Asexual Propagation Of Plants:
Asexual propagation is the multiplication of plant material without passing through the seed cycle. It is accomplished by “mitosis”: the cell division process by which the plants grow. Plants are produced using material from a single parent and as such there is no exchange of genetic material, therefore, daughter plants are identical to their parent plant. Asexual reproduction uses plant parts such as roots, stems and leaves.
Responsible Factors of Asexual Propagation:
The major reasons for propagating plants by vegetative means are as follows.
- Inability to produce seeds by non-flowering plants.
- Non-viable seeds of flowering plants.
- To increase the seed of reproduction and growth.
- To develop immunity to pests.
- Top working of undesirable varieties.
- To perpetuate a particular form of plant.
Methods of Asexual Propagation:
Techniques for asexual propagation of plants include cuttings, grafting, division, layering etc.
Many types of plants, both woody and herbaceous, are frequently propagated by cuttings. A cutting is a vegetative plant part which is separated from the parent plant in order to regenerate itself when placed under proper environment, forming a whole new plant. The different techniques of propagation of plant by cuttings are as follows.
Stem cuttings of plants must be taken in the fall or in the dormant season. New roots must develop at the base of the cutting before a newplant will be formed. The stem cuttings are further divided into following classes;
a. Hardwood cuttings:
There are two types of hardwood cuttings. Deciduous hardwood cuttings taken from deciduous plants (such as mulberry, grape, apple, peach etc.) and evergreen hardwood cuttings taken from evergreen plants such as olives.
Hardwood cuttings are taken from deciduous plants in early winter after the plants have dropped their leaves.
b. Simple cuttings:
The simple cutting is done on a stem which usually contains 4-6 buds and planted immediately. The shoots will then grow from the buds above the soil and roots will grow from the modes in the soil.
c. Torn cuttings:
This cutting is performed at the bottom portion of the stem where there is a union with the mother plant.
d. Hammer cuttings:
In this case, a piece of twig is cut together with the stem. This twig helps to develop root system.
e. Semi-wooden cuttings:
These types of cuttings are taken from woody evergreen plants, during the growing season. They are cut off before the wood hardens and turns brown. This propagation technique is used for the reproduction of coffee; mango, litchi etc.
f. Herbaceous stem cutting
These cuttings are rooted in flat form. Sugarcane cuttings are a good example of this technique.
2. Leaf cuttings:
Plants with think and fleshy leaves are generally propagated by this method. Leaves of most plants will either produce a few roots but no plant, or just decay. The various classes/ techniques of propagation by leaf cuttings are as follows:
a. Whole Leaf with Petiole:
Detach the leaf and upto 2 inches of petiole and place it in a medium. One or more new plants will be formed at the base of the petiole.
b. Whole leaf without petiole:
Insert the cutting vertically into the medium. A new plant will form the ancillary bud. The leaf may be removed when the new plant has its own roots.
c. Split Vein:
Detach a leaf from the stock plant. Slit its views on the lower leaf surface. Lay the cutting on the medium. New plants will form at each cut.
3. Root cuttings:
Root cuttings are usually taken from 2-3 year old plants in their dormant season when they have a large carbohydrate supply. Root cuttings of some species produce new shoots which in turn form their own root system, which root cuttings of other plants develop root system before producing new shoots.
a. Plants with large Roots:
Make a straight top cut and store the root at 40F in sand for 3 weeks. Remove from storage. Insert the cutting vertically with the top approximately lend with the surface of the medium.
b. Plants with small Roots:
Take 1-2 inch section of roots. Insert the cutting horizontally about ½ inch below the surface of the medium.
4. Propagation by Layering:
This method is used on the tip of the branch, when stems are usually less than one year of age. Stems still attached to their parent plants may form roots where they touch a rooting medium. Several from the parent plant, the rooted stem become a new plant. This method of propagation is called layering, promotes a high rate of success because it prevents the water stress and carbohydrate shortage that plague cuttings.
Layering is enhanced by wounding one side of the stem or by bending it very sharply. The rooting medium should always provide aeration and a constant supply of moisture. Different layering techniques of propagation are:
a. Tip Layering:
Dig a hole 3 to 4 inches deep. Insert the shoot tip and cover it with soil. The tip grows downward first, then bends sharply and grows upward. Roots form at the bend, and the recurred tip becomes a new plant.
b. Simple layering:
Bend the stem to the ground. Cover part of it will soil, leaving the last 6-12 inches exposed. Bend the tip into a vertical position and stake in place. The sharp bend will inches rooting.
c. Mound Layering:
Cut the plant back to 1 inch above the ground in dormant season. Mound soil over the emerging shoots in the spring to enhance their rooting.
d. Compound Layering:
This method works with plants having flexible stems. Bend the stem to rooting medium and alternately cover and expose stem sections.
5. Asexual Propagation by Grafting:
Grafting is the joining of plant parts so that they will grow as one plant. This technique is used to propagate cultivars that do not root well as cuttings or whose own root systems are inadequate.
The portion of the cultivar that is to be propagated is called scion. It consists of a piece of shoot with dormant buds that will produce the stem and branches. The rootstock provides the new plant’s root system and sometimes the lower part of the stem. The cambium is a layer of cells located between the wood and bask of a stem from which new bark and wood cells originate.
Four conditions must be met for grafting to be successful. The scion and rootstock must be compatible; each must be at proper physiological stage; the cambial layers of the stock and scion must meet; and the graft union must be kept moist until the wound has healed. The methods of grafting are:-
a. Cleft Grafting:
Cleft grafting is often used to change the cultivar or top growth of a shoot or a young tree. Collect scion wood and make a 15 cm vertical cut through the centre of the bark. Keep his cut wedged apart. Insert the scion piece into the wedge of the rootstock. Clip them together for 8-10 days until they become one plant.
b. Whip or tongue Grafting:
The scion and rootstock are usually of same diameter. This strong graft heals quickly and provides excellent cambial contact. Cut the scion and stock, interlock them and waxes the union.
6. Asexual Propagation by Budding:
Budding, or bud grafting, is the union of one bud and a small piece of bark from the scion with a rootstock. It is useful when scion material is divided. It is faster and also forms a strong union than grafting.
a. Patch budding:
Remove a rectangular piece of bark from the rootstock. Cover this wound with a bud of scion.
b. Chip Budding:
This method can be used when the bark is not slipping. Remove a shallow piece of rootstock and fill it with the bud of scion and wrap the wood until it becomes part of the plant.
c. T- Budding:
This is the most commonly used budding technique. When the bark is slipping, make a vertical cut through the bark of the rootstock, avoiding any buds on the stock. Make a horizontal cut at the top of the vertical cut (in a T-Shape) and remove the piece of Woodstock.
Prepare a T-Shape piece of bud, bark and a thin section of wood of scion. Push the shield under the loosened stock bark. Wrap the union, leaving the bud exposed.
Asexual Propagation By Division:
Plants with more than one rooted crown may be divided and the crowns planted separately.
Advantages of Asexual Propagation:
- Simple, cheap and convenient method.
- Less technical skill required.
- Rapid multiplication of scion within a short period of time.
- Requires less space.
- Transportation of material easier.
- Vegetatively propagated fruit plants are true to type and uniform in growth, yield and fruit type.
- Vegetative propagation is ideal to plants which are more prone to seed dormancy e.g., peach, online etc.
- Vegetative plants are less vigorous and can easily be managed as compared to seeding plants.
- Manipulation of fruit quality and tree size is possible.
- A “Clone” can easily be perpetuated through asexual propagation.
- Damaged parts of the plant can be repaired by grafting or budding.
- A number of varieties can be grown on a single plant.
Disadvantages of Asexual Propagation:
- Asexually propagated plants are short lived.
- No new verities can be evolved by vegetative propagation.
- Requires specialized skills/techniques.
- More expansive than sexual propagation.
- Failure in certain plants like pineapple etc.
- Usually more prove to insects/pests & diseases.