Plant Growth and Development

Class 11 Biology: Quick Revision Notes

Chapter: Plant Growth and Development (पादप वृद्धि एवं विकास)

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### Concept 1: Plant Growth (पादप वृद्धि)

• Definition: Growth is defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is accompanied by metabolic processes (anabolism and catabolism) that consume energy.
• Indeterminate Growth: Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems at certain locations.
• Plant Growth is Measurable: Growth can be measured by increase in fresh weight, dry weight, length, area, volume, and cell number.
  • Example: A single maize root apical meristem can give rise to more than 17,500 new cells per hour.

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### Concept 2: Phases of Growth (वृद्धि की अवस्थाएं)

The period of growth is generally divided into three phases:

1. Meristematic Phase (विभज्योतकीय अवस्था):
   • Cells at the root apex and shoot apex represent this phase.
   • Cells have rich protoplasm, large conspicuous nuclei, and thin cellulosic cell walls with abundant plasmodesmata.
2. Elongation Phase (दीर्घीकरण अवस्था):
   • Cells proximal to the meristematic zone represent this phase.
   • Characterized by increased vacuolation, cell enlargement, and new cell wall deposition.
3. Maturation Phase (परिपक्वन अवस्था):
   • Cells further away from the apex represent this phase.
   • Cells attain their maximal size in terms of wall thickening and protoplasmic modification.

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### Concept 3: Growth Rates (वृद्धि दरें)

• Growth Rate: The increased growth per unit time is called growth rate.
• Arithmetic Growth (अंकगणितीय वृद्धि):
  • Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.
  • Formula: L_t = L_0 + rt
    • L_t = length at time 't'
    • L_0 = length at time zero
    • r = growth rate / elongation per unit time
  • Curve: Linear curve when plotting length against time.
• Geometric Growth (ज्यामितीय वृद्धि):
  • The initial growth is slow (lag phase), and then it increases rapidly at an exponential rate (log/exponential phase).
  • Formula: W_1 = W_0 * e^(rt)
    • W_1 = final size (weight, height, number, etc.)
    • W_0 = initial size at the beginning of the period
    • r = growth rate
    • t = time of growth
    • e = base of natural logarithms
  • Curve: Sigmoid curve (S-curve). Comprises Lag phase, Log/Exponential phase, and Stationary phase.

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### Concept 4: Differentiation, Dedifferentiation, and Redifferentiation (विभेदन, अ-विभेदन और पुनः विभेदन)

• Differentiation (विभेदन): Cells derived from root apical meristems, shoot apical meristems, and cambium differentiate and mature to perform specific functions. (Loss of capacity to divide).
• Dedifferentiation (अ-विभेदन): The living differentiated cells that have lost the capacity to divide can regain the capacity to divide under certain conditions.
  • Example: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
• Redifferentiation (पुनः विभेदन): Dedifferentiated cells/tissues once again lose the capacity to divide and mature to perform specific functions.
  • Example: Secondary cortex, cork.

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### Concept 5: Plant Growth Regulators (PGRs) / Plant Hormones (पादप वृद्धि नियामक)

PGRs are simple, small molecules of diverse chemical composition. They can be broadly divided into two groups based on their functions in a living plant body:

1. Plant Growth Promoters (पादप वृद्धि प्रमोटर - involved in growth activities)

• Auxins (ऑक्सिन):
  • Discovery: Isolated by F.W. Went from tips of coleoptiles of oat seedlings.
  • Types: Indole-3-acetic acid (IAA), Indole butyric acid (IBA) [Natural]; NAA, 2,4-D [Synthetic].
  • Functions: Cell elongation in stem and roots, apical dominance, induces parthenocarpy (e.g., in tomatoes), prevents premature leaf and fruit drop, used as herbicides (2,4-D kills dicot weeds).
• Gibberellins (जिबरेलिन):
  • Discovery: E. Kurosawa reported 'Bakane' (foolish seedling) disease of rice caused by fungal pathogen Gibberella fujikuroi.
  • Types: GA1, GA2, GA3, etc. (GA3 is most studied).
  • Functions: Internode elongation (bolting in beet, cabbages), delays senescence, increases stalk length in grapes, breaks seed dormancy.
• Cytokinins (साइटोकाइनिन):
  • Discovery: F. Skoog and co-workers discovered kinetin (from autoclaved herring sperm DNA). Zeatin was isolated from corn kernels.
  • Functions: Promotes cell division (cytokinesis), helps in overcoming apical dominance, promotes nutrient mobilization, delays leaf senescence (Richmond-Lang effect).

2. Plant Growth Inhibitors (पादप वृद्धि संदमन - involved in wound response and stress)

• Abscisic Acid (ABA) (एब्सिसिक एसिड):
  • Functions: Acts as a general plant growth inhibitor and stress hormone. Inhibits seed germination, stimulates closure of stomata (stomatal closure during water stress), induces abscission (shedding of leaves/fruits), induces seed dormancy.
• Ethylene (एथिलीन):
  • Note: A gaseous PGR.
  • Functions: Promotes ripening of fruits, horizontal growth of seedlings, swelling of axis (triple response), breaks seed and bud dormancy, promotes rapid internode/petiole elongation in deep water rice plants, initiates flowering and synchronizing fruit-set in pineapples.

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### Concept 6: Photoperiodism and Vernalisation (दीप्तिकालिता और वसन्तीकरण)

• Photoperiodism (दीप्तिकालिता): The response of plants to periods of day/night (photoperiod) expressed in terms of flowering.
  • Short Day Plants (SDP): Flower when exposed to light periods less than a critical day length (e.g., Chrysanthemum).
  • Long Day Plants (LDP): Flower when exposed to light periods exceeding a critical duration (e.g., Radish, Henbane).
  • Day-Neutral Plants (DNP): No correlation between exposure to light duration and induction of flowering (e.g., Tomato, Cucumber).
• Vernalisation (वसन्तीकरण): Dependence of flowering either quantitatively or qualitatively on low temperature exposure. It prevents premature reproductive development in the growing season and enables the plant to have sufficient time to reach maturity (e.g., Winter varieties of wheat, barley).
• Seed Dormancy (बीज प्रसुप्ति): A condition in which seeds fail to germinate even under favorable environmental conditions due to endogenous conditions (hard seed coat, chemical inhibitors like ABA, immature embryo).