Chapter 15 — Plant Growth And Development

Fig 1 — Educational Diagram: Growth: definition and characteristics

Definition: Growth is a permanent, irreversible and progressive increase in size, mass, volume or number of cells of an organism or part of an organism. In plants growth results from cell division, cell enlargement and cell differentiation occurring mainly in meristems.

What is measured: Length, area, volume, fresh weight, dry weight (biomass), number of cells or number of organs (e.g., leaves).

Key processes that produce growth:

• Cell division (mitosis) in meristems — increases cell number.
• Cell enlargement/expansion — increases cell volume (often major contributor to plant growth).
• Cell differentiation and maturation — qualitative change producing tissues and organs (part of development).

Characteristics of growth:

• Irreversible: Once growth has occurred it cannot be reversed to the previous size.
• Permanent: Results in lasting structural change.
• Quantitative: Can be measured (length, biomass, cell number).
• Qualitative: Accompanied by differentiation and development (forming specialized cells/tissues).
• Localized: In plants, growth is localized to meristems (apical, intercalary, lateral).
• Directional: Growth has direction (e.g., apical growth of roots and shoots, radial growth from cambium).
• Variable rates: Rate of growth changes with age and conditions — not constant.
• Pattern types: Arithmetic (linear), geometric (exponential) and sigmoid (S-shaped) growth patterns occur.
• Determinacy: Organs are often determinate (e.g., leaves), whole plants may be indeterminate (continuous apical growth).
• Regulated: Controlled by genetic factors and environmental influences (light, water, nutrients, hormones).

Growth versus Development: Growth is a quantitative increase (size/number) while development is the sequence of qualitative changes (differentiation, morphogenesis, maturation). Both are integrated.

Practical measurement: Commonly measured as change in dry weight over time (absolute growth rate) or as relative growth rate (per unit mass or per unit time) to compare growth efficiency.

Fig 2 — Educational Diagram: Measurement and analysis of growth

Growth in plants is an irreversible increase in size and mass resulting from cell division and expansion. Measurement and analysis of growth quantify how fast and in what pattern a plant (or plant part) increases over time. Accurate measurement requires choosing a suitable growth parameter (length, area or mass), a sampling method (destructive or non‑destructive) and appropriate time intervals.

Common parameters and methods

• Linear measurements: length of root, shoot or organ; useful for apical growth (measured with ruler, calipers).
• Area measurements: leaf area (planimeter, leaf-area meter, graph paper method); used for photosynthetic surface and LAI estimates.
• Biomass (mass): fresh weight and dry weight (oven-dried to constant mass) — most common for whole-plant growth studies (usually destructive sampling).
• Cellular measurements: cell number and cell size (microscopy) to distinguish growth by division vs expansion.
• Non-destructive methods: repeated measures of the same plant (height, leaf count, image analysis).

Sampling design and sources of error: use replicates, randomization and consistent time intervals. Destructive sampling requires multiple identical plants; non‑destructive sampling reduces variance but may disturb plants. Control environment (light, water, nutrients, temperature) since growth is highly responsive to conditions.

Growth patterns and their interpretation

• Plant growth over time typically follows a sigmoid (S‑shaped) curve with four phases: lag (slow), exponential/log (rapid), deceleration (slowing), and stationary (plateau).
• Different analysis transforms highlight different aspects: plotting mass vs time shows the overall S-curve; plotting ln(mass) vs time is linear during the exponential phase — useful for estimating relative growth rate.

Derived growth rates and indices (used to compare plants or treatments):

• Absolute Growth Rate (AGR): increase in size or mass per unit time. Useful to know how much biomass is added each day.
• Relative Growth Rate (RGR): rate of increase per unit existing biomass (a measure of efficiency). During exponential growth, RGR is constant.
• Crop Growth Rate (CGR): biomass produced per unit ground area per unit time (important in agronomy).
• Specific Leaf Area (SLA) and Leaf Area Ratio (LAR): link leaf area to dry mass and show how leafiness contributes to whole‑plant growth.

Practical uses: breeders and agronomists use RGR and CGR to select high-performing genotypes, horticulturists monitor nursery growth, ecologists compare species’ strategies (fast vs slow growers), and farmers use growth analysis to schedule fertilizer and irrigation.

Fig 3 — Educational Diagram: Meristems and growth zones

Definition: Meristems are regions of actively dividing, undifferentiated cells (meristematic cells) that produce new cells for growth. Growth zones in roots and shoots are spatial regions where division, elongation and differentiation occur.

Characteristics of meristematic cells: small cells with thin primary walls; dense cytoplasm and large nucleus; little or no vacuole; high mitotic activity; ability to remain undifferentiated and to produce new tissues.

Classification (by position):

• Apical meristems — found at shoot and root tips; responsible for primary growth (increase in length). Primary meristems derived from apical meristems: protoderm (forms epidermis), ground meristem (forms cortex and pith), procambium (forms primary xylem and phloem).
• Intercalary meristems — located at internodes or leaf bases (common in grasses and some monocots); allow rapid elongation and regrowth after damage (mowing, grazing).
• Lateral meristems — include vascular cambium and cork cambium (phellogen); responsible for secondary growth (increase in girth) in dicots and gymnosperms.

Root and shoot apex: growth zones:

• Root tip zones (from tip upward): root cap (protection, secretion of mucilage), meristematic zone (zone of cell division; contains quiescent centre — region of slowly dividing cells that can replace damaged meristem), zone of elongation (cells increase in length, pushing root tip forward), zone of maturation/differentiation (cells differentiate into root hairs, xylem, phloem, cortex).
• Shoot apical meristem zones: central zone (stem cells, low division), peripheral zone (gives rise to leaf primordia), and rib zone (contributes to stem tissues). Above these, leaf primordia and young leaves differentiate.

Secondary growth: Vascular cambium forms a continuous cylinder in stems and roots of woody plants; it produces secondary xylem (wood) toward inside and secondary phloem toward outside. Cork cambium produces periderm (protective outer layer). Repeated activity produces annual rings (in temperate trees) and increases stem/root diameter.

Physiological sequence of growth: cell division (mitosis) in meristems → cell enlargement (water uptake, cell wall loosening) → cell differentiation and maturation. Growth is irreversible increase in size and dry mass.

Practical significance: understanding meristems helps in vegetative propagation (cuttings, grafting), pruning (removal of apical meristem breaks apical dominance and stimulates axillary buds), agriculture (regrowth in grasses due to intercalary meristems), and forestry (managing secondary growth for timber).

Fig 4 — Educational Diagram: Cell division, enlargement and differentiation

Overview
Plant growth and development at the cellular level involve three linked processes: cell division (production of new cells), cell enlargement (increase in cell volume), and cell differentiation (specialization of cells to perform specific functions). These processes occur mainly in meristematic regions and are regulated by plant hormones, environmental cues and genetic control.

1. Cell division

Where it occurs: Meristems — apical meristems (root and shoot tips) for primary growth, lateral meristems (vascular cambium and cork cambium) for secondary growth, and intercalary meristems (at bases of leaves or internodes in monocots).

How it occurs: Most mitotic cell divisions in plants follow the eukaryotic cell cycle (G1 → S → G2 → M). Mitosis (M phase) is subdivided into prophase, metaphase, anaphase and telophase, followed by cytokinesis (cell plate formation in plant cells).

Significance: Increases cell number, provides cells for growth and differentiation, contributes to organ formation and repair.

Key points of regulation

• Cell cycle checkpoints (G1/S and G2/M) ensure DNA integrity and proper conditions for division.
• Mitotic index (a diagnostic measure) estimates the proportion of cells undergoing mitosis.

2. Cell enlargement (expansion)

Mechanism: Cell enlargement in plants is mainly due to vacuole enlargement by osmotically driven water uptake and loosening of the cell wall to allow expansion. Important steps:

1. Accumulation of solutes (e.g., K+, organic acids) lowers cell osmotic potential.
2. Water enters by osmosis; turgor pressure increases.
3. Cell wall loosening by enzymes (expansins, cellulases, pectinases) and incorporation of new wall material allows irreversible expansion.

Hormonal control: Auxins (IAA) stimulate cell wall loosening and proton pump activity (acid-growth hypothesis); gibberellins also promote cell elongation.

Importance: Cell enlargement accounts for a large part of plant growth (size increase) after cell division provides new cells.

3. Cell differentiation

Definition: Process by which similar daughter cells become structurally and functionally distinct (forming tissues like xylem, phloem, epidermis, parenchyma).

Mechanisms: Differential gene expression, positional information (signals from neighboring cells), hormonal gradients (auxin, cytokinin), and environmental signals determine fate.

Examples of differentiation events: Lignification and programmed cell death to form mature xylem vessels and tracheids; formation of root hairs from epidermal cells; stomatal guard cell differentiation.

Integration of the three processes

In root apices and shoot tips there is a spatial organization: zone of cell division (meristem), zone of elongation (cells enlarge), and zone of differentiation (cells mature into tissues). Proper timing and coordination (e.g., balance of auxin and cytokinin) are required for normal organogenesis and responses such as wound healing or organ regeneration.

Special phenomena

• Secondary growth: Lateral meristems (vascular cambium) divide to produce secondary xylem (wood) and phloem — causes increase in girth and formation of growth rings.
• Dedifferentiation and redifferentiation: Some mature plant cells can dedifferentiate (become meristematic) — basis of callus formation and plant tissue culture; then redifferentiate to form organs.
• Programmed cell death (PCD): Essential in xylem maturation and aerenchyma formation.

Controls and signals

Major hormones: auxins, cytokinins, gibberellins, ethylene, abscisic acid. Nutrients, light, temperature and water availability also influence cell division, expansion and differentiation.

Practical importance: Understanding these processes underlies agriculture and horticulture practices — pruning, use of growth regulators, tissue culture, grafting, improving yield by manipulating cell division and enlargement (fruit size, stem thickness).

Fig 5 — Educational Diagram: Seed structure and germination

1. Overview

A seed is a matured ovule that contains a young plant (embryo) and stored food, protected by a seed coat. Germination is the sequence of events by which a seed resumes metabolic activity, grows and produces a seedling when environmental conditions are favourable.

2. Structure of a typical dicot seed (e.g., pea/bean)

• Testa (seed coat): protects the embryo; derived from integuments. Often has a thin inner layer (tegmen).
• Micropyle: a small pore through which water and oxygen can enter; also marks pollen tube entry.
• Hilum: point of attachment to the funiculus (seed stalk).
• Embryo: the young plant — includes
  • Radicle (embryonic root) — becomes primary root
  • Hypocotyl — region below cotyledons; lifts radicle/plumule in some types
  • Cotyledons — one or two seed leaves that may store food (in many dicots) or absorb endosperm
  • Plumule (embryonic shoot)
• Endosperm (if present): nutritive tissue produced by double fertilization (triploid in angiosperms). In many legumes endosperm is consumed during seed development and food is stored in cotyledons.

3. Structure of a typical monocot seed (e.g., maize/wheat)

• Pericarp + testa: fused outer coverings.
• Aleurone layer: living outer layer of the endosperm in cereals; produces enzymes (e.g., amylases) on germination.
• Starchy endosperm: main storage tissue.
• Embryo components unique to monocots:
  • Scutellum: a single cotyledon that absorbs food from endosperm
  • Coleoptile: sheath protecting the plumule as it grows up
  • Coleorhiza: sheath protecting the radicle

4. Seed formation (brief)

Double fertilization: one sperm fuses with egg to form diploid embryo; the other fuses with two polar nuclei to form triploid endosperm (storage tissue).

5. Conditions required for germination

• Water: imbibition (rapid water uptake) hydrates tissues, reactivates metabolism.
• Oxygen: for respiration (ATP production) — anaerobic respiration supports only limited activity.
• Suitable temperature: species-specific optimal range for enzyme activity.
• Light or darkness: some seeds require light (photoblastic +), others darkness (photoblastic -) or are indifferent.

6. Phases of germination

1. Phase I — Imbibition: rapid water uptake; seed swells and testa softens.
2. Phase II — Lag/activation: metabolic reactivation, synthesis of enzymes (e.g., amylase), respiration increases; stored food is mobilized.
3. Phase III — Radicle emergence: cell elongation and division cause radicle to break through the testa; visible germination is recorded.

7. Biochemical events

• Stored macromolecules (starch, proteins, lipids) are hydrolysed by enzymes (amylases, proteases, lipases).
• In cereals, embryo releases gibberellins (GA) that stimulate aleurone to release alpha-amylase to break down starch to sugars.
• Respiration (glycolysis, TCA cycle) provides ATP for growth.

8. Types of germination

• Epigeal germination: hypocotyl elongates, cotyledons pushed above ground (become photosynthetic) — example: many beans (Phaseolus).
• Hypogeal germination: epicotyl grows while cotyledons remain below ground (serve as reserve) — example: pea (Pisum sativum).
• Monocot (typical cereal): coleoptile protects the shoot; coleorhiza protects radicle — example: maize, wheat.

9. Seed dormancy and breaking methods

• Types of dormancy
  • Physical dormancy: impermeable seed coat (e.g., some legumes).
  • Physiological dormancy: internal inhibitors or hormonal balance (high abscisic acid, low gibberellin).
  • Morphological dormancy: immature embryo requiring further development.
• Breaking dormancy: scarification (mechanical/acid) to break seed coat, stratification (cold treatment) for temperate seeds, dry storage (after-ripening), gibberellin application in some cases.

10. Seed viability and tests

• Germination test: count % germination under standard conditions.
• Tetrazolium (TZ) test: stains living tissues red (viability assessment).

11. Importance (real-life relevance)

• Agriculture: seed quality (viability, vigour, dormancy) determines crop establishment and yield.
• Food: many edible seeds (beans, cereals) are germinated as sprouts for nutrition.
• Conservation and forestry: understanding dormancy and germination helps in propagation of trees and endangered species.

Concise summary: Seed structure (protective coat, embryo, storage tissue) is closely linked to the mechanism of germination. Germination requires water, oxygen, suitable temperature and involves imbibition, metabolic activation and radicle emergence; hormonal control (GA vs ABA) and enzyme-mediated mobilization of reserves are key.

Fig 6 — Educational Diagram: Seed dormancy and breaking dormancy

Definition: Seed dormancy is a temporary physiological state in which viable seeds do not germinate even under favourable conditions. It prevents germination at inappropriate times and synchronises seedling emergence with optimal conditions.

Types of dormancy (CBSE-level classification):

• Primary dormancy: Imposed during seed development on the mother plant.
• Secondary dormancy: Induced after dispersal when previously non-dormant seeds encounter adverse conditions.
• By cause/mechanism:
  • Physical dormancy (seed coat-imposed) – impermeable testa prevents water/gas entry (e.g., many legumes).
  • Physiological dormancy – embryo is dormant due to internal chemical signals (hormonal control; common in many temperate seeds).
  • Morphological dormancy – embryo underdeveloped at dispersal and requires time to grow.
  • Morphophysiological dormancy – combination of underdeveloped embryo + physiological block.

Causes and internal control:

• Hormones: Abscisic acid (ABA) promotes and maintains dormancy; gibberellins (GA) promote germination. The balance (ABA vs GA) determines the switch.
• Water impermeability and mechanical constraints from hard seed coats.
• Light sensitivity mediated by phytochrome (some seeds require red light to germinate; others are inhibited by light).
• Temperature and oxygen availability influence dormancy release.

Ecological significance: Dormancy spreads germination over time (bet-hedging), allows seeds to survive adverse seasons (winter, drought), and promotes dispersal/colonisation (e.g., fire-cued germination in Mediterranean ecosystems).

Breaking dormancy (major methods):

• Scarification (removes/reduces seed coat impermeability): mechanical (sandpaper, nicking), thermal (hot water), chemical (concentrated sulfuric acid briefly). Used for hard-coated legumes (e.g., Acacia, many beans).
• Stratification (cold or warm moist treatment): moist chilling at low temperatures for weeks/months breaks physiological dormancy in many temperate species (e.g., apple, pear, many temperate trees).
• After-ripening (dry storage at room temperature): physiological dormancy gradually lost during dry storage (e.g., cereals like wheat; many weeds).
• Hormonal treatments: application of gibberellic acid (GA3) can substitute for natural cues and promote germination in some dormant seeds.
• Light or temperature cues: exposure to red light (activates phytochrome Pfr) or alternating temperatures can trigger germination for photodormant or thermo-dormant seeds (e.g., lettuce requires light).
• Smoke/heat cues: smoke-derived chemicals (karrikins) or fire-related heat break dormancy in fire-adapted species (Banksia, some Proteaceae, serotinous cones in some conifers).
• Leaching: rinsing away inhibitors (e.g., some seeds contain water-soluble inhibitors removed by rain).

Practical notes for experiments: Compare treatments by measuring germination percent over time, use controls, replicate samples, and record days to first germination and mean germination time. Safety: chemical scarification (H2SO4) must be done with care and appropriate lab controls.

Summary: Dormancy is an adaptive pause in germination controlled by seed structures and hormones (mainly ABA vs GA), and it can be overcome by physical, chemical, hormonal or environmental treatments designed to remove the barrier imposed by coat, embryo immaturity or inhibitory signals.

Fig 7 — Educational Diagram: Phases of plant growth and development

Overview: Plant growth and development occur in ordered phases at both the cellular level and the whole-plant level. Growth denotes irreversible increase in size/biomass; development denotes progression through stages (e.g., germination, vegetative growth, flowering, fruiting).

Cellular (ontogenetic) phases of growth

• Cell division – Mitosis in meristematic tissues (apical, lateral, intercalary). This increases cell number. Cytokinins and auxins promote division.
• Cell enlargement / elongation – Cells expand by water uptake, loosening of cell wall (expansins), and vacuole growth. Gibberellins and auxins stimulate elongation.
• Cell differentiation and maturation – Cells acquire specialized structures and functions (xylem, phloem, stomata, epidermis) and may undergo secondary wall formation or lignification.

Whole-plant growth phases (sigmoid pattern)

When total plant mass (W) is plotted versus time (t), growth typically follows a sigmoid (S-shaped) curve composed of phases:

• Lag / initial phase – Slow growth after germination or transplanting while systems establish (rooting, initial leaf expansion).
• Exponential / log phase – Rapid, near-exponential increase in biomass; resources and meristems active. RGR is highest here.
• Deceleration phase – Growth rate slows as limitations (nutrients, light, space) and allocation shifts (to reproduction) occur.
• Plateau / stationary phase – Net growth ceases; senescence or maintenance dominates.

Regulation and factors

Internal regulators: plant hormones (auxin, gibberellin, cytokinin, ethylene, ABA), genetic programming and source–sink relationships. External factors: light (quality, duration), temperature, water, nutrients, CO2, and biotic interactions.

Measurement concepts (brief)

Growth is quantified by metrics such as Absolute Growth Rate (AGR), Relative Growth Rate (RGR), Net Assimilation Rate (NAR) and Crop Growth Rate (CGR) — these allow comparison across times and plants.

Interpretation tip: Plotting ln(W) vs time yields a straight line during exponential phase (constant RGR). Comparing AGR and RGR shows that a large plant can have high AGR but low RGR (growth per unit mass).

Fig 8 — Educational Diagram: Photoperiodism, vernalization and flowering

Overview
Plants use environmental cues — day length (photoperiod) and prolonged cold (vernalization) — to time flowering so that reproduction occurs at favorable seasons. Flowering involves three major events: floral induction (leaf perceives signal), floral initiation (shoot apical meristem switches identity) and floral organ formation.

1. Photoperiodism — definition and types
Photoperiodism is the physiological reaction of organisms to the relative lengths of light and dark periods. Plants are classified by their photoperiodic flowering response:

• Short-day plants (SDP): Flower when day length is shorter than a critical day length (equivalently when night length is longer than a critical night). Examples: chrysanthemum, poinsettia, many varieties of rice and soybean.
• Long-day plants (LDP): Flower when day length exceeds a critical day length (night shorter than critical night). Examples: spinach, radish, lettuce, some barley and wheat cultivars, Arabidopsis (some ecotypes).
• Day-neutral plants (DNP): Flower irrespective of day length; flowering controlled by age or other cues. Examples: tomato, cucumber, sunflower.

Mechanism: phytochrome system and critical night
The pigment system that measures light is phytochrome, which exists in two interconvertible forms: Pr (absorbs red ~660 nm) and Pfr (absorbs far-red ~730 nm). Light converts Pr <=> Pfr. Pfr is the biologically active form in many responses. The key idea for photoperiodic flowering is that plants measure the uninterrupted dark period (night length):

• In SDPs flowering is triggered when night length > critical night; in LDPs flowering when night length < critical night.
• Brief flashes of red or far-red light during the night can prevent or permit flowering by changing Pfr/Pr ratios (classic experiments by Garner & Allard; night-break experiments).

Molecular integration (brief)
Leaves sense photoperiod and regulate genes: CONSTANS (CO) integrates circadian clock and light signals to induce FLOWERING LOCUS T (FT) in long-day plants. FT protein (a mobile signal called 'florigen') moves via phloem to the shoot apical meristem and activates floral identity genes (e.g., APETALA1), initiating meristem conversion to a flower.

2. Vernalization — definition and role
Vernalization is the induction of flowering by exposure to a prolonged period of low temperature (cold). It ensures flowering in spring after winter in species from temperate climates.

• Obligate vernalization: flowering will not occur without cold treatment (e.g., many winter wheat cultivars, biennials like sugar beet and carrot).
• Facultative vernalization: cold accelerates flowering but is not absolutely required.

Mechanism (molecular and epigenetic basis)
In Arabidopsis and related species, vernalization causes stable repression of a floral repressor gene FLOWERING LOCUS C (FLC) via chromatin modifications (histone methylation and formation of repressive chromatin). Repression of FLC permits activation of FT/other flowering promoters. In cereals (wheat, barley), VRN genes are involved: cold induces VRN1 and reduces VRN2 (repressor), enabling flowering.

3. Florigen and hormonal influence
Florigen (FT protein) produced in leaves is the long-distance floral signal traveling to the shoot apex. Plant hormones also influence flowering: gibberellins often promote flowering (notably in some long-day plants and non-photoperiodic species), while cytokinins can contribute to floral induction in some contexts.

4. Interaction between photoperiod and vernalization
Plants may require both cues: vernalization permits a plant to become competent to flower; photoperiod (or other cues) then determine the timing. Example: winter wheat requires vernalization and long days for flowering.

5. Classic experiments
Garner & Allard (1920s): showed that photoperiod controls flowering by growing plants under different day-lengths; night-break experiments demonstrated that the dark period is critical. Chailakhyan (1936): proposed the concept of 'florigen' (mobile flowering signal).

6. Importance and applications
Understanding these processes helps agriculture: manipulating light (greenhouses, day-neutral cultivars), vernalization (cold treatment of seeds/seedlings), and breeding for photoperiod/vern-dependent varieties allows control of flowering time and crop adaptation to climates.

Summary points

• Photoperiodism: flowering response to relative day/night length; plants are SDP, LDP or day-neutral.
• Phytochrome (Pr/Pfr) + circadian clock measure night length; Pfr level and timing regulate flowering genes.
• Vernalization: prolonged cold suppresses floral repressors epigenetically, enabling flowering in spring.
• Florigen (FT) produced in leaves travels to SAM to trigger floral development.

Fig 9 — Educational Diagram: Plant growth regulators (phytohormones)

Definition: Plant growth regulators or phytohormones are naturally occurring organic substances produced in very small amounts in one part of a plant and translocated to other parts where they modify physiological processes at very low concentrations.

Major classes and brief functions:

• Auxins (e.g., IAA, IBA): Synthesised mainly in apical meristems, young leaves and developing seeds. Promote cell elongation, apical dominance, root initiation, vascular differentiation and tropic responses. Transported polarly (polar auxin transport) from shoot tip downward.
• Gibberellins (GAs): Produced in shoot tips, young leaves and seeds. Promote stem elongation (cell division and elongation), seed germination (mobilise food reserves), flowering (in some species) and fruit growth (parthenocarpy).
• Cytokinins (e.g., zeatin): Synthesised in roots, transported upward by xylem. Promote cell division, delay senescence, stimulate shoot formation, and act antagonistically with auxin in organogenesis.
• Abscisic acid (ABA): Produced in mature leaves, roots and seeds. Often called the stress hormone: induces stomatal closure during drought, promotes seed dormancy and inhibits germination.
• Ethylene (C2H4): A gaseous hormone produced in ripening fruits, senescing tissues and stressed tissues. Promotes fruit ripening, leaf abscission, triple response in seedlings and senescence.

Mechanism of action (overview): Phytohormones act at low concentrations by binding to specific receptors (membrane or intracellular), triggering signalling cascades that change gene expression and cellular metabolism leading to altered cell division, elongation or differentiation. Hormone effects depend on concentration, tissue sensitivity, and interactions with other hormones.

Interactions: Hormones rarely act alone. Important interactions include auxin–cytokinin ratios (control organogenesis in tissue culture: high auxin:cytokinin favors roots; high cytokinin:auxin favors shoots), auxin–gibberellin synergy in stem elongation, and ABA–GA antagonism in seed dormancy and germination.

Physiological phenomena explained by phytohormones (examples):

• Phototropism: Uneven distribution of auxin causes cells on the shaded side to elongate more, bending the shoot toward light.
• Apical dominance: Auxin from the shoot apical bud suppresses lateral bud growth; removing the apex reduces auxin and lateral buds grow.
• Leaf abscission and senescence: Reduced auxin from aging leaves plus increased ethylene and ABA promote abscission.
• Fruit ripening: Ethylene stimulates enzymes that convert starches to sugars and degrade cell walls; used commercially to ripen climacteric fruits.
• Seed dormancy and germination: ABA maintains dormancy; GAs break dormancy and stimulate hydrolytic enzymes for mobilising food reserves in endosperm.

Applied uses in agriculture and horticulture:

• Auxins (IBA, NAA) as rooting powders to promote cutting root formation.
• Gibberellins (GA3) to increase fruit size (e.g., seedless grapes), to promote malting in barley and to break seed dormancy.
• Cytokinins in tissue culture to induce shoot proliferation (micropropagation).
• Ethylene or ethephon (an ethylene-releasing compound) to ripen fruits uniformly or induce flowering/degreening.
• Growth retardants (e.g., paclobutrazol) to limit gibberellin biosynthesis and control plant height in ornamentals.
• Auxin-type herbicides (2,4-D, dicamba) that cause uncontrolled growth in weeds (selective weed control).

Key points to remember:

• Phytohormones act at very low concentrations and effects are concentration- and tissue-specific.
• Transport routes: auxins (polar transport in phloem/parenchyma), cytokinins (xylem), gibberellins and ABA (xylem/phloem), ethylene diffuses as a gas.
• Physiological responses often result from relative levels and cross-talk among hormones rather than absolute amounts of a single hormone.

Fig 10 — Educational Diagram: Tropisms and nastic movements

Overview

Tropisms are directional growth responses in which the direction of the stimulus determines the direction of movement (growth). They are usually slow and result from differential cell growth. Nastic movements are non-directional responses where the direction of movement is independent of the direction of stimulus; they are often reversible and caused by changes in turgor pressure or growth.

Tropisms — types and mechanism

• Phototropism — response to light. Shoots show positive phototropism (bend toward light); many roots show little or negative phototropism. Mechanism: differential distribution of auxin (IAA) to the shaded side of the coleoptile or stem, causing increased cell elongation on the shaded side (acid-growth hypothesis) and bending toward light.
• Geotropism (Gravitropism) — response to gravity. Roots show positive geotropism (grow downward), shoots show negative geotropism. Mechanism: gravity is perceived by statoliths (amyloplasts) in root columella and shoot endodermal cells, causing auxin redistribution. In roots, higher auxin on the lower side inhibits elongation (root bends down); in shoots, higher auxin on the lower side stimulates elongation (shoot bends up).
• Hydrotropism — growth toward moisture (roots toward water).
• Chemotropism — response to chemicals (e.g., pollen tube growth toward ovule; root growth toward nutrients).
• Thigmotropism — response to touch/contact (climbing plant tendrils coil around support).
• Other tropisms: electrotropism (electric fields), thermotropism (temperature gradients), etc.

Key experimental evidence

• Darwin and Darwin (1880): coleoptile tip required for phototropic bending.
• Boysen-Jensen: a diffusible chemical from the tip passes through gelatin but not through impermeable barriers — evidence for a mobile chemical (auxin).
• Frits Went: Agar block bioassay from a shaded-side coleoptile tip caused curvature in test oat coleoptiles — identification of "auxin" effect.

Cellular mechanism (brief)

Auxin stimulates plasma membrane H+-ATPases, lowering apoplastic pH, activating expansins and cell-wall-loosening enzymes. Water uptake and cell-wall loosening on one side cause cell elongation there and curvature. In roots, high auxin inhibits elongation — tissue-specific response.

Nastic movements — types and mechanism

• Definition: Non-directional movements produced by changes in turgor or growth; direction is independent of stimulus direction.
• Common types:
  • Seismonasty (thigmonasty): response to touch or shock — e.g., Mimosa pudica leaflets close rapidly.
  • Thigmonasty: rapid closure of Venus flytrap lobes in response to touch (mechanical stimulation triggers action potentials and rapid changes).
  • Photonasty: flower movements due to changes in light intensity (some flowers open at day and close at night).
  • Nyctinasty: sleep movements/leaf folding driven by circadian rhythms (legume leaflets show nyctinasty via pulvini).
  • Thermonasty: movement in response to temperature changes (some flowers open on warm days).
• Mechanism: Usually reversible changes in turgor pressure in specialized motor organs (pulvini). Ion fluxes (K+, Cl-) across membranes change osmotic potential, water moves, and cells on one side lose or gain turgor — causing bending. Rapid movements (Mimosa, Venus flytrap) involve action potentials and very fast ion flux and turgor changes; slower nastic movements may involve differential growth.

Comparison: Tropism vs Nastic

• Tropism = directional (movement direction depends on stimulus direction); nastic = non-directional (movement independent of stimulus direction).
• Tropism usually involves growth (irreversible or slow); nastic often involves reversible turgor change (can be rapid).
• Examples: phototropism (shoot bending toward light) vs photonasty/nyctinasty (flower opening/leaf folding at day/night).

Relevance and biological significance

• Optimize light capture (phototropism), water and nutrient acquisition (hydrotropism, chemotropism), anchorage and support (thigmotropism), and pollination timing (nastic flower movements).
• Rapid nastic responses are defensive (e.g., Mimosa closing) or carnivorous trapping (Venus flytrap).

Short notes for students

• Remember: auxin = mobile growth regulator essential in tropisms; its effect depends on tissue type and concentration.
• Statoliths (amyloplasts) = gravity sensors in roots/shoots.
• Pulvini = motor organs for nastic leaf/leaflet movements in legumes.

References for further reading

Darwin (1880) on phototropism; classical experiments by Boysen-Jensen and Went; CBSE Class 11 NCERT Chapter: Plant Growth and Development.

Fig 11 — Educational Diagram: Senescence and abscission

Senescence is the genetically programmed and environmentally influenced process of ageing in plants that leads to the controlled degradation of cellular components and nutrient remobilisation before organ death. Abscission is the process by which a plant sheds an organ (leaf, flower, fruit, or seed) through the formation and action of a specialised abscission zone (AZ) at the organ base.

Key features of senescence

• Triggered by developmental cues (e.g., seed maturation, ageing of leaves, post-pollination) and environmental stresses (drought, shading, pathogens, low temperature).
• Physiological and biochemical events: chlorophyll degradation (leaf yellowing), decline in photosynthesis, protein hydrolysis, lipid peroxidation, increased activity of hydrolytic enzymes (proteases, RNases), increased respiration and reactive oxygen species (ROS) production, and nutrient (N, P, K) mobilisation to other parts (young leaves, seeds).
• Hormonal control: promoted by ethylene, abscisic acid (ABA), jasmonic acid (JA) and reduced cytokinin levels. Cytokinins and some auxins delay senescence.

Key features of abscission

• Occurs at a pre-determined layer of cells (abscission zone) composed of small, dense cells with thin walls located at the base of the organ.
• Phases of abscission: (1) AZ differentiation, (2) acquisition of competence to respond, (3) activation of cell wall hydrolytic enzymes (polygalacturonase, cellulase, pectinmethylesterase) leading to middle lamella degradation and cell separation, and (4) formation of a protective (periderm) layer on the retained plant body.
• Hormonal regulation: auxin supplied from the distal organ (leaf/fruit) normally suppresses sensitivity of AZ to ethylene. When auxin flow falls below a threshold (e.g., due to ageing or fruit ripening), AZ becomes sensitive and ethylene promotes expression of cell-wall-degrading enzymes leading to abscission.

Mechanism — hormonal interplay (simplified)

• High auxin flow from the organ → AZ insensitive to ethylene → no abscission.
• Reduced auxin (organ age, damage, detachment, pollination changes) → AZ becomes ethylene-sensitive → ethylene induces cell-wall hydrolases → cell separation and abscission.
• ABA and jasmonates can promote senescence and sensitize tissues to abscission; cytokinins delay senescence and abscission.

Biochemical events in AZ

• Upregulation of polygalacturonase, cellulase and other pectinases that hydrolyse middle lamella (pectin-rich) and primary cell wall components.
• Deposition of a protective suberised/cork layer on the proximal side to prevent water loss and infection after organ drop.

Agricultural and ecological importance

• Fruit drop at ripening — ethylene-mediated abscission (important for timing harvest; ethephon is used to manipulate ripening/abscission).
• Leaf fall in autumn aids nutrient recycling and reduces transpiration in temperate plants.
• Flower drop and fruit thinning — natural or induced for crop management (mechanical or chemical thinning).
• Manipulation of senescence (cytokinin sprays, genetic approaches) is used to prolong shelf life (stay-green crops) and delay leaf yellowing.

Summary model

Senescence and abscission are controlled processes integrating developmental signals, environmental cues and hormonal balances (especially auxin and ethylene). Senescence mobilises nutrients away from ageing tissues; abscission is the physical detachment achieved by programmed cell separation at the abscission zone.

Fig 12 — Educational Diagram: Tissue culture and totipotency

Definition: Tissue culture (plant tissue culture) is the aseptic culture of plant cells, tissues or organs on a nutrient medium under controlled conditions to regenerate a whole plant. Totipotency is the ability of a single plant cell to divide and produce all differentiated cell types of the plant and therefore regenerate an entire organism.

Historical notes: The idea of cellular totipotency was proposed by Gottlieb Haberlandt (1902). Practical methods and media (notably Murashige and Skoog medium, 1962) and the role of growth regulators (Skoog & Miller) established reproducible plant regeneration in vitro.

Basic principles:

• Explants (small pieces of tissue such as shoot tip, leaf, stem, anther) are cultured under sterile conditions on a nutrient medium that supplies minerals, vitamins, carbon source and solidifying agent.
• Plant growth regulators (auxins and cytokinins) in specific concentrations and ratios direct cell division, callus formation, organogenesis or somatic embryogenesis.
• Totipotency: any viable differentiated cell (or protoplast) can be induced to dedifferentiate, divide, form a callus and subsequently give rise to a whole plant under suitable hormonal and nutritional conditions.

Typical steps in plant tissue culture:

1. Selection of explant (juvenile tissues often give better response).
2. Surface sterilization of explant (e.g., 0.1% mercuric chloride or 70% ethanol briefly; thorough rinsing in sterile water).
3. Preparation of culture medium (e.g., MS medium) with sucrose (carbon source) and agar (0.7–0.8%) and adjustment of pH ~5.8.
4. Inoculation of explant onto medium in aseptic conditions (laminar flow hood).
5. Callus induction (often using higher auxin or balanced auxin:cytokinin).
6. Organogenesis or somatic embryogenesis by changing hormone balance (e.g., high cytokinin for shoots, high auxin for roots).
7. Rooting of regenerated shoots and transfer to soil after acclimatization (hardening).

Medium composition and physical conditions (typical): MS salts (macro- and micro-nutrients), vitamins, 3% sucrose, agar 0.7–0.8%, pH ~5.8, temperature ~25 ± 2 °C, light/dark cycles depending on stage. Aseptic technique is essential.

Role of growth regulators:

• Auxins (IAA, NAA, 2,4-D): promote cell division, callus formation, root initiation (higher auxin relative to cytokinin favors root formation or callus).
• Cytokinins (BAP, kinetin): promote shoot formation and cell division (higher cytokinin relative to auxin favors shoot formation).
• Relative balance (ratio) of auxin : cytokinin determines morphogenic fate: high auxin : cytokinin → roots; high cytokinin : auxin → shoots; intermediate/equal → callus.

Applications:

• Micropropagation / mass clonal multiplication of elite plants (orchids, banana, sugarcane, potato).
• Production of disease-free or virus-free planting material (e.g., virus-free potato, banana).
• Somaclonal variation and breeding; somatic hybridization by protoplast fusion.
• Conservation of endangered plants, germplasm storage and synthetic seeds (encapsulated somatic embryos).
• In vitro selection and production of secondary metabolites.

Limitations and precautions: Contamination by microbes is a major problem; somaclonal variation can cause unwanted genetic changes in clones; acclimatization (hardening) is critical for survival after transfer to soil.

Connection to curriculum (Class 11): Tissue culture illustrates the totipotency concept and shows how external factors (nutrients and hormones) control plant growth and development, linking cellular potential to whole-plant regeneration.

Fig 13 — Educational Diagram: Practical experiments and applications

Practical experiments and applications in Plant Growth and Development focus on measuring growth, testing environmental and hormonal effects, and applying results to agriculture, horticulture and biotechnology. Well-designed experiments identify variables (independent, dependent, controlled), use appropriate replicates and controls, record data at regular intervals, calculate growth indices and present results as tables and graphs for interpretation.

Common practical objectives

• Measure and compare growth (length, fresh/dry weight, leaf area) over time.
• Quantify rate of growth using absolute and relative indices and plot growth curves.
• Test effects of environmental factors: light (photoperiod, intensity), temperature, water, oxygen and nutrients on germination and growth.
• Test effects of plant growth regulators (auxins, gibberellins, cytokinins, ethylene, abscisic acid) on elongation, rooting, germination, dormancy and senescence.
• Demonstrate tropisms (phototropism, geotropism) and practical rooting/propagation techniques.

Typical experimental design and methods

• Choose easily measurable parameters: shoot/root length, number of leaves, fresh weight, dry weight (after oven-drying), leaf area (graph paper or digital image analysis).
• Use multiple time points (for example daily or every 2–3 days) to capture the growth curve phases (lag, exponential, deceleration, stationary).
• Keep conditions constant for controls (same pot size, soil, water regime, temperature) and vary only the independent factor (light, hormone dose, etc.).
• Use statistical replication (≥3 replicates) and calculate mean ± standard deviation; if available, perform basic statistical tests to confirm significance.
• Convert fresh weight to dry weight for comparisons requiring biomass (dry weight removes water-content variability).
• When growth is exponential, use natural log-transformed data to obtain a straight line and compute Relative Growth Rate (RGR).

Interpretation and applications

• Growth curves help identify phases when management (fertilizer, water) is most effective.
• Hormone experiments guide practical uses: auxins for rooting cuttings, gibberellins to break dormancy or increase stem elongation, ethylene for synchronizing fruit ripening.
• Agricultural applications include breeding for desired growth habit (dwarf vs tall), manipulating photoperiod to control flowering, and using growth retardants or stimulants to optimize yield and plant form.
• Biotechnological applications: tissue culture protocols (optimizing cytokinin:auxin ratios for organogenesis), seed priming, dormancy-breaking treatments (stratification, GA3 application).

Notes on accuracy and safety

• Measure at the same time of day to avoid diurnal variation in water content and turgor.
• For biomass studies, oven-dry samples at 60–80°C until constant weight is reached to obtain dry weight.
• Handle hormones and chemical inhibitors with care; follow safety instructions and use minimal effective concentrations.