DEFINATIONS:
The following broad definitions are needed to understand grafting techniques and their underlying principles.
Grafting: the technique of connecting two pieces of living plant issue together so that they will unite and form a functional plant (and subsequently grow as one plant)
Scion: the aerial part of a tree that will form the crown of the new plant. This part contains the dormant buds of the tree whose desired characteristics need to be multiplied.
Rootstock: The belowground or lower part of a tree, sometimes including part of the stem and some branches that will form the root system of the new plant. This part may also contain dormant buds which should not be allowed to develop in the new plant since they do not have the desired characteristics that need to be multiplied.
Vascular cambium: a thin layer of meristematic cells between a tree’s bark (phloem) and wood (xylem). Meristematic cells are capable of dividing into new cells that may differentiate into new tissues and organs.
REASONS FOR GRAFTING:
To obtain a tree which combines both the good characteristics of one tree and the rootstock of another e.g. yellow passion fruit is tolerant to collar rot while Kawanda hybrid is high yielding.
Hastening the reproductive maturity. (Gestation period is reduced).
Repair damaged parts of trees. (Bridge grafting, inarching).
Perpetuating clones that cannot be readily maintained by cuttings, layers, division or other asexual methods.
To change cultivars of established plants (top working).
To detect viral diseases. In some commercially important species e.g citrus, virus disease is a severe constraint. If symptoms are not immediately visible. The insertion of a test bud onto a clean, highly susceptible indicator plant can show the symptoms and prevent large scale spreading of the virus.
Disadvantages of grafting:
- Requires skill
- Viruses can be transmitted to vegetatively propagated offspring.
REQUIREMENTS FOR GRAFTING (HEALING CONDITIONS)
For successful graft union to be established, several factors are important:
A - Stock and scion must be compatible (must be capable of uniting).
B - The cambium region of the scion must be placed in intimate contact with that of the stock. The cut surfaces should be held together tightly by wrapping or tying. Rapid healing of the graft union is necessary so that the scion may be supplied with water and nutrients from the stock by the time the buds open.
C - The grafting operation must be done at a time when the stock and scion are in proper physiological stage – scion buds are dormant (to avoid excessive desiccation associated with newly flushing growth of stem) while the cut tissues at the graft union are capable of producing the callus tissue necessary for healing of the graft.
D - Immediately after grafting operation is completed all cut surfaces must be protected from desiccation – by covering with tape or grafting wax.
This is best done using a high humid chamber (this is a chamber constructed in such a way so as to hold high humidity in the air, nearly airtight it reduces transpiration almost zero when humidity in the air is above 90%. Passion fruit take off % in such conditions is between 95 and 100%.
E - The high humidity and temperatures that are required for successful grafting are also conducive to bacterial and fungal growth. It is therefore imperative that almost care and cleanliness be practiced when grafting.
Kawanda % takes off records without the humid chamber and after the construction of the humid chamber for various crops.
CROP WITHOUT CHAMBER WITH CHAMBER
Passion fruits 60 - 70% 95 – 100%
Mangoes 40 – 50% 85 – 95%
Citrus 30 – 40% 70 - 80%
F - Proper care must be given the grafts for a period of time after grafting – shoots from below graft should be removed.
N.B. Preserve scions in humid conditions after cutting to prevent moisture loss as much as possible. Remember to label the scions and grafts as per.
ROOTSTOCK / SCION SELECTION:
Rootstock selection:
Majority of Rootstocks are raised from seed but can be propagated from cuttings and layering. It should meet the following:
- Seed should be obtained from fruits of disease – free high quality trees.
- Should be suited to the soil and climatic characteristics of the planting site.
- Be compatible with scion
Scion selection:
In general, scions should be in phase of growth approaching a new flush, with buds swollen but not open. They should be from high quality and disease – free plants.
METHODS OF GRAFTING (TECHNIQUES)
Whip or splice
Is useful particularly for grafting relatively small materials. A simple slanting cut of the same length and angle is made in both the stock and scion.
These are placed together and tied.
Cleft or wedge (Fig 2)
This is a technique in which the cut surfaces on the scion form a wedge that is inserted into a vertical slit cut into the stock.
Approach (Fig 1)
It is done on two independent, self-sustaining plants. It consists in taking a scion still attached to the parent and uniting is with a young stock plant. After a union has occurred, the top of the stock plant is removed above the graft and the base of the scion is removed below the graft.
GRAFTING TOOLS AND ACCESSORIES:
- Secateur – cutting back the rootstock, scion collection
- Grafting knife
- Sharpening stone – for sharpening the grafting knife
- Polythene – once the stock and scion are fitted tightly together, they must be tightly bound together to permit a strong union and prevent desiccation during the period of graft union formation.
- Disinfectant e.g JIK
FACTORS INFLUENCIG THE HEALING OF GRAFT UNION
Incompatibility
Kind of plant and method used
Temperature
Growth activity of stock plant
Propagation technique
Virus contamination, insect and diseases
GRAFTING OF PASSION FRUITS:
When the rootstock is grown in ideal medium, it should get ready for grafting in 3 – 4 months time. Graft them at least one foot above the ground as this will prevent the susceptible scion part of grafted plant to pick the infection from the soil manly due to rain splash. The whip method is the most commonly applied. In 15 days time, plants should be got out of the chamber for hardening in preparation for the field. Plants lose a lot of chlorophyll when they over stay in the humid chamber.
LAYERING AS A FORM OF VEGETATIVE PROPAGATION
Layering is a form of vegetative propagation in which roots are formed while the stem is still attached to the mother plant. Only after the root formation, the layer is detached and planted as a separate plant.
Layering is often used in species that are particularly difficult to root from cuttings. As layering methods are often used with species that are otherwise difficult to root, it can take several months until root have formed on the layer.
The most common layering techniques are:
Mound layering:- Soil is mounded over the base of a shrubby plant and roots are allowed to form. After rooting, branches are cut off from the mother plant and new branches are allowed to form.
For a single stemmed plant, the plant is cut off 3 cm above the ground to force it to produce several new shoots. After the shoots are 8 – 12 cm, the plant is mounded with soil and when the shoots have rooted, they are cut from the mother plant.
Tip layering: Used to propagate plants whose lower branches can be bent to contact the soil without breaking. The stems are bent to the ground and a section of bark and phloem removed. This part of the stem is then covered until rooting occurs.
Air layering: Involves the wounding of the stem, ( a ring of bark of about 1 – 5 cm is removed) covering the wound with a moist rooting medium and then wrapping around with a polythene sleeve to hold the medium in place and keep it moist. Roots grow into the media and the stem is then cut and planted. It can be done any where on the plant, even on tall branches as long as the diameter of the branch does not exceed 1.5 cm.
A suitable rooting media should be light in weight, porous to allow sufficient oxygen around the wound but yet wit a high water holding capacity. Examples include: vermiculate, saw dust or mixtures of soil with any of the mentioned substrates.
Disadvantages:
Not suitable for large scale operations
Produces fewer new plants than cuttings
Takes long to form roots.