Chapter 8: Plant Growth and Development

"

Introduction

Development and Germination

• Development = Growth + Differentiation.

• Plant develops from zygote to mature form with roots, leaves, 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 emerge above soil), Hypogeal (cotyledons remain underground).

• Photoblastic seeds require light to germinate.

• Seed viability = ability 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 above soil; Hypogeal = cotyledons below soil.

• Photoblastic seeds need light to germinate.

Quick Checkpoints

• What are the two components of development?

• Difference between epigeal and hypogeal germination?

• What factors affect seed germination?

Growth

Definition and Types

• Growth = irreversible, permanent increase in size of organ, part, or cell.

• Accompanied by metabolic processes requiring energy.

• Plant growth is indeterminate (continuous) and occurs at 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 can enlarge 350,000 times.

• Pollen tube growth measured by length; leaf growth by surface area.

Memory Aids

• Growth = size increase + metabolism.

• Primary = length; Secondary = girth.

• Meristems = growth centers.

Quick Checkpoints

• Define growth in plants.

• What is the difference between primary and secondary growth?

• Where are meristems located?

Phases of Growth

Growth Phases

• Meristematic phase: active cell division at root and shoot apices; cells have dense protoplasm, large nuclei, thin primary walls.

• Elongation phase: cells enlarge by vacuolation and new wall deposition; enlargement may be isodiametric or linear.

• Differentiation/maturation phase: cells develop specialized structures and functions; wall thickening and protoplasmic changes occur.

Memory Aids

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

Quick Checkpoints

• What characterizes the meristematic phase?

• What happens during elongation?

• How do cells change in differentiation?

Growth Rates

Types and Measurement

• Growth rate = increase in growth per unit time.

• Arithmetic growth: one daughter cell divides, other differentiates; linear increase; e.g., root elongation.

• Geometrical growth: both daughter cells divide; exponential increase; shows lag, log (exponential), and stationary phases.

• Sigmoid (S-curve) growth typical in natural conditions.

• Geometrical growth formula: final size depends on initial size and growth rate.

• Absolute growth rate: total growth per unit time.

• Relative growth rate: growth per unit initial size per unit time.

Memory Aids

• Arithmetic = linear growth; Geometrical = exponential growth.

• Growth rate = absolute or relative.

• S-curve = lag, log, stationary phases.

Quick Checkpoints

• Difference between arithmetic and geometrical growth?

• What is absolute vs relative growth rate?

• Describe phases of sigmoid growth curve.

Conditions for Plant Growth

Essential Factors

• Nutrients: macro and micronutrients for cell building and energy.

• Water: maintains cell turgidity, enables enzyme activity.

• 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, temperature, oxygen, gravity = key growth factors.

Quick Checkpoints

• List essential factors for plant growth.

• Why is water important for growth?

• How does gravity affect plant growth?

Differentiation, Dedifferentiation and Redifferentiation

Differentiation

• Permanent changes in cell structure, chemistry, and function.

• Controlled by gene activation or repression.

• Example: Tracheary elements lose protoplasm and develop strong lignocellulosic secondary walls for water conduction.

Dedifferentiation

• Reversal of specialization; cells regain division ability.

• Dedifferentiated tissues act as meristems (e.g., vascular cambium, cork cambium, wound cambium).

• In culture, parenchyma cells form dividing callus mass.

Redifferentiation

• Specialization of cells from dedifferentiated meristems.

• Similar to primary meristem differentiation.

• Produces secondary phloem, xylem, cork, and cortex.

Commitment or Determination

• Same apical meristem cells form diverse tissues (xylem, phloem, parenchyma, sclerenchyma, fibers, collenchyma).

• Determined by cell location and signal reception.

• Example: Root apical meristem distal cells form root cap; peripheral cells form epidermis, cortex, endodermis, pericycle, vascular tissues.

Memory Aids

• Differentiation: Permanent specialization via gene control.

• Dedifferentiation: Cells revert to dividing state (meristem-like).

• Redifferentiation: New specialization from dedifferentiated cells.

• Commitment: Location-based cell fate decision.

Quick Checkpoints

• What defines differentiation in plant cells?

• How does dedifferentiation enable tissue regeneration?

• What tissues arise from redifferentiation?

• How does commitment influence cell type formation?

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 developmental stages: seed germination → seedling → juvenile phase → maturation → flowering → seed formation → senescence.

• Phase conversion is development, e.g., leaf initiation to expansion, vegetative to flowering phase.

• Development occurs at cellular level, e.g., chloroplast appearance in sunlight-exposed cells.

• Development ends with senescence and death.

• Controlled by intrinsic factors (genes, growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition).

• Plants show plasticity: different structures develop in response to growth phases and environment.

• Example: heterophylly—different leaf forms in juvenile and mature plants or due to environment.

This image shows the developmental sequence in a plant cell from meristematic cell to cell death:

• Meristematic cell divides to form new cells.

• Cells grow in size (plasmatic growth).

• Cells differentiate to become specialized.

• Cells expand or elongate.

• Cells mature fully.

• Mature cells undergo senescence (aging).

• Cells die naturally.

Examples of heterophylly:

• Larkspur: juvenile leaves are simple; mature leaves are complex.

• Buttercup: leaves differ by habitat—broad flat leaves on land, divided finely branched leaves in water.

• Shows plant adaptation via development to growth stages and environment.

Memory Aids

• Development = Formation + Growth + Differentiation + Maturation + Senescence + Death.

• Plasticity = Ability to form different structures based on environment or growth phase.

• Heterophylly = Different leaf types on same plant due to age or environment.

Quick Checkpoints

• What stages constitute plant development?

• How do intrinsic and extrinsic factors influence development?

• Define plasticity in plant development.

• Give examples of heterophylly and their significance.

• Describe the sequence of cellular development from meristematic cell to death.

Plant Growth Regulators

Overview

• Plant growth regulators (PGRs) are small molecules regulating growth, differentiation, and development in plants at low concentrations.

• Also called plant hormones or phytohormones.

• Produced naturally and translocated to regulate physiological reactions.

• Divided into two groups: growth promoters and growth inhibitors.

Memory Aids

• PGRs = Plant growth controllers at low doses.

• Promoters: auxins, gibberellins, cytokinins; Inhibitors: abscisic acid, ethylene.

Quick Checkpoints

• What are the two main groups of plant hormones?

• Define phytohormone.

Auxins

Nature 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).

Location and Bioassay

• Found in shoot apices, leaf primordia, developing seeds.

• Synthesized from amino acid tryptophan.

• Bioassays: Avena curvature test, root growth inhibition test.

Physiological Functions

• Stimulate cell division in cambium and elongation in shoots.

• Induce early xylem and phloem differentiation.

• Initiate rooting in stem cuttings; inhibit root growth at high concentration.

• Inhibit axillary bud growth; promote apical dominance.

• Promote flowering in pineapples and fruit enlargement.

• Retard senescence and early abscission; promote abscission in mature organs.

• Induce parthenocarpy (fruit without fertilization) and feminizing effect in some plants.

• Synthetic auxins act as selective herbicides (weedicides).

• Differential auxin distribution causes phototropism and geotropism.

• Stimulate respiration and metabolism by mobilizing resources.

Memory Aids

• Auxins = Apical dominance + rooting + fruit growth.

• High auxin inhibits roots; low auxin promotes roots.

• 2,4-D = synthetic auxin and herbicide.

Quick Checkpoints

• What is the main natural auxin?

• How do auxins affect apical dominance?

• What bioassays test auxin activity?

Gibberellins

Nature and Discovery

• Weakly acidic hormones with gibberellin ring structure.

• Discovered by Kurosawa (1926) from fungus Gibberella fujikuroi.

• GA₃ is the most studied gibberellin.

• Over 100 gibberellins found in fungi and plants.

Location and Bioassay

• Produced in apical shoot buds, young leaves, root tips, developing seeds.

• Synthesized from mevalonic acid (from acetyl Co-A).

• Transported by diffusion and conducting channels.

• Bioassays: Barley endosperm test, dwarf pea test.

Physiological Functions

• Enhance seed germination by increasing α-amylase synthesis, overcoming dormancy.

• Induce seed germination even in dark (photoblastic seeds).

• Cause bolting (rapid internode elongation) in rosette plants.

• Overcome genetic dwarfism, promoting stem elongation.

• Promote maleness in female plants.

• Delay senescence, extending fruit market period.

• Used in brewing to speed malting.

• Increase grape stalk length and improve fruit shape.

• Increase sugarcane stem length and yield.

• Hasten maturity and seed production in conifers.

Memory Aids

• Gibberellins = Germination + Growth + Male promotion.

• GA₃ breaks dormancy and causes bolting.

Quick Checkpoints

• What is the main function of gibberellins in seeds?

• How do gibberellins affect dwarf plants?

• Which bioassays are used for gibberellins?

Cytokinins

Nature 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.

• >

• First cytokinin: kinetin (synthetic), natural cytokinin: zeatin from unripe maize and coconut milk.

Location and Bioassay

• Found in regions of rapid cell division: root apices, shoot buds, young fruits.

• Bioassays: Tobacco pith culture, retardation of leaf senescence, radish cotyledon expansion.

Physiological Functions

• Enhance cell division and morphogenesis (initiation of plant organs).

• Increase seedling width, new leaves, chloroplast formation.

• Promote lateral and adventitious shoot growth.

• Overcome apical dominance.

• Induce femaleness in male plants.

• Retard senescence (Richmond-Lang effect) and abscission.

• Increase mobilization of food materials.

• Prolong life of cut shoots and flowers; keep vegetables fresh.

Memory Aids

• Cytokinins = Cell division + Shoot growth + Delay senescence.

• Zeatin and kinetin are key cytokinins.

Quick Checkpoints

• What is the role of cytokinins in apical dominance?

• Which bioassays test cytokinin activity?

• How do cytokinins affect senescence?

Ethylene

Nature and Discovery

• Only gaseous phytohormone stimulating transverse growth and inhibiting longitudinal growth.

• Synthesized from amino acid methionine.

• Discovered by H.H. Cousins (1910) and identified as ethylene by R. Gane (1934).

• Recognized as plant hormone by Crocker et al. (1935).

Location and Bioassay

• Present in all plant parts in minimal amounts; maximum during fruit ripening and senescence.

• Bioassays: Triple response in etiolated pea plants, gas chromatographic assay.

Physiological Functions

• Causes swelling, horizontal growth, and apical hook formation 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 in peanut seeds and potato tubers.

• Increases female flowers and fruit set in cucumber.

• Used to initiate flowering and synchronize fruit set in pineapple and mango.

• Promotes internode elongation in deep water rice, helping shoots stay above water.

• Enhances root growth and root hair formation.

• Commercially used as ethephon to hasten fruit ripening and abscission.

• Formed in roots under waterlogged conditions, inducing adventitious roots.

Memory Aids

• Ethylene = Gas hormone for ripening and abscission.

• Ethephon = Ethylene source in agriculture.

Quick Checkpoints

• What is the triple response caused by ethylene?

• How does ethylene affect fruit ripening?

• What is the agricultural use of ethephon?

Abscisic Acid

Nature and Discovery

• Mildly acidic growth inhibitor, also called stress hormone.

• Production increases under drought, waterlogging, and stress.

• First isolated from cotton bolls by Addicott et al. (1963).

Location and Bioassay

• Found throughout plant, abundant in chloroplasts of green cells.

• Synthesized from mevalonic acid or xanthophyll.

• Bioassays: Rice seedling growth inhibition, inhibition of α-amylase synthesis in barley endosperm.

Physiological Functions

• Induces abscission of plant parts.

• Induces dormancy in seeds and buds, limiting germination.

• Inhibits flowering in long-day plants; stimulates in some short-day plants.

• Causes partial stomatal closure under drought, acting as antitranspirant.

• Important in seed development, maturation, and desiccation tolerance.

• Promotes leaf senescence.

• Counteracts many effects of gibberellins (GA₃).

• Acts antagonistically to auxins, gibberellins, and cytokinins in growth regulation.

• Induces dormancy; gibberellins overcome dormancy.

• Inhibits cambial activity and fruit development promoted by other hormones.

• Causes stomatal closure; cytokinins cause opening.

Memory Aids

• ABA = Abscission + Dormancy + Drought response.

• Opposes gibberellins and cytokinins.

Quick Checkpoints

• What role does ABA play in seed dormancy?

• How does ABA affect stomata under drought?

• Which hormones act antagonistically to ABA?

"