What is Grafting Incompatibility?

Grafting and budding is an art of joining two different plants together, in such a manner that they unites and continues their growth as simple plant. In case of building single bud is inserted in to the stuck, where as in grafting a bud stick consisting two or more buds is inserted into the stuck. Stock is a lower portion of the graft union, whereas, scion is the upper portion a place at which both unites is termed as scion or graft union.

                                The ability of two different plants when grafted together to produce a successful union and also develop into one composted plant is termed as ‘Compatibility’. The inability of two different plants to do so when grafted together as often defined as grafting incompatibility. Incompatibility is more often observed for inter-specific than intra-specific grafts and might results die back of graft. The distinction between a compatible and incompatible graft union is not clear cut. On one hands, stocks and scions of closely related plants unite readily and grows as one plant. On the other hands stocks and scions of unrelated plants grafted together are likely to fail completely in union. Much graft combination lie between these extremes varieties, compatible to incompatible and therefore the character-ization of incompatibility is not distinct. Certain scion and rootstock combinations show an incompatibility reaction,   which may occur after grafting or may take 10 to 20 years to develop. Graft incompatibility can induce undergrowth or over growth of scion, which can lead to decreased water and nutrient flow through the graft union and cause the wilting of the plant. Grafting incompatibility usually occurs at early stages, when vascular connections are forming, but it can appear as late as the fruiting stage, when the plant has a high demand for water and nutrients. It has been reported that the supply of warm water to the graft union of grafted tomato and eggplant plants improved the storage quality, reducing water stress at the beginning of the storage when vascular tissues of the rootstock and scion are not connected yet. Grafting incompatibility in plants has been classified in two types:-
·         Trans-located incompatibility:-This type of grafting incompatibility includes those cases in which the incompatible condition is not overcome by the insertion of a mutually compatible inter-stock because, apparently, some labile influence can move across it. This type involves phloem degeneration, and can be recognized by the development of a brown line or necrotic area in the bark.

·         Localized incompatibility:-This type of grafting incompatibility includes combinations in which the incompatibility reactions apparently depend upon actual contact between stock and scion, separation of the components by insertion of mutually compatible inter-stock overcomes the incompatibility symptoms.
SYMPTOMS OF GRAFT INCOMPATIBILITY:-Graft union malformation resulting grafting incompatibility which usually express the following external symptoms:-
1.     Failure to form a successful graft or bud union with a high percentage of success.
2.    Yellowing of leaves in the latter part of the growing season followed by early defoliation accompanied by decline in vegetative growth. Appearance of shoot dies back and general in health of tree.
3.    Premature death of the trees which may live only a year or two in the nursery.
4.    Marked difference in the growth rate or vigor of scion and stock.
5.    Over growth at, above or below the graft union.
6.    If cutting vertically through the graft union there will be found a dark line or corky tissues following the contours of the union between the rootstock and scion.
   CAUSES 0F GRAFTING INCOMPATIBILITY:-There are many causes of grafting incompatibility but the measure reasons are listed and explained as follows :-
1.     Main cause grafting incompatibility is vascular system. If there is poor vascular connection, vascular discontinuity and phloem degeneration at unit area that cause the vascular incompatibility. This results in disturbance of water and nutrients transfer by phloem in another direction.
2.    Different growth characteristics of the stock and scion.
3.    Inability of scions and root stocks to establish a close cell contact at graft union to
maintain water and nutrient balance with each other.
4.    Physiological and biochemical differences between stock and scion.
5.    Few callus differentiation into cambium and vascular tissues and most of the part of the callus are not differentiate.
6.    Poor formation of the graft union due to anatomical mismatching.
7.    Virus or phytoplasma (mycoplasma like organism) infection.
8.    When a weak cultivar is grafted onto an excessively vigorous rootstock.
9.    Genetic incompatibility or biochemical reaction in the rootstock and scion.
10.  Cambium of scion and rootstock were not meeting properly.
11.  The union is girdled because the bindings or label were not released in time.
12.  The graft shaded too much after growth began.
Mechanism of Grafting Incompatibility:-Although grafting incompatibility is clearly related to genetic differences between rootstock and scion, the mechanism by which particular cases are expressed are not clear. The mechanism of grafting incompatibility is not clearly distinct but it had been studied in various plants in various way of research through which we can get knowledge about mechanism of grafting incompatibility.                 
                  One of the possible mechanisms is that different growth characteristics of the stocks and scion occur. That is, if marked differences occur in vigor or in the time of starting or completing vegetative growth for the season, incompatibility may occur.
 Another the possible mechanism of grafting incompatibility is abnormalities caused by viruses which have arisen directly as a result of the grafting, that is, by invasion of cells one of the graft partners by proteins of the other. Latent viruses, virus complexes and mycoplasma like  bodies do occur in particular graft combinations, in which a susceptible cultivar is introduced to such a latent organism in its graft partner, rapid decline of the entire plant takes place. This is a serious situation since it is well known that many clones are widely infected with latent viruses.eg:-occurs in apple. Tristeza is another example of viral infection. It is a viral disease of budded sweet orange that is grafted onto infected sour orange rootstock.
                     Another possible mechanism is disturbance in vascular system. When there is poor vascular connection, vascular discontinuity and phloem degeneration during grafting operation at unit area that causes the vascular incompatibility. This creates the problem in distribution of water and nutrients between rootstock and scion. As a result of this transfer of water and nutrient in another direction through phloem occur that cause the lack of nutrition and food in plant which ultimately cause the grafting incompatibility.
                 Another possible mechanism is physiological and biochemical differences between rootstock and scion. This is supported by studies with incompatible combination of certain pear cultivars on quince rootstocks and derived following conclusions;-
Ø  A reduction of level the sugar reaching the quince roots leads to further decomposition of prunasin, liberating hydrocyanic acid and killing large areas of the quince phloem.
Ø  The presence of the hydrocyanic acid leads to a lack of cambial activity at the graft union, with pronounced anatomical disturbances in the phloem and xylem at the union resulting. The phloem tissues are gradually destroyed at and above the graft union. Conduction of water and materials is seriously reduced in both xylem and phloem.
Ø  A water-soluble and readily diffusible inhibitor of the action of pear enzyme occurs in the various pear cultivars, although they differ in their content of the inhibitor. This may explain why certain pear cultivars are compatible and the others incompatible with quince rootstock.
  Correction:-
The grafting incompatibility can be removed or corrected by various operations which are as follows:-
Þ     Remove and top-work the rootstock.
Þ     Bridge graft with a mutually compatible root stock.
Þ     Inarch with a seedling of compatible rootstock.
Þ     Double working