal.
Timing note: In subtropical USDA zone 9 (e.g., central Florida), graft January–March ideally; mid-March–early April if sap flows and buds swell. Late March is borderline—success drops with active growth and heat.

Bark, Veneer, and Other Grafting Methods

Bark grafting, also referred to as rind grafting, is a technique particularly suited for top-working large-diameter trees or branches where the bark readily slips, such as in citrus during spring when sap flow facilitates separation of the bark from the wood.[78] The process involves making a clean horizontal cut at the desired height on the rootstock trunk or branch, followed by one or more vertical incisions through the bark to create flaps that are gently lifted to expose the underlying cambium layer; wedge-shaped scions with matching cambium are then inserted beneath the flaps and secured with ties or wax to promote union.[79] This method allows for multiple scions per site on sizable stocks, enabling efficient variety changes on mature trees while minimizing structural disruption.[5] Veneer grafting involves carefully peeling back a thin layer of wood to expose the cambium without fully severing the bark, providing a precise alignment for scion insertion and is especially effective for conifers with their rigid tissues. The rootstock is prepared by making a shallow, angled cut along the side or top to remove a veneer strip, revealing a flat cambium surface onto which the similarly shaped scion is placed and bound; a common variant, side-veneer grafting, utilizes small potted seedlings as rootstocks for propagating species like spruce (Picea) or fir (Abies), often in controlled nursery settings to produce compact or dwarf forms.[80][52] This approach achieves high success rates in evergreens by preserving vascular continuity and is typically performed in late winter or early spring before bud break. Among other specialized methods, stub grafting facilitates top-working for tree rejuvenation by cutting branches or trunks back to short stubs—typically 4 to 6 inches long—and applying cleft or bark grafts to multiple scions on each stub, promoting vigorous regrowth in older fruit trees like apples or walnuts.[81] The four-flap graft, sometimes called the "banana" graft, is adapted for nut trees such as pecans, where four longitudinal bark flaps are created around the stock by shallow cuts, lifted, and the scion inserted in the center before flaps are replaced and secured; it is ideal for stocks 3/8 to 1 inch in diameter and yields strong unions due to extensive cambium contact.[82][83] Awl grafting employs a simple tool like a sharpened screwdriver or awl to drill or slit a T-shaped incision into the bark without penetrating the cambium deeply, allowing quick field insertion of a wedged scion, making it suitable for rapid propagation in orchards or frameworking fruit trees with minimal equipment.[84] A niche application, bridge grafting, addresses girdled trunks damaged by rodents or machinery by inserting multiple elongated scions horizontally across the injured area to span from healthy cambium above to below the girdle, thereby restoring phloem transport and preventing tree death in species like fruit trees.[85] The scions, often sourced from compatible material, are shaped with angled ends for secure placement in incisions on both sides of the wound and firmly tied, typically applied in early spring on dormant trees to maximize survival rates exceeding 70% in responsive older specimens.[86][87]

Specialized Forms

Natural Grafting

Natural grafting, also known as inosculation, refers to the spontaneous fusion of plant tissues in wild or cultivated settings without human intervention, where adjacent stems, branches, or roots form a functional union through prolonged physical contact. This process typically begins when mechanical forces, such as wind-induced friction or root entanglement, abrade the outer bark, exposing the underlying cambium layers and allowing them to come into direct contact. Over time, the plants' wound healing responses initiate cellular proliferation and differentiation, leading to the development of a shared vascular system that enables the exchange of water, nutrients, and signals between the fused parts.[88][89] The mechanisms of natural grafting rely on contact-induced unions, often triggered by branch abrasion in leaning or intertwined trees or by root proximity in dense stands. In species like maples (Acer spp.), exposed cambium from rubbing branches heals through callus formation, eventually merging the vascular tissues into a single conduit. Similarly, English ivy (Hedera helix) exhibits natural grafting when its climbing stems press against host trees or other vines, where physical pressure facilitates stem fusion without wounding, resulting in interconnected growth. Root grafting, a common variant, occurs when underground roots touch and align, promoting fusion via similar healing processes; this is prevalent in trees like poplars and maples, where soil movement or growth brings roots into contact. Unlike artificial grafting, these natural unions develop slowly over years or decades, are less predictable due to environmental variables, and succeed more reliably when involving genetically identical or closely related individuals, minimizing compatibility barriers.[90][89][91] In ecological contexts, natural grafting plays key roles in forest stability and resource dynamics, particularly through the formation of clonal colonies. For instance, quaking aspen (Populus tremuloides) frequently engages in root grafting, creating vast interconnected networks like the Pando clone in Utah, which spans over 100 acres and consists of genetically identical ramets linked by fused roots for shared water and nutrient distribution. These unions enhance resilience by allowing healthy plants to support stressed neighbors, buffering against drought or herbivory, and providing mechanical reinforcement against windthrow in dense stands. However, as of 2025, the Pando clone is experiencing decline due to intensified deer browsing, drought, and climate impacts, despite its interconnected structure.[92][91][93][94][95]

Herbaceous and Inarching Grafting

Herbaceous grafting involves joining the stems of non-woody, soft-tissued plants, such as annual vegetables, to combine desirable traits from scion and rootstock varieties. This technique is commonly applied to crops like tomatoes (Solanum lycopersicum) and cucurbits (e.g., cucumbers, melons, and squash in the Cucurbitaceae family), where it allows growers to graft disease-susceptible scions onto resistant rootstocks.[56][96] The primary method for herbaceous grafting is splice grafting, performed in controlled greenhouse environments to ensure precision and sterility. Seedlings are grown to a stem diameter of at least 1.5–3.0 mm, typically 2–4 weeks after sowing, during active growth phases when tissues are turgid and cambium layers are easily aligned. Procedures include making angled cuts (approximately 45 degrees) on both scion and rootstock for maximum surface contact, securing the union with specialized plastic clips, and maintaining sterile conditions using sanitized tools to prevent infection. Post-grafting, plants are placed in healing chambers with high humidity (85–95%) and temperatures of 72–85°F (22–29°C) for 48–72 hours to promote callus formation, followed by gradual acclimation over 4–8 days.[56][97] Key benefits include avoidance of soilborne pathogens, such as Verticillium wilt or nematodes in tomatoes, and transfer of resistance traits like tolerance to carmine spider mites in cucurbits from interspecific rootstocks (e.g., Lagenaria to Cucurbita scions). Additionally, it enhances hybrid vigor through improved nutrient uptake, abiotic stress tolerance (e.g., salinity or heat), and yield increases of 12–38% in grafted plants compared to non-grafted ones. Success factors emphasize rapid union formation, typically within 1–2 weeks under optimal conditions, with high humidity above 90% critical to minimize desiccation of soft tissues; failure rates drop below 10% when using uniform seedlings and avoiding temperatures exceeding 100°F (38°C).[96][56][98] Inarching, also known as approach grafting, is a supportive technique where a scion is grafted onto an adjacent rootstock while both remain rooted, creating a permanent bridge for structural repair or propagation. It is particularly common in tropical and subtropical regions for fruit trees, such as citrus or mango, to bypass damaged trunk sections caused by pests, diseases, or mechanical injury.[99][100] The procedure requires positioning a potted scion near the rootstock during active growth (when bark slips easily, often in spring or wet seasons), making matching incisions to expose cambium layers, binding the joined parts with tape or ties, and leaving them connected until union forms (2–4 weeks). Once fused, the scion's original root is severed, and the rootstock's if needed, integrating the plants fully; sterile tools and non-stressed material are essential to avoid contamination. In tropical applications, inverted T-bud or chip budding variants are used for hanging scions on larger trees.[101][99] Success in inarching depends on cambium alignment, high humidity (over 90%) to support healing, and timing with warm temperatures (above 70°F or 21°C) for callus development within 1–2 weeks; compatibility between scion and rootstock, along with fresh, disease-free material, yields union rates exceeding 80% in compatible tropical species.[99][100] Recent innovations as of 2025 include automated grafting systems to enhance efficiency in large-scale production and advanced interspecific rootstocks designed for greater resilience to climate-induced stresses, such as extreme temperatures and salinity. Studies have reported yield increases up to 63% in grafted tomatoes cultivated in polyhouses under stressed conditions, underscoring the technique's evolving role in sustainable agriculture.[102][27][103]

Bridge grafting and inarching

Bridge grafting, also known as repair grafting, is a technique used to restore vascular continuity in trees that have been girdled or suffered bark damage at the trunk base, such as from rodent injury, mechanical damage, or diseases like Phytophthora collar rot in apples. The method bridges the damaged area by grafting scions or using existing live shoots to reconnect the phloem flow from leaves to roots. Inarching is a variant of bridge grafting that uses uninjured suckers or rooted shoots already growing from the base of the tree (rootstock suckers in grafted trees). This approach requires only one graft union (at the upper end into healthy scion bark above the damage), as the lower end remains attached to its roots. It is simpler and often recommended for apples with partial girdling (e.g., less than 50% circumference affected), where the tree remains vigorous. Extension services (e.g., UConn IPM, WVU Extension, Intermountain Fruit) note its applicability to apples, pears, walnuts, and cherries for Phytophthora-induced collar rot when less than 30–50% of the trunk is damaged. Success depends on stopping further rot through cultural practices (improved drainage, exposed base) and performing the graft in early spring when bark slips.

Steps for inarching with suckers

  1. Select a healthy, vigorous sucker (1/4–1/2 inch diameter) near the damage, tall enough to reach above it.
  2. Prepare healthy cambium on the trunk above the dead zone.
  3. Bend the sucker to the contact point, trim to a bevel, and insert into a slot or flap in the trunk bark for cambium alignment.
  4. Secure with nails, tape, or wax; use multiple (2–4) for better reconnection.
  5. Aftercare: Keep base dry, monitor callus formation; grafts may take 1–2 seasons to heal.
This technique can extend tree life in partial damage cases, though full girdling often requires replacement. It leverages the tree's natural suckering response under stress.

Biological Mechanisms

Graft Union Formation

Graft union formation is the biological process by which the scion and rootstock tissues heal and integrate following the wounding associated with grafting, enabling the establishment of a functional connection. This process is essential for the long-term success of the graft and involves a series of coordinated cellular events that restore continuity between the two plant parts. In compatible grafts, the healing occurs through regeneration rather than simple scarring, allowing for the reestablishment of vascular continuity.[104] The process begins with the wound response, characterized by a necrosis phase where cells at the cut surfaces die to form a protective necrotic layer. This layer, composed of dead cells and lignified tissues, seals the exposed surfaces and prevents desiccation and pathogen entry, typically forming within the first few days after grafting. The necrosis phase is followed by adhesion of the scion and rootstock surfaces, where the necrotic tissues begin to bridge the gap between the partners.[104][7] Next comes callus proliferation, during which parenchyma cells adjacent to the wound site dedifferentiate and proliferate to form callus tissue—a mass of undifferentiated, thin-walled cells. This phase involves rapid cell division and expansion, filling the space between the scion and rootstock with a callus bridge that provides structural support and facilitates tissue fusion. The callus arises from cambial and parenchymal initials, creating a disorganized layer that eventually organizes into structured tissues.[7][105] The final stage is vascular differentiation, where the callus tissue develops new vascular elements, reconnecting the xylem and phloem systems between the scion and rootstock. Phloem reconnects first, enabling nutrient transport to support healing, followed by xylem vessels that provide mechanical support and restore water and mineral conduction. This reconnection occurs through the differentiation of callus cells into vascular cambium, which generates secondary xylem and phloem, ensuring the graft's physiological integration. Genetic compatibility between the scion and rootstock is crucial for successful progression through these stages, as incompatible combinations often fail at the callus or vascular phases.[7][106] At the cellular level, the proliferation of undifferentiated parenchyma cells during callus formation is key, as these totipotent cells divide mitotically to produce the bridging tissue. Schizogenous spaces—gaps formed by the separation of living cells without lysis—emerge within the callus, allowing for the organized development of new tissues and preventing excessive pressure buildup. Adhesion between the scion and rootstock is further strengthened by the role of calcium ions, which cross-link unmethyl-esterified homogalacturonan (a pectin component) in the cell walls and middle lamella, promoting stable cell-to-cell contact at the interface.[107][108] The timeline for graft union formation varies by plant type but generally spans several weeks to 3-4 months for the initial union, marked by callus bridging and early vascular reconnection in woody plants, with full structural strength and vascular integration achieved over multiple growing seasons as secondary growth solidifies the connection. In herbaceous model systems like Arabidopsis, initial healing can occur in 7-12 days, while woody species require longer due to slower growth rates.[7] Influencing factors include auxin gradients, which direct the polarity of vascular tissue formation within the callus. Auxin, transported polarly from the scion, accumulates at the graft interface and canalizes the development of new vascular strands, ensuring alignment with existing tissues in the rootstock. This gradient-mediated polarity is critical for oriented cell division and differentiation, preventing disorganized healing.[109][110] Recent studies as of 2025 have further elucidated these mechanisms, including the induction of autophagy to promote callus formation and wound healing, enhancing tissue connectivity during grafting.[111]

Communication and Vascular Integration

Following the formation of the graft union, communication between the scion and rootstock occurs through signal transduction pathways that enable coordinated growth and development. Hormones such as gibberellins and cytokinins play key roles in this process, translocating across the graft interface to regulate cell division, elongation, and differentiation.[112] These phytohormones move symplasmically through plasmodesmata, facilitating bidirectional signaling that synchronizes physiological responses between the grafted tissues.[113] Additionally, mobile RNAs, including messenger RNAs and small interfering RNAs, traffic via plasmodesmata and the phloem, influencing gene expression and epigenetic modifications in recipient tissues to promote compatibility and long-term integration.[114] This RNA movement contributes to systemic signaling, ensuring that developmental cues from one partner propagate to the other for unified plant function.[115] Vascular integration establishes the structural and functional continuity essential for nutrient and water exchange post-grafting. The phloem reconnects first, typically within days to weeks depending on the plant type, enabling the transport of photosynthates and signaling molecules from source leaves in the scion to sink tissues in the rootstock.[116] Xylem integration follows, restoring upward flow of water and minerals, often guided by auxin gradients that canalize vessel alignment across the junction.[117] In mismatched grafts, such as those between incompatible species, vascular bottlenecks can arise due to incomplete reconnection or mismatched cambial activity, leading to restricted flow and potential graft failure.[11] Successful integration results in a fully functional vascular network, where phloem and xylem strands bridge the scion-rootstock boundary without significant impedance. These interactions yield reciprocal effects that extend beyond basic survival to influence overall plant architecture and productivity. The scion can alter rootstock architecture by exporting hormones that promote lateral root formation or inhibit apical dominance, enhancing nutrient uptake capacity.[118] Conversely, the rootstock modulates scion traits, such as fruit quality, by supplying altered levels of minerals, hormones, or secondary metabolites that affect flavor, size, and shelf life; for instance, certain rootstocks delay scion leaf senescence, extending photosynthetic activity and improving yield in crops like cotton and tomato.[119] These effects underscore the long-term functionality of the graft, where ongoing signal exchange optimizes resource allocation and stress resilience. Detection of communication and vascular integration relies on non-invasive and targeted methods to visualize and quantify flow dynamics. Dye tracing with fluorescent compounds like carboxyfluorescein diacetate (CFDA) monitors phloem connectivity by tracking symplastic movement across the graft junction, often revealing reconnection timelines as early as 3-5 days post-grafting in herbaceous systems.[110] For xylem flow, acid fuchsin or similar dyes highlight vessel continuity under microscopy. Isotopic labeling, using stable isotopes like ¹³C or radioactive tracers such as ¹¹C, quantifies long-distance transport in phloem and xylem, allowing assessment of photosynthate partitioning and potential bottlenecks through mass spectrometry or positron emission tomography (PET) imaging.[120] These techniques provide empirical evidence of integration success, guiding improvements in grafting practices.

Advanced Concepts

Graft Chimeras

Graft chimeras are composite plants formed through grafting where tissues of genetically distinct scion and rootstock integrate to produce shoots containing cells from both partners, resulting in a mosaic of genotypes within the same organism. Unlike natural chimeras arising from somatic mutations during development, graft chimeras originate specifically from the fusion at the graft interface, often involving callus proliferation that mixes cellular contributions from each component. These structures are distinguished by their layered or sectoral organization, reflecting the tunica-corpus model of shoot apical meristems, and they exhibit phenotypes that combine traits from both genotypes.[121][122] The primary types of graft chimeras include periclinal, sectorial, and mericlinal forms. Periclinal chimeras feature distinct, parallel layers of tissue from different genotypes, such as the outermost L1 layer (epidermis) from one parent and inner L2/L3 layers (cortex and vascular tissues) from the other, leading to uniform but composite organs. Sectorial chimeras display wedge-shaped sectors of contrasting genotypes extending longitudinally through the shoot, often resulting in striped or patchy appearances. Mericlinal chimeras, an intermediate type, have partial layering where one genotype occupies only part of a cell layer, making them less organized and prone to instability. Periclinal types are generally the most stable among graft chimeras due to their complete stratification.[123][124][125] Formation of graft chimeras occurs through uneven fusion of the cambium layers during graft union development, where proliferative callus tissue at the interface incorporates cells from both the scion and rootstock, leading to chimeric meristems that generate layered or sectoral shoots. This process requires precise alignment and healing, but incomplete separation allows mixed cellular contributions to persist in new growth. Once established, these chimeras can only be propagated vegetatively by grafting, as the composite structure does not survive sexual reproduction.[121][89] Notable examples include variegated ornamental plants such as certain poinsettia cultivars, where periclinal chimeras with mutations or grafts confined to the epidermal layer produce distinctive colored bracts. In citrus, graft chimeras like those between Citrus sinensis (sweet orange) and C. natsudaidai demonstrate layered differences in epicarp coloration and juice sac quality, enhancing fruit aesthetics and traits. The 'Zaohong' navel orange represents a natural periclinal chimera with L1 from Satsuma mandarin and inner layers from 'Robertson' navel orange, illustrating how such structures arise and persist.[126][121][127] Despite their utility, graft chimeras face stability challenges, including reversion where one genotype dominates and displaces the other through uneven cell division or environmental stress, potentially losing desirable traits. In citrus variegated chimeras, for instance, reversion to uniform green tissue is common over time. Breeding is further complicated by non-Mendelian inheritance, as gametes derive from single layers, preventing true-to-type transmission via seeds and necessitating clonal propagation to maintain the chimera.[125][128][124]

Scientific and Research Uses

Grafting has emerged as a valuable experimental tool in plant biology for investigating gene flow, systemic signaling pathways, and systemic acquired resistance. By joining genetically distinct scions and rootstocks, researchers can track the movement of genetic material and signaling molecules across graft unions, revealing mechanisms of long-distance communication in plants. For instance, grafting wild relatives onto domesticated varieties facilitates trait mapping by isolating the effects of specific genetic backgrounds on phenotype expression, allowing precise identification of loci involved in stress responses or development. This approach has been instrumental in dissecting how mobile signals, such as hormones and RNAs, propagate to coordinate organ responses.[117][129][130] In biotechnology, grafting enables the creation of virus-resistant plants through rootstock barriers that limit pathogen spread to the scion. Compatible rootstocks harboring resistance genes can sequester viruses in the root system, preventing systemic infection via vascular connections, as demonstrated in studies with cucurbit crops and transgrafting in stone fruits where non-transgenic scions gained protection from plant viruses such as Plum pox virus.[131][132] Additionally, graft chimeras serve as vehicles for CRISPR delivery, where mobile CRISPR-Cas9 ribonucleoproteins from a transgenic rootstock edit the wild-type scion genome without integrating transgenes, producing heritable, transgene-free mutants in a single generation. This method has been successfully applied in tomato and Arabidopsis, offering a non-invasive alternative to traditional transformation techniques for precise genome editing.[133] Research on climate adaptation leverages grafting to enhance crop resilience by pairing drought-tolerant rootstocks with elite scions, improving water use efficiency and maintaining yield under water-limited conditions. For example, grafting tomato or eggplant onto wild, drought-resistant rootstocks alters root architecture and hydraulic conductance, reducing transpiration while sustaining photosynthesis, thereby mitigating the impacts of projected climate-induced droughts. These interventions have shown up to 50% improvements in survival rates and biomass under deficit irrigation in field trials, highlighting grafting's potential for breeding climate-smart varieties.[134][135][136] As a model system, Arabidopsis thaliana grafts provide insights into molecular underpinnings of graft incompatibility, where interspecies unions fail due to mismatched gene expression in vascular reconnection. Micrografting techniques in Arabidopsis enable high-throughput screening of mutants, revealing that incompatibility often stems from disrupted auxin signaling or lignin deposition at the union interface, as identified through transcriptomic analyses comparing compatible and incompatible pairs. These studies have pinpointed key regulators like NAC domain transcription factors, informing strategies to overcome barriers in economically important crops.[137][138][139] Recent advances since 2020 have expanded grafting's utility in microbiome transfer and controlled environment optimization. Grafting facilitates the inheritance of beneficial rootstock microbiomes, altering rhizosphere communities to suppress pathogens and enhance nutrient cycling; for instance, grafted plants inherit microbial legacies that mitigate negative soil feedback, improving yield stability through enriched beneficial bacteria. In vertical farming, automated grafting protocols integrated with machine learning optimize seedling light regimes, boosting success rates by over 8% and enabling scalable production of stress-resilient plants in soilless systems. As of 2025, further progress includes detailed molecular mechanisms of graft healing, such as regulatory pathways involving NAC transcription factors, and applications of double grafting for stacking multiple traits like disease resistance and stress tolerance. These developments underscore grafting's role in sustainable biotechnology for microbiome engineering and precision agriculture.[140][141][108][142]

Examples

Conifers and Woody Plants

Grafting in conifers and woody plants addresses the unique challenges posed by their long lifespans, slow growth rates, and complex physiological barriers, enabling the propagation of superior genotypes for forestry and timber production. In white spruce (Picea glauca), veneer grafting is commonly employed to establish seed orchards that produce genetically improved seeds for reforestation. This technique involves inserting a thin sliver of scion wood containing dormant buds onto a rooted stock, achieving success rates of 60-70% when performed in late winter or early spring under controlled greenhouse conditions. The method accelerates the production of elite seed stock, reducing the time from selection to seed harvest compared to seed-based propagation, which can take decades due to the species' juvenile phase. For other conifer species, such as pines (Pinus spp.), side-veneer grafting facilitates the rapid dissemination of desirable traits in breeding programs aimed at enhancing disease resistance and growth vigor. This approach, where a side incision on the stock receives a matching scion sliver, has been instrumental in programs like those for loblolly pine (Pinus taeda), allowing clonal propagation of selected trees to create high-performing plantations. Success in these applications often exceeds 50%, though it requires precise alignment of cambial layers to overcome the conifers' thick bark and resinous tissues. Resin flow, a common inhibitor in conifers, can be mitigated by performing grafts during periods of low sap activity, such as dormancy, to prevent callus formation disruption. In woody deciduous species like oaks (Quercus spp.), cleft grafting is utilized to improve timber quality by combining superior scion wood with robust rootstocks, thereby bypassing the extended juvenile periods that delay flowering and fruiting for 20-50 years in seedling-grown trees. This technique splits the stock vertically to insert wedge-shaped scions, promoting union formation and enabling the propagation of straight-grained, rot-resistant varieties for sustainable forestry. Grafting success in oaks typically ranges from 40-60%, influenced by the need for seasonal dormancy synchronization between scion and stock to ensure compatible physiological states during healing. Such synchronization is critical, as mismatches can lead to graft failure due to asynchronous bud break or vascular incompatibility.

Fruit Trees and Vines

Grafting plays a pivotal role in the cultivation of fruit trees and vines, enabling growers to optimize orchard and vineyard productivity through controlled tree size, earlier fruiting, and improved resistance to pests like phylloxera. In apple and pear production, the use of dwarfing rootstocks such as M27 results in trees that reach only about 30-40% of standard size, making them ideal for intensive management.[89] This dwarfing effect supports high-density planting configurations, with spacings that accommodate up to approximately 2,000 trees per hectare, thereby maximizing fruit output per unit area while reducing labor for pruning and harvesting.[143] For grapevines, grafting onto American rootstocks like Vitis riparia proved essential in resolving the phylloxera crisis that devastated European vineyards in the late 19th century. These rootstocks provide strong resistance to the phylloxera aphid, allowing susceptible Vitis vinifera scions to thrive without the insect destroying the root system.[31] This practice, originating from North American species, has become standard in global viticulture, preserving heirloom varieties and sustaining wine production in phylloxera-prone regions.[144] In citrus cultivation, bud grafting techniques are commonly applied to create multi-variety trees on a single rootstock, particularly in subtropical climates where space and pollination needs are optimized. This method involves inserting buds from different citrus types—such as oranges, lemons, and grapefruits—onto compatible rootstocks like trifoliate orange, resulting in diversified fruit production from one plant and enhanced overall yields through better resource utilization.[145] Such multi-graft systems improve harvest efficiency in warm, humid environments. Overall, these grafting applications in fruit trees and vines yield notable productivity gains, including 20-50% increases in output per hectare due to denser plantings and superior vigor control, alongside earlier bearing times of 2-3 years compared to 5 or more years for seedling-grown trees. Additionally, grafting confers disease resistance benefits, such as phylloxera tolerance in vines, further bolstering long-term orchard viability.

Historical and Cultural Context

Ancient Origins in the Fertile Crescent and China

The Fertile Crescent, a region spanning ancient Mesopotamia, Assyria, and surrounding areas, witnessed some of the earliest documented horticultural innovations around 2000 BCE, coinciding with advancements in irrigation that supported intensive fruit cultivation. Assyrian texts and bas-reliefs from this period illustrate artificial pollination of date palms (Phoenix dactylifera), where male pollen was manually transferred to female flowers to enhance fruit set and yield, a technique that laid foundational principles for later vegetative propagation methods. This practice, essential for oasis agriculture in arid environments, is evidenced in royal palace carvings from Nimrud and Nineveh, dating to the 9th–7th centuries BCE, though the underlying methods likely originated earlier during the Bronze Age. Early vegetative propagation techniques in the region focused on figs (Ficus carica) and olives (Olea europaea), crops domesticated by 3000 BCE, using methods such as cuttings and layering to maintain desirable traits, though evidence for grafting remains limited and speculative. Archaeological evidence from sites like Jericho and Tell es-Sultan shows cultivated fig trees from 11,000 BCE, with propagation shifting from seeds to cuttings by the 2nd millennium BCE to combat variability and disease. These methods were integral to the economic and ritual life, symbolizing fertility in Mesopotamian iconography, such as the sacred tree motifs in Assyrian art representing abundance tied to agricultural mastery.[146] In ancient China, grafting emerged as a key propagation tool by the 1st century BCE, enabling the clonal reproduction of fruit trees amid the agricultural expansion of the late Zhou Dynasty and later periods. The Bamboo Annals, a historical chronicle compiled around the 4th century BCE but documenting events from the 3rd millennium BCE onward, references early fruit tree cultivation practices vital for economic activities, such as mulberry (Morus spp.) for sericulture in sustaining the silk industry.[147] These techniques allowed farmers to preserve superior varieties, shortening maturation times and boosting yields in terraced fields supported by hydraulic engineering. Grafting's adoption reflected China's emphasis on harmony between human intervention and nature, with mulberry propagation specifically enhancing leaf production for silkworm rearing, a cornerstone of economic and cultural exchange along early trade routes. The earliest indirect reference to grafting appears in the Fan Sheng-Chih Shu (1st century BCE), discussing agricultural practices including vegetative methods.[148] The Qimin Yaoshu (Essential Techniques for the Welfare of the People), authored by Jia Sixie in 544 CE during the Northern Wei Dynasty, serves as a seminal compilation of pre-existing knowledge, devoting sections to grafting methods like inarching and cleft grafting for pears, apricots, and persimmons. This treatise details rootstock selection, timing (spring for optimal cambial alignment), and binding with silk or bark to promote vascular integration, achieving survival rates through empirical observations of sap flow and wound healing.[149] Culturally, grafting held mythological resonance, evoking themes of renewal and transformation in Taoist texts, where it paralleled alchemical processes of merging essences; agriculturally, it underpinned treatises promoting sustainable farming, influencing imperial policies on food security and tribute systems.[150]

Developments in Europe, Rome, and the Americas

In the Greco-Roman era, spanning approximately 300 BCE to 500 CE, grafting practices were systematically documented and refined, building on earlier Eastern traditions. Theophrastus, a pupil of Aristotle often regarded as the father of botany, detailed various propagation methods in his treatise Enquiry into Plants (ca. 300 BCE), including inarching and budding techniques for olives and grapevines, emphasizing the importance of cambial alignment for successful union.[144] Cato the Elder further advanced these ideas in his agricultural manual De Agri Cultura (160 BCE), providing practical instructions on cleft grafting—where a scion is inserted into a split stock—for olives and vines, highlighting its role in rejuvenating unproductive trees and improving yields in Mediterranean climates.[151] These texts underscore grafting's integration into Roman estate management, where it supported viticulture and olive cultivation essential to the empire's economy. During the medieval period in Europe (c. 500–1500 CE), monastic communities became central to the preservation and dissemination of grafting knowledge, particularly for pome fruits like apples and pears. Benedictine and Cistercian monks maintained extensive orchards, using grafting to propagate superior varieties from wild or seedling stocks, thereby ensuring consistent fruit quality amid feudal agriculture's limitations.[152] For instance, 12th-century Cistercian foundations across England and France planted grafted orchards as symbols of paradise, experimenting with techniques to adapt trees to local soils.[153] Urban guilds, such as those of fruiterers in medieval London and Paris, imposed regulations on grafting practices by the 14th century to standardize propagation, prevent fraud in variety sales, and protect communal orchard resources, reflecting the technique's growing economic significance.[154] The introduction of grafting to the Americas occurred primarily through Spanish colonial efforts from the 16th to 19th centuries, transforming mission landscapes into productive orchards. Franciscan and Jesuit missionaries established self-sustaining complexes in regions like Alta California and the Southwest, planting grafted olives, figs, citrus, and stone fruits sourced from Spain and Mexico to support indigenous converts and supply chains.[155] At sites such as Mission San Gabriel (founded 1771), extensive orchards utilized cleft and whip methods to multiply varieties, with records indicating over 4,000 fruit trees by the early 1800s, blending European techniques with local irrigation systems.[156] Native American communities, including the Ohlone and Pima, observed natural grafting phenomena in indigenous flora—such as inosculated mesquite or oak roots—providing early insights into plant fusion that paralleled introduced practices. Key innovations in the 18th century further refined grafting for European and colonial pomology, with French agronomist Henri-Louis Duhamel du Monceau leading advancements in whip grafting. In his Traité de la culture des arbres fruitiers (1768), Duhamel described precise whip-and-tongue cuts to maximize cambial contact, demonstrating through experiments that this method increased union success rates for apples, pears, and cherries by promoting faster callus formation compared to earlier cleft techniques.[157] These refinements, disseminated via pomological societies, influenced transatlantic practices, enabling more reliable propagation in diverse climates from French chateaus to American mission fields.[158]

Impact of the French Wine Pandemic

The phylloxera crisis, triggered by the accidental introduction of the aphid-like insect Daktulosphaira vitifoliae to Europe in the 1860s, devastated French vineyards and nearly collapsed the wine industry. First detected in southern France around 1863, the pest rapidly spread, feeding on vine roots and causing widespread die-off. By 1900, approximately 2.5 million hectares of French vineyards—nearly half the total—had been destroyed, leading to massive economic losses, unemployment among growers, and a sharp decline in wine production that turned France into a net importer.[159][160] The crisis spurred the widespread adoption of grafting as the primary solution, involving the propagation of European Vitis vinifera scions onto phylloxera-resistant American rootstocks, such as Vitis riparia and hybrids derived from North American species. These rootstocks, which had co-evolved with the pest and developed natural defenses like thick, corky bark, allowed vines to survive while preserving the desired fruit qualities of European varieties. This approach, pioneered by entomologist Charles Valentine Riley and propagated through shipments from U.S. nurseries, enabled the replanting of over two-thirds of French vineyards by 1900 and ultimately saved the industry from total extinction, with grafted vines becoming the global standard for viticulture.[161][162] Despite its success, implementing grafting faced significant challenges, including strong initial resistance from French growers who viewed American rootstocks with suspicion due to their "foxy" flavors and perceived inferiority. Regulatory bans delayed adoption—grafting was prohibited in Bordeaux until 1881 and in Burgundy until 1887, forcing desperate vintners to defy authorities. Technical issues compounded the difficulties, with graft union failures occurring in up to 30-40% of early attempts due to incompatibility between scion and rootstock, alongside reports of subtle flavor alterations in wines, such as increased vigor leading to less concentrated fruit profiles.[163][162][164] The legacy of the phylloxera pandemic profoundly shaped modern viticulture, prompting the establishment of international quarantine laws to prevent future pest incursions. The 1881 Bern Conference marked the first global phytosanitary agreement, with seven European nations coordinating measures against phylloxera, laying the groundwork for organizations like the International Plant Protection Convention. Additionally, the crisis accelerated breeding programs for resistant hybrids, as French and American scientists crossed V. vinifera with resilient North American species to develop rootstocks and direct hybrids that balanced disease resistance with quality, influencing ongoing efforts to reduce chemical dependency in grape cultivation.[165][166][167]

Modern Cultivation Practices

Commonly Grafted Crop Plants

Grafting is widely employed in the propagation of major crop plants, particularly in the Rosaceae, Vitaceae, and Solanaceae families, to enhance economic viability through improved yield, disease management, and adaptation to challenging growing conditions. The majority of commercial fruit and nut trees worldwide are regularly grafted, enabling the commercialization of high-value cultivars on rootstocks that optimize performance and reduce production risks. This practice underpins a global fruit and nut industry that generates over $100 billion annually in economic value from key grafted commodities like apples, grapes, and stone fruits. In the Rosaceae family, apples (Malus domestica) are commonly grafted onto Malling series rootstocks, such as M.9 or MM.106, primarily for precise size control that facilitates high-density orchards and earlier fruiting. These dwarfing rootstocks reduce mature tree height to 2-4 meters, allowing mechanical harvesting and intensive management while maintaining scion vigor and fruit quality.[168] Cherries (Prunus avium and P. cerasus), including sweet and sour varieties, are frequently grafted onto rootstocks like Gisela 5 for dwarfing and precocious bearing or Colt for vigor and tolerance to some soil conditions, though root-knot nematodes (Meloidogyne spp.) can devastate yields in infested soils by up to 50% without intervention. Resistant rootstocks such as Mazzard or Mahaleb are preferred in nematode-prone areas. Similarly, peaches (Prunus persica) rely on nematode-resistant rootstocks such as Flordaguard or Nemaguard, which protect against Meloidogyne incognita and related species prevalent in sandy, warm-climate regions, thereby sustaining commercial viability in nematode-prone areas like the southeastern United States.[169] Grapes (Vitis vinifera) from the Vitaceae family are almost universally grafted onto rootstocks like 101-14 Millardet et de Grasset (101-14 Mgt), a hybrid of V. riparia and V. rupestris valued for its moderate to high vigor and adaptability to diverse soil types, including heavy clays and wet conditions. This rootstock promotes balanced canopy growth and phylloxera resistance, enabling cultivation across varied terroirs from California to Europe without compromising vine productivity.[170] Within the Solanaceae family, tomatoes (Solanum lycopersicum) are increasingly grafted onto interspecific rootstocks like 'Maxifort' (a hybrid involving S. lycopersicum and S. habrochaites) to provide robust resistance to Fusarium wilt (Fusarium oxysporum f. sp. lycopersici races 1 and 2), a soilborne pathogen that causes vascular clogging and up to 100% crop loss in non-resistant systems. This is particularly beneficial in hydroponic production, where 'Maxifort' enhances root development and nutrient uptake, boosting yields by 20-30% in soilless media while minimizing fumigant use.[171] Other notable examples include avocados (Persea americana), grafted onto salt-tolerant rootstocks such as Dusa or West Indian selections (e.g., 'Simmonds') to mitigate chloride uptake in saline-irrigated orchards, where ungrafted trees may suffer 40-60% growth reduction under electrical conductivity levels above 2 dS/m.[172] Walnuts (Juglans regia) utilize the Paradox hybrid rootstock (J. regia × J. hindsii), prized for its exceptional vigor that accelerates tree establishment and increases early yields by 20-50% compared to standard English walnut seedlings, supporting large-scale orchards in regions like California.[173]

Contemporary Innovations and Challenges

Recent advancements in grafting technology have focused on automation to address labor shortages in commercial horticulture. In Japan, automated grafting robots such as the ISEKI GR800 series achieve grafting efficiencies of up to 800 plants per hour with success rates exceeding 95% for solanaceous crops like tomatoes, enabling precise splice grafting under controlled conditions.[174] Similarly, systems like the ROBO-GRF from Kusakabe Kikai report over 95% success rates for tomato and eggplant grafting, incorporating machine vision for seedling sorting to minimize errors.[175] In 2025, further automation breakthroughs, such as high-speed grafters allowing tomato grafting above cotyledons, have reduced labor needs by up to 80%.[176] These innovations integrate into precision agriculture, reducing manual intervention while maintaining high throughput, though they require initial investment and skilled calibration.[27] Biotechnological integrations, particularly CRISPR-based approaches, have revolutionized trait enhancement through grafting. Mobile CRISPR/Cas9 systems delivered via rootstock grafting allow targeted genome editing in the scion without incorporating transgenes into the final plant, facilitating trait stacking for disease resistance and yield improvement in crops like tomatoes.[177] This method exploits natural RNA mobility across graft unions to create heritable edits, bypassing traditional genetic engineering limitations and producing non-transgenic outcomes suitable for regulatory approval.[178] Such chimeras enable the combination of desirable traits from diverse genotypes, enhancing adaptability in challenging environments.[179] Climate change poses significant challenges to grafting practices, particularly in arid regions where drought and heat stress threaten crop viability. For pistachio cultivation, grafting onto UCB-1 rootstock—a hybrid of Pistacia atlantica and P. integerrima—confers improved drought tolerance through enhanced osmotic adjustment and water-use efficiency, addressing yield gaps in water-scarce areas like California's San Joaquin Valley.[180] Studies show UCB-1 grafted plants exhibit greater physiological resilience under water stress compared to traditional rootstocks, with deeper root systems and reduced stomatal closure impacts.[181] However, inconsistent rootstock-scion compatibility in extreme conditions remains a hurdle, necessitating ongoing breeding for region-specific adaptations.[182] Sustainability efforts in grafting emphasize eco-friendly materials to mitigate plastic waste from traditional wraps. Biodegradable grafting tapes, such as the bio-based Tape2Grape developed by Fraunhofer IGB, decompose naturally post-union formation while providing secure, stretchable binding for fruit trees, reducing environmental persistence of non-degradable polymers.[183] Products like EONBON's biodegradable PE tapes further support this shift, offering self-adhesive properties without pollution.[184] Ethical concerns arise with genetically modified rootstocks, including potential biodiversity risks from gene flow and long-term ecological impacts, prompting debates on regulatory frameworks for their deployment in organic systems.[185] Emerging trends leverage grafting to bolster soil health and urban agriculture. Microbiome-enhanced grafting promotes beneficial rootstock-scion interactions, where synthetic microbial communities from grafted rhizospheres improve nutrient cycling and pathogen suppression in crops like watermelon, enhancing overall plant vigor and soil fertility.[186] In urban settings, community grafting workshops and starter kits facilitate fruit tree propagation in gardens, fostering local food security and education on sustainable practices.[187] Recent research as of 2025 has also highlighted autophagy's role in promoting callus formation and wound healing during grafting, advancing biological understanding.[111] These approaches integrate grafting into regenerative agriculture, though scalability in diverse microbiomes and urban constraints presents ongoing challenges.[188]

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