Unit 3: Plant Propagation

1. Introduction to Plant Propagation

Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs, and other plant parts. It is a fundamental practice in horticulture, ensuring the multiplication of species and the preservation of desirable genetic traits.

Propagation is broadly classified into two categories:

1. Sexual Propagation: Reproduction through seeds (formed by the fusion of male and female gametes).
2. Asexual (Vegetative) Propagation: Reproduction using vegetative parts (leaves, stems, roots) to create clones.

2. Sexual Propagation (Seed Propagation)

Merits (Advantages)

• Genetic Diversity: Offspring result from genetic recombination, providing variation essential for developing new varieties (breeding).
• Longevity: Seedling plants usually possess a strong, deep taproot system, making them longer-lived and more firmly anchored than vegetatively propagated plants.
• Disease Filtering: Many viruses and mycoplasmas are not transmitted through seeds (with exceptions like certain bean viruses), allowing for the production of virus-free stock.
• Cost-Effective: It is generally the cheapest and easiest method for mass propagation (e.g., cereals, vegetables, papaya).
• Rootstocks: Hardy rootstocks for grafting are raised via seeds (e.g., Citrus, Mango).

Demerits (Disadvantages)

• Loss of Genetic Purity: Progeny are not "true-to-type" due to genetic segregation; the offspring may not resemble the parent plant in fruit quality or yield.
• Long Juvenile Phase: Seedling trees take a much longer time to reach flowering and fruiting maturity (precocity is low).
• Large Canopy: Seedling trees grow very large, making orchard management (pruning, harvesting, spraying) difficult.
• Dormancy Issues: Some seeds have complex dormancy requirements that must be overcome before they can germinate.

3. Seed Biology: Germination and Dormancy

A. Seed Germination

The resumption of active growth of the embryo that results in the rupture of the seed coat and the emergence of a young plant.

Phases of Germination:

1. Imbibition: The physical uptake of water by the seed, causing swelling and rupturing of the seed coat.
2. Lag Phase (Metabolic Activation): Enzymatic activity increases. Stored food (starch/proteins) is hydrolyzed into simpler forms for the embryo. Respiration rate increases.
3. Radicle Emergence: Cell division and elongation occur. The radicle (root) emerges first to anchor the plant, followed by the plumule (shoot).

Types of Germination:

• Epigeal: Cotyledons are pushed above the soil surface (e.g., Bean, Tamarind).
• Hypogeal: Cotyledons remain below the soil surface (e.g., Pea, Mango, Maize).

B. Seed Dormancy

A state in which viable seeds fail to germinate even when provided with favorable environmental conditions (moisture, temperature, oxygen). It is a survival mechanism to prevent germination during adverse seasons.

1. Types of Dormancy

• Exogenous Dormancy (External): Caused by the seed coat.
  • Physical: Seed coat is impermeable to water (e.g., Legumes).
  • Mechanical: Seed coat is too hard for the radicle to penetrate (e.g., Olive, Peach pits).
  • Chemical: Inhibitors present in the fruit pulp or seed coat prevent germination.
• Endogenous Dormancy (Internal): Caused by the embryo.
  • Morphological: Embryo is immature or underdeveloped at fruit harvest (e.g., Palm, Orchid).
  • Physiological: An imbalance of growth hormones (low Gibberellins, high Abscisic Acid). Requires specific temperature treatment.

2. Methods to Break Dormancy

• Scarification (For Exogenous Dormancy): Altering the seed coat to make it permeable.
  • Mechanical: Rubbing against sandpaper, filing, or cracking.
  • Acid: Soaking seeds in concentrated Sulfuric Acid ().
  • Hot Water: Soaking in boiling water effectively softens the coat.
• Stratification (For Endogenous Dormancy): Subjecting seeds to moist chilling or warming to mimic seasonal changes and mature the embryo.
  • Cold Stratification: Storing seeds at 0°C–5°C in a moist medium (sand/peat) for weeks to simulate winter.
• Leaching: Soaking seeds in running water to wash away chemical inhibitors.
• Chemical Treatment: Application of Gibberellic Acid () or Potassium Nitrate ().

4. Asexual (Vegetative) Propagation

Merits (Advantages)

• True-to-Type: Offspring are genetically identical clones of the mother plant, ensuring uniformity in fruit size, quality, and yield.
• Precocity: Vegetatively propagated plants mature and fruit significantly earlier than seedlings.
• Dwarfness: Combining a scion with a dwarfing rootstock produces smaller trees, facilitating high-density planting.
• Seedless Varieties: The only method to propagate seedless plants (e.g., Banana, Pineapple, Seedless Grape).
• Repair: Grafting can be used to repair damaged trees (Bridge grafting).

Demerits (Disadvantages)

• Short Lifespan: Clonal plants generally have a shorter life span than seedling trees.
• Shallow Root System: Many vegetatively propagated plants (especially layers and cuttings) are shallow-rooted and prone to windthrow.
• Disease Transmission: Viral diseases are easily transmitted from the mother plant to the offspring.
• Skill Required: Techniques like grafting and budding require specialized technical skills.

5. Methods of Asexual Propagation

A. Cuttings

Detaching a vegetative part from the mother plant and placing it in a medium to regenerate the missing parts.

• Stem Cuttings:
  • Hardwood: Mature, dormant wood (e.g., Grape, Fig, Pomegranate).
  • Semi-hardwood: Partially mature wood (e.g., Citrus, Olive).
  • Softwood: Succulent, new growth (e.g., Ornamental shrubs).
• Leaf Cuttings: Leaf blade used to generate plantlets (e.g., Bryophyllum, Begonia, Snake plant).
• Root Cuttings: Root pieces buried in soil (e.g., Blackberry, Breadfruit).
• Hormonal Aid: Auxins like IBA (Indole-3-butyric acid) and NAA (Naphthaleneacetic acid) are commonly used to induce rooting in cuttings.

B. Layering

Rooting a stem while it is still attached to the mother plant.

• Simple/Ground Layering: Bending a low branch to the ground, covering part of it with soil, and allowing it to root (e.g., Guava, Jasmine).
• Air Layering (Marcottage/Gootee): Bark is removed (girdled) from an aerial shoot; moist moss is wrapped around the wound and covered with polyethylene. Roots form at the girdled site (e.g., Litchi, Pomegranate, Citrus).
• Mound (Stool) Layering: The plant is cut back to the ground; new shoots are covered with soil at the base to induce rooting (e.g., Apple rootstocks).

C. Grafting

Joining two plant parts so they grow as one.

• Scion: The upper part that produces the shoot/fruit.
• Stock (Rootstock): The lower part that provides the root system.
• Key Principle: The vascular cambium of the stock and scion must align for a successful union.
• Types:
  • Whip (Tongue) Grafting: For stock and scion of equal diameter.
  • Cleft Grafting: For older stocks with larger diameters than the scion.
  • Approach Grafting (Inarching): Both plants remain on their own roots until the graft heals (e.g., Mango, Sapota).

D. Budding

A form of grafting where the scion is reduced to a single bud with a small piece of bark.

• T-Budding (Shield Budding): A T-shaped cut is made on the stock bark, and the bud is inserted (e.g., Rose, Citrus).
• Patch Budding: A rectangular patch of bark is removed from the stock and replaced with a bud patch (e.g., Walnut, Pecan).
• Chip Budding: A chip of wood with a bud is placed into a matching notch on the stock (e.g., Grape).

E. Specialized Vegetative Structures

• Runners: Strawberry.
• Suckers: Banana, Pineapple.
• Bulbs: Onion, Tulip.
• Tubers: Potato.
• Rhizomes: Ginger, Turmeric.

6. Stock-Scion Relationship

When two distinct plants are grafted together, they interact and influence each other’s growth. This interaction is critical for orchard performance.

A. Effect of Stock on Scion

1. Vigor (Size Control): Rootstocks can determine the height of the tree.
   • Example: Apple on 'Malling 9' (M9) rootstock becomes a dwarf tree; on seedling rootstock, it becomes a standard giant tree.
2. Precociousness: Certain rootstocks induce early fruiting.
3. Fruit Quality: Rootstocks influence sugar content, acidity, and skin color of the fruit (e.g., Citrus on Trifoliate orange usually has higher acidity).
4. Winter Hardiness & Disease Resistance: A rootstock can impart cold tolerance or resistance to soil-borne pathogens (e.g., Grape rootstocks resistant to Phylloxera).

B. Effect of Scion on Stock

1. Root Vigor: A vigorous scion (lots of leaves/photosynthesis) produces more auxin, stimulating vigorous root growth.
2. Root Morphology: In some cases, the scion type can alter the distribution or depth of the root system.

C. Graft Incompatibility

Not all plants can be grafted together. Incompatibility is the inability of the stock and scion to form a successful, long-lasting union.

• Symptoms of Incompatibility:
  • Yellowing of foliage.
  • Premature death of the tree.
  • Significant difference in growth rate (diameter) at the graft union (overgrowth).
  • Clean breaking of the graft union under mechanical stress (wind).
• Types:
  • Localized: Incompatibility at the contact point (can be overcome by using an intermediate stock or "interstock").
  • Translocated: Caused by a toxin or virus moving through the phloem (cannot be overcome by an interstock).

7. Propagation Structures

Structures are used to control environmental factors (temperature, humidity, light) to ensure high survival rates of propagules.

A. Greenhouse (Glasshouse/Polyhouse)

• Description: A framed structure covered with transparent material (glass or polyethylene).
• Function: Creates a controlled microclimate. It traps solar radiation (Greenhouse Effect), raising the temperature. Modern greenhouses control humidity, temperature, and levels automatically.
• Use: Year-round propagation, acclimatization of tissue culture plants.

B. Mist Chamber

• Description: A structure equipped with fine misting nozzles and timers.
• Function: Sprays water intermittently to maintain near 100% relative humidity and creates a film of water on leaf surfaces.
• Mechanism: The water evaporation cools the leaves and reduces transpiration, allowing softwood and leafy cuttings to root without drying out.

C. Hotbeds

• Description: Low box-like structures with a transparent lid, provided with supplementary bottom heat.
• Function: Heat is supplied via electric cables or decomposing manure at the base.
• Use: Bottom heat stimulates rapid root development in cuttings and early seed germination before the outdoor season warms up.

D. Cold Frames

• Description: Similar to hotbeds but without a heating source. They rely on solar heat.
• Function: Used for Hardening Off—the process of gradually acclimating greenhouse-grown plants to outdoor conditions before transplanting.

E. Lath House (Shade House)

• Description: Structures covered with laths (strips of wood) or shade nets.
• Function: Reduces light intensity and temperature; protects young nursery plants from scorching sun and heavy rain.
• Use: Storing shade-loving plants or newly potted plants that need to recover from transplant shock.