What are the eight major factors affecting the survival rate of grafting?
In the past, walnut grafting was relatively difficult and the survival rate was low, until around 2000 when the problem was solved, the grafting survival rate increased to over 90%, and grafting workers everywhere could master this skill proficiently. Overall, there are many factors affecting walnut grafting survival, and in production special attention should be paid to the following aspects:
1. Quality of rootstock and scion. From the mechanism of graft survival, only when both the rootstock and the scion can produce sufficient callus tissue, and the callus tissue differentiates to form connecting tissue, can a new plant finally be formed. This requires both the rootstock and the scion to have strong vitality, especially the quality of the scion, because the scion must rely entirely on its own stored nutrients to survive for a period. If the scion quality is poor, the survival rate will be greatly reduced. Therefore, in production, avoid using poor-quality scions. For stem grafting, the pith of the scion should be small and the buds should be plump; for bud grafting, the scion should be lignified or semi-lignified. The buds at the base and tip of the branch are of poor quality and cannot be used for grafting.
For bud grafting, rootstock seedlings are generally sown in spring of the first year, cut back to the base in spring of the second year, and grafted from late May to mid-June after new shoots emerge, requiring the thickness of the rootstock at the grafting site to reach 1~2.5 cm. If grafting is done on a 2-year-old part, the survival rate is greatly reduced. Some people vividly call the grafted walnut seedling the "three-bend seedling": the first bend is the 2-year-old part of the rootstock, the second bend is the new shoot that grows in the current year, and the third bend is the grafted variety.
Scions for bud grafting should be cut during the period from late May to mid-June. At this time, the growth amount of walnut new shoots is generally not very large, the buds are low in maturity, and there are not many usable buds on a branch. In order to obtain higher-quality scions, the mother plant can be planted in a greenhouse or plastic tunnel so that it sprouts earlier. When grafting can be done in the field, because the scion branches have grown for a longer time, the growth amount is greater, the buds are plump, the maturity is high, and the survival rate after grafting is high. On the same vegetative branch, the buds in the middle and lower parts develop well and can be used as scions, while the buds at the top and base develop poorly and generally cannot be used. Note that branches with sprouting buds cannot be used as scions for stem grafting. Scions for stem grafting are best collected during the dormant period, generally cut before bud break.
2. Bleeding sap. Bleeding is an important factor restricting walnut graft survival. Bleeding sap causes oxygen deficiency at the graft union and inhibits the respiration of the rootstock and scion, thereby preventing the formation of callus tissue. In the past, walnut grafting was mainly stem grafting in spring, and bleeding in walnut during the dormant period is particularly obvious, and bleeding increases under adverse conditions of low temperature, high humidity, and abundant rain. Generally, grafting in summer and autumn reduces bleeding. During spring grafting, making several cuts below the union near the ground as "water-release openings", or cutting the rootstock in advance, leaving water-pulling branches, and delaying the grafting period can all alleviate the occurrence of bleeding, but it is difficult to avoid it completely. Lifting out the seedling stock for indoor grafting produces no bleeding, which is also the theoretical basis for the previous indoor grafting method for walnut.
3. Phenolic substances. This is mainly the influence of tannins, because walnut branches contain more tannins, and the union often has tannin substances precipitated to form an isolation layer, hindering the union of rootstock and scion. When grafting from late May to early June, there are fewer phenolic substances, and the graft survival rate is greatly improved.
4. Environmental factors
(1) Temperature. The formation of callus tissue during walnut grafting is the key to the success or failure of grafting, and the production of callus tissue is closely related to temperature, humidity, and so on. According to experiments, the optimal temperature for walnut callus tissue differentiation is 29°C, and the ambient temperature during grafting is suitable at 25~30°C. Therefore, spring grafting should not be too early, and is generally carried out during the bud break and leaf expansion period. Stem grafting is earlier, and bud grafting is later.
(2) Humidity. The microenvironment at the union is crucial to the formation of callus tissue, mainly by maintaining a certain humidity. A dry union easily causes the parenchyma tissue to dry out and die, and callus tissue cannot differentiate; if too humid, ventilation is poor, and callus tissue cannot be produced. Therefore, in production, plastic film is often used to wrap the union to prevent water loss. In the past, indoor grafting with vine wax and cleft grafting with wet soil embedding were both used to increase humidity.
(3) Rainfall. The formation of callus tissue requires relatively high humidity, but rainfall will seriously reduce the graft survival rate. During spring stem grafting, rainfall will increase the occurrence of bleeding; during summer bud grafting, rainfall will cause insufficient oxygen at the grafting site and make it prone to mildew and rot.
5. Grafting period. The key to ensuring the survival rate of bud grafting is to choose a good grafting period. Late May to early June is the most suitable, and seedlings can be produced in the same year as grafting. The next is supplementary grafting in August; after the graft survives, the grafted bud does not sprout in the same year, and seedlings can be produced the following year. For stem grafting, grafting after leaf expansion is better, when there is less bleeding and the graft survival rate is high.
6. Grafting technique. Skilled grafting technique can shorten the time during the grafting process that the cut surfaces of the rootstock and scion are exposed to the air, reducing the oxidation of tannin substances. At the same time, a smooth cut surface allows the rootstock and scion to contact tightly, and callus tissue is easily connected. In addition, whether the binding is firm and sealed during grafting will also affect graft survival. In terms of grafting methods, square bud grafting has the highest survival rate, reaching over 95%, while stem grafting has a lower survival rate. Whether stem grafting or bud grafting, those with a large contact area between rootstock and scion have a high survival rate, and vice versa.
7. Post-grafting management. Many grafting failures are caused by neglect of management after grafting. Stem grafting should be tied to a support to prevent the new shoots from being broken by wind after they grow out. For bud grafting, attention should be paid to timely loosening the binding and removing the plastic strips to reduce obstruction of water and nutrient supply. Plastic strips that are not removed inhibit the growth of the bound part, making the bound part too thin, and the grafted seedling is easily broken from the constriction mark at the graft union. Regardless of bud grafting or stem grafting, sprouting rootstock buds and shoots need to be removed in time to concentrate nutrients for the growth of the grafted bud, which also helps increase the survival rate.
8. Rootstock-scion compatibility. Graft compatibility is the ability of both the rootstock and the scion to connect normally and form a new plant. Usually, common walnut as rootstock grafted with common walnut, and iron walnut as rootstock grafted with bubble walnut, have strong compatibility. Several other rootstocks such as hickory, Chinese wingnut, and Manchurian walnut are more distantly related to common walnut and have poorer compatibility. For example, Chinese wingnut often shows the "big foot" phenomenon after grafting, the retention rate after survival is low, and it will die after growing for several years or several decades, with a shorter lifespan.