External symptoms of graft incompatibility

Graft union deformity resulting from incompatibility can usually be correlated with some external symptoms. The following symptoms are associated with incompatible graft combination:

1. Failure to form successful graft or bud union in many cases.
2. Yellowing of leaves at the end of growth season followed by premature defoliation, reduced vegetative growth, death of shoots
3. Premature death of trees which survive only for one or two years in the nursery
4. Marked difference in the growth rate of scion and rootstock
5. Overgrowth above or below the graft union
6. Growth of suckers on the rootstock
7. Clean breakage of the graft union

Types of Incompatibility

1) Localized (non-translocated) incompatibility and

2) Translocated incompatibility.

3) Delayed incompatibility

4) Pathogen-induced incompatibility

1) Localized (non-translocated) incompatibility

• Graft combinations in which the mutually compatible inter-stock overcomes the incompatibility of the scion and rootstock.
• The inter-stock prevents physical contact of the rootstock and scion and normally affects the physiology of the incompatible scion and rootstock.
• A good example of this is Bartlett pear on quince rootstock. When the mutually compatible Old Home or (Beurre Hardy) is used as the inter-stock the three graft combinations are fully compatible and produce satisfactory tree growth.

2) Translocated incompatibility

• It involves certain graft/rootstock combinations in which inserting a mutually compatible interstock does not remove the incompatibility.
• It can be identified by developing a brown line or necrotic area in the bark at the rootstock interface.
• As a result, the movement of carbohydrates from the scion to the rootstock at the graft union is restricted.
• Hale’s Early peach grafted onto Myrobalan-B plum rootstock is an example of transferred 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 persist, 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 components.

3) Delayed incompatibility

• Some apricot varieties are grafted onto Myrobalan plum rootstock, which does not break through the graft union until the trees are fully grown and fruitful.
• Graft incompatibility may take up to 20 years to occur. Other examples are conifers, oaks and cherries on pazza (Prunus cerasoides) rootstock.

4) Pathogen-induced incompatibility

• These graft union failures resemble the symptoms of incompatibility caused by pathogens such as viruses or phytoplasma. Virus-induced incompatibility tristeza in citrus is an important example. Failure of sweet orange (Citrus sinensis) to bud on sour orange (Citrus aurantium) rootstocks is caused by a toxin secreted by sweet oranges, and is lethal to sour orange rootstocks.
• Other examples are the black line in English walnut (Juglans regia), which infects susceptible walnut rootstocks, necrosis and decline in apple unions, and the brown line of prunes, which is caused by tomato mosaic virus transmitted by soil-borne nematodes to the rootstock and then to graft unions.
• Pear decline is caused by phytoplasma rather than viruses.

Causes of incompatibility

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

1. Genetic incompatibility: Distantly related plant species or genera often fail to form successful graft unions.
2. Physiological differences: Differences in growth rates, nutrient transport, or hormonal signals may cause failure.
3. Biochemical barriers: The presence of inhibitory substances in the graft union may prevent proper tissue formation.
4. Physical mismatch: Poor alignment of vascular tissues (xylem and phloem) reduces nutrient and water transport.
5. Pathogenic involvement: Certain diseases or latent infections in the stock or scion may contribute to incompatibility.

Predicting incompatible combination: It would be very valuable to predict whether the components of proposed scion-rootstock combinations are compatible before grafting. The various methods used are:

1) Electrophoresis test: This test is being used to test the cambial peroxidase banding pattern of scion and rootstock of chestnut, oak and maple. Peroxidases mediate lignin production. Incompatible grafts have increased peroxidase activity compared to compatible auto grafts. Adjacent rootstock and scion cells must produce similar lignin and have similar peroxidase enzyme patterns to ensure the development of a functional vascular system in the graft union. With electrophoresis if the peroxidase bands match then the union may be compatible, if they do not match then incompatible can be predicted.

2) Magnetic resonance imaging (MRI): Magnetic resonance imaging can be used to detect vascular imbalance in the apple bud union. High magnetic resonance imaging signal intensity is associated with water bound in living tissue and the establishment of vascular continuity between the rootstock and scion. Magnetic resonance imaging may be useful to detect graft incompatibility due to poor vascular connection.

Correcting incompatible combination: This is not a practical, cost-effective way to correct large-scale plantings of incompatible grafting partners. The plants are usually unwanted and discarded. Perhaps with a few individual specimen trees, if the incompatibility is discovered before the tree dies or breaks on the union, a bridge grafting can be done with a mutually compatible rootstock. A more expensive option is to inarch with a compatible rootstock seedling. The inarched plant will eventually form a main root system.

 

Stock -Scion Relationships

A grafted or budded plant may produce abnormal growth patterns which may be different from what would be seen when each part of the graft such as rootstock and scion is grown separately or when it is grafted or budded into another type of rootstock. Some of these are of great importance in horticulture. How this different aspect of the rootstock will affect the performance of the scion variety or vice versa is known as the stock-scion relationship.

Effect of stocks on scion cultivars

1. Size and growth habit

• In apples, rootstocks can be classified as dwarf, semi-dwarf, vigorous, and very vigorous rootstocks based on their effect on a scion cultivar.
• If a scion is grafted on dwarf rootstocks (e.g. M.9), the scion grows less vigorously and remains dwarf only. On the other hand, if the same scion is grafted on a very’ vigorous rootstock (e.g. M2) the scion grows very vigorously.
• In citrus, trifoliate orange is considered to be the most dwarfing rootstock for grapefruit and sweet oranges. On the other hand, in mango, all plants of a given variety are known to have the same characteristic canopy shape of variety despite the rootstocks being of seedling origin.
• But the rootstocks of mangoes like Kalapade, Olour provide dwarfism in scion varieties. Guava varieties grafted on Psidium pumilum are found to be dwarf in stature. The ‘Pusa Srijan’ guava rootstock also imparts dwarfism to the commercial cultivar of guava Allahabad Safeda.

2. Precocity in flowering and fruiting

• The time taken from planting to fruiting i.e., precocity is influenced by rootstocks. Generally, fruit precocity is associated with dwarfing rootstocks and slowness to fruiting with vigorous rootstocks.
• Mandarin, when grafted on Jambhiri rootstock is more precocious than those grafted on sweet orange or orange or acid lime rootstocks.

3. Fruit set and yield

• In oriental persimmon (Diospyros kaki Hachiya), rootstock directly influences flower formation and fruit set. When it is grafted on D. lotus, it produces more flowers but only a few mature into fruits. However, when Diospyros kaki is used as a rootstock, the fruit set is very high.
• The effect of rootstock on the yield performance of many fruit cultivars has been well documented. Acid lime on rough lemon increases the yield by about 70 percent compared to Troyer citrange, Rangpur lime, or budding on its own rootstock. Sweet orange variety Sathuguri grafted on Kichili rootstock gives higher yield as compared to Jambhiri or own rootstock

4. Fruit size and quality

• Sathuguri sweet orange grafted onto Gajanimma rootstock produces larger but poorer quality fruits while those grafted on their own rootstock produce higher juice content and quality fruits.
• Physiological disorder ‘granulation’ is very less in sweet orange on Cleopatra mandarin rootstock, on the other hand, rough lemon rootstock induced maximum granulation.
• Physiological disorder black end is not observed when Pyrus communis rootstock is used in Bartlett pear. When P. pyrifolia is used as rootstock, the appearance of this disorder affects the quality of the fruits.

5. Nutrient status of scion

Rootstocks also affect the nutritional status of the scion. The leaves of Sathuguri sweet lime tree have a better nutrient status of all nutrients when grafted on C. volkarimariana rootstock as compared to its rootstock or Cleopatra mandarin rootstock.

6. Winter hardiness

Grapefruit scion on Rangpur lime rootstocks withstands winter injury better than rough lemons or sour oranges. On trifoliate rootstock, sweet orange, and mandarin are more cold tolerant.

7. Disease resistance

In citrus, considerable variability exists among rootstocks in their response to diseases and nematodes. For example, the rough lemon rhizome is tolerant to ttristeza, xyloporosis and exocortis, but susceptible to gummosis and nematode. On the other hand, Troyer citrange is tolerant to gummosis but susceptible to exocortis virus disease. Similarly, guava varieties grafted on Chinese guava (Psidium friedrichsthalinum) rootstock are resistant to wilt disease and nematodes.

8. Ability to resist soil adverse conditions

In citrus rootstock, trifoliate orange shows poor tolerance, while Sweet orange, Sour orange, and Rangpur lime rootstock show a moderate ability to withstand excess salt in the soil. Similarly, in pome fruits, variation exists between rootstocks to resist excess soil moisture or excess boron in the soil. Myrobalan plum rootstocks generally tolerate more boron and moisture than Mariana plum rootstocks or other rootstocks such as peaches, apricots, or almonds.

 Effect of scion on rootstock

1. Vigour of the rootstocks:

In apple, it has been found that if apple plants were grafted with ”Red Astrachan’ apple, the rootstock produced a very fibrous root system with few taproots. On the other hand, if the scion ‘Goldenburg’ was grafted onto the plants, they produced deep roots with two or three branches without a fibrous root system. In citrus, if the scion variety is less vigorous than the rootstock, the rate of growth and the final size of the tree are determined more by the scion than by the rootstock.

2. Cold tolerance of rootstock

The cold tolerance of citrus rootstocks is affected by scion variety. Sour orange plants grafted to ‘Eureka’ suffered more winter injury than non-grafted plants.

3. Precocity in flowering

Young mango rootstock plants (6 months to one year old) showed early flowering when inarched from branches of older trees, which may be attributed to the effect of scion on rootstock.