Class 11 Biology Notes Chapter 15 (Chapter 15) – Examplar Problems (English) Book

Alright class, let's delve into Chapter 15: Plant Growth and Development from your NCERT Exemplar. This chapter is crucial not just for your Class 11 exams but also forms a significant part of syllabi for various government exams where Biology is a subject. Pay close attention to the definitions, the roles of plant hormones, and phenomena like photoperiodism and vernalisation.

Chapter 15: Plant Growth and Development - Detailed Notes

1. Growth

• Definition: Growth is an irreversible, permanent increase in the size of an organ or its parts, or even of an individual cell. It is generally accompanied by metabolic processes (both anabolic and catabolic) occurring at the expense of energy.
• Plant Growth Characteristics:
  • Indeterminate: Plants exhibit unique, unlimited growth throughout their life due to the presence of meristems (regions of active cell division) at specific locations (root apex, shoot apex, vascular cambium, cork cambium).
  • Determinate: Leaves, flowers, and fruits show limited growth.
  • Measurable: Growth can be measured by various parameters like increase in fresh weight, dry weight, length, area, volume, and cell number.
• Phases of Growth: A typical growth region (like a root tip) shows three phases:
  • Meristematic Phase: Cells are actively dividing (mitosis), rich in protoplasm, have large conspicuous nuclei, and thin primary cell walls. Found at root and shoot apices.
  • Elongation Phase: Cells proximal (just behind) the meristematic zone undergo enlargement, vacuolation increases, and new cell wall material is deposited. This phase accounts for the maximum increase in size/length.
  • Maturation Phase: Cells further away from the apex attain their maximal size, cell walls thicken, protoplasmic modifications occur, and cells differentiate to perform specific functions.
• Growth Rates: The increased growth per unit time is termed the growth rate.
  • Arithmetic Growth: Following mitosis, only one daughter cell continues to divide while the other differentiates and matures. Results in a linear increase in growth. Example: Root/shoot elongation at a constant rate. Equation: L_t = L₀ + rt (L_t = length at time t, L₀ = initial length, r = growth rate). Graph: Linear.
  • Geometric Growth: Initial growth is slow (lag phase), followed by a rapid exponential growth phase (log phase), and finally slows down due to limited resources (stationary phase). Both progeny cells following mitosis retain the ability to divide. Example: Growth of unicellular organisms in culture, early embryonic development, growth of organs like leaves/fruits. Equation: W_t = W₀e^rt (W_t = final size, W₀ = initial size, r = relative growth rate, t = time, e = base of natural logarithms). Graph: Sigmoid or S-shaped curve.
  • Absolute Growth Rate: Measurement and comparison of total growth per unit time.
  • Relative Growth Rate: Growth of a given system per unit time expressed on a common basis (e.g., per unit initial parameter).
• Conditions for Growth: Water (turgidity for cell enlargement, medium for enzymatic activity), Oxygen (for aerobic respiration, energy release), Nutrients (macro & micro, synthesis of protoplasm, energy source), Temperature (optimum range required for enzymes), Light & Gravity (affect certain phases/directions of growth).

2. Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: The process by which cells derived from apical meristems (root and shoot) and cambium mature, undergo structural changes in their cell walls and protoplasm, and become specialized to perform specific functions. Example: Formation of tracheary elements (loss of protoplasm, strong lignocellulosic secondary wall), parenchyma, sclerenchyma, collenchyma. These cells lose the capacity to divide.
• Dedifferentiation: The process by which permanent, differentiated cells regain the capacity to divide mitotically under certain conditions. Example: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells during secondary growth.
• Redifferentiation: The process by which dedifferentiated cells (like cambium cells) divide and produce cells that once again lose the capacity to divide and mature to perform specific functions. Example: Secondary xylem and secondary phloem formed from vascular cambium cells; secondary cortex and cork formed from cork cambium cells.

3. Development

• Definition: Development is the sequence of events or changes that occur in the life history of a plant cell, organ, or organism. It includes all changes from germination of the seed to senescence. It encompasses both growth and differentiation.
• Sequence: Meristematic cell -> Plasmatic growth -> Differentiation -> Maturation -> Senescence -> Death.
• Plasticity: Plants follow different pathways in response to the environment or phases of life to form different kinds of structures. This ability is called plasticity.
  • Heterophylly: Occurrence of different forms of leaves on the same plant. Examples:
    • Environmental Heterophylly: Buttercup (Ranunculus) - leaves produced in water are different from those produced in air.
    • Developmental Heterophylly: Cotton, Coriander, Larkspur - leaves of the juvenile plant are different in shape from those in mature plants.

4. Plant Growth Regulators (PGRs) / Phytohormones

These are small, simple molecules of diverse chemical composition, produced in one part of the plant and transported to other parts to regulate growth and development even in low concentrations.

• Classification:

  • Growth Promoters: Involved in growth-promoting activities like cell division, cell enlargement, flowering, fruiting, etc. (Auxins, Gibberellins, Cytokinins).
  • Growth Inhibitors: Involved in growth-inhibiting activities like dormancy, abscission, response to stress. (Abscisic Acid, Ethylene - though Ethylene fits largely here, it has some promoting effects too).

• Detailed Study of PGRs:

  • Auxins (Greek 'auxein': to grow)

    • Discovery: Charles Darwin and Francis Darwin observed phototropism in canary grass coleoptiles. F.W. Went isolated auxin from Avena (oat) coleoptile tips.
    • Chemical Nature: Indole compounds. Natural: Indole-3-acetic acid (IAA), Indole butyric acid (IBA). Synthetic: Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D).
    • Site of Synthesis: Growing apices of stems and roots.
    • Functions:
      • Apical dominance (suppression of lateral bud growth by apical bud).
      • Initiate rooting in stem cuttings (used in plant propagation).
      • Promote flowering (e.g., in pineapples).
      • Prevent premature fruit and leaf drop.
      • Promote abscission of older leaves and fruits.
      • Induce parthenocarpy (fruit development without fertilization, e.g., in tomatoes).
      • Used as herbicides (2,4-D kills dicot weeds, does not affect mature monocots).
      • Control xylem differentiation.
      • Help in cell division.

  • Gibberellins (GAs)

    • Discovery: E. Kurosawa observed 'bakanae' (foolish seedling) disease in rice caused by the fungus Gibberella fujikuroi. The active substance was identified as gibberellic acid.
    • Chemical Nature: Terpenes (Gibberellic Acids). Over 100 GAs reported (GA₁, GA₂, GA₃...). GA₃ was one of the first discovered and is most studied. All GAs are acidic.
    • Site of Synthesis: Young leaves, roots, developing seeds, buds.
    • Functions:
      • Increase in length of axis (used to increase grape stalk length, elongate and improve apple shape, increase sugarcane stem length -> yield).
      • Delay senescence (fruits can be left on the tree longer).
      • Speed up the malting process in the brewing industry (promote α-amylase synthesis).
      • Promote bolting (internode elongation just prior to flowering) in rosette plants (e.g., beets, cabbages).
      • Break seed and bud dormancy.
      • Promote fruit growth.
      • Substitute for long days or cold treatment (vernalisation) for flowering in some plants.

  • Cytokinins (CKs)

    • Discovery: F. Skoog and co-workers observed that callus proliferated only if the medium contained extracts of vascular tissues, yeast extract, coconut milk, or DNA, in addition to auxin. Miller et al. later identified and crystallized the active substance from autoclaved herring sperm DNA and termed it Kinetin (a modified adenine). Zeatin was isolated from corn kernels and coconut milk.
    • Chemical Nature: Adenine derivatives (N⁶-furfurylaminopurine - Kinetin; Zeatin).
    • Site of Synthesis: Synthesized mainly in root tips, developing shoot buds, young fruits – where rapid cell division occurs.
    • Functions:
      • Promote cell division (cytokinesis), especially in presence of auxins.
      • Help produce new leaves, chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.
      • Overcome apical dominance (promote lateral bud growth).
      • Delay leaf senescence (Richmond Lang effect).
      • Nutrient mobilization.

  • Ethylene (C₂H₄)

    • Discovery: Cousins confirmed that ripened oranges released a volatile substance that hastened the ripening of stored, unripe bananas. Later identified as ethylene.
    • Chemical Nature: Simple gaseous PGR.
    • Site of Synthesis: Synthesized by tissues undergoing senescence and ripening fruits.
    • Functions:
      • Promotes senescence and abscission of plant organs (leaves, flowers).
      • 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 (helps leaves/upper parts remain above water).
      • Promotes root growth and root hair formation (increases absorption surface).
      • Initiates flowering and synchronizes fruit set in pineapples. Induces flowering in mango.
      • Promotes female flower development in cucumbers (increases yield).
      • Ethephon: Most widely used source of ethylene. It's an aqueous solution readily absorbed and transported, releasing ethylene slowly. Used to hasten fruit ripening (tomatoes, apples), accelerate abscission (thinning cotton, cherry, walnut), promote female flowers in cucumbers.

  • Abscisic Acid (ABA)

    • Discovery: Independently discovered as 'inhibitor-B', 'abscission II', and 'dormin'. Later, all three were found to be chemically identical and named Abscisic Acid.
    • Chemical Nature: Carotenoid derivative. Synthesized in chloroplasts.
    • Site of Synthesis: Leaves, stems, roots, fruits. Levels increase during stress.
    • Functions:
      • General plant growth inhibitor and inhibitor of plant metabolism.
      • Inhibits seed germination.
      • Stimulates closure of stomata (response to water stress, hence called the 'stress hormone').
      • Increases tolerance of plants to various stresses.
      • Plays a major role in seed development, maturation, and dormancy (helps seeds withstand desiccation).
      • Induces dormancy in buds and seeds.
      • Promotes abscission of leaves, flowers, and fruits (though primarily ethylene is involved in abscission).
      • Acts as an antagonist to Gibberellins (GA).

• Interaction of PGRs: PGRs can act synergistically (e.g., Auxin + Cytokinin for cell division) or antagonistically (e.g., ABA inhibits GA-induced germination). The physiological effect depends on the relative concentration of different PGRs, the sensitivity of the tissue, and developmental stage.

5. Photoperiodism

• Definition: The effect of photoperiods (duration of light/dark) on plant growth and development, especially flowering.
• Discovery: Garner and Allard (working on 'Maryland Mammoth' tobacco).
• Classification based on Photoperiodic Response for Flowering:
  • Short Day Plants (SDPs): Flower when the photoperiod is less than a critical duration. They actually require a continuous dark period longer than a critical length. (e.g., Tobacco, Soybean, Chrysanthemum, Rice - some varieties). If the dark period is interrupted by a brief flash of light, they won't flower.
  • Long Day Plants (LDPs): Flower when the photoperiod is more than a critical duration. They require a continuous dark period shorter than a critical length. (e.g., Spinach, Radish, Wheat, Barley, Sugar beet). Interruption of the dark period with light promotes flowering.
  • Day Neutral Plants (DNPs): Flowering is independent of the photoperiod. (e.g., Tomato, Cucumber, Cotton, Sunflower, Maize).
• Mechanism:
  • Site of perception of light/dark duration is the leaves.
  • A hormonal substance (hypothesized as florigen) migrates from leaves to shoot apices to induce flowering.
  • Phytochrome: A pigment system (proteinaceous, blue-green) involved in detecting light. Exists in two interconvertible forms: P_r (absorbs red light, ~660nm) and P_fr (absorbs far-red light, ~730nm). P_fr is the physiologically active form. During the day (red light abundant), P_r converts to P_fr. In the dark, P_fr slowly reverts to P_r. The relative amount of P_fr determines the flowering response in SDPs and LDPs.

6. Vernalisation

• Definition: The phenomenon where flowering is quantitatively or qualitatively dependent on exposure to low temperatures. It prevents precocious reproductive development late in the growing season, enabling the plant to reach vegetative maturity before flowering.
• Examples:
  • Winter varieties of cereals (Wheat, Barley, Rye): Planted in autumn, germinate and overwinter as small seedlings, resume growth in spring, and flower mid-summer. Spring varieties planted in spring flower normally without cold treatment.
  • Biennial plants (Sugarbeet, Cabbages, Carrots): Monocarpic plants that normally flower and die in the second season after a period of cold exposure during the first winter.
• Mechanism: Site of perception of cold stimulus is the apical meristems or mature stem/embryo tip. The stimulus is perceived as stable changes (likely epigenetic) that 'prepare' the plant to flower when conditions become favourable. Can sometimes be replaced by Gibberellin treatment.

7. Seed Dormancy

• Definition: A state in which viable seeds fail to germinate even when provided with optimal environmental conditions (water, oxygen, temperature). It's a protective mechanism.
• Causes:
  • Exogenous Dormancy (Seed Coat related):
    • Impermeability of seed coat to water or gases (e.g., legumes).
    • Mechanically resistant/hard seed coat (prevents embryo expansion).
  • Endogenous Dormancy (Embryo related):
    • Presence of chemical inhibitors (e.g., Abscisic acid, phenolic acids, para-ascorbic acid) in the embryo or endosperm.
    • Immature or undeveloped embryo.
• Methods to Break Dormancy:
  • Scarification: Weakening or rupturing the seed coat mechanically (abrasion with sandpaper, nicking with a knife) or chemically (using acids, solvents).
  • Stratification: Subjecting moist seeds to low temperatures (chilling) for a specific period to break physiological dormancy or leach inhibitors.
  • Changing environmental conditions like light (e.g., lettuce seeds are light-sensitive) or specific temperature treatments.
  • Application of certain chemicals like Gibberellic acid (GA) or nitrates.
  • Leaching out inhibitors by washing.

Multiple Choice Questions (MCQs)

1. Which plant growth regulator is primarily responsible for apical dominance?
   a) Gibberellin
   b) Cytokinin
   c) Auxin
   d) Ethylene

2. The phenomenon of 'bolting' (internode elongation prior to flowering) in rosette plants like cabbage is promoted by:
   a) Abscisic Acid
   b) Gibberellin
   c) Cytokinin
   d) Auxin

3. Which of the following is a gaseous plant hormone involved in fruit ripening?
   a) IAA
   b) ABA
   c) C₂H₄
   d) GA₃

4. The S-shaped or sigmoid growth curve is characteristic of:
   a) A cell growing in culture under limited nutrients
   b) A leaf elongating at a constant rate
   c) Arithmetic growth phase only
   d) Plant growth measured by cell number only

5. Photoperiodic stimulus is perceived by which part of the plant?
   a) Flower buds
   b) Shoot apex
   c) Leaves
   d) Roots

6. Vernalisation refers to the promotion of flowering by:
   a) High temperature treatment
   b) Long day exposure
   c) Low temperature treatment
   d) Short day exposure

7. Which PGR acts as a 'stress hormone' by inducing stomatal closure during water scarcity?
   a) Auxin
   b) Cytokinin
   c) Ethylene
   d) Abscisic Acid

8. The process where differentiated cells regain the capacity to divide is called:
   a) Redifferentiation
   b) Dedifferentiation
   c) Differentiation
   d) Plasticity

9. Synthetic auxins like 2,4-D are commonly used as:
   a) Fruit ripening agents
   b) Herbicides for dicot weeds
   c) Agents to break seed dormancy
   d) Promoters of lateral bud growth

10. Cytokinins were first discovered as Kinetin from:
    a) Corn kernels
    b) Coconut milk
    c) Autoclaved herring sperm DNA
    d) Fungus Gibberella fujikuroi

Answer Key:

1. c) Auxin
2. b) Gibberellin
3. c) C₂H₄ (Ethylene)
4. a) A cell growing in culture under limited nutrients (Represents geometric growth with eventual stationary phase)
5. c) Leaves
6. c) Low temperature treatment
7. d) Abscisic Acid
8. b) Dedifferentiation
9. b) Herbicides for dicot weeds
10. c) Autoclaved herring sperm DNA

Make sure you revise these notes thoroughly. Understand the specific functions of each hormone and the conditions influencing phenomena like flowering and dormancy. These concepts are frequently tested. Good luck with your preparation!