Lecture on Plant Growth and Development

Introduction

• Last chapter of Class 11th Botany
• Growth: Irreversible increase in size, mass, volume, or number of cells. Intrinsic in living organisms.
• Focus on plant growth:
  • Indeterminate (open type, occurs throughout lifespan)
  • Determinate (fixed, height is restricted)

Characteristics of Plant Growth

1. Indeterminate Growth
   • Occurs in roots and shoots due to lateral meristematic tissue.
   • No secondary growth in leaves, flowers, fruits.
2. Growth is Measurable and Indirect
   • Measured by parameters like weight, volume, number of cells.
   • Example: Maize root apical meristem can produce 17,500 new cells/hour.

Phases of Growth

1. Meristematic Phase
   • Cells actively divide, thin cell walls, dense cytoplasm.
2. Elongation Phase
   • Increase in cell size and wall thickness.
3. Maturation Phase
   • Complete cell wall thickening, cells mature and specialize.

Growth Rates

• Arithmetic Growth: Linear increase, one cell remains actively dividing.
  • Equation: Lt = L0 + rt (Linear graph)
• Geometric Growth: Exponential increase, all cells remain active.
  • Equation: Wt = W0 * e^(rt) (Sigmoid graph)
  • Phases: Lag, Log, Stationary*

Conditions for Growth

• Optimal light, water, nutrients, temperature, and oxygen needed.
• Extremes are harmful.

Plant Development

• Sum total of growth and differentiation.
• Plasticity: Deviation from normal development path, observed in plants.
  • Heterophylly: Variability in leaves due to genetics (e.g., cotton, coriander) or environment (e.g., buttercup).

Plant Growth Regulators (PGRs)

• Positive PGRs: Promote growth (Auxin, Gibberellin, Cytokinin)
• Negative PGRs: Inhibit growth (Abscisic Acid, Ethylene)

Positive PGRs

1. Auxin
   • Promotes cell division, rooting, apical dominance, and abscission.
   • Experiment: Discovered by Darwin in canary grass.
2. Gibberellin
   • Promotes stem elongation, breaks dormancy, bolting in rosettes.
   • Discovered by E. Kurosawa in rice.
3. Cytokinin
   • Promotes cell division, lateral growth, delays senescence.
   • Discovered in coconut milk (Skoog) and herring fish sperm (Miller).

Negative PGRs

1. Abscisic Acid (ABA)
   • Stress hormone, promotes senescence, closes stomata, induces dormancy.
2. Ethylene
   • Gaseous, promotes ripening, senescence, stem elongation in rice seedlings.

Photoperiodism

• Response to relative lengths of day and night for flowering.
• Types of plants:
  • Long Day Plants: Require longer light periods (e.g., wheat).
  • Short Day Plants: Require longer dark periods (e.g., tobacco).
  • Day Neutral Plants: Flower irrespective of light/dark period (e.g., tomatoes).
• Phytochrome: Pigment involved in light response, exists as PR and PFR.

Vernalization

• Inducing flowering through exposure to low temperatures.
• Important for winter annuals and biennials.
• Natural Vernalization: Exposure in natural conditions.
• Artificial Vernalization: Artificial exposure to low temperatures.

Conclusion

• All important concepts of plant growth and development covered.
• Completes Class 11th Botany syllabus.