Photosynthesis in Higher Plants

PLANT GROWTH AND DEVELOPMENT

**Definition**: Development is the sum of two fundamental processes: growth and differentiation. It encompasses all changes an organism undergoes during its life cycle from seed germination to senescence, following a precise and highly ordered succession of events.

**Key Concept**: Plant development from a zygote (fertilized egg) produces a complex body organization with roots, leaves, branches, flowers, fruits, and seeds. This process is controlled by intrinsic (internal/genetic) and extrinsic (external/environmental) factors.

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GROWTH

**Definition**: Growth is an irreversible, permanent increase in size of an organ, its parts, or even an individual cell. It is accompanied by both anabolic and catabolic metabolic processes that occur at the expense of energy.

**Example**: Expansion of a leaf is growth; swelling of wood in water is not growth (it is physical absorption, reversible).

Plant Growth is Indeterminate

**Key Feature**: Plants retain unlimited growth capacity throughout their life due to the presence of **meristems** at specific locations.

**Meristem Definition**: Meristems are regions of constantly dividing cells with the capacity to divide and self-perpetuate. Meristematic cells eventually lose their dividing capacity and differentiate into specialized cells forming the plant body.

**Open Form of Growth**: This is when new cells are continuously added to the plant body by meristem activity, allowing indeterminate growth.

**Types of Growth**:

**Primary Growth**: Caused by root apical meristem (RAM) and shoot apical meristem (SAM). Results in elongation of plants along their axis. These meristems are located at the tips of roots and shoots.

**Secondary Growth**: Caused by lateral meristems – vascular cambium and cork cambium. Appear later in life in dicots and gymnosperms. Results in increase in girth/diameter of organs.

Growth is Measurable

**Parameters for Measuring Growth**:

Increase in fresh weight

Increase in dry weight

Increase in length

Increase in area (leaves, surface)

Increase in volume

Increase in cell number

**Examples**:

One maize root apical meristem produces >17,500 new cells per hour (measured as increase in cell number)

Watermelon cells increase in size by up to 3,50,000 times (measured as increase in cell size)

Pollen tube growth measured in terms of length

Dorsiventral leaf growth measured as increase in surface area

Phases of Growth

Plants exhibit three distinct developmental phases, observed clearly in root tips:

**1. Meristematic Phase**:

Region of constantly dividing cells at root apex and shoot apex

Cells rich in protoplasm with large conspicuous nuclei

Primary cell walls – thin, cellulosic with abundant plasmodesmatal connections

Cells small and densely packed

Cells just proximal (away) from the apex represent this zone

**2. Elongation Phase**:

Lies proximal to the meristematic zone

Characterized by increased vacuolation

Cell enlargement occurs

New cell wall deposition happens

Cells increase in size

Rapid growth in this zone

**3. Maturation Phase**:

Lies further away from apex, proximal to elongation zone

Cells attain maximal size

Wall thickening occurs

Protoplasmic modifications

Most tissues and cell types studied represent this phase

Cells cease enlargement

**Visual Recognition**: The parallel line technique on root tips demonstrates these zones – zones closer to apex (A, B, C, D) show maximum elongation.

Growth Rates

**Definition**: Increased growth per unit time is called **growth rate**. It can be expressed mathematically and shows two patterns.

#### Arithmetic Growth

**Definition**: Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. Growth increases at a constant/linear rate.

**Mathematical Expression**:

```

Lt = L0 + rt

```

Where:

Lt = length at time 't'

L0 = length at time zero (initial length)

r = growth rate/elongation per unit time (constant)

t = time

**Graphical Representation**: Linear curve when length is plotted against time.

**Example**: Root elongation at constant rate following the equation above.

#### Geometric/Exponential Growth

**Definition**: Both progeny cells following mitotic division retain the ability to divide and continue dividing. Growth increases exponentially.

**Pattern**:

**Lag Phase**: Initial slow growth

**Log/Exponential Phase**: Growth increases rapidly at exponential rate

**Stationary Phase**: Growth slows due to limited nutrient supply

**Mathematical Expression**:

```

W1 = W0 e^(rt)

```

Where:

W1 = final size (weight, height, number, etc.)

W0 = initial size at beginning

r = growth rate (relative growth rate/efficiency index)

t = time of growth

e = base of natural logarithms (2.718)

**Graphical Representation**: Sigmoid curve (S-shaped) when parameter is plotted against time. This is characteristic of living organisms growing in natural environments.

**Significance**: The final size (W1) depends on initial size (W0). Relative growth rate (r) measures plant's ability to produce new plant material (efficiency index).

**Real-life Example**: Cell growth in culture, most higher plants and plant organs follow sigmoid curves.

Absolute Growth Rate vs. Relative Growth Rate

**Absolute Growth Rate (AGR)**: Total growth per unit time. Compares total growth irrespective of initial size.

**Relative Growth Rate (RGR)**: Growth per unit time expressed on a common basis, e.g., per unit initial parameter.

**Example**: In Figure 13.7, two leaves A and B of different initial sizes both increase area by 5 cm² in given time (same AGR). However, if leaf B was initially smaller, its RGR is higher because it achieved same absolute increase from a smaller initial base.

**Calculation Basis**: RGR = (Final increase)/(Initial size) × (1/time)

Conditions for Growth

**Essential Requirements for Plant Growth**:

**1. Water**:

Essential for cell enlargement

Provides turgidity of cells needed for extension growth

Provides medium for enzymatic activities

Maintains water status of plant

**2. Oxygen**:

Releases metabolic energy through respiration

Essential for growth activities

**3. Nutrients**:

Macro-elements (N, P, K, Ca, Mg, S) and micro-elements (Fe, Mn, Zn, B, Mo, Cu)

Required for synthesis of protoplasm

Act as source of energy

**4. Temperature**:

Every plant has optimum temperature range

Deviations detrimental to survival

Affects growth rate

**5. Environmental Signals**:

Light – affects certain growth phases

Gravity – influences directional growth

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DIFFERENTIATION, DEDIFFERENTIATION, AND REDIFFERENTIATION

Differentiation

**Definition**: The process by which cells derived from apical meristems (root apical meristem, shoot apical meristem) and cambium mature and lose their capacity to divide, acquiring specific structure and function.

**Cellular Changes During Differentiation**:

Structural changes in cell walls

Changes in protoplasm composition

Development of specialized structures

Loss of meristematic capacity

**Example**: Formation of tracheary element:

Cells lose their protoplasm (becomes hollow)

Develop strong, elastic, lignocellulosic secondary cell walls

Specialized to carry water under extreme tension over long distances

Cannot divide after differentiation

**Correlation Principle**: Various anatomical features of plants directly correlate with their functions.

**Open Differentiation**: Even differentiation in plants is "open" because:

Cells from same meristem have different mature structures

Final structure determined by cell's position within organ

**Positional Determination**: Cell position determines its fate

**Examples**:

Cells away from RAM differentiate as root-cap cells

Cells pushed to periphery mature as epidermis

Different cells at different positions in same meristem differentiate differently

Dedifferentiation

**Definition**: Process by which living, differentiated cells that have lost capacity to divide regain the capacity of cell division under certain conditions.

**Key Point**: This demonstrates cellular plasticity in plants – differentiated cells are not permanently "locked" into their state.

**Examples**:

Formation of interfascicular cambium from fully differentiated parenchyma cells

Formation of cork cambium (phellogen) from differentiated cells

Parenchyma cells in plant tissue culture under controlled laboratory conditions

Redifferentiation

**Definition**: Following dedifferentiation, cells again lose the capacity to divide and mature to perform specific functions. They form new tissues with specialized structures.

**Process**: Dedifferentiated cells → New meristematic tissue → Redifferentiation → Specialized tissues

**Examples in Woody Dicots**:

Vascular tissues formed from cambium (products of redifferentiation)

Cork tissues from cork cambium

Secondary xylem and phloem

**Sequence**: Differentiation → Dedifferentiation → Redifferentiation represents cellular regeneration in plants.

**Clinical Correlation**: Understanding dedifferentiation is crucial for understanding tumors (abnormal dedifferentiation without controlled redifferentiation).

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DEVELOPMENT

**Definition**: Development includes all changes an organism undergoes during its life cycle, from germination of seed to senescence. It is the sum of growth and differentiation, following precise and ordered succession of events.

**Developmental Sequence** (Figure 13.8):

Cell Division → Plasmatic Growth → Expansion/Elongation → Maturation → Senescence/Death

**Timing**: Development occurs throughout plant's life from seed stage through maturation to death.

Plasticity in Development

**Definition**: The ability of plants to follow different pathways in response to environment or life phases, producing different kinds of structures.

**Example 1 – Heterophylly (Developmental Plasticity)**:

**Heterophylly Definition**: Development of leaves of different shapes and sizes in the same plant at different developmental stages or in different environments.

**Environmental Heterophylly** (e.g., Buttercup):

Leaves produced in water: Dissected, narrow (submerged leaves)

Leaves produced in air: Broad, serrated (aerial leaves)

Same plant produces different leaf types based on environment

Adaptation to different habitats

**Developmental Heterophylly** (e.g., Cotton, Coriander, Larkspur):

Juvenile plant leaves: Different shape

Mature plant leaves: Different shape from juvenile

Shape change correlates with plant age/maturation

Sequential development of different leaf morphologies

**Significance**: Demonstrates that developmental program is flexible and responsive to both environmental and internal developmental cues.

Relationship Between Growth, Differentiation, and Development

**Interrelationship**: These three processes are very closely related:

**Growth**: Irreversible increase in size (quantitative change)

**Differentiation**: Development of specialized structures and functions (qualitative change)

**Development**: Sum of growth + differentiation (complete biological change)

Control of Development

Development is under control of two factor categories:

**Intrinsic (Internal) Factors**:

**Genetic factors**: DNA/genes controlling developmental programs

**Chemical factors**: Plant growth regulators (auxins, gibberellins, cytokinins, abscisic acid, ethylene)

**Intercellular signals**: Communication between cells

**Extrinsic (External) Factors**:

Light intensity and photoperiod

Temperature

Water availability

Oxygen concentration

Mineral nutrition (nutrient availability)

**Integration**: Final developmental outcome results from complex interaction between intrinsic and extrinsic factors.

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PLANT GROWTH REGULATORS (PGRs)

**Definition**: Plant growth regulators (also called plant growth substances, plant hormones, or phytohormones) are small, simple molecules of diverse chemical composition that regulate growth, development, and responses in plants.

Characteristics of PGRs

**Chemical Nature**: Diverse chemical compositions:

**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)

**Properties**:

Small molecular size

Simple structure

Produced in small quantities

Highly effective at low concentrations

Regulate multiple developmental processes

Classification of PGRs

**Based on Functional Role**:

**Group 1 – Growth Promoters** (Stimulate growth):

**Auxins** (IAA, IBA, NAA, 2,4-D)

**Gibberellins** (GA₃, GA₁, GA₂, etc.)

**Cytokinins** (Kinetin)

Functions:

Cell division

Cell enlargement

Pattern formation

Tropic growth (phototropism, gravitropism)

Flowering

Fruiting

Seed formation

**Group 2 – Growth Inhibitors** (Inhibit growth):

**Abscisic acid (ABA)**

**Ethylene** (partially – mostly inhibitor)

Functions:

Response to wounds and stresses (biotic and abiotic)

Dormancy induction

Abscission (leaf and fruit drop)

Stress tolerance

**Note**: Ethylene is ambiguous – can promote some processes (ripening) but inhibits others (growth elongation).

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DISCOVERY OF PLANT GROWTH REGULATORS

**Historical Context**: Each of the five major PGR groups was discovered accidentally through careful observation and experimentation.

Auxins

**Discovery History**:

**Observation** (Charles Darwin & Francis Darwin): Coleoptiles (grass seedlings) responded to unilateral light by growing toward light source (phototropism)

**Experimental Finding**: Tip of coleoptile was the site of a transmittable influence causing bending of entire coleoptile

**Isolation**: F.W. Went isolated auxin from tips of oat seedlings (Avena coleoptiles)

**Figure 13.10 Demonstration**: Series of experiments with coleoptile tips, blocks, and light showing that:

Cut tips cannot bend toward light

Tip separated by opaque barrier cannot bend

Tip separated by transparent barrier can bend

Conclusion: Chemical diffuses from tip to cause bending

Gibberellins

**Discovery History**:

**Agricultural Disease**: 'Bakanae' (foolish seedling) disease of rice seedlings

**Cause**: Fungal pathogen *Gibberella fujikuroi*

**Discovery** (E. Kurosawa, 1926): Sterile filtrates of fungus produced disease symptoms in healthy rice seedlings

**Identification**: Active substance identified as gibberellic acid (GA₃)

**Significance**: Over 100 gibberellins identified from diverse organisms (fungi and higher plants)

**Notation**: Denoted as GA₁, GA₂, GA₃, etc.

Cytokinins

**Discovery History**:

**Experimental Observation** (F. Skoog & co-workers): Callus (mass of undifferentiated cells) from internodal segments of tobacco stems proliferated only when auxins AND one of the following were added:

Vascular tissue extracts

Yeast extract

Coconut milk

DNA

**Identification** (Miller et al., 1955): Identified and crystallized the cytokinesis-promoting substance called kinetin

**Significance**: Demonstrated interaction between different PGRs in controlling cell division

Abscisic Acid (ABA)

**Discovery History**:

**Period**: Mid-1960s

**Three Independent Discoveries**:

Inhibitor-B

Abscission II

Dormin

**Key Finding**: All three were chemically identical

**Naming**: Named **Abscisic Acid (ABA)**

**Significance**: Represents a growth inhibitor class of PGRs

Ethylene

**Discovery History**:

**Historical Observation** (H.H. Cousins, 1910): Volatile substance released from ripened oranges hastened ripening of stored unripened bananas

**Identification**: Volatile substance identified as ethylene (C₂H₄)

**Significance**: Only gaseous PGR; demonstrates long-distance signaling through air

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PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS

AUXINS

**Definition**: Auxins (Greek 'auxein' = to grow) are indole compounds with growth-regulating properties. **Indole-3-acetic acid (IAA)** is the primary naturally occurring auxin.

**Chemical Types**:

**Natural Auxins**:

Indole-3-acetic acid (IAA) – most common

Indole butyric acid (IBA)

**Synthetic Auxins**:

NAA (Naphthalene acetic acid)

2,4-D (2,4-dichlorophenoxyacetic acid)

**Source and Distribution**:

Produced by growing apices of stems and roots

Synthesized primarily at shoot apex

Migrate from production site to region of action

Concentration decreases acropetally (from apex toward base)

**Physiological Effects**:

**1. Root Initiation**:

Initiate rooting in stem cuttings

Widely used for plant propagation in agriculture and horticulture

Applied as auxin paste to cut ends of cuttings

**2. Flowering and Fruiting**:

Promote flowering in certain plants (e.g., pineapple)

Influence reproductive development

**3. Leaf and Fruit Abscission**:

**Young stage**: Prevent fruit and leaf drop at early development stages (protective function)

**Mature stage**: Promote abscission of older, mature leaves and fruits (senescence promotion)

Concentration and developmental stage determine effect

**4. Apical Dominance**:

Definition: Growing apical (terminal) bud inhibits growth of lateral (axillary) buds

Mechanism: Auxin produced at apex moves down and inhibits lateral bud growth

**Decapitation** (removal of shoot tip): Results in growth of lateral buds (they are no longer suppressed)

**Practical Application**:

Tea plantations – regular pruning to promote branching

Hedge-making – decapitation to create bushier growth

Ornamental horticulture – achieve desired plant shape

**5. Parthenocarpy**:

Induction of fruit development without fertilization

Results in seedless fruits

Example: Tomatoes treated with auxin develop parthenocarpic fruits

**6. Herbicide Application**:

**2,4-D (2,4-dichlorophenoxyacetic acid)**: Widely used synthetic auxin herbicide

**Selective action**: Kills dicotyledonous weeds without affecting mature monocotyledonous plants

**Mechanism**: Causes uncontrolled growth and death in susceptible plants

**Application**: Creating weed-free lawns in gardens

**Advantage**: Does not harm grass (monocot) lawns

**7. Xylem Differentiation**:

Control xylem tissue differentiation

Influence vascular development

**8. Cell Division**:

Promote cell division in conjunction with cytokinins

Essential for callus formation in tissue culture

**Concentration-Dependent Effects**: Auxins show biphasic response:

Low concentrations: Promote growth

High concentrations: Inhibit growth

Different effects at different concentrations

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GIBBERELLINS

**Definition**: Gibberellins are a large group of plant hormones (>100 different types identified) with diverse origins and functions.

**Chemical Characteristics**:

Terpene-based compounds

Denoted as GA₁, GA₂, GA₃, etc. (subscript indicates different structure)

**GA₃ (Gibberellic acid)**: Most studied and commercially important

Isolated from both fungi (original source) and higher plants

**Sites of Synthesis**:

Growing apices (similar to auxins)

Young leaves

Reproductive tissues

Root tips

**Physiological Effects**:

**1. Stem Elongation**:

Primary effect of gibberellins

Increase cell elongation

Promote internode elongation

Result in overall increase in plant height

Effect on both primary and secondary growth

**2. Seed Germination**:

Promote seed germination

Overcome seed dormancy

Act on stored starch in endosperm

Activate enzymes for mobilization of seed reserves

**3. Breaking Dormancy**:

Gibberellins break dormancy of seeds and buds

Counteract inhibitory effects of ABA during dormancy

Allow resumption of growth when conditions are favorable

**4. Flowering and Fruiting**:

Promote flowering in many plants

Induce flowering in long-day plants

Regulate flowering time

Promote fruit development

**5. Parthenocarpy**:

Induce seedless fruit development

Used commercially in crop production

**6. Leaf Growth**:

Increase leaf size

Promote cell division and elongation in leaves

**Example**: Bakanae disease (foolish seedling) disease results from fungal gibberellin production, causing abnormal elongation in rice seedlings.

**Commercial Applications**:

Used to increase crop yield

Enhance fruit and seed production

Break dormancy in nurseries

Improve malting quality in barley for brewing

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CYTOKININS

**Definition**: Cytokinins are adenine derivatives with primary role in promoting cell division (cytokinesis). **Kinetin** (N⁶-furfurylamino purine) is the primary synthetic cytokinin.

**Discovery Correlation**: Skoog's tissue culture experiments showed:

Callus formation required both auxin AND cytokinin

Ratio of auxin to cytokinin determined developmental pathway in culture

**Natural Cytokinins**:

Zeatin (most common natural form)

Other adenine derivatives

**Synthetic Cytokinins**:

Kinetin

6-benzyl amino purine (BAP)

**Physiological Effects**:

**1. Cell Division**:

Promote mitotic cell division

Essential for cytokinesis (actual cell division process)

Work synergistically with auxins for cell division

**2. Callus Formation**:

Promote callus (undifferentiated tissue) formation in plant tissue culture

Require specific auxin:cytokinin ratio

**3. Shoot Initiation**:

**High cytokinin: auxin ratio** → Shoot bud formation (organogenesis)

Promote elongation of lateral buds

Overcome apical dominance

**4. Root Development**:

**High auxin: cytokinin ratio** → Root initiation

Cytokinins alone promote shoots, not roots

**5. Leaf Senescence**:

Delay leaf senescence and aging

Maintain chlorophyll and protein content

Extend leaf lifespan

Counteract effects of ethylene in promoting senescence

**6. Stomatal Opening**:

Promote stomatal opening

Influence gas exchange

**7. Nutrient Mobilization**:

Promote nutrient (nitrogen, phosphorus, potassium) mobilization

Increase nutrient translocation to cytokinins-treated organs

**Tissue Culture Application** (Skoog-Miller Technique):

Callus differentiation controlled by auxin:cytokinin ratio:

High cytokinin : Low auxin → Shoots form

Low cytokinin : High auxin → Roots form

Equal ratio → Callus proliferation continues

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ABSCISIC ACID (ABA)

**Definition**: Abscisic acid is a growth inhibitor hormone, carotenoid derivative, involved in plant stress responses and dormancy.

**Discovery**: Mid-1960s – three independently discovered inhibitors (Inhibitor-B, Abscission II, Dormin) proved chemically identical and named ABA.

**Site of Synthesis**:

Leaves (particularly during stress)

Root cap

Seeds

Growing points

**Physiological Effects**:

**1. Stomatal Closure**:

Primary and most important effect

Closes stomata in response to drought/water stress

Reduces water loss through transpiration

Releases from guard cells upon water stress signal

Increases abscisic acid concentration in guard cells

Causes stomatal closure (reduces transpiration)

Mechanism: Affects ion channels and guard cell turgor

**2. Seed Dormancy**:

Induces and maintains seed dormancy

Prevents premature germination

Accumulates during seed development

Maintains high ABA:GA ratio in dormant seeds

Prevents radical emergence

**3. Growth Inhibition**:

Inhibits cell elongation

Reduces growth rate during stress

Antagonistic to gibberellins (GA) and auxins

**ABA:GA Ratio**: Determines germination

High ABA:GA → Dormancy

Low ABA:GA → Germination

**4. Abscission**:

Promotes abscission of leaves, flowers, and fruits

Increases in response to stress

Ethylene synergistically enhances this effect

**5. Root-to-Shoot Signaling**:

Produced in roots during water stress

Signals shoot to reduce transpiration

Acts as hydraulic signal for water status

**6. Stress Response**:

Increases in response to biotic stresses (pathogen attack)

Increases in response to abiotic stresses (drought, salinity, cold, heat)

Activates protective mechanisms

Induces protective protein synthesis

**7. Antagonism to Other Hormones**:

Antagonistic to gibberellins and auxins

Counteracts growth promotion by antagonistic hormones

Works with ethylene for abscission

**Stress Function**: ABA is the primary stress hormone:

Concentration increases under stress conditions

Induces stress tolerance mechanisms

Reduces growth to conserve resources during stress

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ETHYLENE

**Definition**: Ethylene (C₂H₄) is the only gaseous plant hormone, produced from methionine via 1-aminocyclopropane-1-carboxylic acid (ACC).

**Historical Discovery**: H.H. Cousins (1910) observed volatile substance from ripened oranges hastened ripening of unripened bananas.

**Site of Synthesis**:

Ripening fruits

Senescent tissues

Stress-affected tissues

Root and shoot apices (in small quantities)

**Physiological Effects**:

**1. Fruit Ripening**:

Major effect of ethylene

Promotes ripening of climacteric fruits (apples, bananas, tomatoes, mangoes)

Increases pigmentation (color development)

Increases sweetness (sugar accumulation)

Increases aroma

**Auto-catalytic**: Ripening fruits produce ethylene, which promotes more ripening in nearby fruits

**2. Leaf and Flower Abscission**:

Promotes abscission (shedding) of leaves, flowers, fruits

Works synergistically with ABA for abscission

Response to stress, disease, or senescence

Forms abscission layer

**3. Senescence Acceleration**:

Accelerates aging of tissues and organs

Promotes senescence in leaves and flowers

Shortens lifespan of cut flowers

Interacts with cytokinins (antagonistic relationship)

**4. Growth Inhibition**:

Inhibits elongation growth of stems

Causes radial growth (thickening) instead of elongation

Results in shorter, stockier plants

Effect called **triple response** in seedlings:

Inhibition of elongation

Radial expansion

Horizontal growth (gravity resistance)

**5. Stress Response**:

Production increases under stress conditions

Response to wounding

Response to pathogen attack

Response to environmental stress (hypoxia, temperature extremes)

Activates defense mechanisms

**6. Root Initiation**:

At low concentrations: May promote root initiation

At high concentrations: Inhibits root growth

**7. Flower Development**:

Some promotion of floral development in certain species

Promotes flower opening in some flowers

Promotes flowering under stress conditions

**Dual Role**: Ethylene shows both promotional and inhibitory effects:

**Promoter**: Ripening, senescence, abscission, flowering (in some cases)

**Inhibitor**: Elongation growth, lateral bud growth

**Practical Applications**:

Artificial ripening of fruits (banana ripening chambers)

Preservation of cut flowers (ethylene inhibitors like 1-MCP used)

Control of flowering time in ornamental crops

Degreening of citrus fruits

**Triple Response in Seedlings** (Characteristic ethylene response):

Inhibition of elongation growth

Radial/diametric growth (thickening)

Loss of apical hook (in dicot seedlings)

Horizontal orientation of growth (geotropism reversal)

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SUMMARY OF PGR INTERACTIONS

**Synergistic Effects**:

Auxin + Cytokinin = Cell division

Ethylene + ABA = Abscission

**Antagonistic Effects**:

Gibberellin vs. ABA = Growth vs. Dormancy

Cytokinin vs. Ethylene = Delayed senescence vs. Promoted senescence

**Concentration-Dependent Effects**:

Same hormone can promote or inhibit depending on concentration

Ratio between hormones determines response (e.g., auxin:cytokinin ratio in culture)

**Interactions**: Plant responses result from complex interactions and ratios between different PGRs, not individual hormone action alone.

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SEED GERMINATION (Related to PGRs and Growth)

**Definition**: Germination is the process by which a seed resumes metabolic activity and growth after a period of dormancy or rest, leading to emergence of the radical and development of seedling.

**Dormancy**: Period of suspended growth and reduced metabolic activity when seeds do not germinate despite favorable conditions being present sometimes.

**Conditions for Germination**:

Water (imbibition to activate enzymes)

Oxygen (for respiration and energy)

Favorable temperature (optimum range)

Light (in some species – photoblastism)

Nutrient availability

**PGR Roles in Germination**:

**Gibberellins**: Promote germination, break dormancy

**ABA**: Maintains dormancy in seeds

**Ethylene**: Promotes germination in some species

**Cytokinins**: May promote radicle emergence

**Auxins**: Variable role depending on tissue

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EXAM-IMPORTANT POINTS

1. **Growth Definition**: Irreversible permanent increase in size – distinguish from temporary physical changes

2. **Meristematic Growth**: Plants have unlimited growth due to meristems (indeterminate) – this is unique feature

3. **Three Growth Phases**: Memorize meristematic, elongation, maturation with characteristics of each

4. **Growth Rate Formulas**:

Arithmetic: Lt = L0 + rt (linear)

Geometric: W1 = W0 e^(rt) (exponential/sigmoid)

5. **Differentiation**: Cells lose capacity to divide but gain specialized function and structure

6. **Dedifferentiation**: Mature cells regain capacity to divide (e.g., cambium formation)

7. **Development**: Sum of growth + differentiation throughout plant life cycle

8. **Plasticity**: Plant developmental flexibility based on environment (heterophylly examples)

9. **PGR Discovery**: Each discovered accidentally – auxins from Darwin's observations, gibberellins from Bakanae disease, cytokinins from Skoog's culture studies, ABA from multiple independent discoveries, ethylene from ripening fruit observation

10. **PGR Classification**: Growth promoters (auxins, gibberellins, cytokinins) vs. inhibitors (ABA, ethylene mostly)

11. **Auxin Effects**: IAA from apex, promotes root initiation, overcomes apical dominance, 2,4-D selective herbicide, parthenocarpy

12. **Gibberellin Effects**: Stem elongation, seed germination, break dormancy, promote flowering and fruiting

13. **Cytokinin Effects**: Cell division, delay senescence, auxin:cytokinin ratio determines shoot (high CK) vs. root (high Aux) in culture

14. **ABA Effects**: Stomatal closure in drought, seed dormancy, antagonistic to GA and auxins, stress hormone

15. **Ethylene Effects**: Fruit ripening (auto-catalytic), abscission, senescence, inhibits elongation growth (triple response), stress response

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This comprehensive chapter covers all NCERT content with definitions, processes, formulas, examples, and exam-relevant details sufficient for CBSE board examination preparation.