Plant Growth Stages and Germination

Light and Germination

• Light intensity and light quality influence germination.
• The chemical reaction is controlled by the wavelength of light absorbed by the Phytochrome pigment.

Seedling Stage

• The stage between germination and the juvenile stage.
• Epigeal growth (dicot plants): cotyledons and hypocotyl are raised above ground, cotyledons provide food for the plant.
• Hypogeal growth (monocot plants): plumule and hypocotyl emerge above ground, cotyledons remain below soil.

Juvenile Stage

• Vigorous growth phase where the plant isn't readily induced to reproduce.
• Characterized by:
  • Exponential growth
  • Inability to reproduce
  • Rejuvenation ability
  • Development of morphological characteristics (e.g., thorniness)
  • Non-receptiveness to external stimuli

Transition Stage

• The plant gradually transitions from juvenile to mature characteristics.
• Characterized by:
  • Decrease in growth rate
  • Receptiveness to external conditions
  • Decreased rejuvenation ability

Maturity Stage

• Plant becomes capable of flowering (reproductive phase).

Reproductive Phase

• Microsporogenesis: Formation of microspores (pollen grains)
  • Sporogenous tissue (2n) undergoes meiosis to form microspore tetrads (4n).
  • Microspores develop into pollen grains (male gametes), containing generative and tube nuclei.
  • Pollen grains are released from the anther.
• Megasporogenesis & Megagametogenesis:
  • Megasporocyte (2n) divides meiotically to form four megaspores, three degenerate.
  • The remaining functional megaspore divides mitotically to form the embryo sac (7-celled, 8-nucleated).
  • The embryo sac contains:
    • Egg apparatus (2 synergids, 1 egg cell)
    • 2 polar nuclei
    • 3 antipodal cells
• Pollination: Transfer of pollen from stamen to stigma.
  • Pollen germinates on the stigma, forming a pollen tube.
  • The generative nucleus divides into two male gametes (sperm cells).
• Methods of Pollination:
  • Self-Pollination (augamy): Pollen from stamens to pistils within the same flower or plant (e.g., grapes, peach, tomato).
  • Cross Pollination (heterogamy): Pollen from stamens of one plant to the pistil of another plant (e.g., apple, cabbage, cucumber).
• Fertilization: Fusion of male and female gametes.
  • One sperm cell fuses with the egg cell forming a zygote (embryo).
  • The other sperm cell fuses with the two polar nuclei forming the endosperm nucleus (endosperm of seed).
• Double Fertilization: Two sets of fertilization occur in flowering plants.
• Fate of Floral Parts after Fertilization:
  • Sepals, petals, and stamens wither and drop off.
  • Ovary becomes the fruit:
    • Ovary wall becomes fruit wall
    • Ovule becomes seed
    • Integuments become seed coat (testa)
    • Fertilized egg becomes embryo

Senescence

• Erosive process accompanying aging before death.
• Whole plant/complete senescence: Aging and death of the entire plant except the seeds.
• Organ/partial senescence: Deterioration and death of specific organs (leaves, fruits, flowers).

Seed Dormancy

• Viable seed fails to germinate despite favorable conditions.
• Types of Seed Dormancy:
  • Physiological/Internal dormancy: Due to internal factors like inhibitors (ABA, coumarin) or immature embryo.
  • Physical dormancy: Hard seed coat preventing water and oxygen intake.
  • Double dormancy: Both physical and physiological blocks present.
• Breaking Physical Dormancy:
  • Stratification: Exposing imbibed seeds to specific temperatures:
    • Warm stratification (15-25°C)
    • Cold stratification (5-10°C)
  • Scarification: Altering the seed coat:
    • Mechanical methods (abrasion)
    • Hot water treatment
    • Chemical scarification (sulfuric acid)
  • Dry storage: For seeds not germinating immediately after harvest.
  • Embryo culture: Aseptic growth of excised embryo in artificial media.

Seed Viability

• Measure of how many seeds in a batch are alive and can develop into reproductive plants.
• Determination of seed viability:
  • Germination tests
  • Biochemical tests (quicker, but less accurate than germination tests)

Maintaining Viability of Recalcitrant Seeds

• Cannot withstand drying, sensitive to chilling.
• Store in sawdust moistened to 10% at 7-10°C.
• Moistened charcoal or a mixture of charcoal and sterilized sawdust at low temperature (7-10°C).

Maintaining Viability of Orthodox Seeds

• Can endure extreme freezing and drying.
• Maintenance methods:
  • Proper drying to decrease moisture content.
  • Low-temperature storage.
  • Low oxygen levels.
  • Air-tight containers with desiccants.
• Harrington’s Thumb Rules:
  • Each 1% decrease in moisture content doubles seed storage life.
  • Each 5.6°C decrease in storage temperature doubles seed storage life.

Seed Storage

• Protect seeds with fungicides for long-term storage.
• Use various containers: polyethylene, cellophane, aluminum, vacuum-sealed cans.
• For home storage, use airtight cans or bottles.

Liebig’s Law of Minimum

• Plant growth is limited by the nutrient in shortest supply, even if other nutrients are abundant.
• Example: Insufficient micronutrients can hinder growth even with ample nitrogen and phosphorus.

The Law of Limiting Factors

• Plant growth is controlled by the factor in shortest supply, not necessarily the nutrient in shortest supply.

The Law of Diminishing Returns

• As the amount of a limiting factor increases, the rate of plant growth increases initially, but eventually levels off and may even decrease.