In crop production, different parts of crops can be used to replicate the crop. These parts may include seeds, leaves, stems, roots, etc. The success of crop establishment depends on the cultural practices used for production.
This article will explore methods of propagation for horticultural crops. Some crops have one method of propagation, while others may have more than one.
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Propagation of Crops
Plant propagation is the controlled process of creating new plants from various sources such as seeds and other plant parts. The objectives of plant propagation are to:
i. Increase the number of plants
ii. Preserve the essential characteristics of the plant
Propagation involves the formation and development of new individuals using specialized parts of the plant. These new individuals are used in the establishment of new plantings.
Methods of Propagation of Horticultural Crops
In general, two methods are employed in the propagation of horticultural crops:
1. The use of seed (sexual)
2. The use of vegetative parts of plants (asexual)
1. Sexual Propagation (Seed)
Sexual propagation is the method of increasing the number of plants through seeds formed from the union of gametes during pollination. A seed consists of an embryo with nourishing and protecting tissues.
The embryo is considered a minute plant. Principal parts of the embryo include the plumule (the first growing point of the stem), the radical (the first growing point of the root), and the hypocotyls and epicotyls (which together constitute the first or original stem of the plant). The nourishing tissues are the endosperm and cotyledons.
In well-developed mature seeds, these tissues are packed with stored food such as starch, hemicelluloses, reserved proteins, or fats, depending on the kind of plant. The protecting tissue is the seed coat, which slows transpiration and, in some plants, retards respiration while in storage.
It also protects the delicate embryo from mechanical injury. When exposed to favorable environmental conditions (soil, water, temperature, light, etc.), the seed germinates and produces a plant.
Therefore, a seed can be defined as a minute plant with nourishing and protecting tissues used to produce a similar plant. Sexual propagation is the primary means by which biological variation is generated.
Advantages of Sexual Propagation
- Easy and natural phenomenon – Once the seed is given the right environmental conditions, it will germinate and produce the plants.
- Sources of genetic variability – They provide a means of recombination of genetic material to produce improved varieties.
- Helps in selecting different traits – Desirable traits can be retained while undesirable traits are removed.
Disadvantages of Sexual Propagation
- Not all plants produce viable seeds, e.g., banana, pineapple.
- It is not possible to maintain a true-to-type genotype.
- Sexual propagation takes a longer period of time from seed to harvest, especially with tree crops.
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2. Asexual Propagation (Vegetative Propagation)
Vegetative propagation is the method that uses any part of the plant rather than ‘true’ seeds to produce new plants. Plants have a number of vegetative mechanisms, some of which have been utilized by horticulturists and gardeners to rapidly multiply or clone plants.
Plants produced using vegetative parts have no exchange of genetic material, so they are identical to the parent. Vegetative propagation uses parts such as roots, stems, and leaves. In some plants, seeds can be produced without fertilization, and these seeds contain only the genetic material of the parent plant.
This process is called apomixes. Apomixes is a form of asexual reproduction but not vegetative propagation. Vegetative propagation is essential for raising many economic crops, such as fruit crops, nut crops, many flowering and ornamental crops, and certain vegetable crops.
Advantages of Vegetative Propagation
- Maintenance of true-to-type clones – A clone is a group of plants with the same genetic makeup.
- Uniformity of plant genotype is maintained.
- Only means of propagating certain plants – e.g., banana and plantain, which do not produce viable seeds.
- Earliness of production is achieved.
- More economical for certain crops, such as sweet potatoes.
- Best for seeds that germinate with difficulty.
- Avoids seed-borne diseases.
Disadvantages of Vegetative Propagation
- Planting materials are usually bulky.
- Storage of asexual material is cumbersome and usually short-term.
- No source of genetic variability for recombination of genes.
- Mechanized propagation is often impracticable.
- Requires specialized personnel to handle procedures.
- More expensive than seed propagation.
- Systemic viral diseases can spread through plant materials and tools.
Methods of Vegetative Propagation
The methods of vegetative propagation include:
- Use of specialized vegetative structures, e.g., roots, corms, bulbs, rhizomes, etc.
- Cutting
- Layering
- Grafting
- Budding
Use of Specialized Vegetative Structures
Vegetative propagation can be achieved using specialized vegetative structures other than seeds. These structures include bulbs, corms, rhizomes, tubers, runners, and offshoots.
1. Bulbs: These are shooting stems with thick fleshy leaves. Stem buds develop at the axil of leaf scales to form small buds (bulblets), which are known as offsets when grown to full size (e.g., onion).
2. Corms: Resemble bulbs but have no fleshy leaves. They are solid stem structures with nodes and internodes (e.g., cocoyam).
3. Rhizomes: Horizontal cylindrical stems growing underground with nodes and internodes. They readily produce adventitious roots and may be slender and elongated or thick and fleshy. Propagation occurs by cutting the rhizomes into pieces, each containing a vegetative bud (e.g., ginger).
4. Tubers: Thick portions of underground stems (e.g., Irish potatoes). Nodes and internodes are present, and nodes are arranged spirally.
5. Runners: Aerial stems that develop from the leaf axil at the base or crown of plants. Runners grow horizontally along the ground and form new plants at one of the nodes (e.g., strawberry).
6. Offshoots: Lateral shoots that develop from the stem, which, when rooted, serve to reproduce the plant. These are referred to as offsets, crowns, divisions, or slips (e.g., banana, pineapple).
Plants or crops are usually reproduced through sexual or asexual means depending on the mode best suited for the crop to achieve maximum productivity. While some crops reproduce through one method, others use both.
Crops that reproduce sexually have the advantage of providing genetic variability for modification, while those that reproduce asexually have the advantage of producing true-to-type plants.
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