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PLANT GROWTH AND DEVELOPMENT — Study Material, Revision Notes & PDF Guide

Comprehensive chapter-wise study notes, key formulas, conceptual summaries, and practice resources for PLANT GROWTH AND DEVELOPMENT under the Biology syllabus of NEET UG 2027.

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Chapter 13: PLANT GROWTH AND DEVELOPMENT – NEET UG Complete Master Notes (NCERT Biology Class 11)





1. Introduction to Plant Growth and Development



Development in higher plants is the orderly, progressive sum of two coordinated biological phenomena: Growth and Differentiation.



$$\text{Development} = \text{Growth} + \text{Differentiation}$$




The Life Cycle Trajectory of a Higher Flowering Plant:

Zygote (Single Fertilized Cell)

   │

   ▼

Embryogenesis & Seed Formation

   │

   ▼

Seed Dormancy (Quiescence under unfavourable environmental regimes)

   │

   ▼ [Favourable moisture, oxygen, temperature]

Seed Germination (Emergence of radicle and plumule)

   │

   ▼

Vegetative Growth Phase (Roots, stem, branches, foliage leaves)

   │

   ▼

Reproductive / Flowering Phase (Inflorescences, flowers, pollination, fertilization)

   │

   ▼

Fruiting & Seed Set

   │

   ▼

Senescence (Programmed functional decline)

   │

   ▼

Death


1.1 Dual Regulatory Control of Development



The precise execution of plant development is governed by an interlocking network of two regulatory forces:



  1. Intrinsic (Internal) Factors:


  • Intracellular Controls: Genetic constitution and genomic expression programmes encoded within the DNA of cell nuclei.
  • Intercellular Controls: Biochemical signalling molecules termed Plant Growth Regulators (PGRs) or phytohormones (such as auxins, gibberellins, cytokinins, abscisic acid, and ethylene).


  1. Extrinsic (External / Environmental) Factors:


  • Light (intensity, duration, and wavelength), Temperature, Water availability, Oxygen levels, and Soil nutrients (mineral macro- and microelements).




2. Plant Growth Characteristics & Meristems



2.1 Biological Definition of Growth



Growth is universally regarded as one of the most fundamental, conspicuous, and defining attributes of living organisms.



  • Definition: Growth is an irreversible, permanent increase in the size, volume, surface area, or mass of an organ, part of an organ, or even an individual isolated cell.
  • Metabolic Cost: Biological growth is perpetually accompanied by energetic and metabolic processes, involving both anabolism (synthetic reactions) and catabolism (degradative reactions to generate ATP and metabolic intermediates).
  • Physical Swelling vs Biological Growth: The imbibitional swelling of a dry wooden door or seed when immersed in water is a physical, reversible phenomenon lacking active cellular metabolism; hence, physical swelling alone is not true biological growth.




2.2 Plant Growth is Indeterminate & Open



Unlike animals, which exhibit determinate (closed) body plans with fixed adult dimensions, plants exhibit unique growth characteristics:



  • Indeterminate Growth: Plants retain the singular capacity for virtually unlimited, open-ended growth throughout their entire lifespan.
  • Cytological Basis: This perpetual growth potential is due to the persistence of localized, permanently embryonic cell populations called Meristems. Meristematic cells divide continuously, self-perpetuating their initial pool while producing daughter cells that mature into permanent plant tissues.
  • Open Form of Growth: Because new cohorts of cells are continually and indefinitely added to the plant body through meristematic activity, plant growth is defined as having an open form of growth.




Spatial Classification of Meristems in Seed Plants:

Meristems

├── 1. Primary Meristems (Originate from promeristem; mediate PRIMARY GROWTH / elongation):

│   ├── Root Apical Meristem (RAM): Located at root tips; drives downward axial extension into soil.

│   ├── Shoot Apical Meristem (SAM): Positioned at stem apex; produces leaves and stem axis.

│   └── Intercalary Meristem: Internodal / nodal bases (grasses); regenerates grazed grass blades.

│

└── 2. Secondary (Lateral) Meristems (Appear later in dicots & gymnosperms; mediate SECONDARY GROWTH):

    ├── Vascular Cambium (Fascicular + Interfascicular): Produces secondary xylem and secondary phloem.

    └── Cork Cambium (Phellogen): Produces protective phellem (cork) and phelloderm (secondary cortex).

    └── [Result: Progressive radial thickening and increase in girth of stems and roots]




3. Measurability and Quantitative Parameters of Growth



At the fundamental cellular level, biological growth is principally a consequence of an increase in the amount of protoplasm.
Because the direct measurement of living protoplasmic mass is technically difficult in intact tissues, growth is quantified through diverse measurable morphological and physical parameters directly proportional to protoplasmic expansion:





Growth Parameters and Representative Botanical Examples:

Parameter Measured              Botanical System Exemplifying Parameter

─────────────────────────────────────────────────────────────────────────────────────────────

Cell Number Increase            Maize root apical meristem generates > 17,500 new cells per hour

Individual Cell Volume / Size   Watermelon pulp cells expand up to 3,50,000 times original size

Linear Dimension / Length       Growth of the pollen tube through the style towards the ovule

Surface Area Expansion          Expansion of dorsiventral dicotyledonous leaf blades

Fresh Weight & Dry Weight       Accumulation of stored biomass during seedling establishment


[!NOTE] Fresh Weight vs Dry Weight in Growth Measurement: - Fresh Weight: Highly variable due to transient fluctuations in tissue hydration, cell turgor, and ambient humidity. - Dry Weight: Determined after dehydrating tissues at $70^\circ\text{C}$ to $80^\circ\text{C}$ until constant weight is reached. Dry weight accurately quantifies accumulated structural organic matter and is the most reliable scientific index of growth.




4. The Three Phases of Growth



When examining an actively elongating plant organ (such as a primary root or shoot apex), three continuous, overlapping developmental zones or phases of growth are recognized in basipetal sequence from apex to base:





Zonation of Growth in a Growing Root Apex:

[Root Tip Apex]

      │

      ▼

┌────────────────────────────────────────────────────────────────────────────────────────┐

│ 1. PHASE OF CELL DIVISION (MERISTEMATIC ZONE):                                         │

│    - Actively dividing isodiametric initials and derivatives.                          │

│    - Extremely rich, dense protoplasm; prominent, large, conspicuous nuclei.            │

│    - Thin, primary, cellulosic cell walls lacking secondary thickening.               │

│    - Abundant, dense plasmodesmatal bridges connecting adjacent protoplasts.           │

└────────────────────────────────────────────────────────────────────────────────────────┘

      │

      ▼

┌────────────────────────────────────────────────────────────────────────────────────────┐

│ 2. PHASE OF CELL ELONGATION (ELONGATION ZONE):                                         │

│    - Situated immediately proximal (behind) to the meristematic zone.                  │

│    - Rapid longitudinal cell extension; dramatic increase in cell vacuolation.         │

│    - Deposition of new non-cellulosic matrix and cellulose microfibrils in cell walls.  │

│    - Responsible for the tremendous, rapid elongation of the root through soil.        │

└────────────────────────────────────────────────────────────────────────────────────────┘

      │

      ▼

┌────────────────────────────────────────────────────────────────────────────────────────┐

│ 3. PHASE OF CELL MATURATION (DIFFERENTIATION ZONE):                                    │

│    - Situated proximal to the elongation zone.                                         │

│    - Cells attain their ultimate, maximal dimensions and terminal functional identity. │

│    - Extensive secondary wall thickening (lignification, suberization) occurs.        │

│    - Protoplasmic modifications take place (e.g., loss of nuclei in tracheary elements)│

│    - Epidermal cells sprout unicellular root hairs for water and mineral absorption.   │

└────────────────────────────────────────────────────────────────────────────────────────┘


[!TIP] The Parallel Line Technique for Zone Detection: By marking equidistant parallel lines with waterproof India ink on a germinating root tip at $1\text{ mm}$ intervals, observation after 24 hours demonstrates that marks in the zone immediately behind the apex (Zone of Elongation) separate dramatically, confirming it as the site of maximal length extension.




5. Growth Rates and Mathematical Kinetics



The increased growth generated per unit time is defined as the Growth Rate. Organisms and plant tissues expand through two distinct kinetic mathematical models: Arithmetic Growth and Geometric Growth.





5.1 Arithmetic Growth



In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide, while the other daughter cell ceases division, differentiates, and matures into permanent tissue.



  • Kinetics: Constant, steady growth rate over time.
  • Graphical Representation: A plot of organ length against time produces a strictly straight linear curve.
  • Botanical Example: Elongation of a primary root or shoot axis growing at a constant speed.


$$\text{Mathematical Expression for Arithmetic Growth:}$$

$$L_t = L_0 + rt$$


Where:



  • Lt = Length / dimension of organ at time t
  • L0 = Initial length / dimension at time zero (t = 0)
  • r = Growth rate or elongation increment per unit time (slope of the linear plot)
  • t = Duration of growth time




5.2 Geometric (Exponential) Growth



In geometric growth, following mitotic cell division, both daughter cells retain the capacity to divide and continue proliferating exponentially.



  • Kinetics: Cell numbers double in geometric progression (1 \rightarrow 2 \rightarrow 4 \rightarrow 8 \rightarrow 16 \dots 2^n).
  • Botanical Example: Early stages of zygotic embryo development, unicellular alga multiplying in culture, or fresh bacterial inoculations.


$$\text{Mathematical Expression for Geometric Growth:}$$

$$W_1 = W_0 e^{rt}$$


Where:



  • W1 = Final size (weight, height, cell number, surface area) at time t
  • W0 = Initial size at the beginning of the growth period
  • r = Relative growth rate, also designated the Efficiency Index (measuring the intrinsic capacity of the plant system to generate new organic material)
  • t = Time of growth
  • e = Base of natural logarithms (\approx 2.71828)




5.3 The Classical Sigmoid (S-Shaped) Growth Curve



In natural biological systems, geometric growth cannot proceed unchecked indefinitely because space, nutrients, light, and water eventually become limiting, and toxic metabolic by-products accumulate.
Consequently, when the parameter of growth of an intact plant, organ, or cell population is plotted against time, a characteristic Sigmoid (S-shaped) Curve is generated.





The Classical Three-Phase Sigmoid Growth Curve:



Size / 

Biomass 

  ▲

  │                                     ┌───────────── Stationary Phase

  │                                    ╱                (Growth ceases / plateau)

  │                                  ┌┘

  │                                 ╱

  │                                ╱ ◄── Log / Exponential Phase

  │                               ╱       (Maximal rapid acceleration)

  │                             ┌┘

  │                            ╱

  │  Lag Phase ───────────────┘

  │  (Initial slow adaptation)

  └────────────────────────────────────────────────────────► Time (t)


The Sigmoid Curve comprises three distinct phases:



  1. Lag Phase: The initial phase of very slow growth where cells adapt, activate enzymes, and accumulate energy reserves prior to division.
  2. Log / Exponential Phase: Rapid, maximal, exponential proliferation where both cell division and cell enlargement operate at peak metabolic velocity with surplus nutrients.
  3. Stationary / Plateau Phase: Nutrient depletion, physiological limitation, and senescence slow growth down until net biomass addition reaches equilibrium (zero net growth).




5.4 Absolute Growth Rate (AGR) vs Relative Growth Rate (RGR)



Quantitative comparisons between growing biological structures require distinguishing total growth from relative growth efficiency:



  • Absolute Growth Rate (AGR): The measurement and comparison of total net growth achieved per unit time.


$$\text{AGR} = \frac{\Delta \text{Growth}}{\Delta t}$$


  • Relative Growth Rate (RGR): The growth increment per unit time expressed on a common baseline, specifically per unit of initial parameter.


$$\text{RGR} = \frac{\Delta \text{Growth}}{\Delta t \times \text{Initial Growth Parameter}}$$




NCERT Leaf Area Comparison Matrix (Leaf A vs Leaf B):

Attribute                       Leaf A (Small Leaf)         Leaf B (Large Leaf)

─────────────────────────────────────────────────────────────────────────────────────────────

Initial Surface Area            5 cm²                       50 cm²

Final Surface Area (after Δt)   10 cm²                      55 cm²

Absolute Growth Rate (AGR)      10 - 5 = 5 cm² per Δt       55 - 50 = 5 cm² per Δt  [Identical AGR]

Relative Growth Rate (RGR)      5 / 5 = 1.0 (100% increase) 5 / 50 = 0.1 (10% increase)

NEET Conclusion                 Both leaves show IDENTICAL Absolute Growth Rates (5 cm²),

                                but Leaf A exhibits a 10-fold HIGHER Relative Growth Rate!




6. Conditions Essential for Plant Growth



Plant growth cannot occur in a physical vacuum; it depends upon a suite of internal and external permissive factors:



  1. Water:


  • Cell Enlargement: Plant cell extension depends directly on water uptake creating hydrostatic turgor pressure that stretches the loosened primary cell wall.
  • Enzymatic Medium: Water provides the essential aqueous solvent matrix for all metabolic and biocatalytic reactions.


  1. Oxygen:


  • Essential for aerobic mitochondrial cellular respiration to oxidize organic substrates and generate high-energy ATP required for biosynthetic processes.


  1. Nutrients (Macro- and Micronutrients):


  • Carbon, nitrogen, phosphorus, sulfur, and trace minerals provide structural building blocks for protoplasm, membranes, nucleic acids, and enzymatic prosthetic groups.


  1. Temperature:


  • Every plant species possesses a specific cardinal temperature spectrum (minimum, optimum, and maximum). The optimum range, typically $20^\circ\text{C}$ to $35^\circ\text{C}$, ensures peak enzymatic catalytic rates without thermal denaturation.


  1. Light and Gravity:


  • Light drives photosynthesis and photomorphogenesis, while gravity directs the directional orientation of roots (positive geotropism) and shoots (negative geotropism).




7. Differentiation, Dedifferentiation and Redifferentiation



The life cycle of every vegetative plant cell involves transitions between dividing meristematic states and specialized non-dividing mature states:





Transitions in Plant Cell Developmental States:

[Meristematic Cell] (Actively dividing, undifferentiated)

        │

        ▼  [DIFFERENTIATION: Loses division power, undergoes structural specialisation]

[Primary Mature Cell] (e.g., Cortical parenchyma cell, tracheary element)

        │

        ▼  [DEDIFFERENTIATION: Re-acquires division capacity under specific stimulus]

[Secondary Meristem] (e.g., Interfascicular cambium, Cork cambium / phellogen)

        │

        ▼  [REDIFFERENTIATION: Divides, then progeny lose division power to mature again]

[Secondary Mature Tissue] (e.g., Secondary xylem, Secondary phloem, Cork / phellem)


7.1 Differentiation



  • Definition: The process whereby cells derived from root apical meristem, shoot apical meristem, or cambium undergo structural and biochemical modifications to achieve terminal functional specialization.
  • Example: Formation of tracheary elements (vessels and tracheids). The developing cell loses its protoplasm entirely (forming a hollow lumen) and deposits robust, elastic, lignocellulosic secondary wall thickenings to withstand extreme negative water tension.


7.2 Dedifferentiation



  • Definition: The phenomenon whereby living, fully differentiated cells that have lost the capacity to divide regain mitotic division potential under specific developmental or wounding triggers.
  • Examples:
  • Formation of interfascicular cambium from fully differentiated interfascicular medullary ray parenchyma cells.
  • Formation of cork cambium (phellogen) from mature collenchyma or parenchyma cells of the cortex.
  • Formation of unorganized callus from differentiated parenchymatous explants in plant tissue culture media.


7.3 Redifferentiation



  • Definition: The process wherein cells produced by secondary (dedifferentiated) meristems lose the ability to divide once again and mature to carry out permanent specialized functions.
  • Examples:
  • Derivatives of vascular cambium maturing into secondary xylem (wood) and secondary phloem.
  • Derivatives of cork cambium maturing into phellem (cork) externally and phelloderm (secondary cortex) internally.


[!IMPORTANT] Open Nature of Differentiation: Just as plant growth is open, differentiation in plants is also open, meaning cells originating from the exact same apical meristem adopt completely different structural fates depending upon their spatial position in the organ: - Cells pushed to the outer periphery differentiate into epidermal cells or root hairs. - Cells pushed toward the central interior differentiate into vascular elements. - Cells positioned distally at the extreme apex of the root meristem differentiate into protective root-cap cells.




8. Development and Plasticity



8.1 The Developmental Pathway of a Higher Plant Cell





Sequence of the Developmental Process in a Plant Cell:

Meristematic Cell

      │

      ▼

Plasmatic Growth (Synthesis of proteins, nucleic acids, and cell constituents)

      │

      ▼

Cell Division (Mitosis producing derivative cells)

      │

      ▼

Cell Expansion / Elongation (Vacuolation and turgor-driven wall enlargement)

      │

      ▼

Differentiation (Structural and chemical specialization)

      │

      ▼

Mature Cell (Performs physiological vegetative or transport function)

      │

      ▼

Senescence (Programmed deterioration of structures and metabolic decline)

      │

      ▼

Cell Death




8.2 Plasticity and Heterophylly



  • Plasticity: The intrinsic capacity of plants to alter their developmental pathways, growth habits, or structural morphology in response to diverse environmental cues or different phases of life.
  • Heterophylly: The occurrence of different types or shapes of leaves on the same individual plant. Heterophylly is the classic, high-yield botanical expression of plasticity.




Two Types of Heterophylly (Plasticity):

Heterophylly Modalities

├── 1. Intrinsic / Developmental Heterophylly (Phase-Dependent):

│   ├── Leaves of the juvenile seedling phase differ morphologically from mature adult leaves.

│   └── Classic NCERT Examples: Cotton (Gossypium), Coriander (Coriandrum), Larkspur (Delphinium).

│

└── 2. Extrinsic / Environmental Heterophylly (Habitat-Dependent):

    ├── Morphology changes drastically according to the surrounding physical medium.

    └── Classic NCERT Example: Buttercup (Ranunculus flabellaris / sceleratus):

        ├── Submerged Underwater Leaves: Highly dissected, ribbon-like, feathery (minimizes water resistance).

        └── Emergent Aerial Leaves: Broad, expanded, lobed or entire blades (maximizes sunlight capture).




9. Plant Growth Regulators (PGRs) – General Architecture



Plant Growth Regulators (also termed phytohormones or plant hormones) are small, simple organic molecules of diverse chemical constitution produced in minute concentrations that modulate physiological events.





Chemical Diversity and Biosynthetic Precursors of the Five Major PGRs:

PGR Class           Chemical Constitution               Biosynthetic Precursor

─────────────────────────────────────────────────────────────────────────────────────────────

Auxins              Indole derivatives (IAA, IBA)       Amino acid L-Tryptophan (requires Zn²⁺)

Cytokinins          Adenine (Purine) derivatives        Isopentenyl pyrophosphate (IPP) + Adenine

Gibberellins        Terpenes / Diterpenoids             Mevalonic acid / Geranylgeranyl pyrophosphate

Abscisic Acid (ABA) Carotenoid / Sesquiterpene deriv.   Violaxanthin / Carotenoid pathway

Ethylene            Simple gaseous olefin hydrocarbon   Amino acid L-Methionine


9.1 Functional Grouping of PGRs



Phytohormones are divided into two broad physiological camps:



  1. Plant Growth Promoters: PGRs primarily involved in growth-promoting activities such as cell division, cell enlargement, pattern formation, trophic growth, flowering, and fruit-set.


  • Representatives: Auxins, Gibberellins, and Cytokinins.


  1. Plant Growth Inhibitors: PGRs involved in responses to abiotic and biotic stresses, wounding, dormancy induction, and organ abscission.


  • Representative: Abscisic Acid (ABA).
  • Ethylene: A gaseous regulator that can participate in promotion (e.g., pineapple flowering, potato sprouting), but functions largely as a growth inhibitor and promoter of senescence and abscission.




10. Accidental Discoveries of the Five Major PGRs



Every single one of the five major classes of plant growth regulators was discovered through serendipitous observation:





Timeline of PGR Discoveries:

1. AUXIN:

   ├── Charles Darwin & Francis Darwin (1880): Observed canary grass (Phalaris canariensis) coleoptiles

   │   bending towards unilateral illumination (phototropism); concluded the tip transmits an influence.

   ├── Boysen-Jensen (1910-1913): Proved transmittable signal passes across gelatin/agar block, not mica plate.

   ├── Paal (1919): Demonstrated asymmetrical tip placement causes curvature in complete darkness.

   └── F.W. Went (1928): Isolated the active substance from oat (Avena sativa) coleoptile tips into agar;

       developed the standard Avena Curvature Bioassay; named the substance "Auxin" (Greek auxein = to grow).

       [Auxin was first chemically isolated from human urine of pellagra patients by Kögl and Haagen-Smit].



2. GIBBERELLIN:

   ├── "Bakanae" (Foolish Seedling) Disease of Rice: Infected seedlings grew abnormally tall, thin, and sterile.

   ├── Pathogen: Soil-borne ascomycete fungus Gibberella fujikuroi (Fusarium heterosporum).

   └── E. Kurosawa (1926): Filtered sterile fungal cultures; applied filtrate to uninfected seedlings,

       reproducing symptoms. Active compound subsequently crystallized and named Gibberellic Acid (GA).



3. CYTOKININ:

   ├── F. Skoog & Coworkers (1950s): Culturing tobacco stem pith parenchyma explants; observed callus proliferated

   │   only when auxin was supplemented with vascular tissue extract, yeast extract, coconut milk, or DNA.

   ├── Miller, Skoog, et al. (1955): Isolated and crystallized active cytokinesis-promoting purine from

   │   autoclaved herring sperm DNA; named it Kinetin (6-furfurylaminopurine). Kinetin is not natural to plants.

   └── D.S. Letham (1963): Isolated the first naturally occurring plant cytokinin, Zeatin, from immature

       corn kernels (Zea mays) and liquid endosperm of coconut.



4. ABSCISIC ACID (ABA):

   └── Mid-1960s: Three independent research groups simultaneously purified three distinct growth inhibitors:

       ├── Inhibitor-B (Bennet-Clark and Kefford)

       ├── Abscission II (Addicott and coworkers from cotton bolls)

       └── Dormin (Wareing and coworkers from Acer leaves)

       Chemical analysis proved all three were chemically identical; renamed Abscisic Acid (ABA).



5. ETHYLENE:

   ├── H.H. Cousins (1910): Confirmed that volatile gases released from ripe Jamaican oranges hastened the

   │   premature ripening of unripened bananas stored in close proximity.

   └── R. Gane (1934): Established analytically that the active ripening volatile gas is Ethylene (C2H4).




11. Comprehensive Physiological Effects & Applications of PGRs





11.1 Auxins



Auxins are predominantly synthesized in the actively growing apices of stems and roots (shoot and root apical meristems), exhibiting polar basipetal transport in stems (moving downwards from apex toward base).





Auxin Types and Chemical Classification:

Auxins

├── Natural Auxins (Endogenous plant compounds):

│   ├── Indole-3-Acetic Acid (IAA): Primary, most abundant natural auxin.

│   └── Indole-3-Butyric Acid (IBA): Endogenous auxin isolated from plants.

│

└── Synthetic Auxins (Man-made chemical analogues):

    ├── Naphthalene Acetic Acid (NAA)

    ├── 2,4-Dichlorophenoxyacetic acid (2,4-D)

    └── 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) [Component of Agent Orange]


Physiological Actions & Agricultural Uses of Auxins:



  1. Root Initiation in Cuttings: Application of synthetic auxins (IBA, NAA) to base of stem cuttings stimulates rapid, vigorous adventitious root formation; widely exploited in vegetative plant propagation and horticulture.
  2. Apical Dominance: The presence of the terminal apical shoot bud suppresses the growth and emergence of lateral (axillary) buds.


  • Decapitation Effect: Removal of the terminal shoot tip removes the endogenous auxin source, releasing lateral buds from inhibition and promoting dense bushy branching.
  • Commercial Applications: Extensively practiced in tea gardens (to maximize tender leaf plucking) and in ornamental hedge-trimming.


  1. Prevention vs Promotion of Abscission: Auxins exert a dual concentration-dependent influence:


  • Prevents premature drop of young developing leaves, floral buds, and immature fruits.
  • Promotes the abscission and falling of older, senescent mature leaves and ripe fruits by triggering abscission layer formation.


  1. Flower Induction: Induces uniform, synchronized flowering in pineapples and litchi.
  2. Parthenocarpy: Exogenous application of auxins induces seedless fruit development without fertilization in tomatoes.
  3. Selective Herbicide / Weedicide: 2,4-D is widely sprayed as a selective broad-leaved (dicot) weedkiller in monocotyledonous cereal crop fields (wheat, rice, maize) and golf lawns without harming mature monocot plants.
  4. Xylem Differentiation & Cambial Division: Controls the differentiation of procambium into tracheary elements and stimulates cell division in the vascular cambium.




11.2 Gibberellins



More than 100 distinct gibberellins (\text{GA}_1, \text{GA}_2, \text{GA}_3 \dots \text{GA}_{100+}) have been isolated from fungi and higher plants. All gibberellins are weakly acidic diterpenoid acids. $\text{GA}_3$ (Gibberellic acid) was the first to be structurally elucidated and remains the most intensively investigated form.



Physiological Actions & Agricultural Uses of Gibberellins:



  1. Stem and Axis Elongation: Stimulates dramatic subapical internodal cell elongation.


  • Grape Stalk Elongation: Exogenous $\text{GA}_3$ spray significantly lengthens grape flower pedicels, reducing bunch compactness and preventing fungal infections.
  • Fruit Shape Improvement: Elongates and enhances the symmetry and shape of apples (improves market grade).


  1. Sugarcane Yield Enhancement: Sugarcane stores sucrose in the parenchyma of stem internodes. Spraying sugarcane crops with gibberellins elongates stem internodes, increasing cane yield by up to 20 tonnes per acre.
  2. Bolting in Rosette Plants: Rosette plants (cabbage, beet, turnip) exhibit compressed internodes during vegetative growth. Gibberellin treatment induces rapid internodal elongation just prior to flowering, a phenomenon known as Bolting.
  3. Hastening Conifer Maturity: Spraying juvenile conifer seedlings with gibberellins shortens the juvenile vegetative phase, accelerating cone and early seed production in forestry tree-breeding programmes.
  4. Malting in Brewing Industry: $\text{GA}_3$ speeds up the malting process during barley grain germination by de-repressing genes in the aleurone layer to synthesize hydrolytic enzymes (α-amylase, proteases, ribonuclease) that mobilize starch into maltose.
  5. Delay of Senescence: Delays fruit rind senescence, allowing fruits to remain fresh on parent trees longer, thereby extending the commercial market window.




11.3 Cytokinins



Cytokinins exert profound, specific effects on cytokinesis (cytoplasmic division). Natural cytokinins are synthesized predominantly in plant zones exhibiting active mitotic division: root apical meristems, developing shoot buds, young leaves, and developing seeds/fruits.





Cytokinin Forms:

Cytokinins

├── Synthetic / Animal-Derived:

│   └── Kinetin (6-furfurylaminopurine): Isolated from autoclaved herring sperm DNA; not found in plants.

│

└── Natural Phytohormones:

    ├── Zeatin (6-(4-hydroxy-3-methylbut-trans-2-enylamino)purine): Discovered in Zea mays & coconut milk.

    └── Isopentenyladenine (IPA): Endogenous active plant purine derivative.


Physiological Actions & Agricultural Uses of Cytokinins:



  1. Promotion of Cell Division: Works obligatorily in concert with auxin to drive mitosis in cambial and meristematic tissues.
  2. Organogenesis (Morphogenesis in Tissue Culture):


  • High Cytokinin-to-Auxin ratio $\rightarrow$ Promotes Shoot differentiation (caulogenesis).
  • High Auxin-to-Cytokinin ratio $\rightarrow$ Promotes Root differentiation (rhizogenesis).
  • Intermediate / Balanced Auxin and Cytokinin ratio $\rightarrow$ Proliferation of undifferentiated Callus.


  1. Overcoming Apical Dominance: Exogenous application of cytokinins directly stimulates the outgrowth and branching of lateral axillary buds, counteracting the inhibitory influence of shoot apical auxin.
  2. Delay of Leaf Senescence (Richmond-Lang Effect): Cytokinins actively stimulate the mobilization of amino acids, minerals, and nutrients towards treated tissues, preserving chlorophyll content and delaying yellowing and foliar senescence.
  3. Chloroplast Development: Stimulates the transformation of proplastids into mature chloroplasts in leaves and promotes de novo leaf surface expansion.




11.4 Ethylene



Ethylene (\text{C}_2\text{H}_4) is a unique, simple, naturally occurring gaseous hydrocarbon phytohormone. It is synthesized in large quantities by tissues undergoing senescence and by climacteric ripening fruits.





The Classic Seedling "Triple Response" to Ethylene:

Ethylene Exposure on Germinating Etiolated Dicot Seedlings

├── 1. Inhibition of elongation with pronounced HORIZONTAL GROWTH of seedling axis.

├── 2. Radial swelling and lateral THICKENING OF THE HYPOCOTYL AXIS.

└── 3. Exaggerated APICAL HOOK FORMATION at the plumule tip to protect delicate meristem.


Physiological Actions & Agricultural Uses of Ethylene:



  1. Fruit Ripening & Respiratory Climacteric: Ethylene triggers rapid ripening in climacteric fruits (banana, mango, apple, tomato), accompanied by a dramatic surge in respiration rate called the Respiratory Climacteric.
  2. Abscission and Senescence: Accelerates the yellowing, senescence, and abscission of leaves, flowers, and ripe fruits.
  3. Breaking Dormancy: Breaks seed and vegetative bud dormancy; initiates germination in peanut seeds and triggers sprouting of potato tubers.
  4. Deep-Water Rice Internode Elongation: In flooded deep-water rice varieties (Oryza sativa), submergence stimulates ethylene synthesis, triggering rapid internodal and petiole elongation so foliage stays above water to prevent asphyxiation.
  5. Root Growth & Root Hair Production: Promotes adventitious rooting and stimulates tremendous proliferation of root hairs, vastly expanding the absorptive surface area.
  6. Flowering Induction: Promotes uniform flowering and fruit synchrony in pineapples, and induces off-season flowering in mango.
  7. Commercial Aqueous Agent (Ethephon):


  • Pure ethylene is an explosive, diffusable gas difficult to apply in fields. Farmers use Ethephon (2-chloroethylphosphonic acid).
  • Ethephon in aqueous spray is absorbed and translocated inside plant vascular tissues, slowly releasing ethylene gas at alkaline cellular pH.
  • Agricultural Applications: Hastens ripening of tomatoes and apples; promotes fruit thinning in cotton, cherry, and walnut; increases the proportion of female flowers in cucumbers, boosting fruit yield.




11.5 Abscisic Acid (ABA)



Abscisic Acid is a sesquiterpenoid derivative historically identified through its role in regulating abscission and bud dormancy. It acts as a general plant growth inhibitor and an inhibitor of plant cellular metabolism.



Physiological Actions & Agricultural Uses of Abscisic Acid:



  1. The "Stress Hormone" & Stomatal Regulation: Under severe water stress (drought), roots synthesize ABA and transport it through the xylem to leaves. ABA induces rapid efflux of $\text{K}^+$ and malate ions from guard cells, causing loss of turgor and immediate stomatal closure to minimize transpirational water loss.
  2. Seed Maturation, Desiccation Tolerance & Dormancy:


  • ABA accumulates during late embryogenesis, promoting the synthesis of heat-shock proteins and dehydration-protective osmoprotectants that allow seeds to withstand severe desiccation.
  • Imposes seed dormancy, preventing premature viviparous sprouting on the parent plant during wet weather.


  1. General Growth & Germination Inhibition: Inhibits seed germination by suppressing GA-induced transcription of α-amylase in aleurone layers.
  2. Antagonism with Gibberellins: In almost all major developmental checkpoints, ABA and Gibberellins act in diametrical opposition:


  • $\text{GA}$ breaks dormancy, promotes seed germination, promotes stem elongation, and delays senescence.
  • $\text{ABA}$ imposes dormancy, inhibits germination, retards elongation, and promotes senescence.
  • The physiological ratio of GA to ABA determines whether a seed remains dormant or germinates.




12. Plant Responses to Light & Temperature (Extrinsic Factors)





12.1 Photoperiodism



The physiological response of plants to the relative lengths of light (day) and dark (night) periods in a 24-hour cycle is known as Photoperiodism (discovered by Garner and Allard in 1920 in Maryland Mammoth tobacco).





Photoperiodic Categories of Flowering Plants:

Category                    Photoperiodic Requirement for Flowering                Representative Plants

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Short Day Plants (SDP)      Require photoperiod SHORTER than critical day length;   Soybean, Tobacco,

[Long Night Plants]         require uninterrupted, continuous dark period.         Chrysanthemum, Xanthium

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Long Day Plants (LDP)       Require photoperiod LONGER than critical day length;    Wheat, Radish, Spinach,

[Short Night Plants]        short dark periods permissive for flowering.            Sugarbeet, Henbane

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Day Neutral Plants (DNP)    Flowering is completely INDEPENDENT of day length;     Tomato, Cucumber,

                            flowers upon reaching developmental maturity.          Maize, Sunflower, Pea






Photoperiodic Dark-Interruption Dynamics in Short-Day Plants:

[Day Length < Critical Duration] + [Continuous Dark > Critical Night] ──► FLOWERING OCCURS

[Day Length < Critical Duration] + [Dark Interrupted by Brief Flash of Red Light] ──► NO FLOWERING!

Conclusion: Short-Day Plants are more accurately termed LONG-NIGHT PLANTS because an uninterrupted dark

duration is absolute and obligatory for flowering.


  • Perception Site: The primary organ that perceives the light/dark photoperiodic stimulus is the Leaf (not the shoot apex).
  • The Florigen Concept: Upon perception of inductive photoperiods in leaves, a transmissible chemical hormonal signal designated Florigen (associated with the protein product of the Flowering Locus T gene, FT protein) migrates through the phloem from leaves to the shoot vegetative apex, transforming it into a reproductive floral meristem.
  • Defoliated Plant Behavior: A plant completely stripped of all its leaves (defoliated) cannot perceive photoperiodic signals and fails to flower even when exposed to optimal photoperiods.




12.2 Vernalization



Certain plants require a qualitative or quantitative exposure to low temperature, chilling at $1^\circ\text{C}$ to $6^\circ\text{C}$, to induce, accelerate, or permit subsequent flowering. This chilling requirement is termed Vernalization (coined by T.D. Lysenko in 1928).



Ecological & Agronomic Significance:



  1. Winter vs Spring Varieties:


  • Spring Varieties: Sown in spring; flower and produce grain before the end of the growing season.
  • Winter Varieties: Sown in autumn; if sown in spring, they fail to flower within the growing season. Their seedling phase experiences the winter cold, which vernalizes the shoot apex, permitting normal flowering and grain harvest the following summer.
  • Classic examples: Wheat, Barley, and Rye.


  1. Biennial Plants:


  • Biennials (cabbage, sugarbeet, carrot) grow vegetatively in their first season, overwinter through cold temperatures (vernalization), and bolt, flower, and set seed in their second season.
  • Exogenous application of Gibberellins can substitute for the cold requirement, inducing premature bolting and flowering in biennials without cold treatment.


  1. Site of Cold Perception: The actively dividing meristematic cells of the Shoot Apical Meristem or embryo tips perceive cold temperatures, generating a postulated transmissible stimulus called Vernalin.




12.3 Seed Dormancy



Seed dormancy is an innate physiological or physical condition where viable seeds fail to germinate even when provided with permissive environmental conditions (favourable moisture, oxygen, and temperature).





Mechanisms and Remedies of Seed Dormancy:

Causative Mechanism                     Botanical Basis / Compounds             Remedial Method to Break Dormancy

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Hard, Impermeable Seed Coat             Thick suberin, lignin, cutin coats      Mechanical scarification (abrasion,

                                        (Legumes, Malvaceae)                    sandpaper) or concentrated H2SO4 dip

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Chemical Germination Inhibitors         Accumulation of Abscisic acid (ABA),    Leaching with running water, chilling,

                                        phenolic acids, para-ascorbic acid      or application of GA3 / Nitrates

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Immature / Rudimentary Embryo           Morphologically unorganized embryo      Stratification (moist chilling)

                                        requiring after-ripening period         allowing embryonic maturation




13. Master Comparison Table of the Five Major Plant Growth Regulators





Comprehensive Phytohormone Comparative Matrix:

Regulator       Chemical Family      Precursor       Major Synthesis Sites         Hallmark Physiological Roles

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

AUXIN           Indole               L-Tryptophan    Shoot & root apical           Apical dominance, rooting in

                derivatives                          meristems, young leaves       stem cuttings, 2,4-D weedicide

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

GIBBERELLIN     Acidic               Mevalonic acid  Young leaves, shoot apices,   Bolting in rosette plants,

                diterpenes                           germinating embryos           sugarcane yield (+20 t/acre)

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

CYTOKININ       Purine (Adenine)     IPP + Adenine   Root apices, active dividing  Overcomes apical dominance,

                derivatives                          buds, developing fruits       Richmond-Lang effect (delays senescence)

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

ETHYLENE        Gaseous olefin       L-Methionine    Ripening fruits, senescent    Respiratory climacteric, fruit

                hydrocarbon                          tissues, flooded roots        ripening, breaks seed/bud dormancy

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

ABSCISIC ACID   Sesquiterpene        Violaxanthin /  Leaves under water stress,    Stress hormone, closes stomata,

(ABA)           derivative           Carotenoids     maturing seeds, root caps     imposes seed dormancy, GA antagonist




14. NEET High-Yield Traps, Mnemonics & Misconceptions Matrix





NEET High-Yield Trap Matrix:

Concept / Question Trap         Common Student Misconception            Correct NEET Examination Fact

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Auxin in Root vs Shoot          Students assume auxins promote          High auxin concentrations PROMOTE shoot growth

Growth                          both root and shoot elongation.         but INHIBIT root elongation (roots are far

                                                                        more sensitive to auxin than shoots).

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Kinetin Natural Occurrence      Students write kinetin is an            Kinetin was isolated from AUTOCLAVED HERRING

                                endogenous natural plant hormone.       SPERM DNA and does NOT occur naturally in plants!

                                                                        Zeatin is the natural plant cytokinin.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Site of Photoperiodic           Students think shoot apical bud         The LEAF is the sole site of photoperiod

Perception                      perceives light duration.               perception. Defoliated plants NEVER flower.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Short-Day vs Long-Day Plants    Students focus solely on day length.    Short-Day Plants are functionally LONG-NIGHT

                                                                        plants; a single light flash interrupting the

                                                                        dark period completely abolishes flowering.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

Ethylene: Promoter or           Students strictly classify ethylene     Ethylene is primarily an INHIBITOR (senescence,

Inhibitor?                      as a growth promoter due to fruit.      abscission), but promotes specific events like

                                                                        pineapple flowering and potato sprouting.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────

2,4-D Weedicide Specificity     Students assume 2,4-D kills all         2,4-D selectively kills broad-leaved DICOT

                                types of weeds indiscriminately.        weeds while leaving mature MONOCOTS intact.