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Reproductive Biology in Crop Plants: Part One

Reproduction is one of the fundamental features of life on earth. Reproduction means ‘to produce’. It is the way of producing offspring. In this article, basic ideas about plant reproduction including its modes and the significance of these modes will be discussed.

Reproduction in Plants

Plant reproduction is a biological process by which plants produce new individuals or offsprings. Reproduction enables the continuity of plant species, generation after generation.

Modes of Reproduction

Modes of reproduction in crop plants mainly divided into two categories:

1. Asexual reproduction.

2. Sexual reproduction.

Asexual Reproduction

Asexual reproduction is a type of reproduction process that does not involve fertilization, i.e. the fusion of gametes. Therefore, the offsprings produced are genetically identical to the parent.

  • Asexual reproduction may occur through vegetative reproduction or through apomixis.

Vegetative Reproduction

In vegetative reproduction, new plants develop from vegetative parts of parent species. 

  • There are two types of vegetative reproduction method, natural method and artificial method.

Natural Methods of Vegetative Reproduction

Naturally, new plants develop from a portion of the plant body. These occur through specialized vegetative structures such as underground stems, sub-aerial stems and bulbils.

Underground stems: Generally, the modified underground stems work as storage organs and contain many buds. These buds develop into shoots and produce plants after rooting.

Example:

Tubar Potato (Solanum tuberosum).

Bulb Onion (Allium cepa), Garlic (Allium sativum).

Rhizome Ginger (Zingiber officinale), Turmeric (Curcuma domestica).

Corm Bunda (Colocasia antiquorum), Arwi (Colocasia esculenta).

Sub-aerial stems: These modifications include runner, stolon, sucker etc.

Example: Mint (Mentha sp.), Date plam (Phoenix dactilifera) etc.

Bulbils: Modified flowers that develop into plants directly without fertilization and seed formation are known as bulbils.

Example: Garlic (Allium sativum).

Artificial Methods of Vegetative Reproduction

In many crop species, vegetative reproduction does not occur naturally. In that case, some artificial techniques are used for propagation. Some processes are described below:

Cutting: A process that involves cutting a part of a plant, such as stem, branch or root and placed into a growth medium to develop into a new plant.
Example: Sugarcane, lemon, grape, rose etc.

Grafting: Process in which part of a plant known as ‘scion’ is attached to the root system of another closely related plant species is known as the stock. They unite and form a new plant.
Example: Pomato (Stock- Potato, Scion- Tomato).

Fig: A. Grafting and B. Layering; Source here.

 

Layering: In this process, part of a plant is bent down and and covered with the soil. This buried part can generate new root system and therefore, an entirely new plant.

Example: Lemon, rose, jasmine, grapevine etc.

Micropropagation: Process in which part of a plant is placed in a plant culture medium and provided with all the hormones and nutrients it needs to develop new plants.

Fig: Micropropagation, Source here.

 


Apomixis

In apomixis, reproduction occurs through seeds but embryos develop without fertilization.

  • Apomixis may be facultative or obligate.
Fig: T. S. of a typical ovule, Source here.

Types of Apomixis

A simplified classification of Apomixis is given below:

1. Adventitious embryony: Embryo develops directly form vegetative cells of ovule such as nucellus, integument, chalaza etc. In this case, development of embryo does not involve production of embryo sac.

Example: Mango (Mangifera indica), Citrus.

2. Apospory: Embryo sac produced from any diploid cell except megaspore mother cell either directly by meiosis or after interrupted meiosis. The embryo may develop from the egg cell or some other cell of this embryo sac. 

Example: Some species of Hieraceum, Malus, Crepis, Ranunculus etc.

3. Diplospory: Embryo sac is produced from megaspore. Generally the meiosis is so modified that the megaspore remains diploid. 

  • Diplospory leads to parthenogenesis or apogamy.

i. Parthenogenesis: Embryo develops from egg cell. Depending upon whether the embryo sac is haploid or diploid, parthenogenesis is termed as haploid or diploid parthenogenesis.

Example: Haploid parthenogenesis occurs accidentally in Solanum nigrum, Nicotiana, Crepis and maize. Diploid parthenogenesis occurs in many grasses, e.g. Taraxacum.

ii. Apogamy: Embryo develops either from synergids or antipodal cells of embryo sac. Apogamy may also be haploid or diploid depending on the haploid or diploid state of embryo sac.

Example: Diploid apogamy occurs in Antennaria, Alchemilla, Allium etc.


Significance of Asexual Reproduction

The importance of asexual reproduction is extremely significant. Some of them are given below:

  • Large number of genetically identical individuals can be obtained through asexual reproduction.
  • Asexual reproduction is not complex instead it requires less energy compared to the sexual reproduction.
  • It can be applied for multiplication for improved varieties of plants that cannot produce seeds.
  • As the offsprings produced are genetically identical to the parent so it can be used to save endangered species.
  • Numerous offspring can be produced within short period of time in smaller area through asexual reproduction.

So respectively asexual reproduction is an easy and much more productive process.


Reference:

  • Plant Breeding Principles and Methods by B. D. Singh (4th edition; 1990).

 

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