Asexual Reproduction in Plant - Artificial Vegetative Reproduction

• Artificial vegetative propagation is a type of plant reproduction that involves human intervention.
• Apart from the natural methods of vegetative reproduction, a number of methods are used in agriculture and horticulture to propagate plants from their parts. Such methods are said to be artificial propagation.
• Some of the artificial propagation methods have been used by man for a long time and are called conventional methods. Nowadays, technology is being used for propagation to produce a large number of plants in a short period of time. Such methods are said to be modern methods.

Professor F.C. Steward (1932) of Cornell University showed that the mature phloem parenchyma cells removed from the carrot were placed in a suitable medium under controlled conditions, could be stimulated to start dividing again to produce a new carrot plant. These cells were described as totipotent. The genetic ability of a plant cell to produce the entire plant under suitable conditions is said to be totipotency. This characteristic feature of a cell is utilized in horticulture, forestry, and industries to propagate plants. The growth of plant tissue in a special culture medium under suitable controlled conditions is known as tissue culture.

1. Micropropagation: The regeneration of a whole plant from a single cell, tissue, or small pieces of vegetative structures through tissue culture is called micropropagation. This is one of the modern methods used to propagate plants.
   

Advantages of modern methods:

1. Plants with desired characteristics can be multiplied rapidly in a short duration.
2. Plants produced are genetically identical.
3. Tissue culture can be carried out in any season to produce plants.
4. Plants which do not produce viable seeds and seeds that are difficult to germinate can be propagated by tissue culture.
5. Rare and endangered plants can be propagated.
6. Disease-free plants can be produced by meristem culture.
7. Cells can be genetically modified and transformed using tissue culture.

Disadvantages of modern methods:

1. It is labour intensive and requires skilled workers.
2. Sterile condition must be maintained which adds to the cost.
3. Since the clones are genetically identical, the entire crop is susceptible to new diseases or changes in environmental conditions that will wipe out the species.
4. Sometimes, callus undergoes genetic changes which are undesirable for commercial use.

The common methods of conventional propagation are cutting, grafting, and layering.

1. Cutting: It is the method of producing a new plant by cutting the plant parts such as root, stem, and leaf from the parent plant. The cut part is placed in a suitable medium for growth. It produces roots and grows into a new plant. Before planting, cuttings are occasionally treated with hormones to stimulate root formation. Depending upon the part used it is called root cutting (Malus), stem cutting (Hibiscus, Bougainvillea, and Moringa), and leaf-cutting (Begonia, Bryophyllum). Stem cutting is widely used for propagation.  
   

   Root cutting - Malus

   Stem cutting - Hibiscus

   Leaf-cutting - Begonia

2. Grafting: In grafting, parts of two different plants are joined so that they continue to grow as one plant. Of the two plants, the plant which is in contact with the soil is called stock, and the plant used for grafting is called the scion. Over time, the tissue systems of the cutting graft into or integrate with the tissue systems of the base plant. Examples are Citrus, Mango, and Apple. 
   

   Grafting

   
   Types of grafting:
   There are different types of grafting based on the method of uniting the scion and stock. They are bud grafting, approach grafting, tongue grafting, crown grafting, and wedge grafting.
   
   i) Bud grafting: A T-shaped incision is made in the stock and the bark is lifted. The scion bud with little wood is placed in the incision beneath the bark and properly bandaged with tape. 
   

   Bud grafting

   
   ii) Approach grafting: In this method both the scion and stock remain rooted. The stock is grown in a pot and it is brought close to the scion. Both of them should have the same thickness. A small slice is cut from both and the cut surfaces are brought near and tied together and held by a tape. After 1-4 weeks the tip of the stock and base of the scion is cut off and detached and grown in a separate pot.
   

   Approach grafting

   
   iii) Tongue grafting: A scion and stock having the same thickness is cut obliquely and the scion is fit into the stock and bound with a tape.
   

   Tongue grafting

   
   iv) Crown grafting: When the stock is large in size scions are cut into a wedge shape and are inserted on the slits or clefts of the stock and fixed in position using graft wax.
   

   Crown grafting

   
   v) Wedge grafting: In this method, a slit is made in the stock, or the bark is cut. A twig of the scion is inserted and tightly bound so that the cambium of the two is joined.
   

   Wedge grafting

3. Layering: In this method, the stem of a parent plant is allowed to develop roots while still intact. When the root develops, the rooted part is cut and planted to grow as a new plant. Examples: Ixora and Jasminum.
   
   Types of layering:
   Mound layering and Air layering are a few types of layering.
   
   i) Mound layering: This method is applied for plants having flexible branches. The lower branch with leaves is bent to the ground and part of the stem is buried in the soil and the tip of the branch is exposed above the soil. After the roots emerge from the part of the stem buried in the soil, a cut is made in the parent plant so that the buried part grows into a new plant.
   

   Mound layering

   
   ii) Air layering: In this method the stem is girdled at nodal region and hormones are applied to this region which promotes rooting. This portion is covered with damp or moist soil using a polythene sheet. Roots emerge in these branches after 2-4 months. Such branches are removed from the parent plant and grown in a separate pot or ground

4. Suckering: Suckering is the process through which suckers adhere to a parent plant and form a dense, compact mat. Excess suckers are trimmed because too many can result in reduced crop size. Mature suckers are separated from the parent plant and transplanted to a new location where they produce new plants. Suckering serves two purposes: it grows new shoots and removes nutrient-sucking buds that prevent the main plant from expanding. 

   Suckering - Banana tree

Advantages of conventional methods:

1. The plants produced are genetically uniform.
2. Many plants can be produced quickly by this method.
3. Some plants produce little or no seeds; in others, the seeds produced do not germinate. In such cases, plants can be produced in a short period by this method.
4. Some plants can be propagated more economically by vegetative propagation. Example: Solanum tuberosum.
5. Two different plants with desirable characters such as disease resistance and high yield can be grafted and grown as a new plant with the same desirable characters.

Disadvantages of conventional methods:

1. Use of virus-infected plants as parents produces viral-infected new plants.
2. Vegetative structures used for propagation are bulky and so they are difficult to handle and store.