Crops which are propagated asexually or by vegetative means are known as asexually propagated or vegetatively propagated or clonal crops. There are some agricultural (Sugarcane, Potato, Sweet Potato, etc) and horticultural (Banana, mango, citrus, pears, peaches, litchi, etc) crops that propagates by asexual means. The main reasons of asexual reproduction are
1) Reduced flowering and seed set,
2) Non flowering in many cases.
3) To avoid inbreeding depression in certain crops and
4) Apomixis in some species.
Asexual reproduction produces the progeny, which are exactly identical to their parent in genotype, because the progeny is derived from the vegetative cells through mitosis. Therefore, the main advantage of asexually reproduction is that it preserves the genotype of an individual indefinitely.
Characteristics of Asexually Propagated Crops:
1. Most of these plants are perennial and annual. E.g Sugarcane, potato, sweet potato.
2. Many of them shows reduced flowering and seed set and many varieties do not flower at all.
3. They are invariably cross-pollinated.
4. They are nightly heterozygous and show severe inbreeding depression.
5. Many species are interspecific hybrid. E.g Banana, sugarcane.
6. Many species shows wider adaptation.
7. These crop consist of a large number of clones. i. e. progeny derived from single plat through asexual reproduction.
Clonal crops are generally improved by crossing two or more desirable clones, followed by selecting in the F1 progeny and in the subsequent clonal generations. Once the F1 has been produced, the breeding procedure is essentially the same as clonal selection. The improvements through hybridization involved three steps: 1) Selection of parents, 2) Production of F1 progeny, and 3) Selection of superior clones.
Selection of Parents:
The selection of parents to be used in hybridization is very important since the value of F1 progeny would depend upon the parents used. Parents are generally selected on the basis of their known performance both as varieties and as parents in hybridization programmes. The performance of a clone hybridization programme depends on its propotency and general combining ability. It would be highly desirable to know the relative values of GCA and SCA in the crops to be improved. If GCA is more important, a small number of parents with good GCA should be used in hybridization programmes. On the other hand , when SCA is more important ,a large number of parents should be used to produce a large number of F1 families.
Production of F1 Progeny:
Generally clonal crops are cross pollinated and they may show self- incompatibility. The selected parents may be used to produce single crosses involving two parents or an equivalent of a Polycross involving more than two parents.
Selection among F1 Families:
When the breeding value of parents is not known and the relative contribution of GCA and SCA is not available, a large number of crosses have to be made in order to ensure that at least some of the crosses would produce outstanding progeny in F1. This is particularly true in a species, where crop improvement has not been done or has been done at a small scale. The general worth of individual F1 families or populations is estimated visually. The presence of outstanding individuals in these families is also noted. Inferior families are eliminated. Promising families with outstanding individuals are then grown at a much larger scale for selection. The procedure is designed to save time, space and labour by planting only small populations of large number of crosses at the preliminary stage.
Selection within F1 Families:
The selection procedure within F1 families is essentially the same as that in the case of clonal selection.
First Year:
Clones to be used as parents are grown and crosses are made to produce F1 progeny.
Second Year:
Sexual progeny from the cross i.e seedlings obtained from seeds are grown. Undesirable plants are eliminated. Few hundreds to few thousand desirable plants are selected.
Third Year:
Clones from individual plants are grown separately. Poor are clones are eliminated. Upto 200 superior clones may be selected for preliminary yield trial.
Fourth Year:
a replicated preliminary yield trial is conducted. A suitable check is included for comparison. Few outstanding clones are selected for trails at several locations.
Fifth to Seventh Year:
Replicated yield trials are conducted at several locations. A suitable check is included for comparison. One or few outstanding clones are identified and released as new varieties.
Eighth Year:
The clones released as varieties are multiplied and distributed among farmers.
Natural Vegetative Propogation
Vegetative propagation is a form of asexual reproduction of a plant. Only one plant is involved and the offspring is the result of one parent. The new plant is genetically identical to the parent.
New plants grow from parts of the parent plant. They include:
Stems
Runners are stems that grow horizontally above the ground. They have nodes where buds are formed. These buds grow into a new plant.
Roots
New plants will grow out of swollen, modified roots called tubers. Buds develop at the base of the stem and then grow into new plants.
Leaves
Leaves of some plants will grow into a new plant if they become detached from the parent plant. Other plants grow small plants called plantlets on the edge of their leaves.
Bulbs
A bulb contains an underground stem. Leaves are attached to the stem. These leaves contain much stored food. At the centre of the bulb is an apical bud. Also attached are lateral buds. The apical bud will produce leaves and a flower while the lateral buds will produce new shoots. As the plant grows and develops it will form a new bulb underground.
Artificial Vegetative Reproduction
Horticulturists and farmers use artificial means to produce plants that are identical to the parent plant. Some of the methods used are:
Cuttings
Cuttings are part of the plant that is cut off of the parent plant. Shoots with leaves attached are usually used. New roots and leaves will grow from the cutting. The shoot is cut at an angle. A growth promoter may be used to help with the growth of the roots.
Grafting
In grafting 2 plants are used to develop a new plant with combined traits from the 2 parent plants. In grafting the scion is the above ground part of one plant. The scion is attached to the stock which is the rooted part of the second plant.
Layering
In layering a shoot of a parent plant is bent until it can be covered by soil. The tip of the shoot remains above ground. New roots and eventually a new plant will grow. These plants can then be separated.
