Asexual & Sexual Reproduction
Trench Layering
Description:
Common propagation method for woody plants.
Particularly suitable for plants exhibiting strong apical dominance.
Process:
New plants are planted at an angle of 30^{\circ} \text{ to } 45^{\circ}.
Once established, the plants should be laid flat on the soil.
Wire or pegs can be used to hold the plants in place.
The release of apical dominance initiates the development of lateral shoots.
New shoots should be covered with growing media (e.g., sawdust, peat moss) to induce etiolation and promote rooting.
Cut the rooted shoots for propagation.
Examples of plants suitable for trench layering: Cherry, apple.
Mound Layering
Description:
Performed during the dormant season.
Involves cutting the parent plant near ground level (approximately 15-20 \text{ cm} above ground).
Process:
When new shoots begin to grow, the base of the plant is mounded or covered with growing media to facilitate rooting.
Rooted plants are subsequently separated from the mother plant for propagation.
Examples of plants suitable for mound layering: Apple rootstock, cherry rootstock.
Compound Layering
Description:
Similar to simple layering techniques.
Requires a flexible branch.
Process:
The stem or branch is laid horizontally, and sections of it are alternately covered with soil and exposed along its length. Rooting occurs in the covered sections, while shoots develop from the exposed portions.
Examples of plants suitable for compound layering: Bougainvillea, muscadine grape.
Propagation Through Specialized Structures
Description:
Refers to vegetative propagation structures capable of producing new plants.
Specialized structures include:
Bulbs, tubers, rhizomes, suckers, runners, offsets, slips, crowns, etc.
Rhizomes and Suckers
Description of Rhizomes:
A specialized stem structure that grows horizontally below ground.
Segmented into nodes and internodes.
Suckers:
New plants developed from rhizomes.
Propagation:
Rhizomes should be divided for propagation, and each division must have at least one bud for new plant production.
Example: Banana.
Types of banana suckers:
Sword suckers: Commonly used for propagation.
Water suckers: Less commonly used.
Runners (Stolons)
Description:
A specialized horizontal stem (stolon) originating from the crown of the plant.
Propagation Process:
Each node produces new plants.
When new plants develop a well-established root system, they are separated and transplanted.
Examples: Strawberry, some blueberry varieties.
Offshoots/Suckers
Description:
Developed from the base of the fruits or from the base of leaves/stem.
Example: Pineapple. - Pineapple can be propagated through its crown, suckers, or slips.
Root Suckers
Description:
Generated from adventitious buds on the roots.
Propagation Process:
Later suckers are separated and can be used as planting material.
Example: Guava.
Apomixis
Description:
A form of asexual propagation where embryo formation occurs without fertilization.
Process:
Embryo formation occurs without meiosis and fertilization, resulting from diploid cells of other tissues like the nucellus, integument, or unfertilized egg.
Characteristics:
The new plant is genetically identical to the mother plant due to the absence of meiosis and fertilization.
The seedlings produced are termed apomictic seedlings.
Examples: Nucellar embryo of citrus, mango, apple.
Micropropagation/Tissue Culture
Historical Context:
Strawberry was the first fruit plant on which micropropagation technology was standardized.
Process:
Plants are grown under aseptic conditions in a controlled environment with a specified growing medium.
Small parts of parent plants (termed explants), such as meristematic tissue, callus, axillary buds, embryo, protoplast, or pollen grains, are utilized for tissue culture.
With appropriate growth regulators and optimal growth conditions, the explants develop roots and shoots.
Once plantlets are established, they are transferred to soil and grown under normal conditions (acclimatization).
Limitations:
This method may not be applicable for plants with high phenolic compounds.
Advantages of Micropropagation:
Generates disease-free, healthy plants.
Requires very small amounts of propagation material.
Rapidly produces large quantities of new plants.
Particularly useful for plants that do not propagate easily (e.g., orchids).
Disadvantages of Micropropagation:
Expensive.
Requires skilled labor.
Seedlings generated in laboratories may not adapt well to harsh open climatic conditions, necessitating plant hardening or acclimatization.
Sexual Propagation
Process:
Involves pollen transfer from anthers to stigma, leading to fertilization and seed development.
Seeds produced are utilized to propagate new plants.
Characteristics:
New plants resulting from sexual propagation are not identical to parent plants and are referred to as seedlings.
Advantages of Sexual Propagation
Benefits:
Generally cheaper method.
Simpler propagation process.
Facilitates the development of hybrid varieties more readily through seed propagation.
New plants can potentially be superior to parent plants.
New plants often exhibit better rooting systems compared to adventitious rooting systems.
Sexual propagation can lead to a longer lifespan for fruit trees than asexually propagated trees.
Identical plants to parents may be obtained through polyembryony seeds or nucellar seedlings (a form of apomixis).
Disadvantages of Sexual Propagation
Challenges:
New plants may not be identical to parent plants.
The pre-bearing period (time from planting to fruiting) is typically longer than for asexually propagated plants.
Generally larger in size.
Benefits of specific rootstocks cannot be utilized.
Seeds can have reduced viability over short storage durations.
Seeds may experience dormancy and require specific treatments to break dormancy.
Types of Seeds Based on Length of Storage
Categories:
Different seeds can be stored for varying durations without affecting their viability.
Based on longevity in storage, seeds are classified into three types:
Orthodox seeds
Recalcitrant seeds
Intermediate seeds
Orthodox Seeds
Characteristics:
Can be dried to a moisture content below 10\%.
Can be preserved at freezing temperatures after proper drying.
Can be stored for extended periods.
Examples: Apple, cherry, guava.
Recalcitrant Seeds
Characteristics:
Cannot endure extensive desiccation like orthodox seeds.
Can sustain moisture content between 30\%-70\%.
Loss of viability commences between 30\%-65\% moisture levels.
Not suitable for storage at freezing temperatures since lethal ice crystals can form.
Typically stored for few months to few years.
Examples: Chestnut, maple (temperate), mango, avocado, rambutan, and other tropical fruit crops.
Intermediate Seeds
Characteristics:
Exhibit a tolerance to desiccation that is better than recalcitrant seeds but poorer than orthodox seeds.
Not compatible with conventional freezing for storage.
Can be stored for several years.
Examples: Papaya, macadamia nuts.