What Is Plant Grafting? Understanding Grafting Principles in One Article

Grafting:

The advantage is that it allows you to obtain varieties with excellent genetic traits. Grafting is the process of joining the branch or bud of one plant onto the stem or root of another plant. The former is called the scion, and the latter is called the rootstock or stock. The scion generally uses a seedling with 2 to 4 buds, and after grafting it becomes the upper or top part of the plant; the rootstock after grafting becomes the root system part of the plant. Whether grafting succeeds depends first on the compatibility between the scion and the rootstock. Generally speaking, the closer the genetic relationship between the plants, the stronger the compatibility; it also depends on the regeneration capacity of the cambium cells on the cut surfaces of both, as well as the skill level of the grafting technique and other factors. Grafting is commonly used in the cultivation of superior fruit trees, such as apples, pears, and citrus.

A Brief Introduction to Grafting Techniques for Bonsai Plants

What Is Grafting?

Grafting is the process of joining part of one plant's organs, such as a bud or branch, onto the trunk or root of another plant, so that the two parts can heal together and grow into one complete plant. The bud or branch that is joined on is called the scion (commonly known as the "coded piece"), and the plant that receives the scion is called the rootstock (commonly known as the "mother piece," stock, or foot wood). The rootstock mainly serves to absorb water and inorganic salts. The seedling obtained through grafting is called a grafted seedling.

What Is the Principle Behind Successful Grafting?

Grafting is a propagation method that utilizes the regenerative capacity of plants. The place where a plant's regenerative capacity is most vigorous is the cambium. It is located between the xylem and phloem of the plant. It can absorb water and minerals from the outer phloem and the inner xylem, allowing itself to continuously divide, producing xylem inward and phloem outward, causing the plant's branches and trunk to continuously thicken. Grafting is the process of bringing the cambium of the cut surfaces of the scion and rootstock
into close contact with each other. Due to the wound, callus tissue differentiates, and the developed callus tissue combines with each other, filling the gap between the scion and rootstock, connecting the conducting tissues, so that nutrients can be conducted between them, forming a new plant.

What Are the Internal Factors Affecting Grafting Success?

The compatibility between rootstock and scion refers to the similarity in internal tissue structure, genetic and physiological characteristics between the rootstock and scion, and the ability to combine with each other through grafting and grow normally. Compatibility mainly depends on the genetic relationship between the rootstock and scion. Generally, the closer the genetic relationship, the stronger the compatibility. Within the same genus and species, the genetic relationship is close, and grafting easily succeeds. For example, grafting double-petaled peony onto single-petaled peony, walnut scion onto walnut, rose onto rose, and ornamental peach onto mountain peach are most likely to succeed. This type of grafting combination is called "common stock."

Between different genera of the same family, grafting compatibility is relatively low, and grafting is not easy to succeed. Between different families, compatibility is even lower, and grafting is very difficult to succeed.

What Are the External Factors Affecting Grafting Success?

1.    Temperature

The optimal temperature for callus tissue growth is around 20 to 25°C.

2.    Humidity at the Graft Union

Maintaining relatively high humidity is beneficial to callus tissue growth. Generally, plastic bags and wax sealing methods are used to maintain the humidity of the scion and rootstock.

3.    Light

Light has a clear inhibitory effect on callus tissue. Under dark conditions, callus tissue grows well, so after grafting, light should be blocked. Generally, soil mounding or wrapping with opaque materials is used to create dark conditions.

4.    Quality of Rootstock and Scion

The rootstock and scion should have strong vitality. Generally, well-lignified crown branches are selected, not growing branches, because growing branches easily lose water.

5.    Growth Habits of Rootstock and Scion

The growth habits of the rootstock and scion should be compatible. Generally, if the rootstock sprouts first, the survival rate is high; if the scion sprouts first, the survival rate is low, because when the scion sprouts first, it easily loses water and dies. For example, when the terminal bud of persimmon begins to sprout, grafting onto Diospyros lotus (round jujube) results in a high survival rate, regardless of whether the Diospyros lotus has sprouted.

6.    Grafting Period

Grafting is closely related to air temperature, soil temperature, and the active state of the rootstock and scion. Therefore, the grafting period should be chosen according to the characteristics of the flower species.

7.    Grafting Technique

The cut surfaces at the graft union are required to be smooth, and the operation should be fast and skillful.

When Is Grafting Suitable?

Branch grafting is mainly carried out in early spring before the flowering trees sprout. At this time, the tissues of the rootstock and scion are full, and the temperature and humidity are also favorable for cambium division and callus tissue growth. Softwood grafting is best done during the plum rain season in summer.

Bud grafting is mainly carried out in summer and autumn, when the cambium cells are still very active, the tissues for bud grafting are full, the graft heals in the same year, and after overwintering, the bud sprouts and survives the following spring, which is very suitable.
suitable.
Generally, branch grafting is carried out in February to March, approach grafting in May to June, and bud grafting in July to September. In cold regions, the time for bud grafting should be advanced to avoid frost damage.

How to Graft

1. Cut Grafting

Cut grafting is the basic grafting technique, suitable for rootstocks with a root collar of 1 to 2 cm for ground-level grafting. The specific operation steps are as follows:

(1) Select rootstock and scion. Use young, tender branches with full tissue development as the scion. One-year-old branches facing the sun with short internodes are best. Do not use diseased or pest-infested branches, flower bud branches, secondary branches, or weak branches. Deciduous trees can have scions collected and stored after dormancy (can be stored in sand, refrigerator, or wax-sealed). Roses (monthly roses) and others are not easy to store and survive, so they should be grafted as soon as collected. Evergreen trees are generally not stored and can be grafted as soon as collected. The rootstock should be robust, with a well-developed root system, free of pests and diseases, with high nutrient accumulation, and attention should be paid to the compatibility with the scion.

(2)    Cut the scion. Cut a section from the middle of the branch, about 10 to 12 cm long, as the scion. Keep 3 to 4 buds on the scion. The top cut should be slightly higher than the topmost bud to protect the bud from being damaged. The bottom cut should be 3 to 4 cm below the lowest bud. The scion length is 5 to 8 cm, with 3 to 4 buds. Cut the scion into two cut surfaces, one long and one short. The long surface should remove more than 1/3 of the xylem, forming a 30-degree slope, with a cut surface length of 2.3 to 2.5 cm. The lowest bud should remain on both sides of the cut surface; on the opposite side of the long surface, cut a horseshoe-shaped small slope at 45 degrees, about 0.5 cm long. These two cut surfaces should be flat and smooth.

(3)    Cut the rootstock. Cut off the rootstock branch, cutting the rootstock 3 to 4 cm above the ground. Trim the rootstock end flat, then make a light cut on one side of the rootstock cut, then cut vertically downward, with a cut surface length of about 2 to 3 cm, striving for smoothness. Such a cut surface has a large cambium area and is easy to align and closely contact with the scion cambium.

(4)    Join. Insert the prepared scion into the rootstock cut. The long cut surface of the scion should face inward, and the short one outward. When inserting, stop as soon as the scion just touches the lower end of the rootstock cut. Do not press down with force. The upper end should be left exposed by a line (commonly known as "leaving white," about 0.3 cm), leaving room for callus tissue growth.

(5)    Tie. The joint between the rootstock and scion should be tied carefully. When tying, do not move the aligned position of the scion cambium, and the tightness should be moderate. Tying from top to bottom or from bottom to top is both acceptable. Plastic strips 0.5 cm wide are commonly used as tying material, which has good moisture retention. For flowers and trees that are difficult to survive, grafting wax should also be applied.

(6)    Management. For general flowers and trees, open-field grafting and planting can be done. It is only necessary to cover the cut grafting area with fine soil, with the soil 1 cm higher than the scion, exposing only the terminal bud. Camellia can be planted after bagging. However, hotbed cultivation can promote healing and improve the survival rate. Afterwards, water continuously from the base to keep the soil mound moist until budding. After the cut-grafted seedling survives and unfolds leaves, do not rush to remove the tying material; it can be kept for one to two months.
After 1 to 2 months of branch grafting, buds can emerge. After the buds emerge from the soil, gradually remove the covering soil to expose the graft union.

2.    Cleft Grafting

When the rootstock is thick and the scion is thin, cleft grafting is used.

(1)    Rootstock preparation. Cut off the rootstock branch, cut the rootstock at the grafting site, and trim the rootstock end flat. It is required that there be no knots within 6 cm below the cut. Make a vertical split from near the center of the rootstock cross-section. The knife should be sharp and slightly thick, and this cut should split the rootstock

into a wedge shape. The split depth is 3 to 4 cm.

(2)    Scion cutting method. The scion is also cut into a wedge shape, with two symmetrical cut surfaces, each 3 to 5 cm long.

(3)    Join. Use a cleft knife to pry open the rootstock split, insert the scion, and gently remove the cleft knife, so that the cambium of both sides closely contacts. When the rootstock is thick, you can align only one side of the cambium, or graft one scion on each of the left and right sides of the rootstock split, and after survival, keep the one that develops well. When inserting the scion, the scion cut surface should be exposed by 0.5 cm.

(4)    Tie. After grafting, tie with wool or hemp rope and mound soil. For softwood cleft grafting of camellia, pine, and similar plants, bags can be used to retain moisture (softwood is mostly cleft grafted).

(5)    Other operating essentials are basically the same as cut grafting.

3.    Approach Grafting

The approach grafting method is mostly used for species that are difficult to propagate by other grafting methods, but are precious and difficult to graft, such as osmanthus, maple, camellia, bergamot, and other citrus. This is because during approach grafting, the rootstock and scion are both brought together without leaving the mother plant. Only after survival is the scion cut from the mother plant. Therefore, propagation by approach grafting is the most reliable.

The time for approach grafting is mostly during the period of vigorous flower growth in summer.

For flowers and trees propagated by approach grafting, the mother plant must first be planted next to the rootstock, or planted in the same flower pot. The rootstock can also be hung on the scion mother tree or placed beside it. The rootstock should be a branch or trunk more than 2 years old, with a diameter of 1 to 1.5 cm; the scion should be a 1- to 2-year-old branch with lateral branches from a mature plant, with a diameter of 0.6 cm and a length of 15 cm.

A portion is removed from both the rootstock and scion. Align the cambium of the rootstock and scion, leaving no gap, so that the cambium of the wound surfaces closely adheres to each other. Then tie firmly with plastic strips or hemp rope, and prevent rainwater from entering. After successful grafting, cut off the scion below the graft union, and cut off the upper part of the rootstock in batches, making it an independent plant. Generally, it is required that the scion mother tree has a larger crown and the rootstock tree has a smaller crown. Four or five branches can be approach grafted at one time.

After approach grafting, no fertilizer is applied, but the pot soil should be kept moist at all times. In 1 to 2 months, the cut will heal, and then the scion and mother plant are cut apart, making it an independent plant.

4. Side Grafting

Side grafting, also called root-collar grafting, is a branch grafting method that does not cut off the rootstock crown. It is generally grafted at the root collar on the side of the rootstock. It is suitable for the propagation of five-needle pine and Japanese white pine by grafting. It is also used for needle juniper, dragon juniper, and Oriental arborvitae. In recent years, it has also been tried on rhododendron and camellia.

There are many specific methods of side grafting. In flower and tree production, cut side grafting, bark grafting, and sub-bark side grafting are the most widely used.

(1) Cut side grafting. This is the traditional side grafting method and is the most commonly used, generally carried out in early spring. The specific steps are as follows (taking five-needle pine as an example):

①    Cut the scion: Select a 1- to 2-year-old robust, pest-free five-needle pine branch as the scion, 5 to 8 cm long. Remove the needles from the lower 2 to 3 cm, make a diagonal cut about 2 cm long, and cut a small slope on the opposite side about 0.5 cm long. The cutting method is the same as cut grafting.

②    Cut the rootstock: Select a 2- to 3-year-old black pine as the rootstock (Masson pine is also acceptable, but the survival rate is lower than black pine). After digging it up with a soil ball, cut off some branches and leaves, trim the root system, and at the straight, smooth part of the stem at the root collar, make a diagonal cut 2.5 cm long, reaching the xylem, forming a shield-shaped opening. You can also cut vertically slightly into the xylem. The cut should be slightly longer than the scion cut surface, otherwise close contact is difficult.

③    Join and tie: After the rootstock and scion are cut, quickly insert the scion into the rootstock cut and align the cambium, then tie with plastic film strips. Put a small plastic bag over the scion and graft union to prevent water loss. Remove the plastic bag after the scion buds sprout needles.

④    Management: For side-grafted five-needle pine, plant in a high, dry location, with the graft union facing north, and cover the graft union with soil to retain moisture. In the early healing stage (about 〖 months), it is best to provide shade. Afterwards, provide filtered light. In hot summer, shade is required. Just after needles emerge, avoid direct sun exposure.

(2) Sub-bark side grafting. This is an improved side grafting method, suitable for grafting during the growing season (June to September). The scion is taken from current-year semi-lignified branches, so it is also called softwood side grafting. This method has a high survival rate. The scion cutting and operation are the same as cut side grafting. The rootstock is cut into a T shape: first make a horizontal cut, then make a vertical cut in the center of the horizontal line, lift the bark, expose the cambium, and finally join and tie.

5. Water Branch Grafting

Water branch grafting is also called water insertion grafting and hanging bottle grafting. It is used for camellia and rhododendron. During the process of callus healing and survival, the scion replenishes water from the bottle and is not easy to wither.

(1)    Carve the rootstock. The rootstock for water branch grafting is a perennial old trunk or curved thick root. First plant it in a pot to survive, cut it into a stump, and let a few sparse lateral branches sprout. At the upper end of the rootstock, use a round-blade carving knife to cut diagonally into a groove, with the upper end slightly reaching the xylem, about 2.5 cm long.

(2)    Cut the scion. The scion is about 25 cm long, with 3 to 4 leaves retained at the upper end. Cut the middle into a convex V shape, about 3 cm long. The cut must reach deep into the xylem. Otherwise, growth will be poor.

(3)    Tie. The scion cut surface should be exposed 0.5 cm above the rootstock cross-section, which is beneficial to callus tissue formation. At the joint, the scion should be slightly higher than the rootstock (about 3 to 4 mm), and should not sink into the groove. Use tying material to wrap the entire rootstock.

(4) Hang a bottle. Finally, hang a small bottle, fill it with water, and fix the small bottle at the base of the scion. When the callus heals and survives, remove the hanging bottle, cut off the scion below the graft union, and place it under a shade shed for care. The tying material can be removed after 1 month. Water branch grafted seedlings grow rapidly.

Why Should Suckers Be Pruned Away Promptly?

"Suckers" mostly refer to branches growing from the base of the rootstock of a grafted seedling. For example, when a rose is grafted onto a rose rootstock, the branches growing from the rose rootstock are called suckers. This situation also occurs when lilac is grafted onto privet, red maple onto green maple, and so on. If the suckers on the rootstock are not pruned away promptly, they will inevitably take nutrients from the scion, affecting its growth and development, and in severe cases causing the scion to wither and die.

Learning Grafting Propagation Techniques for Flowering Plants

The following are grafting propagation technique learning resources for flowering plants that I collected from the internet. I thank the author here. (When reposting, I could not see who the author was, so I can only silently thank them.) I believe that the illustrated grafting propagation techniques for plants here will be helpful to everyone, so that in future plant grafting propagation, you can avoid many detours.

What Is Grafting
01. Grafting: That is, joining the branch or bud of one plant onto the stem or root of another plant, so that the two joined parts grow into one complete plant. In short, grafting uses a plant'svegetative organpart and transfers it onto another plant body. One of the artificial vegetative propagation methods of plants
Grafting: That is, people purposefully join the branch or bud of one plant onto the branch, trunk, or root of another plant, so that they heal and grow together, forming one new plant.
02. Related Terms:
The seedling after successful grafting (the seedling cultivated through grafting) is called agrafted seedling。
The branch or bud used for grafting (that is, the branch joined on) is called thescion; the bud used is called thegraft bud.(Both are commonly known as the "coded piece")
The plant that receives the scion (that is, the plant being grafted onto) is called therootstockor stock (commonly known as the foot tree).

03. Advantages of Grafted Seedlings:
①Grafted seedlingscan maintain the traits of the superior variety scion, and grow fast, with strong tree vigor and early fruiting. Therefore, they help accelerate the promotion and application of new varieties.
② Certain traits of the rootstock can be utilized, such as drought resistance, cold resistance, waterlogging tolerance, salt-alkali tolerance, and pest and disease resistance, to enhance the adaptability and stress resistance of cultivated varieties, expand the cultivation range, or reduce production costs.
③ In fruit tree and flower and tree production, the rootstock can be used to regulate tree vigor, making the tree dwarf or standard, to meet different needs in cultivation or consumption.
④ Most rootstocks can be propagated by seed, so the propagation coefficient is large, which is convenient for large-scale promotion in production.

04. Grafting Methods: Divided intobranch graftingandbud graftingtwo major categories. The former is best carried out in spring and autumn, especially in spring with a higher survival rate; the latter is best carried out in summer.
When grafting, the cambium of the scion and rootstock should be closely combined to ensure the survival of the scion.

For the scion, generally a seedling with 2 to 4 buds is selected. After grafting, it becomes the upper or top part of the plant, and the rootstock after grafting becomes the root system part of the plant.

Grafting Principles:
Grafting propagation is a method of propagating superior clones, that is, a type of vegetative reproduction within asexual propagation. It is commonly used for plum blossom, monthly rose, and others.
Grafting utilizes the healing ability that plants have after being wounded. Grafting can demonstratethat the cell nucleusis the center of genetic control.
When grafting, the cambium of the two wound surfaces is brought into close proximity and tied tightly together. As a result, due to cell proliferation, they heal with each other and become a whole connected by vascular tissue. This is the principle of grafting.The key to graftingis to make the scion cambiumclosely adhere tothe rootstock cambium.
The principle of successful grafting is that between two plants with compatibility, at the joint,the cambiumproduces a healing phenomenon, causing the vessels andsieve tubesto interconnect, forming a new individual.
Representation of grafting: The symbol "+" is used for grafting, that is, rootstock + scion; it can also be represented by "/", but its meaning is exactly the opposite of what "+" represents.
Generally, the scion is placed before the "/". That is:Scion/rootstock == rootstock + scion，
Osmanthus/privet == privet + osmanthus；Cultivated walnut/wild walnut == wild walnut + cultivated walnut.

Influencing factors：
The main factor affecting the survival of grafting is thescionandrootstockcompatibility; next is the grafting technique and the management after grafting.
The so-calledcompatibilityrefers to the ability of the scion and rootstock to be identical or similar to each other in internal tissue structure, physiology and genetics, so that they can combine with each other.
The higher the compatibility, the higher the survival rate of grafting; conversely, the lower the survival rate.
Generally speaking, the closer the genetic relationship between plants, the stronger the compatibility. For example:
Apple grafted onto sand apple; pear grafted onto birchleaf pear,Ussurian pear; persimmon grafted ontoblack date; walnut grafted onto Manchurian walnut, etc., all have very good compatibility.

Significance of grafting：
In production practice, there are also very many examples of grafting increasing economic value, such as:
Ordinarydawn redwood, worth one yuan; while through the means of grafting, after being cultivated intogolden-leaf dawn redwood, the economic value increases by more than 20 times;
Ordinarylarge-leaf privettrees, worth several jiao; while through the means of grafting, after being cultivated intocolorful-leaf Zelkovatrees, their economic value is as much as nearly a hundred times higher.
From this it can be seen that grafting is of very important significance to the improvement of varieties and the increase of economic value.

Grafting is of great significance for the propagation of some fruit trees that do not produce seeds (such as persimmon,citrussome varieties).
Grafting can not only maintain the excellent traits of the scion variety, but also make use of the beneficial characteristics of the rootstock, achieving early fruiting, enhancing cold resistance, drought resistance, and resistance to pests and diseases, and can also economically use propagation materials and increase the number of seedlings.
It is commonly used in the propagation of fruit trees, forest trees, and flowers; it is also used in the seedling cultivation of melon vegetables.

Function enhancement 01. Enhancing plant disease resistance. Cucumbers grafted onto black-seeded pumpkin can effectively controlcucumber Fusarium wilt; at the same time,
it can also delaydowny mildewthe occurrence period; after using CRP (spiny eggplant) and tomato as rootstocks to graft eggplants, the occurrence ofVerticillium wiltcan basically be controlled.
02. Improving the low-temperature tolerance of plants. Because the rootstock root system is well developed and has strong stress resistance, grafted seedlings are obviously tolerant to low temperatures. For example:
Cucumbers grafted onto black-seeded pumpkin have good root elongation at low temperatures, and the root system can still grow normally at a soil temperature of 12-15°C and an air temperature of 6-10°C.
03. Helping to overcome continuous cropping hazards. The cucumber root system is fragile and unsuitable for continuous cropping. Cultivation in solar greenhouses is extremely susceptible to damage from soil salt accumulation and harmful substances.After switching toblack-seeded pumpkin roots, the harm from soil salt accumulation and harmful substances can be greatly reduced.
04. Expanding the scope and capacity of root absorption. The root system of grafted plants increases severalfold compared with own-root seedlings. On the same area, it can absorb about 30% more nitrogen and potassium and more than 80% more phosphorus than own-root seedlings, and can also utilize phosphorus in deep soil layers.
05. Helping to increase yield. Grafted seedlings have thick stems and large leaves, which can increase yield by more than 40%.
For tomato, usinglate-maturing varietiesas the rootstock andearly-maturing varietiesas the scion not only retains earliness, but can also greatly shorten the fruiting period and increase total yield.

Grafting site

Grafting is best carried out in a greenhouse:
In hot seasonsuse shade nets or straw curtains for shading, avoiding direct strong light that causes excessive wilting of seedlings and affects survival. For example,deep-winter cropeggplant grafting in July coincides with the high-temperature period, and heat prevention and cooling are key.
In low-temperature seasons(such as cucumber and pumpkinoverwintering cropgrafting at the end of September to early October), heat preservation should be the main focus, as low temperatures are not conducive to wound healing; when grafting, the suitable temperature should be 24-28°C, and the relative air humidity should be above 75%. When humidity is insufficient, use a sprayer to spray water into the air or onto the walls to increase humidity.
Grafting tools 01. Blade: that is, an ordinary double-edged shaving blade. When grafting, break it into two halves, which both saves blades and facilitates operation.
02. Bamboo stick: one type is used for inserting holes in the rootstock during splice grafting, and its thickness is consistent with the thickness of the young stem of the scion seedling: one end is whittled into a wedge shape; the thickness of the other end is not strictly required. One end is whittled into a single-sided wedge, and it is used to remove the pumpkin growing point during approach grafting.
03. Grafting clip: used to fix the scion and rootstock. Grafting clips sold on the market include: one type iseggplant grafting clips, and one type ismelon grafting clips。
Old grafting clips must first be soaked in 200-fold formaldehyde solution for 8 hours for disinfection. Operators' fingers, blades, and bamboo sticks should be sterilized by applying 75% alcohol (medical alcohol), and disinfected once every 1-2 hours to prevent miscellaneous bacteria from infecting the wounds. However, blades and bamboo sticks wiped with alcohol cotton balls must wait untildrybefore use, otherwise the survival rate will be seriously affected.
04. Grafting machine: Becausethe problem of continuous cropping obstacles in melonsis becoming more and more prominent, vegetable grafting technology has attracted people's attention; specialized seedling-raising households and seedling companies have also emerged as the times require.
For specialized seedling-raising households and seedling companies, if they rely on manual grafting, due to low work efficiency and low grafting technology level, it is easy to delay the grafting timing.
It is hoped to adoptgrafting machinesfor operation. Small and semi-automatic grafting machines are welcomed in the market due to their low prices.
The Taiwan Suonuo grafting machine is a semi-automatic grafting machine, with a maximum productivity of about 310 plants/hour and a grafting success rate of 90%. Due to its simple structure, easy operation, and low cost, it has a certain sales volume both abroad and in our country, and is suitable for semi-mechanized operation of fruit tree branches, watermelon, cucumber and other melon vegetable seedlings.

Grafting process
After the scion is grafted onto the rootstock, on the surface of the wounds of the rootstock and scion, a brown film is formed from the residues of dead cells, covering the wounds.
Subsequently, under the stimulation of wound hormone, the cells around the wounds and the cambium cells divide vigorously, and cause the brown film to rupture, formingcallus tissue。
The callus tissue continuously increases. After the gap between the scion and rootstock is filled, the parenchyma cells of the callus tissue of the rootstock and scion connect with each other, connecting the cambium of the two.
The callus tissue continuously differentiates, forming new xylem inward and new phloem outward, thereby also making the vessels and sieve tubes communicate with each other, so thatrootstock-scioncombine intoa unified body, forming 1 new plant.
Compatibility between rootstock and scion: Grafting compatibility refers to the ability of the rootstock and scion to heal after grafting and grow normally. Specifically, it refers to the similarity of the internal tissue structure, genetic and physiological characteristics of the rootstock and scion, the ability to survive through grafting, and mutual physiological adaptation after survival.
Whether grafting can succeed, compatibility is its most basic condition. The stronger the compatibility, the better the grafting healing, the higher the survival rate, and the more normal the growth and development.
There are many manifestations of the phenomenon of rootstock-scion incompatibility or low compatibility, such as:
① Poor healing: after grafting, it cannot heal and does not survive; or the healing ability is poor and the survival rate is low.
Some can heal, but the grafted bud does not germinate; or the healing firmness is very poor, and it is extremely easy to break after germination.
② Abnormal growth and fruiting: although it can grow after grafting, the branches and leaves are yellowed, the leaves are small and clustered, and the growth is weak, so that it dies.
Some form a large number of flower buds early, or the fruit develops abnormally, the flesh quality deteriorates, and the fruit is deformed.
③ The growth above and below the rootstock-scion junction is uncoordinated, causing "big foot", "small foot" or "strangulation" phenomena (as shown in the figure below).

④ Late-stage incompatibility: some grafted combinations have good junction healing and can grow and fruit normally, but after several years they show severe incompatibility. For example, peach grafted onto hairycherryOn the rootstock, shortly after entering the fruiting stage, leaf yellowing and scorched shoot tips appear, and the branches, or even the entire plant, age and die.
The strength of compatibility depends on how closely the rootstock and scion are related. Generally, the closer the relationship, the stronger the compatibility.
Compatibility is strongest between the same species or the same cultivar, such as Chinese chestnut grafted onto Chinese chestnut, or Nanguo pear grafted onto Ussurian pear (Pyrus ussuriensis); compatibility between different species of the same genus isstrongerthan between different families and genera.
In addition, the tissue structure, metabolic status, and physiological and biochemical characteristics of the rootstock and scion have a great bearing on grafting compatibility. For example, when Chinese chestnut (Castanea mollissima) is grafted onto Japanese chestnut (Castanea crenata), incompatibility occurs because the latter absorbs more inorganic salts, whereas when Chinese chestnut is grafted onto Chinese chestnut, compatibility is good. When grafting persimmon or walnut, survival is often affected because of the large amount of tannins present.

Grafting time: The success or failure of grafting is closely related to air temperature, soil temperature, and the active state of the rootstock and scion.
The appropriate grafting time should be chosen according to the characteristics of the tree species and the requirements of the method. Grafting in the rainy season or in windy weather is not good. Generally, 20-25 degrees C is suitable.

Temperature: Different tree species and grafting methods have different temperature requirements. For example:Walnutthe optimum temperature for callus formation after grafting is 26-29 degrees C;
Grapethe optimum temperature for indoor grafting is 24-27 degrees C; above 29 degrees C the callus formed is tender and easily damaged during planting; below 21 degrees C the healing tissue forms slowly.
Humidity: Maintaining relatively high humidity at the graft union helps callus formation, but do not immerse it in water.
Oxygen: Callus formation requires sufficient oxygen, especially for certain tree species that need more oxygen; for example, when grafting grapes by the hardwood method, the graft union should be tied loosely and does not need waxing.
Light: Light inhibits the growth of callus tissue.

Grafting methods

According tothe scion(that is,the material taken）) differencesgrafting can be divided intobud grafting、stem grafting、root grafting3 major categories;

01. Bud grafting: A method of grafting in which a bud patch (with or without a little wood) is cut from a branch as the scion, inserted into a cut made on the rootstock, and tied so that the two unite and heal closely.
Bud grafting should be carried outduring the slow-growth periodbecause at this time thecambiumcells are still very active, and the tissues of the bud have also become well filled.
Grafted this year, it heals, and next spring it sprouts into a seedling, which is very suitable.
Grafting too earlycauses the bud to sprout in the same year, and in winter it cannot lignify and is easily damaged by frost;grafting too latemakes the bark of the rootstock difficult to peel.
Climatic conditions also affect grafting; the cambium andcallus tissuecan only become active at a certain temperature.
When air humidity is close to saturation, it is most suitable for healing; when grafting outdoors, even more attention should be paid to weather conditions.
Because bud graftingdiffers in formit is divided into different methods such as bud-patch grafting, shield budding, tube budding, and bud-eye grafting, among which bud-patch grafting is the most widely used. Its advantages are:
the operation is simple, grafting is fast, both rootstock and scion are used economically, one-year-old rootstock seedlings can be grafted, and it heals easily, joins firmly, has a high survival rate, and produces seedlings quickly, making it suitable for mass propagation of nursery stock.
The period suitable for bud grafting is long, and the rootstock is not cut off at the time of grafting; if one grafting fails to take, it can be re-grafted. The main bud grafting methods are introduced below.

A. "T"-shaped budding: Because the cut on the rootstock looks very much like the letter "T", it is also called "T"-shaped budding. And because the bud patch cut off is shield-shaped, it is also calledshield budding。
"T"-shaped budding is a widely used grafting method in fruit tree propagation, and is also the method with simple operation, high speed, and the highest grafting survival rate.
The bud patch is 1.5-2.5 cm long and about 0.6 cm wide; the rootstock diameter is between 0.6-2.5 cm. If the rootstock is too thick and the bark thickens, survival is instead affected. Specific operation:
Cutting the bud: Hold the scion in the left hand and the grafting knife in the right hand. Select a plump bud on the scion, first make a horizontal cut 0.5 cm above the bud, cutting through the bark, with the horizontal cut about 0.8 cm long.
Then, 1-1.2 cm below the bud, make an upward slanted cut, going from shallow to deep, penetrating into the xylem, and meeting the horizontal cut above the bud. Then use the right hand to pry out the shield-shaped bud patch.
Opening the rootstock: Select arootstock5-6 cm above the ground at a smooth, branch-free place and make a horizontal cut, with the depth appropriate to cut through the bark and reach the xylem. Then, from the middle of the horizontal cut, make a vertical downward cut 1-1.5 cm long.
Joining: Use the tip of the budding knife to lift open the bark of the rootstock, insert the bud patch into the "T"-shaped cut, and align and fit the horizontal cut of the bud patch with the horizontal cut of the rootstock.
Tying: Tie with a plastic strip. First tie one tight band above the bud, then tie one tight band below the bud, then wrap around three or four more times and tie a slip knot. Be careful to expose the petiole; whether the bud is exposed or not is acceptable.
B. Chip budding: For tree species whose shoots have ridges or grooves, such as Chinese chestnut and jujube, or when other plant material rootstocks and scions do not slip bark, the chip budding method can be used.
Use a knife to make a downward slanted cut 0.8-1 cm above the bud of the scion, penetrating into the xylem, about 1.5 cm long, then make a slanted cut at 30 degrees below the bud 0.5-0.6 cm away to meet the first cut, and remove the inverted shield-shaped bud patch. The cut on the rootstock is slightly longer than the bud patch; after inserting the bud patch, note that the upper end of the bud patch must expose a line of rootstock bark. Finally, tie tightly with a plastic strip.C. Approach grafting:Hang the rootstock close to the mother plant from which the scion is taken, select branches or trunks of similar thickness and smoothness on both sides, cut away 1/3 to 1/2 of the branch thickness on each, with the cut surface 3-5 cm long, align the cambium of the two cuts, tie tightly with plastic film strips, and after the two graft unions heal and survive, cut off the branch of the scion mother plant at one part of the union and cut off the upper part of the rootstock, thus forming a new plant.

D. Insertion grafting:First use a bamboo skewer to remove the true leaves and growing point of the bottle gourd seedling, and at the same time insert the bamboo skewer downward 0.5-0.7 cm deep from the growing point between the cotyledons of the rootstock, then cut the watermelon seedling 1 cm below the cotyledons with a blade into a wedge shape of about 0.5 cm, and while pulling out the bamboo skewer, insert the watermelon seedling; this isstraight insertion. Another insertion grafting method isoblique insertionusing a single-beveled bamboo skewer as thick as the scion, with the flat side of the skewer facing downward, inserting obliquely from the base of one cotyledon of the bottle gourd seedling toward the other side, with the tip of the bamboo skewer pressing against the epidermis of the young stem or piercing through the epidermis, then cutting the watermelon seedling 1 cm below the cotyledons into a slanted surface, and while pulling out the bamboo skewer, quickly insert the slanted young stem of the watermelon seedling downward. After grafting, move it into a shed for strengthened management.

02. Stem grafting: Grafting a branch with several buds or 1 bud onto the rootstock is calledstem grafting. Theadvantagesof stem grafting are a high survival rate and fast growth of the grafted seedling.
When the rootstock is relatively thick and neither rootstock nor scion slips bark, stem grafting is mostly used, such as in spring for re-grafting rootstocks that failed in autumn bud grafting.Root graftingandindoor graftingalso mostly use the stem grafting method.
Disadvantages: The operation technique is not as easy to master as bud grafting, and more scion is used, and the rootstock is required to have a certain thickness. Common stem grafting methods include cut grafting, cleft grafting, bark grafting, side grafting, and tongue grafting.
A. Cut grafting. This method is suitable for rootstocks 1-2 cm thick at the root crownfor field graftingand is one commonly used method in stem grafting.
Cutting the scion: The scion is usually 5-8 cm long, preferably with three or four buds. Cut the lower part of the scion into 2 cut surfaces, 1 long and 1 short; the long surface is on the same side as the lateral bud, cutting away more than 1/3 of the xylem, about 3 cm long, and on the opposite side of the long surface cut a small horseshoe-shaped slanted surface about 1 cm long.
Rootstock treatment: Cut off the rootstock trunk 3-4 cm above the ground. Choose a place where the rootstock bark is thick, smooth, and straight-grained, smooth the cut surface of the rootstock, and then make a vertical downward cut at the edge of the xylem. The width of the cut is equal to the diameter of the scion, and the depth is generally 2-3 cm.
Joining: Insert the scion with the large cut surface facing inward into the cut on the rootstock. Align and fit the cambium of the scion with that of the rootstock. If both sides cannot be aligned, aligning 1 side is also acceptable.
Tying: Wrap tightly with plastic, and wrap the split seam and cut opening completely and firmly. Be careful not to disturb the scion when tying.
B. Cleft grafting. Remove the growing point and heart leaves of the rootstock, and make a vertical downward cut 8-10 mm long between the two cotyledons; cut the scion about 1 cm below the cotyledons with a blade on both sides of the young stem into a double-beveled wedge 8-10 mm long, align the double wedge of the scion with the rootstock cut and insert gently, so that the two cuts fit tightly, and fix with a grafting clip.
It is an ancient grafting method and is very widely used. It can also be used for relatively thin rootstocks and is very suitable for top grafting of fruit trees.
Cutting the scion: Cut the scion into a wedge shape with 2 symmetrical cut surfaces, 3-5 cm long.The outer side of the scionshould be slightly thicker than the inner side. If the rootstock is too thick and the clamping force is too great, the inner and outer thickness can be made equal or the inner side slightly thicker, to prevent crushing the joining surface.The cut surface of the scionis required to be flat and smooth; a rough and uneven cut surface is not easy to join closely.
When cutting the scion, hold the scion firmly with the left hand and push the knife obliquely into the scion with the right hand. The force of pushing the knife should be even and consistent from front to back, and the direction of pushing the knife should be kept consistent with the direction of the cut. If the force is uneven, or the force from front to back is inconsistent, the cut surface will not be smooth, and if the direction deviates upward midway, the cut surface will not be straight. If the cut is uneven, one or two more cuts can be made to bring the cut surface up to standard.
Rootstock treatment: Cut or saw off the rootstock at the grafting site. The position of the cut is very important; the surface of the remaining stump should be smooth and the grain straight, with no scars at least within 6 cm above and below, otherwise the split seam will not be straight and the xylem will split to one side. After the grafting site is selected and cut off, use a splitting knife to make 1 vertical split in the center of the rootstock, with the split opening 3-4 cm deep.
Joining and tying: Use the wedge part of the splitting knife to pry open the split opening of the rootstock, gently insert the scion into the rootstock, with the thick side of the scion facing outward and the thin side facing inward, then gently remove the splitting knife. When inserting, special attention must be paid to aligning the cambium of the rootstock with the cambium of the scion. Generally, the bark of the rootstock is often thicker than the bark of the scion, so the outer surface of the scion should be slightly inward from the outer surface of the rootstock, so that the cambium layers can align with each other. The xylem can also be used as the standard, aligning the xylem surface of the rootstock with that of the scion, and the cambium will then also be aligned.
When inserting the scion, do not insert the entire cut surface; leave about 0.5 cm of the cut surface exposed. In this way, the contact surface between the cambium of the scion and rootstock is larger, and it also facilitates the formation and healing of meristematic tissue. A thicker rootstock can have 2 scions inserted, one on each side. Then, tie tightly with a plastic strip.
C. Tongue grafting. Often used for stem grafting of grapes, generally suitable for grafting when the rootstock diameter is about 1 cm and the rootstock and scion are roughly the same thickness.
Cut a slanted surface about 3 cm long on the back side of the lower bud of the scion; then, at 1/3 of the cut surface from bottom to top, split upward along the branch, with the split opening about 1 cm long, forming a tongue shape.
The rootstock is also cut into a slant of about 3 cm long, with the slant going from top to bottom at l/3, splitting downward along the rootstock, with the split about 1 cm long, matching the slant part of the scion.
Insert the split of the scion into the split of the rootstock so that the rootstock and scion interlock in a tongue-like manner, then align the cambium and push inward firmly.
If the rootstock and scion differ in thickness, simply align the cambium on 1 side. Once joined, tie them together.
D.Machine method:Grafting machines are used for the work; small and semi-automatic grafting machines are popular on the market because of their low price.
Grafting machine application example:

03. Root grafting method: (to be determined)
Development overview
Grafting has a long history in China. Records of grafting date back more than 2000 years, called "transferring flowers and grafting wood”；
People long ago discovered that when branches of trees in the forest rub against each other and are damaged, they become connected when they come close togethernatural graftingphenomenon, called in ancient China "trees joined in harmony". Grafting was inspired by this natural phenomenon and was created as a production technique.
In ancient Europe,Aristotleand the ancient Roman scholar Pliny both mentioned grafting at different times.
In the 5th century,scion graftingand budding techniques were increasingly used in the Mediterranean region.
In the 16th century, England already hadcleft grafting, crown grafting and whip-and-tongue grafting and other scion grafting methods.
The widespread application of budding techniques in Europe came after the 17th century, when it was mainly used to propagate stone fruit trees such as peach, nectarine and apricot.
Early records of grafting in China -- seen in the "Fan Shengzhi Shu", which contains a method of grafting 10 bottle gourd seedlings into one vine to produce a large gourd.
The Northern Wei dynasty "Qimin Yaoshu" gives a detailed description of the selection of rootstock and scion in fruit tree grafting, the grafting period, and how to ensure grafting survival and the effects of grafting.
In the several hundred years from the 6th to 13th centuries, grafting techniques developed greatly in ornamental plants such as peony andchrysanthemumand in fruit trees. In the Southern Song dynastyHan Yanzhipraised the wondrous citrus grafting technique in his work "Ju Lu", saying "human effort can participate in creation in this way". In the 13th century, due to the development of sericulture, mulberry grafting received attention. In the 17th centuryWang Xiangjinin "Qunfangpu" discussed that combining grafting with cultivation can promote plant variation. By the early Qing dynasty, works such as "Hua Jing" further confirmed the effect of grafting in changing plant traits.
Using modern scientific methods, the history of research on grafting is less than 100 years, but it has developed very rapidly.
Practical applications
In fruit tree production, grafting is used not only to maintain the fine characteristics of varieties;
it is also used to bear fruit earlier, overcome the difficulty that some species are not easy to propagate, provide disease resistance and immunity, and prevent insect pests;
in addition, the climatic and soil adaptability of the rootstock can be used to expand the cultivation area and improve yield and quality;
and to make fruit trees dwarf or vigorous (Figure 3).
In the production of ornamental plants and the like, often --
the root grafting method (Figure 3a) is used to restore tree vigor and preserve ancient and famous trees;
thebridge graftingmethod (Figure 3b) is used to save large trees whose trunks have been damaged (see tree protection);
thetop graftingmethod is used to replace the original inferior varieties of large trees and make up for incomplete crowns. Using top grafting to change varieties can also solve the pollination problem of fruit trees that do not set fruit by self-pollination or are dioecious, as well as the propagation and shaping of special ornamental tree varieties such asSophora japonica 'Pendula', weeping peach and weeping plum.
In forest tree breeding, grafting techniques are often used to dwarf mother plants, so as to facilitate hybridization operations and establishseed orchards、scion orchards。
Special rootstocks are used to promote plant flowering, such assweet potatografted onto water spinach ormoonflowerto achieve hybridization purposes, which is even more a method commonly used by breeders.
In addition, grafting is often used as a means to study plant physiology, plant pathology,plant virologyandplant histogenesisand so on.
Many scholars often use grafting to clarify problems such as the local synthesis process of certain substances in individual organs and stage development.
The application of grafting machines, and the effect of the grafting machine -- as shown in the figure below

Development trends
The scope of application of grafting techniques is still constantly expanding. Besides fruit trees and ornamental trees, herbaceous plants such as vegetables as well as forest trees and other economic plants, such as rubber trees and cacao trees, are increasingly using grafting.
As for grafting materials, from ordinary scion grafting and buddingit has developed tosoftwood grafting, leaf grafting, embryo grafting, growing point grafting, bud-eye grafting of bulbs and tubers, and even grafting of inflorescences, stigmas, ovaries and fruits, etc.,almost all parts of the plant can be used for grafting。
Since 1980, on the basis oftissue culturetechnology,micropropagationandmicrografting。
have been developed. In sterilized tissue culture, micrografting with 0.10-0.14 mm isolated shoot tips can cultivatecitrus, apple and other fruit trees'virus-free seedlings, and it has already been applied in China, the United States and Spain. In addition to the continuous improvement and enhancement of grafting techniques, further exploringgraft compatibilitythe essence and the mechanism of rootstock influence will be the focus of future research.
Precautions
01. Choose rootstocks and scions with strong compatibility: compatibility refers to the ability of the rootstock and scion to heal after grafting. In general, the closer the genetic relationship, the stronger the compatibility, and the higher the survival rate of grafting.
02. Choose rootstocks and scions with strong vitality: vitality is related to the nutrients accumulated in the vegetative organs of the rootstock and scion. The more nutrients accumulated in the vegetative organs and the fuller the development, the stronger the vitality. Therefore, before grafting, water and fertilizer management of the rootstock should be strengthened to let it accumulate more nutrients, and branches that are mature and have plump buds should be selected as scions.
03. Choose the best grafting time: generallyscion graftingis suitable in early spring before fruit trees sprout, because at this time the rootstock and scion tissues are full, and temperature and humidity are also favorable for vigorous division of the cambium, speeding up wound healing. Whilebuddingshould be chosen during the slow growth period, so that the graft survives and sprouts into a seedling the following spring.
04. Useplant hormonesto promote healing: treat the scion with plant hormones before grafting, such as soaking in 200-300PPM naphthaleneacetic acid for 6-8 hours, which can promote cambium activity, thereby promoting wound healing and improving the survival rate of grafting.
05. Standardize technical operations. During grafting, movements should be quick, and the rootstock and scion should be cut well strictly according to the technique. The cut surfaces should be smooth, so that the cambium of the rootstock and scion is closely connected, the tying is moderately tight, and the binding is removed in due time.