GROWTH

Definition

• Growth:
  • Irreversible permanent increase in size of an organ, its parts, or an individual cell.

Characteristics

• Fundamental Aspect:
  • Growth is considered one of the most fundamental and conspicuous characteristics of living beings.
• Accompanied by Metabolic Processes:
  • Growth is typically accompanied by metabolic processes, including both anabolic and catabolic activities.
  • These processes occur at the expense of energy.

Examples

• Leaf Expansion:
  • Example: The expansion of a leaf is a form of growth.
• Wood Swelling:
  • Example: The swelling of a piece of wood when placed in water can be described as a growth process.

Indeterminate Plant Growth

• Unique Feature:
  • Plant growth is characterized by its uniqueness – plants retain the capacity for unlimited growth throughout their life.
• Meristems and Unlimited Growth:
  • The presence of meristems at specific locations in the plant body enables continuous growth.
  • Meristematic cells have the ability to divide and perpetuate, contributing to the perpetual growth of plants.
• Open Form of Growth:
  • Growth in plants is often described as an open form where new cells are consistently added to the plant body through meristematic activity.
• Significance of Meristems:
  • Meristems play a crucial role in maintaining the capacity for growth.
  • The cessation of meristematic division would impact plant growth significantly.

Meristems in Primary and Secondary Growth

• Root Apical Meristem and Shoot Apical Meristem:
  • Responsible for primary growth in plants, contributing to elongation along the plant’s axis.
• Lateral Meristems (Vascular Cambium and Cork Cambium):
  • Found in dicotyledonous plants and gymnosperms.
  • Contribute to secondary growth, causing an increase in the girth of organs.

MEASURING GROWTH AT THE CELLULAR LEVEL

Protoplasmic Increase as Basis for Growth

• Principle of Growth:
  • Growth at the cellular level is primarily a consequence of an increase in the amount of protoplasm.
• Challenges in Direct Measurement:
  • Direct measurement of protoplasmic increase is challenging.
  • Instead, various parameters proportional to protoplasmic growth are measured.

Parameters for Measuring Growth

1. Fresh Weight:
   • Measurement of the increase in the overall weight of an organism.
2. Dry Weight:
   • Determination of the weight of an organism after removal of water, indicating cellular material increase.
3. Length:
   • Measurement of the elongation or extension of an organism or its parts.
4. Area:
   • Calculation of the expansion of a surface, applicable to leaves and other planar structures.
5. Volume:
   • Assessment of the three-dimensional space occupied by an organism or its parts.
6. Cell Number:
   • Counting the number of cells, indicating growth through cell division.

Fascinating Examples of Growth Rates

• Maize Root Apical Meristem:
  • Can give rise to over 17,500 new cells per hour, highlighting rapid cellular division.
• Watermelon Cells:
  • Cells may increase in size by up to 3,50,000 times, emphasizing substantial cellular enlargement.

Growth Expression in Different Structures

• Pollen Tube:
  • Growth measured in terms of length.
• Dorsiventral Leaf:
  • Growth is denoted by an increase in surface area.

PHASES OF PLANT GROWTH

1. Meristematic Phase:

• Characteristics:
  • Location: Root and shoot apices.
  • Cell Division: Constantly dividing cells.
  • Protoplasm: Rich in protoplasm.
  • Nuclei: Large and conspicuous.
  • Cell Walls: Primary, thin, cellulosic.
  • Plasmodesmatal Connections: Abundant.

2. Elongation Phase:

• Location: Cells proximal to the meristematic zone.
• Characteristics:
  • Cell Enlargement: Cells exhibit increased vacuolation and enlargement.
  • Cell Wall: New cell wall deposition occurs.

3. Maturation Phase:

• Location: Further away from the apex, proximal to the elongation phase.
• Characteristics:
  • Size: Cells attain maximal size.
  • Wall Thickening: Cell walls undergo thickening.
  • Protoplasmic Modifications: Modifications in protoplasm.

Illustration Using Root Tips:

• Meristematic Phase:
  • Constant cell division at both root and shoot apices.
• Elongation Phase:
  • Cells proximal to the meristematic zone exhibit enlargement and new cell wall deposition.
• Maturation Phase:
  • Cells further away undergo maximal size attainment, wall thickening, and protoplasmic modifications.

GROWTH RATES IN LIVING ORGANISMS

1. Arithmetic Growth:

• Characteristics:
  • Mitotic cell division leads to one daughter cell continuing division.
  • The other differentiates and matures.
  • Linear curve on plotting organ length against time.
• Mathematical Expression:
  • L_t​=L₀​+rt
    • L_t​: Length at time ‘t’
    • L₀​: Length at time ‘zero’
    • r: Growth rate/elongation per unit time.

2. Geometrical Growth:

• Characteristics:
  • Initial slow growth (lag phase) followed by rapid exponential growth.
  • Progeny cells retain the ability to divide continuously.
  • Sigmoid or S-curve on plotting growth parameter against time.
• Mathematical Expression:
  • W₁​=W₀​ert
    • W₁​: Final size (weight, height, number, etc.)
    • W₀​: Initial size at the beginning of the period
    • r: Relative growth rate
    • t: Time of growth
    • e: Base of natural logarithms.

Growth Rate Comparison:

• Absolute Growth Rate:
  • Measurement and comparison of total growth per unit of time.
• Relative Growth Rate:
  • Growth of the system per unit time expressed on a common basis (e.g., per unit initial parameter).
  • The efficiency index is the measure of the plant’s ability to produce new plant material.

CONDITIONS FOR GROWTH IN PLANTS

1. Water:

• Importance:
  • Essential for cell enlargement.
  • Turgidity of cells facilitates extension growth.
  • Medium for enzymatic activities crucial for growth.
• Connection to Growth:
  • Plant growth and development intimately linked to water status.

2. Oxygen:

• Role:
  • Vital for releasing metabolic energy necessary for growth activities.
• Significance:
  • Essential for cellular respiration.

3. Nutrients:

• Types:
  • Macro and micro essential elements.
• Functions:
  • Required for synthesizing protoplasm.
  • Serve as a source of energy.

4. Temperature:

• Optimum Range:
  • Each plant organism has an optimum temperature range for optimal growth.
  • Deviation from this range can be detrimental.

5. Environmental Signals:

• Light:
  • Affects specific phases/stages of growth.
• Gravity:
  • Influences certain aspects of growth.

DIFFERENTIATION, DEDIFFERENTIATION, AND REDIFFERENTIATION

1. Differentiation:

• Definition:
  • The process where cells derived from meristems and cambium undergo structural changes to perform specific functions.
• Changes During Differentiation:
  • Structural modifications in cell walls and protoplasm.
  • Examples: Tracheary elements developing lignocellulosic secondary cell walls.

2. Dedifferentiation:

• Definition:
  • The ability of living, differentiated cells to regain the capacity for division under specific conditions.
• Example:
  • Formation of meristems (interfascicular cambium, cork cambium) from fully differentiated parenchyma cells.

3. Redifferentiation:

• Definition:
  • The process where dedifferentiated cells undergo maturation to perform specific functions.
• Example:
  • Tissues in woody dicotyledonous plants resulting from redifferentiation.

4. Products of Redifferentiation in Woody Dicotyledonous Plants:

• Examples:
  • Vascular tissues (xylem, phloem).
  • Cork and cortex.
  • Wood fibers.

5. Tumour Formation:

• Description:
  • Uncontrolled cell division leading to abnormal tissue growth.
• Plant Tissue Culture:
  • Parenchyma cells induced to divide under controlled laboratory conditions.
  • Resulting cells are similar to those in a tumour.

6. Open Differentiation:

• Characteristics:
  • Differentiation in plants is open.
  • Indeterminate or determinate differentiation.
  • Cells/tissues from the same meristem exhibit different structures at maturity.

7. Influence of Location:

• Example:
  • Cells away from root apical meristems differentiate as root-cap cells.
  • Cells pushed to the periphery mature as the epidermis.

DEVELOPMENT

1. Definition of Development:

• Definition:
  • All changes an organism undergoes during its life cycle, encompassing germination to senescence.
  • Applicable to tissues and organs.

2. Plasticity in Plants:

• Definition:
  • The ability of plants to follow different pathways in response to environmental cues or different phases of life, resulting in the formation of various structures.
• Examples:
  • Heterophylly in cotton, coriander, and larkspur.
  • Variation in leaf shapes in buttercup leaves produced in air vs. water.

3. Heterophylly:

• Definition:
  • The phenomenon where leaves of a plant exhibit different shapes at different stages of development or under varying environmental conditions.
• Examples:
  • Leaves of juvenile plants vs. mature plants in certain species.

4. Relationship Between Growth, Differentiation, and Development:

• Interconnection:
  • Growth, differentiation, and development are closely related events in the life of a plant.
  • Development is considered the sum of growth and differentiation.

5. Factors Influencing Plant Development:

• Intrinsic Factors:
  • Genetic (intracellular) factors.
  • Intercellular factors, including plant growth regulators.
• Extrinsic Factors:
  • Environmental factors such as light, temperature, water, oxygen, and nutrition.

6. Control of Development:

• Intrinsic Factors:
  • Genetic factors within the plant cells.
• Extrinsic Factors:
  • Environmental factors influencing development.
  • Examples: Light, temperature, water availability, oxygen levels, and nutrient availability.

PLANT GROWTH REGULATORS

1. Characteristics of Plant Growth Regulators (PGRs):

• Chemical Composition:
  • Small, simple molecules with diverse chemical compositions.
  • Examples include:
    • Indole compounds: Indole-3-acetic acid (IAA).
    • Adenine derivatives: N6-furfurylamino purine (Kinetin).
    • Carotenoid derivatives: Abscisic acid (ABA).
    • Terpenes: Gibberellic acid (GA3).
    • Gases: Ethylene (C2H4).

2. Nomenclature:

• Synonyms:
  • Also known as plant growth substances, plant hormones, or phytohormones in literature.

DISCOVERY OF PLANT GROWTH REGULATORS

1. Auxins:

• Discovery:
  • Charles Darwin and Francis Darwin observed phototropism in canary grass coleoptiles.
  • F.W. Went (1928) isolated auxin from oat seedling coleoptile tips.
• Physiological Effects:
  • Induces cell elongation, root initiation, and apical dominance.

2. Gibberellins:

• Discovery:
  • ‘Bakanae’ disease in rice caused by Gibberella fujikuroi.
  • E. Kurosawa (1926) identified gibberellic acid as the active substance.
• Physiological Effects:
  • Promotes stem elongation, seed germination, and flowering.

3. Cytokinins:

• Discovery:
  • F. Skoog and co-workers (1954) observed callus proliferation in tobacco stem segments.
  • Miller et al. (1955) identified and crystallized the cytokinesis-promoting substance (kinetin).
• Physiological Effects:
  • Stimulates cell division and influences organ development.

4. Abscisic Acid (ABA):

• Discovery:
  • Mid-1960s, three independent studies reported inhibitors: inhibitor-B, abscission II, and dormin.
  • Later identified as chemically identical and named abscisic acid (ABA).
• Physiological Effects:
  • Induces dormancy and regulates stress responses.

5. Ethylene:

• Discovery:
  • H.H. Cousins (1910) observed the release of a volatile substance from ripened oranges.
  • Identified as ethylene, a gaseous PGR.
• Physiological Effects:
  • Involved in fruit ripening, senescence, and stress responses.

PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS

Auxins:

• Definition: Auxins refer to indole-3-acetic acid (IAA) and compounds with similar growth-regulating properties.
• Sources:
  • Produced in growing apices of stems and roots.
  • Isolated from plants (IAA, IBA) and synthetic auxins (NAA, 2,4-D).
• Applications:
  • Root Initiation: Auxins like IAA and IBA promote rooting in stem cuttings, aiding in plant propagation.
  • Flowering: Induce flowering in plants (e.g., pineapples).
  • Leaf and Fruit Drop: Prevent early leaf and fruit drop but promote the abscission of mature leaves and fruits.
  • Apical Dominance: Inhibit lateral bud growth; removal of shoot tips (decapitation) stimulates lateral bud growth.
  • Parthenocarpy: Induce parthenocarpy (fruit development without fertilization), e.g., in tomatoes.
  • Herbicides: Used as herbicides (e.g., 2,4-D) to control weed growth in gardens and lawns.
  • Xylem Differentiation: Control xylem differentiation and aid in cell division.

GIBBERELLINS: A Catalyst for Plant Growth

• Definition:
  • Gibberellins (GAs) constitute a class of promotory Plant Growth Regulators (PGRs).
  • Over 100 types were reported in fungi and higher plants, denoted as GA1, GA2, GA3, etc.
  • Gibberellic acid (GA3) is a prominent and extensively studied form.
• Physiological Responses:
  • Stem Elongation: GAs induce a notable increase in the length of the stem axis.
  • Fruit Modification: The application of GAs elongates fruits like apples, enhancing their shape.
  • Senescence Delay: GAs contribute to delaying senescence, prolonging the market period for fruits.
  • Industrial Applications:
    • Used in brewing: GA3 expedites the malting process in the brewing industry.
    • Agriculture: Spraying sugarcane with GAs boosts stem length, increasing yield by up to 20 tonnes per acre.
    • Forestry: Accelerates maturity in juvenile conifers, leading to early seed production.
  • Promotion of Bolting: Induces internode elongation (bolting) just before flowering in beets, cabbages, and plants with rosette habits.

CYTOKININS: Orchestrating Cell Division

• Definition:
  • Cytokinins are Plant Growth Regulators (PGRs) with a distinctive impact on cytokinesis.
  • Discovery: Kinetin, a modified form of adenine, was initially identified from autoclaved herring sperm DNA.
  • Natural Source: Zeatin, a naturally occurring cytokinin, was later isolated from corn kernels and coconut milk.
• Physiological Effects:
  • Cell Division Promotion: Cytokinins play a crucial role in promoting cytokinesis, facilitating cell division.
  • Natural Synthesis: Produced in regions with active cell division, such as root apices, developing shoot buds, and young fruits.
  • Stimulating Growth:
    • Induces the formation of new leaves and chloroplasts in leaves.
    • Facilitates lateral shoot growth and the development of adventitious shoots.
  • Overcoming Apical Dominance: Cytokinins contribute to overcoming apical dominance, promoting lateral bud growth.
  • Nutrient Mobilisation: Aids in nutrient mobilization, leading to a delay in leaf senescence.

ETHYLENE: Mastering Plant Physiology

• Definition:
  • Ethylene stands out as a simple gaseous Plant Growth Regulator (PGR) with profound effects on various physiological processes in plants.
• Synthesis:
  • Senescence and Ripening: Synthesized in significant quantities by tissues undergoing senescence and ripening fruits.
• Physiological Effects:
  • Horizontal Growth:
    • Influences the horizontal growth of seedlings.
    • Induces swelling of the axis and apical hook formation in dicot seedlings.
  • Senescence and Abscission:
    • Promotes senescence and abscission of plant organs, particularly leaves and flowers.
  • Fruit Ripening:
    • Highly effective in fruit ripening, enhancing the respiration rate during the climacteric phase.
  • Dormancy and Germination:
    • Breaks seed and bud dormancy.
    • Initiates germination in peanut seeds and sprouting of potato tubers.
  • Internode/Petiole Elongation:
    • Promotes rapid internode/petiole elongation in deep-water rice plants, allowing leaves and upper parts of the shoot to stay above water.
  • Root Growth:
    • Enhances root growth and root hair formation, expanding the absorption surface for nutrients.
  • Flowering and Fruit Set:
    • Used to initiate flowering and synchronize fruit set in pineapples.
    • Induces flowering in mango plants.
• Applications in Agriculture:
  • Ethephon: The widely used compound as a source of ethylene.
    • Readily absorbed and transported within the plant.
    • Releases ethylene slowly.
    • Accelerates fruit ripening in tomatoes and apples.
    • Promotes abscission in flowers and fruits (thinning of cotton, cherry, walnut).
    • Increases the yield by promoting female flowers in cucumbers.

ABSCISIC ACID (ABA): Orchestrating Growth and Stress Responses

• Discovery:
  • Originally identified for its role in regulating abscission and dormancy.
• Diverse Effects on Plant Growth and Development:
  • General Inhibition:
    • Functions as a general plant growth inhibitor.
    • Acts as an inhibitor of plant metabolism.
• Specific Effects:
  • Seed Germination Inhibition:
    • ABA plays a pivotal role in inhibiting seed germination.
  • Stomatal Closure:
    • Stimulates the closure of stomata, regulating water loss and gas exchange.
• Stress Hormone:
  • Stress Tolerance:
    • ABA earns the title “stress hormone” due to its role in enhancing plant tolerance to various stresses.
• Versatile Applications:
  • Seed Dormancy:
    • Plays a crucial role in maintaining seed dormancy until conditions are favorable for germination.
  • Environmental Stress Response:
    • Enhances plant resilience to environmental stresses such as drought, salinity, and temperature fluctuations.
• Integration into Agriculture:
  • Stress Management:
    • ABA application aids in stress management, making plants more resilient to adverse environmental conditions.
• Holistic Impact:
  • ABA, with its multifaceted effects, contributes significantly to plant adaptation, growth regulation, and stress response, making it a key player in plant physiology.