Class 11 Biology Notes Chapter 15 (Plant growth and development) – Biology Book

Detailed Notes with MCQs of Chapter 15: Plant Growth and Development. This is a vital chapter, not just for understanding plant life but also because questions frequently appear from this section in various government exams. Pay close attention to the definitions, the roles of plant growth regulators, and phenomena like photoperiodism and vernalisation.

Chapter 15: Plant Growth and Development - Detailed Notes

1. Growth

• Definition: Growth is an irreversible, permanent increase in size, volume, or mass of an organ, its parts, or an individual cell. It's accompanied by metabolic processes (anabolic and catabolic).
• Plant Growth is Unique:
  • Indeterminate: Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems (apical and lateral) containing actively dividing cells.
  • Determinate: Leaves, flowers, and fruits stop growing after reaching a certain size.
  • Open Form of Growth: New cells are constantly added to the plant body by meristems.
• Growth is Measurable: Can be measured by increase in:
  • Fresh weight
  • Dry weight (more reliable, as water content varies)
  • Length or Height
  • Area or Volume
  • Cell number
• Phases of Growth: Occur primarily at the root and shoot apices.
  • Meristematic Phase: Cells in this zone (root apex, shoot apex) are rich in protoplasm, have large nuclei, thin cellulosic walls, and divide constantly.
  • Elongation Phase: Cells just proximal (away from the tip) to the meristematic zone undergo enlargement, vacuolation increases, and new cell wall material is deposited. This phase accounts for the maximum increase in length/size.
  • Maturation Phase: Cells further away from the apex attain their maximal size, wall thickening, and protoplasmic modifications occur. Cells differentiate and mature to perform specific functions.
• Growth Rates: The increased growth per unit time.
  • Arithmetic Growth: Following mitosis, only one daughter cell continues to divide while the other differentiates/matures. Growth occurs at a constant rate. Example: Root/shoot elongation at a constant rate.
    • Formula: L_t = L₀ + rt (where L_t = length at time t, L₀ = initial length, r = growth rate)
    • Graph: Linear slope.
  • Geometric Growth: Initial growth is slow (lag phase), followed by a rapid exponential increase (log or exponential phase), and finally slows down due to limited resources (stationary phase). Both daughter cells retain the ability to divide. Example: Growth of unicellular organisms in culture, early growth of multicellular organisms/organs.
    • Formula: W_t = W₀ e^rt (where W_t = final size, W₀ = initial size, r = relative growth rate, t = time, e = base of natural logarithm)
    • Graph: Sigmoid or S-shaped curve.
  • Absolute Growth Rate (AGR): Measurement and comparison of total growth per unit time.
  • Relative Growth Rate (RGR): Growth of a system per unit time expressed on a per unit initial parameter basis. (Efficiency index). RGR = (Growth per unit time) / (Initial size).
• Conditions for Growth:
  • Water: Essential for cell enlargement (turgidity), enzymatic activity.
  • Oxygen: Required for respiration to release energy for growth activities.
  • Nutrients: Macro and micronutrients needed as raw materials for synthesis of protoplasm and energy source.
  • Temperature: Optimal range required; deviations can harm enzymes and metabolism.
  • Light & Gravity: Affect certain phases/aspects of growth (discussed under photoperiodism, tropic movements).

2. Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: Process where cells derived from apical meristems (root and shoot) and cambium mature to perform specific functions. Involves major structural changes in cell walls and protoplasm (e.g., tracheary elements lose protoplasm, develop strong lignocellulosic secondary walls).
• Dedifferentiation: Process where differentiated, living cells that have lost the capacity to divide regain it under certain conditions. Example: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
• Redifferentiation: Process where dedifferentiated cells divide and produce cells that again lose the capacity to divide but mature to perform specific functions. Example: Cells formed from interfascicular cambium differentiate into secondary xylem and secondary phloem.

3. Development

• Definition: Includes all changes an organism goes through during its life cycle, from seed germination to senescence. It's the sum of growth and differentiation.
• Sequence: Germination -> Vegetative Growth -> Flowering -> Fruiting -> Senescence.
• Plasticity: Plants follow different pathways in response to environment or phases of life to form different kinds of structures.
  • Heterophylly: Example of plasticity. Occurrence of different leaf forms on the same plant.
    • Environmental Heterophylly: e.g., Buttercup (Ranunculus flabellaris) - leaves produced in water are different from those produced in air.
    • Developmental Heterophylly: e.g., Cotton, Coriander, Larkspur - leaves of the juvenile plant are different in shape from those in mature plants.
• Factors Controlling Development: Both intrinsic (genetic factors, PGRs) and extrinsic (light, temperature, water, oxygen, nutrients).

4. Plant Growth Regulators (PGRs) / Phytohormones

• Definition: Small, simple molecules of diverse chemical composition that regulate plant growth and development even in low concentrations.

• Classification: Based on their functions.

  • Growth Promoters: Involved in cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation. (Auxins, Gibberellins, Cytokinins).
  • Growth Inhibitors: Involved in dormancy, abscission, response to stress. (Abscisic Acid).
  • Note: Ethylene fits into either group but is largely an inhibitor of growth activities.

• Detailed PGRs:

  • A. Auxins (e.g., Indole-3-acetic acid - IAA)

    • Discovery: Charles Darwin & Francis Darwin (coleoptile bending towards light), F.W. Went (isolated auxin from oat coleoptile tips).
    • Occurrence: Synthesized mainly in growing apices (shoot/root tips), young leaves.
    • Types: Natural (IAA, IBA - Indole butyric acid), Synthetic (NAA - Naphthalene acetic acid, 2,4-D - 2,4-Dichlorophenoxyacetic acid).
    • Physiological Effects/Functions:
      • Apical dominance (suppression of lateral bud growth by apical bud).
      • Cell elongation (in shoots).
      • Root initiation in stem cuttings (used for propagation).
      • Promote flowering (e.g., in pineapples).
      • Prevent premature fruit and leaf drop.
      • Promote abscission of older leaves/fruits.
      • Induce parthenocarpy (seedless fruits, e.g., tomatoes).
      • Used as herbicides (2,4-D kills dicot weeds, doesn't affect monocots).
      • Xylem differentiation.

  • B. Gibberellins (e.g., GA₃)

    • Discovery: E. Kurosawa (Bakanae disease - foolish seedling in rice caused by fungus Gibberella fujikuroi).
    • Occurrence: Found in higher plants and fungi. Over 100 GAs reported (GA₁, GA₂, GA₃...). GA₃ is most studied.
    • Chemical Nature: Acidic, Terpenoids.
    • Physiological Effects/Functions:
      • Increase in stem length/axis length (internode elongation, e.g., grapes stalk length).
      • Cause fruits like apples to elongate and improve shape.
      • Delay senescence.
      • Speed up malting process in brewing industry (promote amylase synthesis in barley seeds).
      • Promote bolting (internode elongation just prior to flowering) in rosette plants (e.g., beet, cabbage).
      • Break seed and bud dormancy.
      • Promote fruit set.

  • C. Cytokinins (e.g., Kinetin, Zeatin)

    • Discovery: F. Skoog & Miller (crystallized Kinetin from autoclaved herring sperm DNA; does not occur naturally in plants). Zeatin isolated from corn kernels and coconut milk.
    • Occurrence: Synthesized mainly in root tips, developing shoot buds, young fruits (where rapid cell division occurs).
    • Chemical Nature: Modified forms of adenine (N⁶-furfurylaminopurine - Kinetin).
    • Physiological Effects/Functions:
      • Promote cell division (cytokinesis), especially along with auxins.
      • Help produce new leaves, chloroplasts in leaves, lateral shoot growth, adventitious shoot formation.
      • Overcome apical dominance (promote lateral bud growth).
      • Delay leaf senescence (Richmond Lang effect).
      • Nutrient mobilization.

  • D. Ethylene (C₂H₄)

    • Discovery: H.H. Cousins confirmed release of a volatile substance from ripened oranges that hastened ripening of stored bananas.
    • Occurrence: Synthesized by tissues undergoing senescence and ripening fruits.
    • Chemical Nature: Gaseous PGR.
    • Physiological Effects/Functions:
      • Promotes senescence and abscission (leaves, flowers, fruits).
      • Highly effective in fruit ripening (enhances respiration rate - respiratory climactic).
      • Breaks seed and bud dormancy.
      • Initiates germination in peanut seeds, sprouting of potato tubers.
      • Promotes rapid internode/petiole elongation in deep water rice plants.
      • Promotes root growth and root hair formation (increases absorption surface).
      • Used to initiate flowering and synchronise fruit set in pineapples.
      • Induces flowering in mango.
      • Promotes female flower development in cucumbers (increases yield).
      • Ethephon: Most widely used source of ethylene. Aqueous solution readily absorbed and releases ethylene slowly. Used to hasten fruit ripening (tomatoes, apples), accelerate abscission (thinning cotton, cherry, walnut), promote female flowers in cucumbers.

  • E. Abscisic Acid (ABA)

    • Discovery: Independently discovered as Inhibitor-B, Abscission II, and Dormin. Later identified as chemically identical.
    • Occurrence: Synthesized in stems, leaves, fruits, roots.
    • Chemical Nature: Derivative of carotenoids.
    • Physiological Effects/Functions:
      • General plant growth inhibitor and inhibitor of metabolism.
      • Inhibits seed germination.
      • Stimulates closure of stomata (stress hormone; increases tolerance to stresses).
      • Induces dormancy (buds, seeds).
      • Promotes senescence and abscission (though plays minor role compared to ethylene).
      • Acts as an antagonist to Gibberellins.

• Interaction of PGRs: Many events (dormancy, senescence, abscission, apical dominance, germination) are regulated by the interplay and balance between different PGRs (e.g., Auxin/Cytokinin ratio determines root/shoot differentiation in callus).

5. Photoperiodism

• Definition: Response of plants to periods of day/night (photoperiod) for flowering.
• Discovery: Garner and Allard (1920) on Maryland Mammoth tobacco.
• Critical Photoperiod: The specific duration of light/dark period required for flowering.
• Classification based on Photoperiodic Response:
  • Short Day Plants (SDP): Flower when exposed to photoperiod shorter than their critical length. (Need long nights). E.g., Tobacco, Soybean, Chrysanthemum, Xanthium.
  • Long Day Plants (LDP): Flower when exposed to photoperiod longer than their critical length. (Need short nights). E.g., Spinach, Radish, Wheat, Henbane.
  • Day Neutral Plants (DNP): Flowering is not dependent on photoperiod. E.g., Tomato, Cucumber, Cotton, Sunflower.
• Role of Night Period: It's actually the duration of the uninterrupted dark period that is critical. If the dark period is interrupted by a flash of light, SDPs won't flower, while LDPs might be induced to flower.
• Site of Perception: Leaves perceive the photoperiod stimulus.
• Hormonal Substance: A hypothetical hormone, florigen, is thought to be produced in leaves and transported to shoot apices to induce flowering (though not yet isolated).
• Phytochrome: A photoreceptor pigment (proteinaceous) involved in detecting light. Exists in two interconvertible forms:
  • P_r (absorbs red light, ~660 nm)
  • P_fr (absorbs far-red light, ~730 nm)
  • P_r converts to P_fr in red light (daylight). P_fr converts back to P_r in far-red light or slowly in darkness.
  • P_fr is the physiologically active form, thought to be involved in controlling flowering (inhibits flowering in SDP, promotes in LDP). Also involved in seed germination, chlorophyll synthesis, etc.

6. Vernalisation

• Definition: Phenomenon where flowering is quantitatively or qualitatively dependent on exposure to low temperature. It prevents precocious reproductive development late in the growing season, allowing the plant to reach vegetative maturity.
• Examples:
  • Winter varieties of cereals (Wheat, Barley, Rye): Planted in autumn, germinate and overwinter as seedlings, resume growth in spring, and flower mid-summer. Spring varieties planted in spring flower the same season. Winter varieties if planted in spring normally fail to flower unless vernalised.
  • Biennial Plants (Sugarbeet, Cabbage, Carrot): Monocarpic plants that normally flower and die in the second season after a period of cold treatment (winter).
• Stimulus Perception: Apical meristems or mature stem perceives the cold stimulus.
• Significance: Allows plants to time their flowering correctly according to seasonal cycles. Can be used in agriculture to induce early flowering.

7. Seed Dormancy

• Definition: A state in which viable seeds fail to germinate even when provided with favourable external conditions (water, oxygen, temperature).
• Causes:
  • Impermeable and hard seed coat.
  • Presence of chemical inhibitors (e.g., ABA, phenolic acids, para-ascorbic acid).
  • Immature embryos.
• Breaking Dormancy (Overcoming Dormancy):
  • Scarification: Weakening or rupturing the seed coat mechanically (abrasives like sandpaper) or chemically (acids, solvents).
  • Stratification: Subjecting seeds to chilling temperatures (moist, cold conditions) to break physiological dormancy.
  • Changing environmental conditions (light, temperature).
  • Application of certain chemicals (Gibberellic acid, nitrates).
• Significance: Ensures germination only occurs under favourable conditions, helps seeds survive adverse periods, allows for dispersal.

Multiple Choice Questions (MCQs)

1. Which of the following represents the correct sequence of growth phases in a plant cell?
   a) Elongation -> Maturation -> Meristematic
   b) Meristematic -> Maturation -> Elongation
   c) Meristematic -> Elongation -> Maturation
   d) Maturation -> Elongation -> Meristematic

2. Apical dominance in plants is primarily caused by which plant growth regulator?
   a) Gibberellin
   b) Cytokinin
   c) Auxin
   d) Abscisic Acid

3. The phenomenon of 'bolting' (internode elongation prior to flowering) in rosette plants like cabbage is induced by:
   a) Auxins
   b) Gibberellins
   c) Ethylene
   d) Cytokinins

4. Which plant hormone is known as the 'stress hormone' for its role in stomatal closure during water scarcity?
   a) Ethylene
   b) Auxin
   c) Abscisic Acid (ABA)
   d) Gibberellin

5. Fruit ripening is significantly accelerated by the application of:
   a) Auxin
   b) Gibberellin
   c) Cytokinin
   d) Ethylene

6. A plant requires a photoperiod longer than its critical length to flower. This plant is classified as a:
   a) Short Day Plant (SDP)

   • b) Long Day Plant (LDP)
     c) Day Neutral Plant (DNP)
     d) Intermediate Day Plant

7. The pigment involved in perceiving light for photoperiodism is:
   a) Chlorophyll
   b) Carotenoid
   c) Phytochrome
   d) Xanthophyll

8. Vernalisation refers to the requirement of _______ for flowering in some plants.
   a) High temperature exposure

   • b) Low temperature exposure
     c) Specific light duration
     d) High humidity

9. The process by which differentiated cells regain the capacity to divide is called:
   a) Redifferentiation

   • b) Dedifferentiation
     c) Differentiation
     d) Plasticity

10. Cytokinins are primarily involved in promoting:
    a) Cell elongation
    b) Stomatal closure
    c) Fruit ripening

    • d) Cell division

Answer Key:

1. c
2. c
3. b
4. c
5. d
6. b
7. c
8. b
9. b
10. d

Study these notes thoroughly. Remember the specific functions of each PGR and the definitions of terms like photoperiodism, vernalisation, and the phases of growth. These are common areas for questions. Good luck with your preparation!