Sexual Propagation and Seed Science

Course Metadata (Page 1)

• Subject: Sexual Propagation
• Lecturer: Dr Sbulelo Mwelase (Department of Horticulture)
• Institution: Durban University of Technology (DUT)
• Date: 26 February 2025

Context and Careers (Page 2-3)

• Core Benefits: Sexual propagation promotes genetic diversity/variation, enhances crop resilience, provides mass production, and is cost-effective compared to vegetative methods.
• Key Solving Areas: Seed viability (dormancy/germination), food security, and nursery management.
• Professional Roles: Plant Propagation Specialist, Seed Technologist, Horticulturist, and Agricultural Researcher.

Fundamental Concepts of Sexual Propagation (Page 4-6)

• Sexual Propagation: Propagation from seeds produced through the fusion of male ($1n$) gametes from pollen and female ($1n$) gametes from ovules.
• Pollination: The transfer of pollen from the anther to the stigma.
• Result: Fertilized ovules become seeds; ovaries transform into fruits to aid dispersal.

Floral Morphology and Types (Page 7-9)

• Structure:   - Calyx (Sepals): Protects the flower bud.   - Corolla (Petals): Attracts pollinators.   - Androecium (Stamen): Contains the Anther (pollen) and Filament.   - Gynoecium (Pistil): Includes the Stigma (capture), Style (pathway), and Ovary (contains ovules).
• Classifications:   - Complete: Possesses all four whorls (e.g., Rose).   - Perfect: Includes both male and female organs (e.g., Orange).   - Monoecious: Separate male/female flowers on one plant (e.g., Maize).   - Dioecious: Separate male/female flowers on different plants (e.g., Kiwifruit).

The Angiosperm Life Cycle Phases (Page 10-12)

1. Seed Germination: Imbibition and radicle emergence.
2. Vegetative Growth: Leaf and stem establishment via photosynthesis.
3. Flowering: Development of reproductive organs.
4. Pollination/Fertilization: Zygote formation.
5. Seed/Fruit Development: Maturation and protection.
6. Dispersal: Migration away from parent plant.
7. Dormancy: Inactivity until favorable conditions occur.

Double Fertilization and Embryogenesis (Page 13-15)

• Process: Unique to angiosperms.   - First Fertilization: Sperm ($1n$) + Egg ($1n$) $→$ Zygote ($2n$) (becomes the embryo).   - Second Fertilization: Sperm ($1n$) + Two Polar Nuclei $→$ Endosperm ($3n$) (nutritive tissue).
• Key Outcome: The endosperm forms only upon successful fertilization to ensure resource efficiency.

Physiological Regulation of Flower Production (Page 16-18)

• Environmental Factors:   - Photoperiodism: Short-day (Chrysanthemums), Long-day (Spinach), and Day-neutral (Tomatoes).   - Vernalization: Cold requirements (e.g., Winter Wheat).
• Hormonal Regulation:   - Gibberellins (GA): Promote flowering.   - Cytokinins (CK): Stimulate initiation.   - Abscisic Acid (ABA): Inhibits flowering under stress.
• Anthesis: The opening of the flower for pollination.

Seed Development and Maturation (Page 20-25)

• Embryogenesis: Development of the Radicle (root), Plumule (shoot), and Cotyledons (seed leaves).
• Desiccation: Water content reduces from approximately $80\%$ to $5 \text{ -- } 15\%$ to prepare for dormancy.
• Growth Phases: Lag Phase, Exponential Phase, Stationary Phase, and Decline/Death Phase.

Growth Patterns and Seedling Cycles (Page 26-28)

• Annuals: Complete life cycle in one season (e.g., Sunflowers).
• Biennials: Vegetative growth in Year 1; flowering/seeding in Year 2 (e.g., Carrots).
• Perennials: Live for multiple years with recurring cycles (e.g., Fruit trees).
• Morphological Mapping:   - Ovary $→$ Fruit   - Ovule $→$ Seed   - Integument $→$ Seed Coat (Testa)

Five Stages of Seed and Fruit Development (Page 29-35)

1. Morphological Development: Differentiation of embryo and endosperm.
2. Acquisition of Germination Ability: Activation of metabolic pathways and enzyme production.
3. Reserve Accumulation: Storing starch (cereals), proteins (legumes), and lipids/oils (sunflowers).
4. Maturation and Primary Dormancy: Hardening of the testa and high ABA levels.
5. Ripening and Dissemination: Fruit senescence and dispersal via animals (Zoochory), wind (Anemochory), or self-bursting (Autochory).

Germination Mechanisms and Requirements (Page 36-41)

• Epigeal Germination: Cotyledons are lifted above the soil (e.g., Beans).
• Hypogeal Germination: Cotyledons remain underground (e.g., Maize).
• Environmental Requirements:   - Water: Triggers imbibition and turgor pressure.   - Gases: Oxygen ($20\%$) needed for aerobic respiration; high $CO_2$ can retard growth.   - Temperature: Optimum for most seeds is $15 \text{ -- } 30 \, ^\circ C$ (Max: $40 \, ^\circ C$).

Phases of Germination and Establishment (Page 42-46)

• Phase I (Imbibition): Rapid water uptake.
• Phase II (Lag Phase): Metabolic activation; enzymes like Amylases, Proteases, and Lipases mobilize reserves.
• Phase III (Emergence): Radicle shows positive gravitropism (downward); plumule shows negative gravitropism (upward).

Seed Dormancy and Alleviation (Page 47-51)

• Dormancy Types:   - Exogenous: Physical (hard coat), Chemical (inhibitors), or Mechanical.   - Endogenous: Physiological (ABA/GA balance) or Morphological (underdeveloped embryo).
• Artificial Treatments:   - Scarification: Mechanical (sandpaper), Chemical (acid), or Thermal (hot water).   - Stratification: Cold ($2 \text{ -- } 5 \, ^\circ C$) or Warm exposure.

Seed Quality and Storage Management (Page 52-56)

• Parameters: Genetic, Physical (purity), Physiological (vigor), and Sanitary (pest-free).
• Viability Factors: Maintaining low moisture, low temperature, and low oxygen in airtight containers.

Seedling Production Systems (Page 57-61)

• Direct Seeding: Low transplant stress; used for large-scale crops (Maize).
• Field Nurseries: Allows selection of strong seedlings but requires high labor for transplanting.
• Container Production: Uses protected environments (greenhouses) for high survival rates and resource efficiency.