CBSE Class 11 Biology Chapter 15 Notes: Plant Growth and Development

Chapter Introduction

Plant Growth and Development is Chapter 15 of the Plant Physiology unit in NCERT Class 11 Biology. Growth is a fundamental characteristic of all living organisms, and in plants it involves three sequential processes — cell division, cell elongation, and cell differentiation. Unlike animals, plants exhibit indeterminate growth thanks to meristematic tissues that remain active throughout their life.

This chapter covers the characteristics and measurement of growth, the phases and types of growth (including the sigmoid curve), the concept of differentiation–dedifferentiation–redifferentiation, the five major classes of plant growth regulators (auxins, gibberellins, cytokinins, ABA, and ethylene), photoperiodism, vernalisation, and seed dormancy. Mastering this chapter gives you a strong foundation for understanding how plants adapt, respond to their environment, and complete their life cycle.

Key Definitions

1. Growth — An irreversible, permanent increase in size, volume, and dry mass of an organism or its parts, brought about by cell division, cell elongation, and differentiation. 2. Differentiation — The process by which cells derived from meristems undergo structural and biochemical changes to perform specialised functions (e.g., tracheary elements losing protoplasm and developing lignocellulosic walls). 3. Dedifferentiation — The process by which mature, differentiated cells regain the capacity to divide (e.g., formation of interfascicular cambium and cork cambium from parenchyma cells). 4. Redifferentiation — The process by which dedifferentiated cells lose their capacity to divide and mature once again into specialised cells (e.g., secondary xylem, cork cells). 5. Plant Growth Regulators (PGRs) — Small, naturally occurring organic compounds that influence physiological processes at very low concentrations; classified as growth promoters (auxins, gibberellins, cytokinins) and growth inhibitors (ABA, ethylene). 6. Apical dominance — The phenomenon in which the growing apical bud suppresses the growth of lateral (axillary) buds, controlled by auxin produced at the shoot apex. 7. Photoperiodism — The response of plants to the relative duration of light and dark periods (photoperiod) for the induction of flowering. 8. Vernalisation — The induction of flowering by exposure to low temperatures (0–5 °C) for a prolonged period (weeks). 9. Bolting — The rapid elongation of the stem just before flowering, commonly seen in rosette plants and induced by gibberellins. 10. Senescence — The age-related, progressive deterioration of cells, tissues, or organs leading to their eventual death. 11. Abscission — The shedding of plant parts such as leaves, flowers, and fruits, regulated by the balance of auxin and ethylene.

Detailed Concepts

Growth: Characteristics and Phases

Growth in plants is indeterminate (open form) because meristems — the root apical meristem and the shoot apical meristem — continue to add new cells throughout the plant's life. This is fundamentally different from animals, which show determinate (closed) growth.

Measurement of growth:

• Increase in length or height (e.g., plant height)
• Increase in area (e.g., leaf area)
• Increase in volume
• Increase in fresh weight
• Increase in dry weight (most reliable, as it discounts water content fluctuations)
• Increase in cell number

Growth rate is expressed as:

Growth rate = (Final size − Initial size) / Time

Phases of Growth

During the lag phase, cells synthesise proteins and enlarge but divide slowly. The log phase sees maximum mitotic activity and cell expansion. In the stationary phase, the limited nutrient supply and space cause the rate of new cell formation to balance cell death, giving a net growth of zero.

Types of Growth

The sigmoid curve is the hallmark of plant growth. Growth starts slowly (lag phase), accelerates rapidly (log/exponential phase), then decelerates and plateaus (stationary/deceleration phase). The exponential phase can be expressed mathematically as:

W₁ = W₀e^rt

where W₀ = initial size, r = growth rate, t = time, and e = base of natural logarithms. This equation describes the geometric growth component. However, because resources are finite, the growth rate slows and the curve flattens — giving the characteristic S-shape.

Differentiation, Dedifferentiation, Redifferentiation

Plants exhibit remarkable developmental plasticity through three interconnected processes:

1. Differentiation: Cells produced by meristems mature into specialised types. For example, tracheary elements develop thick lignocellulosic secondary cell walls and lose their protoplasm at maturity to become dead, hollow water-conducting cells. This structural specialisation enables division of labour.

2. Dedifferentiation: Under certain conditions, mature differentiated cells reverse their developmental pathway and regain the capacity to divide. The interfascicular cambium and cork cambium are classic examples — they form from fully differentiated parenchyma cells that revert to a meristematic state.

3. Redifferentiation: The dedifferentiated cells, after dividing for some time, once again lose their capacity to divide and mature into new specialised cell types. Examples include secondary xylem, secondary phloem, and cork cells derived from cambium and cork cambium respectively.

This cycle — differentiation → dedifferentiation → redifferentiation — gives plants the plasticity to adapt to environmental changes and repair damaged tissues, a capability largely absent in animals.

Plant Growth Regulators (PGRs)

Plant growth regulators are small, organic compounds that act as chemical messengers, influencing physiological processes at extremely low concentrations. They are broadly grouped into growth promoters and growth inhibitors.

Comprehensive PGR Comparison Table

Auxins

Discovery: Auxins were discovered through Went's avena curvature test (1928). Went placed agar blocks containing auxin from oat coleoptile tips on one side of decapitated coleoptiles and observed bending towards the side without auxin — proving that auxin promotes cell elongation and is responsible for phototropic curvature.

Natural and synthetic auxins:

• Natural: IAA (Indole-3-acetic acid) — the principal natural auxin
• Synthetic: NAA (Naphthalene acetic acid), 2,4-D (2,4-Dichlorophenoxyacetic acid), IBA (Indole-3-butyric acid)

Mechanism of cell elongation — Acid Growth Hypothesis: 1. Auxin binds to receptors on the plasma membrane 2. Activates proton pumps (H⁺-ATPases) 3. Protons are pumped into the cell wall, lowering its pH 4. Acidic pH activates enzymes (expansins) that loosen cell wall bonds 5. Cell wall becomes flexible; water enters the cell by osmosis 6. Cell expands and elongates

Apical dominance: Auxin produced by the apical bud travels downward (basipetal transport) and inhibits the growth of lateral (axillary) buds. Removing the apical bud eliminates the auxin source, allowing lateral branches to grow. This principle is used in hedge pruning and tea plantation management to produce bushy growth.

Applications:

• Rooting of stem cuttings: IBA and NAA promote root initiation in vegetative propagation
• Weed control: 2,4-D is a selective herbicide that kills broad-leaved dicot weeds in cereal (monocot) crops
• Parthenocarpy: Auxin spray on unpollinated flowers produces seedless fruits
• Prevents premature abscission of leaves and fruits

Gibberellins

Discovery: Gibberellins were discovered from the study of "bakanae" (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi (now Fusarium fujikuroi). Infected seedlings grew excessively tall and then died. The causative substance was isolated and named gibberellic acid (GA).

Key facts:

• Over 100 gibberellins have been identified; GA3 is the most commonly studied and commercially used
• Gibberellins are diterpenoid acids (C₂₀ or C₁₉ compounds)

Bolting: GA induces rapid stem elongation in rosette plants (cabbage, beet, radish) just before flowering. In these plants, the stem is normally very short with leaves arranged in a rosette. GA application causes the internodes to elongate dramatically — this is called bolting.

Applications:

• Sugar cane yield increase: GA sprays increase the length and sugar content of internodes
• Brewing industry: GA stimulates alpha-amylase production in barley aleurone cells, increasing malt yield and reducing brewing time
• Delay of fruit senescence: GA sprays extend the shelf life of fruits
• Parthenocarpy: GA induces seedless fruit formation in grapes and apples
• Breaking seed dormancy: GA overcomes dormancy in seeds that require cold or light treatment

Cytokinins

Discovery: Cytokinins were discovered by Skoog and Miller while studying nutrient media for plant tissue culture. They found that coconut milk (liquid endosperm) and herring sperm DNA contained substances that promoted cell division. The active compound from DNA was named kinetin (6-furfurylaminopurine).

Natural and synthetic cytokinins:

• Natural: Zeatin (most active naturally occurring cytokinin, isolated from corn/maize)
• Synthetic: Kinetin, BAP (6-Benzylaminopurine), TDZ (Thidiazuron)

Cytokinin:Auxin ratio and morphogenesis (Skoog and Miller experiment):

This discovery was foundational for plant tissue culture and micropropagation techniques.

Anti-senescence effect: Cytokinins delay leaf senescence by mobilising nutrients towards the treated area. This is why cytokinin-treated leaves stay green longer. Commercially, cytokinin sprays are used to extend the shelf life of leafy vegetables and cut flowers.

Abscisic Acid (ABA)

Abscisic acid acts as a general plant growth inhibitor and the primary stress hormone. Despite its name suggesting abscission, ABA's most critical role is in stress responses, not abscission (which is primarily ethylene-driven).

Stress response — Stomatal closure: 1. During water stress (drought), ABA levels rapidly increase in leaves 2. ABA binds to receptors on guard cell membranes 3. This triggers efflux of K⁺ ions and anions from guard cells 4. Water follows by osmosis; guard cells lose turgor 5. Stomata close, reducing transpiration and conserving water

Seed dormancy: ABA levels are high in dormant seeds, maintaining them in a state of inactivity even under favourable conditions. During germination, ABA levels decline while gibberellin levels rise — the balance between these two hormones determines whether a seed germinates or remains dormant.

Counteraction of growth promoters: ABA antagonises the effects of auxins (inhibits cell elongation), gibberellins (inhibits bolting and seed germination), and cytokinins (promotes senescence rather than delaying it). This balancing act ensures that growth does not proceed unchecked.

Ethylene

Ethylene is the only gaseous plant growth regulator. Its gaseous nature allows it to diffuse through air and affect not only the tissue producing it but also neighbouring tissues and even nearby plants.

Fruit ripening: Ethylene is the "ripening hormone." It triggers the climacteric rise in respiration rate in ripening fruits. Biochemical changes include:

• Starch → sugars (sweetening)
• Chlorophyll breakdown (degreening)
• Cell wall softening (pectinase activation)
• Production of aroma compounds

Epinasty: Ethylene causes the petiole to grow more on the upper side than the lower side, resulting in downward bending of leaves — a response distinct from tropism because it is not directional relative to a stimulus.

Applications:

• Fruit ripening acceleration: Ethylene or ethephon (ethylene-releasing compound) is used commercially to ripen fruits uniformly
• Latex flow: Ethylene application increases latex flow in rubber trees by stimulating the laticiferous system
• Breaking seed dormancy: Ethylene can overcome dormancy in some seeds
• Promotes horizontal growth of stems in deep-water rice, helping the plant reach the surface

Photoperiodism

Discovery: Photoperiodism was discovered by Garner and Allard (1920) while studying the Maryland Mammoth variety of tobacco, which failed to flower during summer but flowered in winter when days were shorter.

Critical photoperiod: The minimum (for LDP) or maximum (for SDP) duration of light needed to trigger flowering. It varies between species.

Classification of plants based on photoperiodic response:

Critical dark period — the key insight:

It is the uninterrupted dark period (not the light period) that is critical for flowering. This was demonstrated by interrupting the dark period with a brief flash of light (night break):

• A night break inhibits flowering in SDP (because it shortens the uninterrupted dark period)
• A night break promotes flowering in LDP (because it shortens the dark period below the critical length)

Phytochrome system:

The photoreceptor involved is phytochrome, which exists in two interconvertible forms:

• P_r (phytochrome red-absorbing form) — absorbs red light (660 nm), converts to P_fr
• P_fr (phytochrome far-red absorbing form) — absorbs far-red light (730 nm), converts to P_r

P_r ⇌ P_fr

P_fr is the biologically active form. In daylight, P_r converts to P_fr (promoting LDP flowering). In darkness, P_fr slowly reverts to P_r (allowing SDP flowering). A brief red-light flash during the dark period converts P_r to P_fr and inhibits SDP flowering; a far-red flash reverses this effect.

Vernalisation

Vernalisation is the induction of flowering by exposing plants to low temperatures (0–5 °C) for several weeks.

Discovery: The concept was established by Lysenko working on wheat varieties in Russia. Winter wheat required cold treatment to flower, while spring wheat did not.

Key features:

• Biennial plants (sugar beet, cabbage, carrot, radish) grow vegetatively in the first year, undergo vernalisation during winter, and flower in the second year
• The vernalised state is mitotically heritable — cells that have undergone vernalisation pass this state to their daughter cells through division
• Devernalisation: The vernalised state can be reversed by exposure to high temperatures (approximately 30–40 °C) soon after the cold treatment
• Site of perception: The shoot tip (apical meristem) is the site that perceives the vernalisation stimulus, as demonstrated by localised cooling experiments
• Vernalisation and photoperiodism can interact — many vernalisation-requiring plants are also long-day plants that flower only after both cold treatment and long days

Seed Dormancy and Germination

Seed dormancy is a state of inactivity in which a viable seed fails to germinate even when provided with favourable conditions (water, oxygen, suitable temperature). Dormancy is an adaptive strategy that ensures seeds germinate only when conditions are most likely to support seedling survival.

Causes of dormancy:

1. Hard seed coat (physical barrier): Impermeable to water and/or oxygen (e.g., legumes, Xanthium). The hard coat mechanically restricts embryo expansion. 2. Immature embryo: The embryo has not completed development at the time of seed shed (e.g., Ginkgo, certain orchids). 3. Chemical inhibitors: ABA and phenolic compounds present in seed coats or embryo inhibit germination. 4. Light requirement: Some seeds need exposure to light (photoblastic seeds). Positive photoblastic seeds (lettuce, tobacco) need light; negative photoblastic seeds (Phlox, onion) are inhibited by light.

Methods to break dormancy:

Important Diagrams

Practise drawing and labelling the following diagrams for the exam:

1. Sigmoid growth curve — Plot time on the x-axis and growth (size/mass) on the y-axis. Clearly label the lag phase, log/exponential phase, and stationary phase. Mark the inflection point where the curve transitions from exponential to stationary.

2. Arithmetic vs geometric growth curves — Draw two graphs side by side. Arithmetic growth: a straight line (constant slope). Geometric growth: a J-shaped exponential curve. Label axes and key equations.

3. Structure of IAA (Indole-3-acetic acid) — Show the indole ring with the acetic acid side chain at position 3. This is the most frequently asked PGR structure.

4. Photoperiodism — effect of day length on SDP, LDP, DNP — Draw three bars representing day length; shade the dark period. Show which combination triggers flowering for each type.

5. Phytochrome interconversion (P_r ⇌ P_fr) — Show red light (660 nm) converting P_r → P_fr and far-red light (730 nm) converting P_fr → P_r. Indicate that P_fr is the biologically active form.

6. Seed germination sequence — Show imbibition → rupture of seed coat → emergence of radicle → emergence of plumule → establishment of seedling.

Common Mistakes

1. Confusing dedifferentiation with redifferentiation — Dedifferentiation REVERSES maturation (mature cells regain division capacity); redifferentiation MATURES the dedifferentiated cells into new specialised types. Think: de = reverse, re = again into specialisation.

2. Thinking photoperiodism depends on light duration only — It actually depends on the uninterrupted dark period. A brief light flash during the night (night break) inhibits flowering in SDP because it interrupts the critical dark period.

3. Calling ABA a growth promoter — ABA is a growth inhibitor and the primary stress hormone. It closes stomata, promotes dormancy, and antagonises auxins, gibberellins, and cytokinins.

4. Confusing bolting with flowering — Bolting is GA-induced rapid stem elongation that occurs just before flowering, especially in rosette plants. Bolting is a prerequisite for flowering in some species but is not flowering itself.

5. Forgetting that ethylene is a GASEOUS hormone — Unlike all other PGRs, ethylene (C₂H₄) is a gas at room temperature. It diffuses through air and affects nearby tissues and plants. This property is exploited commercially in fruit ripening chambers.

6. Mixing up SDP and LDP conditions — SDP need short days (long nights); LDP need long days (short nights). A useful mnemonic: "Short plants need short days" is wrong — SDP flower in autumn/winter when days are short; LDP flower in late spring/summer when days are long.

7. Writing "geometric growth" when the question asks about the overall sigmoid curve — Geometric growth is only the exponential (log) phase. The overall growth curve is sigmoid, which combines lag, log, and stationary phases.

8. Assuming auxin is only a growth promoter — At high concentrations, auxin (especially synthetic 2,4-D) acts as a herbicide and kills dicot weeds. Concentration determines the effect.

Exam Tips

1. For 5-mark questions on PGRs, always write a comparison table with at least 5 columns (PGR name, example, source, functions, effect). Tables score more than paragraphs.

2. When asked about apical dominance, mention both the mechanism (auxin from apical bud inhibits lateral buds) and the application (hedge pruning, tea plantation management, fruit tree training).

3. Photoperiodism questions often test the critical dark period concept — emphasise that it is the uninterrupted dark period that matters, not the light period. Cite the night-break experiment as evidence.

4. Draw the sigmoid curve and label all three phases — lag, log (exponential), and stationary. Mark the inflection point. This is one of the most frequently asked diagrams in board exams.

5. For vernalisation, mention the biennial plant example (sugar beet, cabbage), the site of perception (shoot tip / apical meristem), and the concept of devernalisation. These details fetch full marks.

6. Use NCERT terminology — "indeterminate growth," "open form of growth," "meristem," "basipetal transport," "climacteric," "epinasty" — examiners check for exact keyword usage.

7. In PGR applications questions, mention at least one agricultural or commercial use for each hormone: auxin (2,4-D weed control, IBA rooting), GA (sugarcane yield, brewing malt), cytokinin (shelf life extension), ABA (stress tolerance research), ethylene (fruit ripening, latex flow).

8. For differentiation/dedifferentiation/redifferentiation, use the specific NCERT examples: tracheary elements (differentiation), interfascicular cambium (dedifferentiation), secondary xylem (redifferentiation). Generic examples lose marks.

Quick Revision

• Growth: Irreversible, permanent increase in size/mass; measured best by dry weight; indeterminate in plants via meristems (root and shoot apical meristems)
• Phases: Lag (slow) → Log/exponential (rapid) → Stationary (plateau) — gives the sigmoid (S-shaped) curve
• Arithmetic growth: Linear (constant rate); Geometric growth: Exponential (accelerating rate); Sigmoid: Combined, most common
• Differentiation → Dedifferentiation → Redifferentiation: Gives plants developmental plasticity
• Auxins: IAA (natural), apical dominance, cell elongation (acid growth hypothesis), phototropism, 2,4-D weed control
• Gibberellins: GA3, bolting in rosette plants, delay senescence, parthenocarpy, sugarcane yield, brewing (alpha-amylase)
• Cytokinins: Zeatin (natural), cell division/cytokinesis, delay senescence, cytokinin:auxin ratio determines morphogenesis
• ABA: Stress hormone, stomatal closure during drought, seed dormancy, inhibits growth, antagonises promoters
• Ethylene: Only gaseous PGR, fruit ripening (climacteric), epinasty, senescence, abscission, latex flow
• Photoperiodism: SDP, LDP, DNP — critical factor is the uninterrupted dark period; phytochrome (P_r ⇌ P_fr) mediates light perception
• Vernalisation: Low temperature (0–5 °C) induces flowering; shoot tip perceives; mitotically heritable; devernalisation possible
• Seed dormancy: Caused by hard seed coat, immature embryo, ABA, light requirement; broken by scarification, stratification, GA treatment, light exposure