Chapter 8: Plant Growth and Development

Introduction

Development and Germination

• Development = Growth + Differentiation.
• Mature plant develops from zygote via ordered events forming roots, leaves, branches, flowers, fruits, seeds.
• Development controlled by intrinsic (internal) and extrinsic (external) factors.
• Seed germination activates embryo growth into seedling under favorable conditions.
• Types of germination: Epigeal (cotyledons above soil), Hypogeal (cotyledons below soil).
• Photoblastic seeds require light to germinate.
• Seed viability = capacity to germinate and develop.
• Favorable conditions: water, oxygen, suitable temperature, food, growth regulators, rest period completion.
• Unfavorable conditions cause suspended growth until favorable.

Memory Aids

• Development = Growth + Differentiation.
• Epigeal = cotyledons emerge; Hypogeal = cotyledons stay underground.
• Photoblastic seeds need light.

Quick Checkpoints

• What are the two components of development?
• Difference between epigeal and hypogeal germination?
• What conditions favor seed germination?

Growth

Definition and Types

• Growth = irreversible permanent increase in size of organ, part, or cell.
• Accompanied by metabolic processes using energy.
• Plant growth is indeterminate (continuous) in higher plants via meristems.
• Open form of growth: new cells added by meristem activity.
• Primary growth: elongation by apical and intercalary meristems.
• Secondary growth: increase in girth by lateral meristems (vascular cambium, cork cambium).
• Growth measurable by fresh/dry weight, length, area, volume, cell number.
• Examples: maize root apical meristem produces 17,500+ cells/hour; watermelon cells enlarge 350,000 times.

Memory Aids

• Growth = size increase + metabolism.
• Primary = length; Secondary = girth.
• Meristems drive growth.

Quick Checkpoints

• Define growth in plants.
• What is primary vs secondary growth?
• Role of meristems in growth?

Phases of Growth

Growth Phases

• Meristematic phase: active cell division, high biosynthesis, thin primary walls.
• Elongation phase: cells enlarge, vacuolation increases, new wall deposition.
• Differentiation/maturation phase: cells specialize, wall thickening, protoplasm modification.

Memory Aids

• Meristematic = divide; Elongation = enlarge; Differentiation = specialize.

Quick Checkpoints

• What happens in meristematic phase?
• How do cells change in elongation phase?
• What defines differentiation phase?

Growth Rates

Types and Measurement

• Growth rate = increase in growth per unit time.
• Arithmetic growth: one daughter cell divides, other differentiates; linear increase.
• Geometrical growth: both daughter cells divide; exponential increase; shows lag, log, stationary phases.
• Sigmoid (S-curve) growth typical in natural conditions.
• Growth expressed as absolute growth rate (total growth/time) and relative growth rate (growth per unit initial size/time).

Memory Aids

• Arithmetic = linear growth; Geometrical = exponential growth.
• Absolute growth rate = total growth/time.
• Relative growth rate = growth per unit initial size/time.
• S-curve = lag, log, stationary phases.

Quick Checkpoints

• Difference between arithmetic and geometrical growth?
• What phases are in sigmoid growth curve?
• Define absolute and relative growth rates.

Conditions for Plant Growth

Essential Factors

• Nutrients: macro and micronutrients for cell building and energy.
• Water: maintains cell turgor, enzyme function, growth.
• Temperature: optimal 28-30°C; extremes inhibit growth.
• Oxygen: required for respiration and energy production.
• Light: needed for pigment synthesis and growth regulation.
• Gravity: guides root and shoot orientation.
• Other factors: salt concentration and mineral availability affect growth.

Memory Aids

• Water, nutrients, light, oxygen, temperature, gravity essential.
• Optimal temperature 28-30°C.

Quick Checkpoints

• List key factors for plant growth.
• Why is water important for growth?
• How does gravity affect plant growth?

Differentiation, Dedifferentiation and Redifferentiation

Differentiation

• Permanent qualitative changes in cell structure, chemistry, and physiology.
• Occurs by activation or repression of specific genes.
• Example: Tracheary elements lose protoplasm and develop strong lignocellulosic secondary walls for water conduction.

Dedifferentiation

• Despecialisation of differentiated cells to regain division capacity.
• Dedifferentiated tissues act as meristems (e.g., interfascicular vascular cambium, cork cambium, wound cambium).
• In culture, parenchyma cells dedifferentiate to form callus (mass of dividing cells).

Redifferentiation

• Specialisation of cells derived from dedifferentiated meristems.
• Similar to differentiation by primary meristems.
• Forms secondary tissues like secondary phloem, secondary xylem, cork, and secondary cortex.

Commitment or Determination

• Same apical meristem cells produce different cell types (xylem, phloem, parenchyma, sclerenchyma, fibres, collenchyma).
• Determined by location and specific signals received.
• Example: Cells distal to root apical meristem form root cap; peripheral cells form epidermis, cortex, endodermis, pericycle, vascular tissues.

Memory Aids

• Differentiation = Permanent specialization.
• Dedifferentiation = Reversal to division-capable state.
• Redifferentiation = New specialization after dedifferentiation.
• Commitment = Location-based cell fate decision.

Quick Checkpoints

• What triggers differentiation in cells?
• How does dedifferentiation enable tissue regeneration?
• What tissues form by redifferentiation?
• What determines different cell types from the same meristem?

Development

Concept and Process

• Development is a sequence of changes in structure and function of an organism, organ, tissue, or cell.
• Includes formation, growth, differentiation, maturation, reproduction, senescence, and death.
• Plant stages: seed germination, seedling, juvenile, maturation, flowering, seed formation, senescence.
• Phase conversion (e.g., leaf initiation to expansion) is also development.
• Occurs at cellular level (e.g., chloroplast appearance in sunlight-exposed cells).
• Development ends in senescence and death.
• Controlled by intrinsic factors (genes, growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition).
• Development pathways vary with environment and life phases, showing plasticity.
• Plasticity example: heterophylly—different leaf forms in juvenile vs mature plants or air vs water leaves.

Memory Aids

• Development = Formation + Growth + Differentiation + Maturation + Senescence + Death
• Plasticity = Ability to form different structures based on environment or growth phase
• Heterophylly = Different leaf shapes in one plant

Quick Checkpoints

• What stages are included in development?
• How do intrinsic and extrinsic factors influence development?
• Define plasticity in plant development.
• Give examples of heterophylly.

Development Sequence in Plant Cell

• Starts with meristematic cell capable of division.
• Cell divides to form new cells.
• Cells grow in size (plasmatic growth).
• Cells differentiate to become specialized.
• Cells expand or elongate.
• Cells mature into fully developed cells.
• Mature cells age (senescence).
• Cells die naturally at cycle end.

Memory Aids

• Meristematic cell → Division → Growth → Differentiation → Expansion → Maturation → Senescence → Death

Quick Checkpoints

• What is the first stage of cell development?
• What happens after cell division?
• Define senescence in cells.

Heterophylly Examples

• Larkspur: juvenile leaves are simple; adult leaves are complex.
• Buttercup: leaf shape varies by habitat—broad flat leaves on land, divided finely branched leaves in water.
• Shows plant adaptation to growth stages and environment.

Memory Aids

• Heterophylly = Different leaf types on one plant
• Environmental heterophylly = Leaf shape changes with habitat

Quick Checkpoints

• What is heterophylly?
• How does Buttercup leaf shape change with environment?
• Give an example of heterophylly in Larkspur.

Plant Growth Regulators

Overview

• Plant growth regulators (PGRs) are small molecules regulating growth, differentiation, and development in plants.
• Also called plant hormones or phytohormones.
• Produced naturally, active in low concentrations, and translocated to target regions.
• Divided into plant growth promoters and inhibitors based on function.

Memory Aids

• Promoters: Auxins, Gibberellins, Cytokinins.
• Inhibitors: Abscisic Acid, Ethylene.

Quick Checkpoints

• What are phytohormones?
• Difference between growth promoters and inhibitors?

Auxins

Characteristics and Discovery

• Weakly acidic hormones with unsaturated ring structure.
• Natural auxins: Indole-3-acetic acid (IAA), Indole-3-butyric acid (IBA).
• Synthetic auxins: NAA, 2,4-D, MCPA.
• Discovered by Darwin (1880) and E.W. Went (1928) via coleoptile experiments.
• Auxin moves basipetally (tip to base).
• IAA synthesized from tryptophan.
• Bioassays: Avena curvature test, root growth inhibition test.

Memory Aids

• Auxin = "Apex Up" (promotes apical dominance).
• IAA = Indole ring + Acetic acid group.

Quick Checkpoints

• What is the role of auxins in apical dominance?
• How does auxin affect root growth?

Physiological functions

Auxin Effects

• Stimulates cell division in cambium and elongation in shoots.
• Induces early xylem and phloem differentiation.
• Initiates rooting in stem cuttings; inhibits root growth at high concentration.
• Inhibits axillary bud growth; promotes apical dominance.
• Decapitation overcomes apical dominance (used in tea plantations, hedge making).
• Promotes flowering in pineapples and early fruit formation.
• Retards senescence and early abscission; promotes abscission in mature organs.
• Induces parthenocarpy (e.g., tomato) and feminizing effect in some plants.
• Synthetic auxins (2,4-D, 2,4,5-T) act as selective herbicides.
• Differential auxin distribution causes phototropism and geotropism.
• Stimulates respiration and metabolism by mobilizing resources.

Memory Aids

• Auxin promotes "Apex Dominance" and rooting.
• 2,4-D = selective dicot weed killer.

Quick Checkpoints

• How does auxin regulate apical dominance?
• What is the effect of auxin on senescence?

Gibberellins

Characteristics and Discovery

• Weakly acidic hormones with gibberellin ring structure.
• Discovered by Kurosawa (1926) from Gibberella fujikuroi fungus.
• GA₃ is the most studied gibberellin.
• Produced in apical shoot buds, young leaves, root tips, developing seeds.
• Synthesized from mevalonic acid (acetyl Co-A derivative).
• Transported by diffusion and conducting channels.
• Bioassays: Barley endosperm test, dwarf pea test.

Memory Aids

• GA = "Grow Axis" (promotes stem elongation).
• GA₃ overcomes dwarfism and dormancy.

Quick Checkpoints

• How does GA₃ affect seed germination?
• What is bolting in plants?

Physiological functions

Gibberellin Effects

• Enhances seed germination by increasing α-amylase synthesis, overcoming dormancy.
• Induces seed germination even in dark (photoblastic seeds).
• Causes bolting (rapid internode elongation) in rosette plants.
• Overcomes genetic dwarfism (e.g., maize, pea).
• Promotes maleness in female plants.
• Delays senescence, extending fruit market period.
• Used in malting industry to speed up process.
• Improves fruit shape and stalk length (e.g., grapes, apples).
• Increases sugarcane stem length and yield.
• Hastens maturity and seed production in juvenile conifers.

Memory Aids

• GA = "Germination Activator".
• Bolting = GA-induced stem elongation.

Quick Checkpoints

• What role does gibberellin play in dormancy?
• How does GA affect fruit development?

Cytokinins

Characteristics and Discovery

• Basic hormones derived from aminopurine or phenyl urea promoting cytokinesis.
• Discovered by F. Skoog et al. observing callus proliferation with yeast extract and DNA.
• Kinetin (synthetic) discovered from degraded herring sperm DNA.
• Natural cytokinin: Zeatin from unripe maize grains and coconut milk.
• Found in regions of rapid cell division: root apices, shoot buds, young fruits.
• Bioassays: Tobacco pith culture, leaf senescence retardation, radish cotyledon expansion.

Memory Aids

• Cytokinins = "Cell Kick-starters" (promote division).
• Zeatin and Kinetin are key cytokinins.

Quick Checkpoints

• Where are cytokinins mainly found?
• How do cytokinins affect apical dominance?

Physiological functions

Cytokinin Effects

• Enhances cell division and morphogenesis.
• Increases seedling width and promotes new leaves and chloroplast formation.
• Stimulates lateral and adventitious shoot growth.
• Overcomes apical dominance.
• Induces femaleness in male plants.
• Retards senescence (Richmond-Lang effect) and abscission.
• Increases mobilization of food materials.
• Prolongs life of cut shoots and flowers; keeps vegetables fresh.

Memory Aids

• Cytokinins = "Cell division and delay senescence".

Quick Checkpoints

• What is the role of cytokinins in senescence?
• How do cytokinins influence shoot growth?

Ethylene

Characteristics and Discovery

• Only gaseous phytohormone.
• Stimulates transverse growth; inhibits longitudinal growth.
• Synthesized from methionine.
• Discovered by H.H. Cousins (1910) and identified as ethylene by R. Gane (1934).
• Produced in minimal amounts throughout plant; peaks during fruit ripening and senescence.
• Bioassays: Triple response in etiolated pea, gas chromatographic assay.

Memory Aids

• Ethylene = "E for Ethephon and Ripening".

Quick Checkpoints

• What is the triple response caused by ethylene?
• How does ethylene affect fruit ripening?

Physiological functions

Ethylene Effects

• Causes swelling and horizontal growth; forms apical hook in dicot seedlings.
• Promotes senescence and abscission of leaves and flowers.
• Increases respiration rate during fruit ripening (respiratory climacteric).
• Overcomes seed and bud dormancy; initiates germination and sprouting.
• Increases female flowers and fruit set in cucumber.
• Used to synchronize flowering and fruit set in pineapple and mango.
• Promotes internode elongation in deep water rice to keep shoots above water.
• Stimulates root growth and root hair formation.
• Commercially used as ethephon for fruit ripening and abscission.
• Induces adventitious root formation in waterlogged roots.

Memory Aids

• Ethylene = "Gas hormone for ripening and abscission".

Quick Checkpoints

• What role does ethylene play in seed dormancy?
• How is ethylene used in agriculture?

Abscisic Acid

Characteristics and Discovery

• Mildly acidic growth inhibitor, also called stress hormone.
• Production increases under drought, waterlogging, and stress.
• Discovered by Addicott et al. (1963) from cotton bolls.
• Found abundantly in chloroplasts; synthesized from mevalonic acid or xanthophyll.
• Bioassays: Rice seedling growth inhibition, α-amylase synthesis inhibition in barley endosperm.

Memory Aids

• ABA = "Abscission and Dormancy Agent".
• Stress hormone that closes stomata.

Quick Checkpoints

• How does ABA induce dormancy?
• What is ABA's role in stomatal regulation?

Physiological functions

Abscisic Acid Effects

• Induces abscission of plant parts.
• Limits seed germination; induces dormancy in seeds and buds.
• Inhibits flowering in long-day plants; stimulates in some short-day plants.
• Causes partial stomatal closure under drought (antitranspirant).
• Important in seed development, maturation, and desiccation tolerance.
• Promotes leaf senescence; antagonizes gibberellin effects.
• Regulates growth synergistically or antagonistically with other hormones.
• Auxins, gibberellins, cytokinins act synergistically; ABA antagonizes them.
• Auxins, cytokinins, ethylene feminize plants; gibberellins promote maleness.
• Cytokinins and auxins prevent senescence; ABA and ethylene induce it.
• Cytokinins open stomata; ABA closes stomata.
• ABA induces dormancy; gibberellins overcome dormancy.
• Auxins, gibberellins, cytokinins promote cambial activity and fruit development; ABA inhibits.

Memory Aids

• ABA = "Antagonist to growth promoters".
• Induces dormancy and senescence.

Quick Checkpoints

• How does ABA interact with gibberellins?
• What hormones regulate dormancy and senescence?