ASEXUAL REPRODUCTION IN PLANTS

PLANT REPRODUCTION

Plants have two main types of asexual reproduction in which new plants are produced that are genetically identical clones of the parent individual.

Vegetative reproduction involves a vegetative piece of the original plant (budding, tillering, etc.) and is distinguished from apomixis, which is a replacement for sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species and also in some non-plant organisms.

AN EXAMPLE OF A  STRAWBERRY PLANT WITH VEGETATIVE REPRODUCTION

Natural vegetative reproduction is mostly a process found in herbaceous and woody perennial plants, and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size.

A plant that persists in a location through vegetative reproduction of individuals constitutes a clonal colony, a single ramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony.

The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons.

In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic.

A major disadvantage to vegetative reproduction, is the transmission of pathogens from parent to offspring; it is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs.

Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweed (Hieracium), dandelion (Taraxacum), some Citrus (Citrus) and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction.

Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring. Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced.

Structures

The rhizome is a modified underground stem serving as an organ of vegetative reproduction; the growing tips of the rhizome can separate as new plants, e.g., Polypody, Iris, Couch Grass and Nettles.

Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns.

Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves.

A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and many members of the Rose family such as Rosa and Rubus.

Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) and tulips (Tulipa) reproduce by dividing their underground bulbs into more bulbs. Other plants like potatoes (Solanum tuberosum) and dahlia (Dahlia) reproduce by a similar method involving underground tubers. Gladioli and crocuses (Crocus) reproduce in a similar way with corms.

Usage

The most common form of plant reproduction utilized by people is seeds, but a number of asexual methods are utilized which are usually enhancements of natural processes, including: cutting, grafting, budding, layering, division, sectioning of rhizomes, roots, tubers, bulbs, stolons, tillers (suckers), etc., and artificial propagation by laboratory tissue cloning.

Asexual methods are most often used to propagate cultivars with individual desirable characteristics that do not come true from seed. Fruit tree propagation is frequently performed by budding or grafting desirable cultivars (clones), onto rootstocks that are also clones, propagated by layering.

In horticulture, a “cutting” is a branch that has been cut off from a mother plant below an internode and then rooted, often with the help of a rooting liquid or powder containing hormones. When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant, genetically identical to the mother plant.

Examples include cuttings from the stems of blackberries (Rubus occidentalis), African violets (Saintpaulia), verbenas (Verbena) to produce new plants. A related use of cuttings is grafting, where a stem or bud is joined onto a different stem. Nurseries offer for sale trees with grafted stems that can produce four or more varieties of related fruits, including apples.

The most common usage of grafting is the propagation of cultivars onto already rooted plants, sometimes the rootstock is used to dwarf the plants or protect them from root damaging pathogens.

Since vegetatively propagated plants are clones, they are important tools in plant research. When a clone is grown in various conditions, differences in growth can be ascribed to environmental effects instead of genetic differences.

Table showing various types of vegetative propagation

Advantages of vegetative reproduction

New plants resemble the parent plant and any good quality in the
parent is retained.
The growth of the new plant is rapid.
The reproductive organ stores plenty of food which the new plant
uses.
It does not involve processes like pollination, fertilization and
dispersal agents are not required.
Large areas can be covered in relatively little time.
It involved only one individual.

Disadvantages

Since new plant grows on its parent, it can lead to crowding.
Shortage of water and mineral salts is likely to occur due to competition.
Diseases of the parent plant can be transmitted to the young ones.
If the parent plant has poor characters, they can be maintained by the young
ones.

ARTIFICIAL VEGETATIVE PROPAGATION

This is a mode of reproduction where man is involved in the propagation
process. It is done in several ways, which include, budding, grafting, layering,
cuttings, etc.

1. Budding

This is the process where a bud is detarched from a plant and grown in suitable
conditions into a new plant.

2. Grafting

This is the insertion of part of one plant onto another plant so as to come into
organic union and to grow as one plant. The part inserted can be a bud or a
shoot of a plant and it is called a scion. The part in the ground on which the
scion is inserted is called a stolk. The scion and stolk should be of different
varieties but same species.

3. Layering

This is where a branch of a plant is bent to touch the ground and allowed to
develop roots. When the roots are developed, it is cut from the plant and it starts
to grow as a separate self-supporting plant.

Advantages of asexual reproduction

1. It is reliable because it is less likely to be affected by adverse environmental
factors like for the case of seeds.
2. It leads to genetic consistence since there is no mixing of genes during
reproduction.
3. It results into early maturity because the organisms produced have enough
food reserve from the parent.
4. It is self-sufficient because it does not rely on external processes like
pollination, fertilization and dispersal.
5. It does not result in indiscriminate and wide spread distribution like in the
case of seeds, which leads to wastage.
6. It does not require formation of sex organs.
7. It is the only means of reproduction in some organisms.

Disadvantages of asexual reproduction

1. It leads to maintenance of bad characters.
2. It does not introduce variations in the offspring since there is no gene
mixing.
3. It easily results into competition between offspring due to overcrowding.
4. It gradually results into reduction of the strength and vigour of the
succeeding generations.
5. There is a high chance of disease transmission from parent to offspring.

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