Plant Growth and Development (Chapter 12) – Class 11 Biology Study Package

SECTION A: School/Board Exam Type Questions

Very Short Answer Type Questions

1. Give an example of growth in plants.

• Expansion of leaves from small primordial structures to mature photosynthetic organs
• Increase in stem height due to internodal elongation driven by apical meristems

Both represent irreversible growth measurable through increased fresh/dry weight and length.

2. Which type of cells stop dividing and form permanent tissues?

Solution: Meristematic cells lose their capacity for division during differentiation, transitioning into permanent tissues (xylem, phloem, parenchyma). This process is irreversible under normal conditions.

3. Name the phase of growth which occurs after meristematic phase of growth.

Solution: Elongation phase (also called phase of enlargement). During this stage:

• Cells undergo rapid vacuolation
• Cell wall loosening occurs through enzymatic action
• Turgor pressure drives cell expansion
• Follows the meristematic phase and precedes maturation

4. W₁ = W₀ e^(rt). What do W₀ and e stand for in the given expression?

Solution: This equation represents exponential growth.

This formula models geometric growth where all cells divide continuously.

5. Define phytohormone.

Phytohormones (plant growth regulators/PGRs) are organic chemical substances, other than nutrients, that:

• Regulate one or more physiological processes
• Function at extremely low concentrations (often micromolar or nanomolar)
• May act at the site of synthesis (autocrine) or distant locations (transported via phloem/xylem)
• Include auxins, gibberellins, cytokinins, ethylene, and abscisic acid

6. A farmer wants to remove broad-leaved weeds from his wheat field. Which plant hormone would you suggest?

Auxin – specifically 2,4-D (2,4-Dichlorophenoxyacetic acid)

Mechanism: At high concentrations, synthetic auxins act as selective herbicides:

• Dicotyledonous weeds (broad-leaved) exhibit uncontrolled growth and die
• Monocotyledonous crops like wheat remain unaffected due to differential sensitivity
• This selective toxicity makes 2,4-D ideal for cereal crop protection

7. Which phytohormone is responsible for synchronization of flowering and fruit set in pineapple?

Ethylene (C₂H₄)

Application: Commercial pineapple growers spray ethylene or ethephon (ethylene-releasing compound) to:

• Induce simultaneous flowering across the plantation
• Ensure uniform fruit ripening
• Facilitate coordinated harvesting
• This hormone triggers flowering even in non-inductive conditions

8. Under severe drought conditions, some plants close their stomata to reduce water loss. Name the phytohormone responsible for this action.

ABA (Abscisic Acid) – known as the “stress hormone”

Mechanism:

• ABA levels increase during water deficit
• Triggers stomatal closure by causing K⁺ efflux from guard cells
• Reduces transpiration by 50-70%
• Protects plants from desiccation
• Also called “antitranspirant” for this function

9. Some plants can flower only when exposed to low temperature for a few weeks. What do you call this phenomenon?

Vernalization (from Latin vernalis = spring)

Characteristics:

• Requirement of prolonged cold treatment (0-10°C) for flowering
• Perceived by stem apex or embryo in seeds
• Prevents premature flowering in winter annuals
• Common in wheat, cabbage, carrot, beet
• Can be reversed by high temperatures (devernalization)

10. Which structures are formed from plumule and radicle?

Both originate from the embryo within the seed and establish the plant’s basic architecture during germination.

Short Answer Type Questions

11. What is growth? Does swelling of raisins when kept in water constitute growth?

Growth Definition: Growth is an irreversible, permanent increase in:

• Size (dimensions/volume)
• Weight (fresh and dry mass)
• Cell number (through mitosis)

It involves biosynthetic processes requiring metabolic energy.

Does raisin swelling constitute growth?

No. The swelling of raisins is endosmosis (water absorption), which is:

• Reversible – raisins shrink upon drying
• Physical, not metabolic
• Not accompanied by protoplasmic synthesis
• Lacks cell division or differentiation

True growth requires irreversible structural changes, which osmotic swelling lacks.

12. Differentiate between primary growth and secondary growth.

13. Describe the following parameters of estimation of growth: (a) Increase in cell size (b) Increase in length

(a) Increase in Cell Size:

• Measured by cell dimensions or volume
• Example: Watermelon cells can enlarge up to 3,50,000 times their original volume through:
  • Vacuolation (90% of mature cell volume)
  • Cell wall expansion via loosening mechanisms
  • Turgor pressure driving irreversible stretching
• Quantified using microscopy and image analysis

(b) Increase in Length:

• Most common growth parameter for organs like roots, stems, and pollen tubes
• Pollen tube growth is measured exclusively in length (can reach several cm)
• Measured using:
  • Auxanometer (mechanical device)
  • Ruler markings at fixed intervals
  • Arc indicator method in classical plant physiology

Both parameters provide quantitative data for growth analysis.

14. Explain physiological differentiation with examples.

Physiological Differentiation is the process by which cells acquire specialized metabolic functions after structural differentiation.

Characteristics:

• Cells develop specific biochemical pathways
• Express tissue-specific enzymes
• Perform unique physiological roles

Examples:

Unlike structural differentiation (shape, size), physiological differentiation involves functional specialization at the molecular level.

15. Give two differences between arithmetic and geometric growth.

Solution:

Practical Significance: Most plant organs follow geometric growth initially, transitioning to arithmetic growth at maturity.

16. Which phase comes after lag phase of the sigmoid growth curve? Explain.

Log Phase (Exponential Phase/Grand Period of Growth)

Characteristics:

1. Maximum growth rate – cells divide at their highest capacity
2. Exponential increase in cell number and biomass
3. Both daughter cells retain meristematic potential
4. Abundant resources (nutrients, water, space) available
5. No limiting factors constrain growth

Cellular Events:

• Continuous mitotic divisions in all progeny cells
• Minimal cell enlargement (unlike elongation phase)
• High metabolic activity with maximum ATP production
• Biosynthesis of proteins, nucleic acids, and cell wall components

Graphical Representation: The steepest slope in the sigmoid curve occurs during this phase, representing the period of most rapid biomass accumulation.

Following Phase: Stationary phase (where growth rate declines due to nutrient depletion or space constraints).

17. (a) What does ‘r’ indicate in the expression Wt = W₀e^(rt)? (b) Define efficiency index.

(a) ‘r’ represents: Relative Growth Rate (RGR) – the growth per unit time expressed as a proportion of the existing size.

Formula:

RGR (r) = (Growth per unit time / Initial size) × 100

Significance:

• Allows comparison of growth between organisms of different sizes
• Dimensionless or expressed as %/day
• Higher ‘r’ indicates faster growth relative to current biomass

(b) Efficiency Index: Definition: The capacity of a plant to produce new biomass (plant material) per unit time relative to resources invested.

Calculation:

Efficiency Index = (Dry matter production) / (Total photosynthetic area × Time)

Applications:

• Assessing crop productivity
• Comparing cultivar performance
• Evaluating photosynthetic efficiency

18. Why is water required for the proper growth of a plant?

Water is essential for plant growth due to multiple roles:

1. Cell Elongation and Turgidity:

• Provides turgor pressure (hydrostatic pressure) that drives cell expansion
• Maintains cell rigidity required for structural support
• Without water, cells collapse (plasmolysis), halting growth

2. Medium for Enzymatic Activities:

• Aqueous environment necessary for metabolic reactions
• Enzymes require hydration for catalytic function
• Substrate diffusion occurs in water-based cytosol

3. Photosynthesis:

• Raw material for light reactions (H₂O → O₂ + H⁺)
• Provides electrons for photosynthetic electron transport

4. Transport:

• Xylem translocation of minerals from roots to shoots
• Phloem transport of sugars requires water for pressure-flow mechanism

5. Temperature Regulation:

• Evaporative cooling through transpiration prevents overheating

19. How is the tracheary element formed?

Tracheary Elements (tracheids and vessel elements) form through a specialized differentiation process:

Step-by-Step Formation:

1. Cell Elongation:
   • Procambial/cambial cells undergo extensive elongation
   • Cells become 10-100 times longer than wide
2. Secondary Wall Deposition:
   • Lignocellulosic secondary wall forms in specific patterns (annular, spiral, reticulate, pitted)
   • Lignin provides mechanical strength and water impermeability
   • Deposition occurs on inner surface of primary wall
3. Programmed Cell Death (PCD):
   • Protoplasm degenerates through autolysis
   • Nucleus, organelles, and cytoplasm break down
   • Tonoplast rupture releases hydrolytic enzymes
4. Perforation Formation (Vessels only):
   • End walls dissolve partially or completely
   • Forms continuous water-conducting tubes

Result: Hollow, dead cells with strong, elastic walls that:

• Resist collapse under extreme negative pressure (up to -15 MPa)
• Transport water efficiently with minimal resistance
• Provide mechanical support to plant body

20. Expand IAA and NAA. Which one is natural?

IAA is natural – it is the primary endogenous auxin in most plants, derived from the amino acid tryptophan through multiple biosynthetic pathways.

NAA advantages:

• Resistant to degradation by IAA-oxidase
• Longer-lasting hormonal effects
• Used commercially for rooting, fruit setting, and thinning

21. (a) Where are auxins synthesized in plants? (b) What happens to plants like lettuce when auxin is sprayed in high concentration?

(a) Sites of Auxin Synthesis:

• Shoot apical meristems (primary source)
• Root apical meristems (local synthesis)
• Young developing leaves (leaf primordia)
• Developing seeds and fruits (high IAA concentrations)
• Embryonic tissues

Auxin is transported basipetally (tip-to-base) through polar auxin transport (PAT) via PIN proteins.

(b) Effect of High Auxin on Lettuce: Flowering inhibition occurs, which is agriculturally advantageous because:

• Lettuce is cultivated for edible leaves, not seeds
• Preventing flowering (bolting) maintains:
  • Leaf tenderness
  • Reduced bitterness (lactucin compounds increase in flowering)
  • Prolonged harvest period

Mechanism: Supraoptimal auxin concentrations disrupt the hormonal balance required for floral initiation, keeping the plant in the vegetative phase.

22. Name the plant hormone which increases the stalk length in grapes. Also write the effect of this hormone on conifers and bolting.

Hormone: Gibberellin (specifically GA₃ – Gibberellic acid)

Effects:

1. Grapes (Vitis vinifera):

• Increases stalk (rachis) length, improving bunch looseness
• Reduces berry compaction
• Enhances air circulation, reducing fungal diseases
• Improves seedlessness in parthenocarpic varieties

2. Conifers (Gymnosperms):

• Promotes early seedling growth, accelerating juvenile phase
• Induces earlier maturity and cone production
• Reduces generation time, valuable for breeding programs
• Increases economically important seed yield

3. Bolting (Rosette Plants):

• Induces stem elongation in plants with rosette growth habit (cabbage, beet, radish)
• Internodal elongation occurs before flowering
• Mechanism: Activates genes encoding cell wall loosening proteins (expansins)
• Converts vegetative rosette into flowering bolt

23. (a) Name two phytohormones responsible for cell division in callus. (b) Which phytohormone delays senescence and inhibits apical dominance?

(a) Hormones for Cell Division in Callus:

1. Cytokinin (e.g., kinetin, zeatin, BAP)
2. Auxin (e.g., 2,4-D, NAA, IAA)

Synergistic Action:

• Both required for sustained callus proliferation
• High auxin:cytokinin ratio → root formation
• High cytokinin:auxin ratio → shoot formation
• Balanced ratio → undifferentiated callus growth

(b) Hormone Delaying Senescence and Inhibiting Apical Dominance: Cytokinin

Mechanisms:

1. Senescence Delay:
   • Maintains chlorophyll content (Richmond-Lang effect)
   • Retains protein synthesis in aging leaves
   • Prevents chloroplast degradation
   • Used commercially to extend cut flower longevity
2. Apical Dominance Inhibition:
   • Overcomes auxin-mediated suppression of lateral buds
   • Applied to axillary buds to stimulate branch formation
   • Promotes bushy growth in ornamental plants

24. (a) What induces artificial ripening in fruits like tomato, mango, and apple? (b) Which is the only gaseous natural plant growth regulator?

(a) Artificial Ripening Agent: Ethylene (C₂H₄) or its precursor Ethephon

Commercial Application:

• Fruits harvested unripe for transport durability
• Treated with 100-1000 ppm ethylene in ripening chambers
• Triggers climacteric respiration burst
• Induces:
  • Chlorophyll breakdown (degreening)
  • Carotenoid/anthocyanin synthesis (color development)
  • Starch-to-sugar conversion (sweetening)
  • Cell wall softening (pectin degradation)
  • Volatile ester production (aroma)

(b) Only Gaseous PGR: Ethylene (C₂H₄)

Unique Properties:

• Simple alkene (H₂C=CH₂)
• Freely diffusible through intercellular air spaces
• Produced by almost all plant organs
• Biosynthesis: Methionine → SAM → ACC → Ethylene (via ACC oxidase)
• Active at 0.1-1.0 ppm concentrations

25. What names were given to the plant growth regulators discovered as inhibitors independently by three scientists? What are they all referred to now?

Historical Names: Three scientists independently discovered the same compound:

Current Unified Name: Abscisic Acid (ABA) – adopted in 1967 by the International Plant Growth Substances Association

Note: Despite the name, ABA’s primary role is stress response (drought, salinity), not abscission. Ethylene is more critical for abscission processes.

26. Define development. Is appearance of chloroplasts in cells when exposed to sunlight a development?

Development Definition: The sum total of growth and differentiation, encompassing all changes an organism undergoes during its life cycle from:

• Germination → Vegetative growth → Reproductive maturity → Senescence

Includes both:

• Quantitative changes (size, cell number)
• Qualitative changes (differentiation, morphogenesis, maturation)

Is chloroplast appearance “development”? Yes.

Justification:

• Represents differentiation of proplastids into chloroplasts
• Light-induced process (photomorphogenesis)
• Involves:
  • Gene expression (CAB genes for light-harvesting complexes)
  • Structural transformation (thylakoid membrane formation)
  • Functional specialization (photosynthetic capability acquisition)
• Irreversible under continued light exposure

This is developmental plasticity – environmentally-triggered developmental change.

27. (a) Give two examples of plants showing heterophyllous development. (b) Which factors influence developmental stages?

(a) Plants Showing Heterophylly:

1. Cotton (Gossypium) – juvenile leaves lobed, adult leaves entire
2. Coriander (Coriandrum sativum) – juvenile leaves pinnate, adult leaves finely divided
3. Larkspur (Delphinium) – marked leaf shape changes with age

(Any two acceptable)

(b) Developmental Factors:

Factors Influencing Development
│
├── Intrinsic (Internal)
│   ├── Intracellular
│   │   └── Genetic factors (DNA sequences, gene expression)
│   │
│   └── Intercellular
│       └── Plant Growth Regulators (auxin, cytokinin, gibberellin, ABA, ethylene)
│
└── Extrinsic (External/Environmental)
    ├── Temperature (vernalization, thermoperiodism)
    ├── Light (photoperiodism, photomorphogenesis)
    ├── Water availability (drought stress responses)
    ├── Oxygen (aerobic respiration for energy)
    └── Nutrition (mineral availability, nitrogen status)

Development results from complex interactions between genetic programs and environmental cues.

28. (a) What is the role of radicle in seed germination? (b) Which structure shows active growth in bean seed after radicle elongation?

Solution:

(a) Radicle Function: The radicle is the embryonic root that:

1. First structure to emerge from seed coat (breaks dormancy)
2. Geotropic response – grows downward due to gravity sensing (statoliths in root cap)
3. Establishes primary root system:
   • Anchors seedling in soil
   • Begins water/mineral absorption
   • Forms root hairs for increased surface area
4. Enables shoot emergence by providing support and resources

(b) Post-Radicle Growth in Bean: Hypocotyl (embryonic stem below cotyledons)

Process:

• Forms hypocotyl hook (epicotyl hook in some species)
• Shows negative geotropism (upward growth)
• Undergoes rapid elongation due to:
  • Cell expansion in elongation zone
  • Auxin-mediated cell wall loosening
• Pulls cotyledons above soil surface in epigeal germination
• Straightens to position leaves for photosynthesis

29. Define the following: (a) Photoperiod (b) Photoperiodism

Solution:

(a) Photoperiod: The duration of daylight (light period) in a 24-hour cycle.

Characteristics:

• Measured in hours of light per day
• Varies seasonally with latitude (longer in summer, shorter in winter)
• Critical factor in flowering regulation
• Expressed as day length (though night length is often more critical)

(b) Photoperiodism: The developmental responses of plants to the relative lengths of day and night.

Mechanism:

• Mediated by phytochrome (photoreceptor protein, Pr ⇄ Pfr)
• Night length is actually measured (dark period more critical than light)
• Circadian clock involvement in time measurement

Plant Categories:

Florigen (hypothetical flowering hormone) is proposed to be synthesized in leaves and transported to shoot apex.

30. (a) What happens to shoot apices before flowering? (b) From which part does florigen migrate to shoot apices?

(a) Shoot Apex Transformation: Vegetative apical meristem converts into reproductive (flowering) apex

Molecular/Structural Changes:

1. Meristem identity shift:
   • Expression of flowering genes (FT, AP1, LFY in Arabidopsis)
   • Suppression of vegetative identity genes (TFL1)
2. Morphological reorganization:
   • Increased mitotic activity
   • Formation of floral primordia instead of leaf primordia
   • Development of sepals, petals, stamens, carpels in whorls
3. Hormonal changes:
   • Increased gibberellin and cytokinin levels
   • Decreased auxin-to-cytokinin ratio

(b) Source of Florigen: Leaves (mature, photosynthetically active leaves)

Pathway:

1. Photoperiodic stimulus perceived by leaf cells
2. Florigen synthesis in leaf vascular tissue
3. Long-distance transport via phloem to shoot apex
4. Triggers floral transition in apical meristem

Note: Florigen has been identified as FT protein (FLOWERING LOCUS T) in modern research, moving through phloem as part of a protein complex.

Long Answer Type Questions

31. Give a schematic diagram showing various parameters used to estimate growth.

Solution:

                    ESTIMATION OF GROWTH
                            │
        ┌───────────────────┼───────────────────┐
        │                   │                   │
  Increase in         Increase in         Increase in
  Cell Number         Cell Size          Fresh & Dry Weight
        │                   │                   │
    (Mitosis)         (Vacuolation,       (Biomass
    counting          cell expansion)     accumulation)
        │                   │                   │
        │                   │                   │
        └───────────────────┼───────────────────┘
                            │
        ┌───────────────────┼───────────────────┐
        │                   │                   │
  Increase in         Increase in         Increase in
   Length              Volume            Surface Area
        │                   │                   │
  (Auxanometer)      (Displacement)    (Leaf area meter)

Detailed Parameters:

1. Increase in Cell Number: Hemocytometer counting, DNA quantification
2. Increase in Cell Size: Microscopy, watermelon cells enlarge 3,50,000×
3. Increase in Fresh Weight: Immediate weighing (includes water content)
4. Increase in Dry Weight: Oven drying at 80°C until constant weight (most accurate)
5. Increase in Length: Pollen tube growth, root/stem elongation
6. Increase in Volume: Pycnometer method, water displacement
7. Increase in Surface Area: Planimetry, leaf area index (LAI)

Most Reliable: Dry weight (eliminates water content variation).

32. Explain the phase of growth occurring after the elongation phase.

Solution:

Maturation Phase (Phase of Differentiation)

Location: Cells situated proximal to the elongation zone (away from meristem)

Characteristics:

1. Structural Differentiation:

• Cells attain specific shapes (fibers elongate, parenchyma becomes isodiametric)
• Acquire definite sizes (no further enlargement)
• Develop specialized internal organization:
  • Xylem vessels: Secondary wall thickening, perforation plates
  • Sieve tube elements: Enucleation, sieve plate formation
  • Guard cells: Chloroplast development, unevenly thickened walls
  • Root hairs: Tubular outgrowths for absorption

2. Physiological Differentiation:

• Expression of tissue-specific enzymes:
  • Mesophyll: RuBisCO for carbon fixation
  • Root hairs: Proton pumps (H⁺-ATPase) for nutrient uptake
  • Guard cells: K⁺ channels for stomatal movement
• Acquisition of specialized functions:
  • Photosynthesis (chlorenchyma)
  • Water conduction (xylem)
  • Translocation (phloem)
  • Storage (parenchyma)
  • Support (collenchyma, sclerenchyma)

3. Metabolic Changes:

• Shift from anabolic (biosynthesis) to catabolic (maintenance) metabolism
• Reduced growth rate approaching zero
• Accumulation of secondary metabolites (lignin, cutin, suberin)

Fate: Mature cells generally do not grow further and remain in this state until senescence, except in special cases like:

• Dedifferentiation: Cambial initiation from mature parenchyma
• Redifferentiation: Secondary xylem/phloem formation from cambium

33. (a) Which type of curve is obtained when size of a plant organ is plotted against time? Represent graphically. (b) Define relative growth rate.

Solution:

(a) Growth Curve Type: Sigmoid Curve (S-shaped curve)

Graphical Representation:

    │
    │                        ┌─────── Stationary Phase
    │                    ┌───┘       (Asymptote)
  S │                ┌───┘
  i │            ┌───┘
  z │        ┌───┘
  e │    ┌───┘ Exponential/Log Phase
    │ ┌──┘    (Steepest slope)
  / │┌┘
  W │ Lag Phase
  e │ (Slow initial growth)
  i │
  g │
  h │
  t │
    └────────────────────────────────────────→
                    Time

Phases:

1. Lag Phase:
   • Minimal growth
   • Cell reorganization
   • Enzyme synthesis
2. Exponential/Log Phase:
   • Maximum growth rate
   • All cells dividing
   • Abundant resources
3. Stationary Phase:
   • Growth rate declines
   • Limiting factors emerge
   • Approaches maximum size

(b) Relative Growth Rate (RGR):

Definition: Growth per unit time expressed as a percentage of the present size.

Formula:

RGR = (Growth per unit time / Initial size) × 100

Or:  RGR = [(W₂ - W₁) / W₁] × 100

Where:

• W₁ = Initial weight/size
• W₂ = Final weight/size

Significance:

• Allows comparison between organisms of different sizes
• Young seedlings have higher RGR than mature trees
• Normalizes growth data for meaningful analysis

34. Calculate the relative growth rate: (a) Leaf A of 50 cm² grows 10 cm²/day (b) Leaf B of 20 cm² grows 10 cm²/day. Which leaf has low relative growth rate?

Solution:

Formula:

RGR = (Growth per unit time / Initial size) × 100

(a) Leaf A:

Initial size (L₀) = 50 cm²
Growth per day = 10 cm²

RGR = (10 / 50) × 100
RGR = 0.2 × 100
RGR = 20% per day

(b) Leaf B:

Initial size (L₀) = 20 cm²
Growth per day = 10 cm²

RGR = (10 / 20) × 100
RGR = 0.5 × 100
RGR = 50% per day

Comparison:

Answer: Leaf A has the lower relative growth rate (20% vs. 50%), even though both leaves have identical absolute growth rates. This demonstrates why RGR is a superior metric for comparing growth across different-sized organs.

35. Write the name of phytohormone: (a) Made up of terpene (b) Fits both promoter and inhibitor categories (c) Studied by Charles Darwin (d) Occurs in rapid cell division areas (e) Known as antitranspirant

Solution:

| (a) Terpene-derived | Gibberellin (diterpenoid structure, 4 isoprene units, ent-gibberellane skeleton) | | (b) Dual role (promoter/inhibitor) | Ethylene (promotes ripening/senescence, inhibits stem elongation in dicots) | | (c) Studied by Darwin | Auxin (coleoptile phototropism experiments, 1880) | | (d) Rapid division areas | Cytokinin (root apices, shoot buds, developing fruits, embryos) | | (e) Antitranspirant | ABA (Abscisic Acid) (induces stomatal closure, reduces transpiration by 50-70%) |

Note: ABA is also terpenoid (sesquiterpenoid), but the classical terpene hormone is gibberellin.

36. (a) Derivation of term “auxin” (b) Explain how auxin was discovered

Solution:

(a) Etymology: “Auxin” derives from the Greek word “auxein” (αὔξειν) meaning “to grow” or “to increase”.

Coined by Frits Went (1928) after isolating the growth-promoting substance from oat coleoptile tips.

(b) Discovery of Auxin:

Historical Timeline:

1. Charles Darwin & Francis Darwin (1880):

• Observed phototropism in canary grass (Phalaris canariensis) coleoptiles
• Key Experiment:
  • Coleoptile tip removed → No bending toward light
  • Tip covered with opaque cap → No bending (stimulus not perceived)
  • Tip covered with transparent cap → Normal bending
  • Base covered, tip exposed → Normal bending
• Conclusion: Stimulus perceived by tip, transmitted downward to cause bending in subapical region

2. Boysen-Jensen (1913):

• Placed mica sheet on shaded side of coleoptile → Normal bending
• Mica sheet on illuminated side → No bending
• Conclusion: Growth substance moves down the dark side

3. Paál (1919):

• Removed coleoptile tip, placed it asymmetrically on decapitated stump
• Stump bent toward the side without tip
• Conclusion: Chemical substance diffuses from tip, causes unequal growth

4. Frits Went (1928):

• Agar block method:
  • Placed coleoptile tips on agar blocks
  • Chemical diffused into agar
  • Agar placed asymmetrically on decapitated coleoptile
  • Curvature proportional to auxin concentration
• Developed auxin bioassay (Avena curvature test)

5. Kögl & Haagen-Smit (1931):

• First isolated auxin from human urine (inactive metabolite)
• Named it “auxin-a” (later found to be heterogeneous mixture)

6. Kögl (1934):

• Isolated pure auxin from oat (Avena sativa) seeds
• Identified as Indole-3-acetic acid (IAA)
• Chemical structure confirmed

Significance: This discovery established the field of plant hormones and demonstrated chemical coordination in plants.

37. (a) Meaning of apical dominance (b) Effect of removing apical buds in tea plants (c) Role of auxin in cell division (d) Hormone counteracting apical dominance

Solution:

(a) Apical Dominance: The phenomenon whereby the presence of the apical (terminal) bud inhibits the growth of lateral (axillary) buds on the same shoot.

Mechanism:

• Auxin from shoot apex moves basipetally
• Suppresses lateral bud outgrowth through:
  • Diverting nutrients to apical meristem
  • Inhibiting cytokinin synthesis in lateral buds
  • Promoting abscisic acid accumulation locally

(b) Tea Plant Pruning: When apical buds are removed (decapitation/pruning):

• Lateral buds activate and develop into branches
• Bushy growth occurs instead of tall single stem
• Increased leaf production:
  • More harvest points
  • Higher tea yield (commercial advantage)
  • Better quality leaf flush

Agricultural Practice: Regular “tipping” maintains productive bush architecture.

(c) Auxin in Cell Division: Auxin promotes cell division in:

• Vascular cambium (secondary growth initiation)
• Cork cambium formation
• Callus tissue (in combination with cytokinin)
• Root initiation from stem cuttings

Molecular Mechanism:

• Activates cyclin-dependent kinases (CDKs)
• Promotes G1 → S phase transition in cell cycle
• Induces expression of cell cycle genes

(d) Hormone Overcoming Apical Dominance: Cytokinin

Mechanism:

• Applied to axillary buds to stimulate outgrowth
• Antagonizes auxin effects locally
• Promotes nutrient mobilization to treated buds
• Used in tissue culture for shoot multiplication
• Commercial application in ornamental plants for branching

38. Explain: (a) Bolting (b) Gibberellin stimulates seed germination (c) Most intensively studied GA form

Solution:

(a) Bolting: Definition: Rapid stem elongation in rosette plants just before flowering.

Characteristics:

• Rosette plants (cabbage, beet, radish, spinach) have:
  • Suppressed internodal growth vegetatively
  • Leaves form compact basal rosette
• Bolting Process:
  • Gibberellin application or favorable photoperiod triggers:
    • Internodal cell elongation (cells grow 10-100× longer)
    • Stem height increase from cm to meters
  • Transition from vegetative to reproductive phase
• Mechanism:
  • GA activates expansins (cell wall loosening proteins)
  • Stimulates xyloglucan endotransglycosylase (cell wall remodeling)
  • Increases cell wall plasticity

Agricultural Significance:

• Desirable in seed crops (facilitate harvesting)
• Undesirable in leafy vegetables (reduces quality)

(b) Gibberellin Stimulates Seed Germination:

Mechanisms:

1. Hydrolytic Enzyme Synthesis:
   • GA₃ induces α-amylase gene expression in aleurone layer
   • Proteases and lipases synthesis activated
   • Enzymes break down stored reserves:
     • Starch → Maltose → Glucose
     • Proteins → Amino acids
     • Lipids → Fatty acids + Glycerol
2. Mobilization of Reserves:
   • Nutrients transported to embryo
   • Provides energy (ATP via respiration)
   • Supplies building blocks for growth
3. Overcoming Dormancy:
   • Breaks seed coat-imposed dormancy
   • Substitutes for cold/light requirements
   • Used commercially to standardize germination

Classical Experiment: Barley aleurone layer + GA₃ → α-amylase secretion (demonstrated by Paleg, 1960)

(c) Most Studied Gibberellin: GA₃ (Gibberellic Acid)

Reasons:

• First discovered from Gibberella fujikuroi fungus (1930s, “bakanae” disease)
• Commercially available and inexpensive
• Highly bioactive across species
• Stable compound for research
• Standard in bioassays and agricultural applications

Note: Over 136 gibberellins (GA₁-GA₁₃₆) have been identified, but GA₃ remains most utilized.

39. (a) Discovery of cytokinin (b) Where is cytokinin found? (c) Which hormone helps cytokinin in cell division?

Solution:

(a) Cytokinin Discovery:

Timeline:

1. F. Skoog (1940s-1950s):

• Studying tobacco pith callus cultures
• Found coconut milk and yeast extract promoted cell division
• Effect beyond auxin alone

2. Miller, Skoog, Strong (1955):

• Tested autoclaved herring sperm DNA
• Powerful cytokinesis-promoting activity
• Isolated active compound:
  • Named Kinetin (6-furfurylaminopurine)
  • Synthetic artifact from DNA degradation
  • Not naturally occurring in plants

3. Letham (1964):

• Isolated natural cytokinin from corn (Zea mays) kernels
• Named Zeatin [(E)-4-hydroxy-3-methylbut-2-enylaminopurine]
• Confirmed widespread natural occurrence

4. Further discoveries:

• Found in coconut milk (liquid endosperm)
• tRNA contains cytokinin derivatives
• Roots are major synthesis sites

(b) Cytokinin Locations: Synthesis Sites:

• Root apical meristems (primary source)
• Developing seeds (high concentrations)
• Young fruits (especially immature coconuts)
• Shoot apical buds (local synthesis)
• Germinating seeds

Transport: Moves acropetally (root-to-shoot) via xylem sap

(c) Hormone Synergizing with Cytokinin: Auxin

Synergistic Effects:

• Both required for sustained callus proliferation
• Cell division maximized with optimal auxin:cytokinin ratio
• Tissue culture applications depend on this interaction

Ratio Effects:

40. (a) Where is ethylene synthesized? (b) Source compound for ethylene (c) Effect of ethylene on unripe fruits (d) How ethylene increases absorption surface

Solution:

(a) Ethylene Synthesis Sites: Nearly all plant organs, with maximum production in:

• Ripening fruits (climacteric fruits especially: banana, mango, tomato, apple)
• Senescing tissues (aging leaves, fading flowers, abscission zones)
• Nodes of stems
• Wounded tissues (injury-induced synthesis)
• Germinating seeds
• Root tips (under stress)

(b) Ethylene Precursor: Ethephon [(2-Chloroethyl)phosphonic acid]

Characteristics:

• Synthetic compound: ClCH₂CH₂PO₃H₂
• Decomposes above pH 4.0 to release ethylene:

  ClCH₂CH₂PO₃H₂ → C₂H₄ (ethylene) + Cl⁻ + H₃PO₄

• Advantages:
  • Stable in solution
  • Easy application (spray/drench)
  • Controlled ethylene release

Natural Biosynthesis:

Methionine → SAM → ACC → Ethylene
           (S-adenosylmethionine) (1-aminocyclopropane-1-carboxylic acid)

(c) Effect on Unripe Fruits: Ethylene treatment causes rapid ripening through:

1. Color Change:

• Chlorophyll degradation (degreening)
• Carotenoid synthesis (yellow/orange/red pigments)
• Anthocyanin accumulation (red/purple colors)

2. Softening:

• Pectinase activation (cell wall degradation)
• Cellulase activity increases
• Texture changes from firm to soft

3. Sweetening:

• Starch hydrolysis to sugars
• Acid reduction (malate, citrate decrease)
• Sugar:acid ratio increases

4. Aroma Development:

• Volatile ester synthesis (banana: isoamyl acetate)
• Flavor compound production

(d) Ethylene Increases Absorption Surface:

Mechanisms:

1. Root Hair Formation:
   • Stimulates root hair initiation from epidermal cells
   • Increases root surface area by 5-10 fold
   • Enhances water and nutrient absorption
2. Root Hair Elongation:
   • Promotes tip growth of existing root hairs
   • Extends absorption zone
3. Lateral Root Production:
   • Induces lateral root primordia
   • Creates branched root architecture
   • Maximizes soil exploration

Ecological Significance: Stress-induced ethylene production helps plants adapt to nutrient-poor soils by enhancing absorption capacity.

41. (a) Which hormone induces seed dormancy? (b) Effect of minute ABA on leaves (c) Why is ABA called inhibitor B?

Solution:

(a) Dormancy-Inducing Hormone: ABA (Abscisic Acid)

Mechanisms:

• Prevents premature germination in developing seeds
• Maintains desiccation tolerance during seed maturation
• Inhibits embryo growth until favorable conditions
• Synthesizes storage proteins instead of growth proteins

Types of ABA-mediated Dormancy:

• Coat-imposed: ABA in seed coat prevents water uptake
• Embryo dormancy: ABA in embryo blocks germination
• Chemical dormancy: ABA inhibits GA synthesis/action

(b) ABA Application on Leaves: When minute quantities (micromolar concentrations) of ABA are applied:

1. Stomatal Closure:

• Rapid response (within minutes)
• Mechanism:
  • ABA binds to guard cell receptors
  • Triggers K⁺ efflux from guard cells
  • Water follows by osmosis
  • Turgor loss → stomatal closure

2. Transpiration Reduction:

• 50-70% decrease in water loss
• Conserves soil moisture
• Reduces irrigation requirements

3. Stress Tolerance:

• Increases drought resistance
• Enhances heat stress tolerance
• Improves water use efficiency

Commercial Application: Used as antitranspirant in:

• Transplanting operations
• Ornamental plant shipping
• Drought management

(c) “Inhibitor B” Designation:

Historical Context:

• Independently discovered by Ohkuma (1963) during dormancy studies
• Called “Inhibitor-B” because it:
  • Acts as general growth inhibitor (opposes auxin, GA, cytokinin)
  • Inhibits plant metabolism in various processes
  • Suppresses seed germination, shoot growth, cell division

Other Early Names:

• Dormin (dormancy inducer – Eagles & Wareing)
• Abscission II (promotes abscission – Addicott)

Modern Understanding: While originally characterized as inhibitor, ABA is now recognized as crucial stress response hormone with positive adaptive roles.

42. (a) Synergistic vs. antagonistic interactions of auxin and cytokinin (b) Two hormones promoting abscission (c) Four events with multiple PGR interactions

Solution:

(a) Auxin-Cytokinin Interactions:

Synergistic (Work Together): Cell Division:

• Both required for sustained mitosis
• Neither alone sufficient for callus proliferation
• Combined action drives cell cycle progression
• Used in tissue culture media (MS medium: NAA + BAP)

Antagonistic (Oppose Each Other): Apical Dominance:

Application: Pruning (removes auxin source) + cytokinin spray = maximum branching

(b) Abscission-Promoting Hormones:

1. Ethylene (primary abscission hormone)
2. ABA (Abscisic Acid)

Mechanisms:

• Induce cellulase and pectinase in abscission zone
• Degrade middle lamella
• Weaken cell wall connections
• Trigger programmed cell death in abscission layer

(c) Four Multi-PGR Events:

1. Seed/Bud Dormancy:

• ABA: Induces/maintains dormancy
• GA: Breaks dormancy, promotes germination
• Ethylene: Can break dormancy in some seeds
• Cytokinins: Overcome dormancy in lettuce seeds

2. Abscission:

• Ethylene: Primary promoter (leaf, flower, fruit drop)
• ABA: Accelerates senescence/abscission
• Auxin: Prevents abscission when high (young organs)
• Cytokinins: Delay senescence, reduce abscission

3. Senescence:

• Cytokinins: Delay/prevent senescence (Richmond-Lang effect)
• Ethylene: Promotes senescence (aging hormone)
• ABA: Accelerates senescence
• Auxin/GA: Slow senescence in some organs

4. Apical Dominance:

• Auxin: Establishes/maintains dominance
• Cytokinins: Release lateral buds
• Strigolactones: Reinforce auxin’s suppression
• GA: Can modify dominance patterns

Principle: Most developmental processes involve hormonal balance rather than single hormone action.

43. (a) Meaning and example of plasticity (b) Requirements for breaking seed dormancy

Solution:

(a) Plasticity:

Definition: The ability of plants to alter developmental pathways and produce different structures/forms in response to environmental signals.

Characteristics:

• Phenotypic flexibility without genetic change
• Adaptive responses to conditions
• Modular construction allows variable morphology

Example: Heterophylly

Definition: Production of different leaf forms on the same plant.

Classic Examples:

Mechanisms:

• Developmental stage changes (juvenile → adult transition)
• Environmental cues (light quality/intensity, humidity)
• Hormonal gradients (changing auxin:cytokinin ratios)

Adaptive Value:

• Juvenile leaves optimized for rapid growth
• Adult leaves optimized for reproduction/stress tolerance

(b) Seed Dormancy Breaking Requirements:

Three Essential Factors:

1. Water (Imbibition):

• Rehydrates desiccated tissues
• Activates enzymes (stored in inactive state)
• Initiates metabolism
• Typically requires 30-50% water uptake
• Turgor pressure drives radicle emergence

2. Oxygen:

• Required for aerobic respiration
• Generates ATP for growth processes
• Enables biosynthesis of proteins, nucleic acids
• Germination fails in waterlogged (anoxic) soils
• Respiration rate increases 10-100 fold during germination

3. Suitable Temperature:

• Provides activation energy for enzymes
• Optimal range: 25-30°C for most species
• Species-specific:
  • Lettuce: 15-20°C
  • Tomato: 20-30°C
  • Rice: 30-35°C
• Stratification (cold treatment) may be required for some seeds

Additional Factors (Species-Dependent):

• Light: Lettuce, tobacco (phytochrome-mediated)
• Scarification: Hard seed coats (mechanical/chemical breakdown)
• After-ripening: Time-dependent changes
• Gibberellin: Breaks dormancy in barley, wheat

44. Draw a well-labeled diagram showing seed germination.

Solution:

Seed Germination Process – Labeled Diagram:

Stage 1 – Imbibition:

• Seed coat (testa) softens
• Water uptake begins
• Radicle swells

Stage 2 – Radicle Emergence:

• Radicle breaks through seed coat
• Positive geotropism – grows downward
• Primary root establishes
• Forms root hairs for absorption

Stage 3 – Hypocotyl Elongation (Epigeal Germination):

• Hypocotyl (below cotyledons) elongates
• Forms hypocotyl hook (protects plumule)
• Pushes upward through soil
• Pulls cotyledons above soil line

Stage 4 – Epicotyl Development:

• Epicotyl (above cotyledons) extends
• Plumule develops into shoot system
• Cotyledons spread and photosynthesize (in some species) or wither
• True leaves emerge
• Epicotyl hook straightens

Structures Labeled:

• Seed coat (testa)
• Cotyledons (seed leaves – 2 in dicots)
• Hypocotyl (embryonic stem below cotyledons)
• Epicotyl (embryonic stem above cotyledons)
• Radicle (embryonic root)
• Primary root
• Root hairs
• True leaves (first photosynthetic leaves)
• Soil line (demarcation point)

Note: Monocots show hypogeal germination (cotyledons remain underground).

45. (a) What are day-neutral plants? Give example. (b) Flowering hormone (c) Low-temperature perception sites (d) Light/dark perception site

Solution:

(a) Day-Neutral Plants:

Definition: Plants that flower independently of photoperiod (day length). Flowering is not regulated by the relative duration of light and dark periods.

Characteristics:

• No critical photoperiod requirement
• Autonomous flowering pathway predominates
• Flower throughout the year if other conditions favorable
• Temperature, age, nutrients are primary flowering triggers

Examples:

Agricultural Advantage: Can be cultivated in multiple seasons without photoperiodic constraints.

(b) Flowering Hormone: Florigen (Flowering Locus T protein/FT protein)

Characteristics:

• Hypothetical for decades, identified in 2000s as FT protein
• Synthesized in leaves in response to photoperiod
• Translocated via phloem to shoot apex
• Protein complex moves through plasmodesmata
• Triggers floral meristem identity genes (AP1, LFY)

(c) Low-Temperature Perception Sites: Two primary locations:

1. Stem Apex (Shoot Apical Meristem):
   • In winter annuals and biennials
   • Direct vernalization of growing apex
2. Embryo (in Seeds):
   • During seed development or early germination
   • Imbibed seeds vernalize more effectively

Molecular Mechanism:

• Cold induces VRN genes (vernalization genes)
• Represses FLC (FLOWERING LOCUS C), a floral repressor
• Chromatin remodeling maintains “vernalized” state

(d) Photoperiod Perception Site: Leaves (mature, photosynthetically competent leaves)

Mechanism:

1. Phytochrome in leaves measures day/night length
2. Circadian clock integration
3. Critical night length determination
4. FT gene expression (in LDP during short nights)
5. FT protein transport to apex
6. Floral induction at shoot apex

Classic Experiment: Grafting experiments (Knott, Hamner) showed single induced leaf can trigger flowering in entire plant.

SECTION B: Model Test Paper

(This section contains additional practice questions following the same format. Due to length constraints, I’ll provide selected high-value solutions.)

Solutions from Section B

Question on Growth Curve Expression

Expression: Lt = L₀ + rt

(a) Type: Arithmetic Growth

(b) Parameter Meanings:

Characteristic: Linear relationship – when plotted, produces a straight line (unlike exponential growth’s curve).

Oxygen as Essential Growth Factor

Explanation:

Role in Growth:

1. Aerobic Respiration:
   • All higher plants require oxygen for cellular respiration
   • Mitochondrial oxidative phosphorylation produces ATP
   • Energy yield: 36-38 ATP per glucose (vs. 2 ATP in anaerobic)
2. Biosynthetic Energy:
   • Anabolic processes (protein synthesis, cell wall formation, DNA replication) require ATP
   • Oxygen availability determines respiratory energy generation rate
   • Limited O₂ → reduced growth rate
3. Root Growth Sensitivity:
   • Roots especially sensitive to oxygen deprivation
   • Waterlogged soils (hypoxic/anoxic conditions) severely limit root growth
   • Aerenchyma formation in aquatic plants is adaptive response

Quantitative Effect:

• Optimal O₂ (21% atmosphere) → maximum growth
• 5% O₂ → 50-70% growth reduction
• <1% O₂ → growth cessation, anaerobic stress

Climacteric Fruits

Climacteric fruits are fleshy fruits exhibiting a sharp, sudden increase in respiration rate and ethylene production at the onset of ripening.

Characteristics:

Practical Significance:

• Harvested unripe for shipping
• Ethylene treatment induces synchronized ripening
• Storage: Reduce ethylene/O₂ to slow ripening

Summary of Concepts

Essential Definitions for NEET

Growth Measurement Comparison

Hormone Classification Table

Exam Strategy Tips

For NEET/Board Exams:

1. Memorize formulas: Arithmetic growth (Lt = L₀ + rt), Geometric growth (Wt = W₀e^rt), RGR formula
2. Diagram practice: Seed germination, sigmoid curve, auxin discovery experiments
3. Differentiate clearly: Primary vs. secondary growth, arithmetic vs. geometric growth, photoperiodism types
4. Hormone applications: Know commercial uses (2,4-D as herbicide, ethylene in ripening, GA in brewing)
5. Exception awareness: Ethylene is only gaseous PGR, ABA called stress hormone despite name

Example: A seedling increasing from 5 cm to 20 cm shows growth, while the transformation of shoot apex into a flower demonstrates development.

Q. What are the five plant growth hormones (phytohormones) and their main functions?

The five major plant growth regulators are:

Q. What is the difference between arithmetic and geometric growth in plants?

Most plant organs initially show geometric growth (rapid exponential phase) but transition to arithmetic growth at maturity when resources become limiting.

Q. What is photoperiodism and what are the three types of plants based on photoperiod?

Photoperiodism is the physiological response of plants to the relative duration of light and dark periods in a 24-hour cycle, particularly affecting flowering.

Three Plant Categories:

1. Short-Day Plants (SDP):

• Flower when day length is shorter than critical photoperiod (actually need long nights)
• Examples: Tobacco, soybean, chrysanthemum, rice, strawberry
• Critical factor: Require uninterrupted long dark period

2. Long-Day Plants (LDP):

• Flower when day length is longer than critical photoperiod (need short nights)
• Examples: Wheat, spinach, radish, lettuce, barley
• Season: Typically summer bloomers

3. Day-Neutral Plants (DNP):

• Flowering independent of photoperiod
• Examples: Tomato, cucumber, cotton, sunflower
• Flower based on age, temperature, nutrition

Mechanism:

• Phytochrome (photoreceptor) measures day/night length in leaves
• Florigen (FT protein) produced in leaves, transported to shoot apex
• Induces floral meristem identity genes for flowering

Q. What is vernalization and why is it important in agriculture?

Vernalization is the requirement of prolonged exposure to low temperature (0-10°C) for a plant to attain flowering competence.

Process:

• Cold treatment (typically 4-8 weeks at 0-5°C)
• Perceived by stem apex or seed embryo
• Induces molecular changes (VRN gene activation, FLC repression)
• Quantitative response: Longer cold = earlier/more flowering

Examples of Plants Requiring Vernalization:

• Winter wheat, barley, rye
• Cabbage, beet, carrot (biennials)
• Certain apple and pear varieties

Agricultural Importance:

1. Prevents Premature Flowering:
   • Winter annuals won’t flower in fall (would die in winter)
   • Ensures flowering in favorable spring conditions

2. Crop Timing:
   • Allows winter wheat to be sown in fall, harvested in summer
   • Maximizes growing season utilization

3. Biennial Management:
   • First year: vegetative growth (edible parts like cabbage head)
   • Second year (after vernalization): flowering and seed production

Practical Application:

• Seeds can be artificially vernalized by cold-moist treatment before planting
• Converts winter varieties into spring varieties

Reversal: Devernalization occurs if plants exposed to high temperatures after vernalization.

Q. How do auxins and cytokinins work together and against each other in plant growth?

Auxins and cytokinins show both synergistic (cooperative) and antagonistic (opposing) interactions depending on the process.

SYNERGISTIC EFFECTS (Work Together):

1. Cell Division:

• Both required for sustained mitotic activity
• Used together in tissue culture media
• Neither alone sufficient for callus proliferation
• Optimal ratio: Varies by tissue and species

Tissue Culture Applications:

ANTAGONISTIC EFFECTS (Work Against Each Other):

1. Apical Dominance:

Mechanism:

• Auxin from shoot tip moves basipetally, inhibiting lateral bud outgrowth
• Cytokinin applied to axillary buds overcomes this suppression
• Removing apex (pruning) eliminates auxin source → lateral buds activate

2. Senescence:

• Auxin: Delays senescence in some organs
• Cytokinin: Strongly delays senescence (Richmond-Lang effect)
• Work through different pathways

Practical Applications:

Agriculture:

• Tea/coffee pruning: Remove apex (↓auxin) → bushy growth
• Ornamental plants: Cytokinin spray → more branches → fuller appearance
• Rooting cuttings: High auxin + low cytokinin → root formation