Plant Propogation and Nursery Management

Types of Incompatibility

Graft incompatibility is of two types 1) Localized (non-translocated) incompatibility and 2). Translocated incompatibility.

Localized (non-translocated) incompatibility

• Graft combination in which a mutually compatible interstock overcomes the incompatibility of the scion and rootstock.
• The interstock prevents physical contact of the rootstock and scion and affects the physiology of the normally incompatible scion and rootstock.
• A good example is Bartlett pear on quince rootstock. When mutually compatible Old Home or (Beurre Hardy) is used as inter-stock the three graft combination is completely compatible and satisfactorily tree growth takes place.
Translocated incompatibility

• It includes certain graft/rootstock combination in which the insertion of a mutually compatible interstock does not overcome incompatibility.
• This can be recognized by the development of a brown line or necrotic area in the bark at the rootstock interface.
• Consequently carbohydrate movement from the scion to the rootstock is restricted at the graft union.
• Hale’s Early peach grafted onto Myrobalan-B plum rootstock is an example of translocated incompatibility.
• The tissues are distorted and a weak union forms. Abnormal quantities of starch accumulate at the base of the peach scion. If the mutually compatible ‘Brompton’ plum is used as interstock between Hale’s Early peach and Myrobalan- B rootstock the incompatibility systems persists, with an accumulation of starch in the Brompton inter-stock.
• Nonpareil almond on Mariana ‘2624’ plum rootstock shows complete phloem breakdown, although the xylem tissue connections are quite satisfactory. In contrast Texas almond on Mariana- 2624 plum rootstock produces a compatible combination. Inserting 15 cm piece of ‘Texas’ almond as an inter-stock between the Nonpareil almond and the incompatibility between these two component.
Delayed incompatibility

• Some apricot cultivars grafted onto Myrobalan plum rootstock will not break at the graft union until the trees are full grown and bearing crops.
• Graft incompatibility can take as long as 20 years to occur. Other examples are conifers, oaks and cherry on pazza (Prunus cerasoides) rootstocks.
Pathogen induced incompatibility
• These graft union failures resemble incompatibility symptoms, but are due to pathogens like virus or phytoplasma. Tristeza is an important example of virus induced incompatibility in citrus. Failure of sweet orange (Citrus sinensis) budded onto sour orange (Citrus aurantium) rootstock is due to toxic substance from sweet orange, but lethal to the sour orange rootstock.
• Other examples are black line in English walnut (Juglans regia), which infects susceptible walnut rootstocks, apple union necrosis and decline and brown line of prune, which is caused by tomato mosaic virus that is transmitted by soil-borne nematodes to the rootstocks and then to the graft union.
• Pear decline is due to a phytoplasma, rather than a virus.
Causes and mechanism of incompatibility

The large number of different genotypes that can be combined by grafting produces a wide range of different physiological, biochemical and anatomical interaction when grafted. Several hypotheses have been advanced in attempts to explain incompatibility.

A. Physiological and biochemical mechanism

• In case of incompatible combination of certain pear cultivars on quince rootstock, the incompatibility is caused by a cyanogenic glucoside, prunasin, normally found in quince, but not in pear tissues.
• Prunasin is translocated from the quince into the phloem of the pear. The pear tissues, breakdown the prunasin in the region of the graft union, with hydrocyanic acid as one of the decomposition products.
• The presence of the hydrocyanic acids leads to a lack of cambial activity at the graft union, with pronounced anatomical disturbances in the phloem and xylem at the resulting union. The phloem tissues are gradually destroyed at and above the graft union.
• Conduction of water and material is seriously reduced in both xylem and phloem. The presence of cynogenic glucosides in woody plants is restricted to a relatively few genera. Hence, this relation cannot be considered a universal cause of graft- incompatibility.
B. Modification of cells and tissue: The lignification process of cell in walls is important in the formation of strong union in pear-quince grafts. Adjoining cell walls in the graft union of incompatible combination contain no lignin and are interlocked only by cellulose fibres. With incompatible apricot/plum (Prunus) grafts some callus differentiation into cambium and vascular tissue does occur, however, a large portion of the callus never differentiates. The union that occurs is mechanically weak.
C. Cellular recognition: “Cellular recognition is defined as the union of specific cellular groups on the surface of the interacting cells that results in specific defined response e.g. pollen-stigma compatibility recognition responses with glycoprotein surface receptors in flowering plants.”
It has been postulated that the critical event deciding compatible and incompatible grafts may occur when the callus cell first touch. There may be cellular recognition that must occur in successful graft union formation.
Predicting incompatible combination: To be able to predict in advance of grafting whether or not the components of the proposed scion-stock combinations are compatible would be of tremendous value. The different methods used are:

1) Electrophoresis test: This test is being used for testing cambial peroxidase banding pattern of the scion and rootstock of chestnut, oak and maple. Peroxidases mediate lignin production. Increased peroxidase activity occurs in incompatible grafts as compared to compatible auto grafts and adjacent rootstock and scion cells must produce similar lignin and have identical peroxidase enzyme pattern to ensure the development of a functional vascular system across the graft union. With electrophoresis if the peroxidase bands match the combination may be compatible, if they do not match incompatible may be predicted.
2) Magnetic resonance imaging (MRI): Magnetic resonance imaging can be used to detect vascular discontinuity in bud union of apple. High magnetic resonance imaging signal intensity is associated with bound water in live tissue and the establishment of vascular continuity between the rootstock and scion. Magnetic resonance imaging may be useful for detecting graft incompatibility caused by poor vascular connection.

Correcting incompatible combination: This is not a practical, cost-effective way to correct large scale planting of incompatible grafting partners. Plants would normally be rogued and discarded. With perhaps some isolated specimen trees of value, if the incompatibility is discovered before the tree die or break off at the union, a bridge graft could be done with a mutually compatible rootstock. Another costly alternative is to inarch with seedling of a compatible rootstock. The marched seedlings would eventfully become the main root system.