Unit3 - Subjective Questions

Difference between Sexual and Asexual Propagation:

Advantages of Sexual Propagation:

1. New Varieties: Hybridization offers a chance to develop new varieties with desirable traits.
2. Hardiness: Seedling trees usually have a deep root system, making them more hardy and resistant to abiotic stress.
3. Rootstocks: Essential for raising rootstocks for grafting and budding.
4. Polyembryony: In citrus and mango, nucellar seedlings can produce true-to-type plants.

Disadvantages of Sexual Propagation:

1. Variation: Progeny are not true-to-type, leading to variation in yield and quality.
2. Long Juvenile Phase: Trees take a long time to come into bearing.
3. Size: Trees grow very tall, making management (pruning/harvesting) difficult.

Definition:
Seed dormancy is a physiological state in which a viable seed fails to germinate even when provided with favorable environmental conditions (moisture, temperature, and oxygen).

Types of Seed Dormancy:

1. Exogenous Dormancy (Coat-imposed):

   • Physical Dormancy: The seed coat is impermeable to water (e.g., Legumes). The hard testa prevents imbibition.
   • Mechanical Dormancy: The seed coat is too hard for the radicle to expand and break through (e.g., Olive, Peach pits).
   • Chemical Dormancy: Inhibitors (like Coumarin, Abscisic acid) present in the seed coat or fruit pulp prevent germination.

2. Endogenous Dormancy (Embryo-imposed):

   • Physiological Dormancy: Caused by physiological mechanisms within the embryo, often requiring a period of after-ripening or chilling (Stratification) to reduce ABA levels and increase Gibberellic Acid ().
   • Morphological Dormancy: The embryo is underdeveloped or rudimentary at the time of fruit dispersal and needs time to grow before germination (e.g., Papaya).
   • Combined Dormancy: A combination of both physical (hard coat) and physiological dormancy.

Stock-Scion Relationship:
When a scion is grafted onto a rootstock, they form a composite genetic system. While the genotype of each part remains distinct, they influence each other's phenotypic expression (growth, yield, quality) through the translocation of water, nutrients, and hormones.

Influence of Rootstock on Scion:

1. Control of Vigour (Size): Rootstocks can significantly alter the height of the tree. For example, Malling-Merton rootstocks in apples can induce dwarfing (M-9) or semi-dwarfing effects.
2. Precosity (Early Flowering): Dwarfing rootstocks generally induce early fruiting compared to vigorous rootstocks.
3. Yield and Quality: Rootstocks influence the size, color, and sugar content (Brix) of the fruit. For example, Rangpur Lime rootstock improves the quality of Mandarin oranges.
4. Nutrient Absorption: Different rootstocks have varying capacities to absorb specific minerals (e.g., Nitrogen, Phosphorus) from the soil.
5. Resistance to Biotic/Abiotic Stress:
   • Disease: Grape rootstocks resistant to Phylloxera.
   • Salinity/Drought: Use of deep-rooted rootstocks for drought tolerance.

Differences between Stratification and Scarification:

Air Layering (Marcottage/Gootee):

This is a method of vegetative propagation where roots are induced to form on the stem while it is still attached to the parent plant.

Procedure:

1. Selection: Select a healthy, pencil-thick branch (usually 1 year old) from the parent plant.
2. Girdling: A ring of bark (approx. 2.5 to 3 cm wide) is removed from the selected branch to interrupt the downward translocation of carbohydrates (Phloem transport).
3. Application: Rooting hormone (e.g., IBA - Indole Butyric Acid) can be applied to the upper cut surface.
4. Covering: The girdled portion is covered with moist sphagnum moss to retain moisture and then wrapped tightly with a polyethylene sheet. It is tied at both ends (looks like a toffee).
5. Separation: After 1-3 months, when roots are visible through the plastic, the branch is cut below the rooted zone and potted.

Examples: Guava, Litchi, Pomegranate, Citrus (Lemon/Lime).

Merits of Asexual Propagation:

• True-to-Type: Offspring are genetically identical to the parent, ensuring uniformity in fruit size, quality, and yield.
• Early Bearing: Vegetative plants bypass the long juvenile phase associated with seed propagation.
• Propagation of Seedless Plants: It is the only method to propagate plants that do not produce viable seeds (e.g., Banana, Pineapple, seedless Grapes).
• Repair: Grafting can be used to repair damaged trees (Bridge grafting).
• Combining Traits: Allows combining a good root system (rootstock) with a superior fruiting cultivar (scion).

Demerits of Asexual Propagation:

• Short Life Span: Plants are generally shorter-lived than seedling trees.
• Disease Transmission: Systemic diseases, especially viruses, easily spread from parent to offspring.
• Cost: It requires skilled labor and specialized structures, making it more expensive than seed propagation.
• Lack of Diversity: Genetic uniformity makes the entire plantation susceptible to new pests or environmental changes.

Definition:
Graft incompatibility is the inability of two different plant parts (stock and scion) grafted together to produce a successful union and develop into a composite plant. It leads to the failure of the graft union immediately or prematurely.

Symptoms of Incompatibility:

1. Low Take: Failure to form a successful union initially.
2. Yellowing: Chlorosis or yellowing of foliage in the later stages.
3. Premature Death: The tree may die after a few years of growth.
4. Overgrowth: Marked difference in growth rate or vigor between stock and scion (e.g., scion overgrowing the stock or vice versa).
5. Clean Break: Physical breaking of the graft union at the point of attachment due to poor structural continuity (lack of vascular connection).
6. Accumulation: Accumulation of starch above the graft union due to phloem blockage.

Phases of Seed Germination:

1. Phase I: Imbibition (Water Uptake):

   • Rapid uptake of water by the dry seed.
   • The seed coat swells and softens.
   • Respiration rate increases slightly.

2. Phase II: Lag Phase (Metabolic Activation):

   • Water uptake stabilizes.
   • Enzyme Activation: Hydrolytic enzymes (Amylase, Protease, Lipase) are activated.
   • Digestion of Reserves: Starch is converted to sugars (by amylase), proteins to amino acids, and lipids to fatty acids.
   • Translocation: These simpler food substances are translocated to the growing points (embryo axis).
   • Hormonal Balance: Ratio of Promoters () to Inhibitors (ABA) increases.

3. Phase III: Radicle Emergence (Growth):

   • Cell division and elongation occur in the embryonic axis.
   • The radicle breaks through the seed coat (first visible sign of germination).
   • Secondary water uptake begins as seedling growth accelerates.

Mist Chamber:

• Definition: A propagation structure where a fine mist of water is sprayed intermittently over the cuttings to maintain high humidity and lower the leaf surface temperature.
• Mechanism: It uses high-pressure nozzles and automated timers/sensors to create mist. This prevents the desiccation (drying out) of softwood and semi-hardwood cuttings which still have leaves attached.
• Advantages:
  • Reduces transpiration rate significantly.
  • Keeps the turgidity of leaf cuttings high, allowing them to carry out photosynthesis.
  • Drastically improves the rooting percentage of difficult-to-root species.
  • Allows for year-round propagation in controlled conditions.
• Use: Widely used for rooting stem cuttings of ornamental plants and fruit crops.

T-Budding (Shield Budding):

This is the most common method of budding used in fruit tree propagation (e.g., Rose, Citrus, Apple).

Procedure:

1. Preparation of Stock: A 'T' shaped cut is made on the bark of the rootstock (a vertical cut of 2.5 cm and a horizontal cut of 1 cm). The bark flaps are gently loosened.
2. Preparation of Scion (The Bud): A single bud along with a shield-shaped piece of bark (and a thin sliver of wood) is sliced off from the bud stick. The length should match the vertical cut on the stock.
3. Insertion: The shield piece containing the bud is inserted downwards under the two flaps of bark on the stock.
4. Wrapping: The union is wrapped tightly with polythene tape or raffia fiber, leaving only the eye of the bud exposed.
5. Aftercare: Once the bud heals and starts sprouting, the rootstock portion above the bud is cut off to divert energy to the new shoot.

Specialized Vegetative Structures:
These are modified plant parts (stems, roots, or leaves) specialized for food storage and reproduction.

1. Bulbs:

• Description: A short, modified underground stem surrounded by fleshy, scale-like leaves that store food. The stem is reduced to a basal plate.
• Types:
  • Tunicate: Have a dry, papery outer covering (e.g., Onion, Tulip).
  • Non-tunicate (Scaly): Lack the papery covering (e.g., Lily).
• Propagation: Small bulbs (bulblets) form at the base and can be separated.

2. Corms:

• Description: A swollen, solid underground stem base that is rounded and flattened. Unlike bulbs, the bulk of the corm is solid stem tissue, not leaves. It has distinct nodes and internodes.
• Propagation: New corms (cormels) develop on top or beside the old corm.
• Examples: Gladiolus, Crocus, Amorphophallus.

External Factors Affecting Seed Germination:

1. Water (Moisture): Essential for imbibition, which activates enzymes and facilitates the breaking of the seed coat. Excessive water can cause rotting (lack of oxygen), while insufficient water prevents germination.
2. Temperature: Every seed has an optimum temperature range.
   • Cool season crops: to .
   • Warm season crops: to .
   • Extreme temperatures denature enzymes.
3. Oxygen (Aeration): Respiration increases rapidly during germination. Seeds require adequate oxygen for aerobic respiration to generate energy (ATP) for growth. Waterlogged soils inhibit germination due to hypoxia.
4. Light:
   • Photoblastic seeds require light to germinate (e.g., Lettuce).
   • Negative photoblastic seeds require darkness (e.g., Onion, Lily).
   • Most crop seeds are non-photoblastic (indifferent to light).

Greenhouse vs. Polyhouse:

Concept:
Micropropagation is the technique of propagating plants by culturing cells, tissues, or organs (explants) in a sterile environment on a nutrient medium. It is based on the principle of Totipotency (the ability of a single plant cell to regenerate into a whole plant).

Steps: Explant selection Sterilization Inoculation Multiplication Rooting Hardening.

Advantages:

1. Mass Multiplication: Thousands of plants can be produced from a small tissue piece in a short time.
2. Disease-Free Plants: Meristem culture allows for the production of virus-free planting material.
3. Round the Year: Production is independent of seasons.
4. Space: Requires very little space compared to conventional nurseries.
5. Germplasm Conservation: Useful for preserving endangered species.

Definition:
A cutting is a method of asexual propagation where a portion of the stem, root, or leaf is cut from the parent plant and induced to form roots and shoots to create a new independent plant.

Classification:

1. Stem Cuttings:
   • Hardwood cuttings: Mature, dormant wood (e.g., Grape, Fig).
   • Semi-hardwood cuttings: Partially matured wood (e.g., Citrus, Olive).
   • Softwood cuttings: Succulent, new growth (e.g., Coleus).
   • Herbaceous cuttings: From soft tissue plants (e.g., Carnation).
2. Root Cuttings: Pieces of roots are used (e.g., Blackberry, Breadfruit).
3. Leaf Cuttings:
   • Whole leaf with petiole: (e.g., African Violet).
   • Leaf blade: (e.g., Begonia, Bryophyllum).

Factors Influencing Rooting of Cuttings:

1. Internal Factors:

   • Stored Food (C:N Ratio): High carbohydrate to nitrogen (C:N) ratio favors root initiation.
   • Auxins: Presence of endogenous auxins is crucial. Application of IBA (Indole Butyric Acid) or NAA typically enhances rooting.
   • Age of Stock Plant: Cuttings from juvenile (young) plants root easier than those from mature plants.
   • Presence of Leaves: In softwood cuttings, leaves produce auxins and carbohydrates that aid rooting.

2. External Factors:

   • Temperature: Optimum temperature ( - ) promotes cellular activity. Bottom heat (warming the soil) helps rooting.
   • Humidity: High humidity is essential to prevent transpiration and desiccation.
   • Light: While the base of the cutting needs darkness (etiolation promotes rooting), the leaves need light for photosynthesis.
   • Media: The rooting medium must be porous, well-drained, and sterile (e.g., perlite, vermiculite, sand).

Hotbed:
A hotbed is a propagation structure, similar to a cold frame, but provided with a source of bottom heat to encourage seed germination and rooting of cuttings. The heat source can be:

• Biological: Decomposing organic matter (like manure).
• Electrical: Heating cables buried in the soil.
• Hydronic: Hot water pipes.

Difference from Cold Frame:

• Heat Source: A Cold Frame relies solely on solar energy (trapped by the glass sash) to warm the soil. A Hotbed has an artificial or supplementary heat source.
• Usage: Hotbeds are used earlier in the season or in colder weather compared to cold frames.
• Function: Hotbeds actively promote growth through heat; cold frames are primarily used for hardening off plants or extending the season.

Methods of Grafting:

1. Whip (Splice) Grafting:

   • Used when stock and scion are of the same diameter (pencil thickness).
   • A slanting cut (approx. 2.5-5 cm) is made on both stock and scion.
   • The cut surfaces are joined together and tied.

2. Tongue (Whip and Tongue) Grafting:

   • Modification of Whip grafting.
   • A second "tongue" cut is made on both the slanting surfaces to interlock them.
   • Provides better structural strength and cambial contact.

3. Cleft (Split) Grafting:

   • Used when the rootstock is thicker than the scion.
   • The stock is split vertically.
   • Two scions are prepared with a wedge shape and inserted into the split on either side, matching the cambium layers.
   • Commonly used for top working old trees.

Runners (Stolons):

• Definition: Specialized aerial stems that grow horizontally along the ground. They form roots and shoots at the nodes.
• Propagation: The daughter plants formed at the nodes can be separated and planted.
• Example: Strawberry. It is the primary commercial method for Strawberry propagation.

Suckers:

• Definition: A shoot that arises from the underground part of the stem or adventitious buds on the roots.
• Types:
  • Sword Suckers: Have narrow leaves and a broad base; vigorous (Preferred for Banana).
  • Water Suckers: Have broad leaves; weak.
• Propagation: Suckers are dug out with a piece of the rhizome/root and planted.
• Examples: Banana, Pineapple, Chrysanthemum, Date Palm.

Definition:
Apomixis is a form of asexual reproduction where seeds are formed without fertilization (fusion of gametes). The resulting embryo is vegetative in origin and genetically identical to the mother plant (clone).

Significance in Horticulture:

1. True-to-Type: Since there is no gene segregation, seedlings are uniform and identical to the parent, similar to vegetative propagation.
2. Virus-Free: Unlike vegetative cuttings which carry viruses, apomictic seeds (especially from the nucellus) are often free from viruses (e.g., in Citrus).
3. Cost-Effective: It allows for the production of clonal rootstocks using seeds, avoiding the labor-intensive process of cuttings or layering.
4. Fixing Hybrid Vigor: If a superior hybrid is apomictic, its hybrid vigor can be maintained indefinitely through seeds.