METHODS OF ARTIFICIAL VEGETATIVE PROPAGATION:
The artificial method of vegetative propagation includes cuttings, layering, grafting and micropropagation.
- Cuttings: Stem cuttings are generally used to obtain new plants. At times, roots are not easily produced in the cuttings and hence, they are treated with growth hormones (IAA, IBA, NAA). Plants like sugarcane, Bougainvillea, rose, china rose, grape etc. are propagated by stem cuttings.
- Layering: Erect stems of many plants, if come in contact with the soil, root readily at the base, at the nodes or near the tips. Several methods of layering are employed. A few of them are as follows:
- Simple layering: In this technique, lower branches of the plants are bent to the ground and covered with soil, with the tips exposed. In a few days, the covered part of the stem produces adventitious roots. At this stage, the branch is cut off from the parent plant and it grows into a new plant. This method is commonly used for propagating jasmine, strawberry, raspberry, rose, grape vine etc.
- Mound layering: In this method, the plant is pruned to stimulate the growth of new shoots, close to the ground. After several new branches have been produced, the base of the plant is covered with soil. The branch tip is kept outside the soil. Within few days, new shoots develop roots. These are separated from the parent and are planted. Gooseberry, apple, currant etc. are propagated by this method.
- Compound layering: This method is suitable for plants producing long, slender and flexible shoots. For example, Jasminum, Clematis. Long and young shoots of these plants are laid in the ground and covered with soil at short intervals. Rooting takes place at each of the node and new shoots develop from the buds at the nodal region. At this stage, the layered branch is cut into sections each consisting of a new shoot and a mass of adventitious roots.
- Air layering: In this method, part of the stem is girdled (a ring of bark is removed). The exposed part is covered with moist moss or cotton and enclosed in a polythene bag to prevent dessication. The root appears after sometime and at that stage, the branch is cut and planted. It grows into a new individual. Litchi, pomegranate, guava, orange, lemon etc are propagated by air layering method.
- Grafting: It is the technique of joining parts of two plants so as to form a composite plant. A new variety is produced by joining parts of two different plants. The rooted shoot of one plant called stock is joined with the piece of another plant known as scion. The grafting ends of both, stock and scion, are cut obliquely and then placed over one another in such a way that the cambia of two come in close contact. The two pieces are firmly held together by tape, rubber tubing, etc. This results in division of cambia and formation of a new vascular tissue. This method has been practised for many economically useful plants such as rose, mango, apple, pear, guava, citrus, rubber etc.
SIGNIFICANCE OF VEGETATIVE PROPAGATION
- It is the best known method of multiplication in seedless varieties and species. Plants like banana, grapes, pineapple, roses, gladioli, chrysanthemums, seedless orange, seedless grapes etc., do not form viable seeds. Thus, vegetative propagation is the only method of reproduction and continuation of species in such plants.
- It is a cheaper, easier and rapid method of multiplication. Many fruits usually require four to five years to bear the fruits when developed from the seeds. The plant developed by vegetative method, however, takes only a year to bear the fruits. Lilies take four to seven years for multiplication through seeds while one to two years are required from bulbs. Similarly, potato crop is raised within three months from tubers while more than one year is required for raising it from the seeds.
- Plants with poor seed set, for example, Cynodon dactylon (Lawn, Dhoob grass or Bermuda grass), poor seed viability or prolonged seed dormancy are mostly propagated vegetatively.
- The survival rate of the new plants is almost 100% in vegetative propagation while it is hardly 1% through the formation of seed.
- Vegetative propagation helps in rapid spread of a plant over an area.
- The greatest advantage of vegetative propagation is that all the plants developed by these methods are genetically similar to the parent plant. Such a genetically uniform population of plant raised from a single parent constitute clone. The plants grown from seeds show variations due to meiosis that brings about genetic recombination and segregation. The vegetative propagation, thus, helps propagating the plant with selected and desirable characters.
- Desirable qualities of two varieties can be obtained through grafting.
- Common infections can be easily removed through pruning, micrografting and micropropagation. Micropropagation is useful in raising disease-free plants, homozygous diploids and those without viable seeds. This is an important method for growing commercially and economically important plants.
- Vegetative organs required for propagation cannot be preserved for long.
- Vegetative propagules are not efficiently protected as the seeds are. They get easily decayed and are prone to various viral, fungal and bacterial diseases.
- Infected parents transfer disease to the daughters.
- Variability is absent. Hence, adaptability to the changed environment decreases.
- The plants may show degeneration due to absence of variation.
- There is no mechanism for dispersal. Vegetative multiplication causes overcrowding and hence, severe competition which can damage most of the plants.
- Good qualities cannot be introduced nor bad characters eliminated in plants multiplied through vegetative propagation.