Plant Growth & Development

Growth = Irreversible increase in cell number, cell size, and organ size over time.

• Irreversible — once grown, cannot reduce back
• Conspicuous/Observable — can be measured and seen
• Involves both Catabolism and Anabolism (synthesis of new material)
• Enzyme-mediated — requires metabolic enzymes
• Can be measured (by length, weight, area, volume, cell count)

• Indeterminate / Unlimited — plant can keep growing throughout life (has meristems)
• Open growth — new cells continuously added
• Localised — growth occurs only at specific locations (meristems), not all over like animals
• Not uniform — unlike animal growth which is uniform
• Plant growth: Vegetative growth + Reproductive growth

• Water — increases cell turgidity, cell expansion (plasmolysis is growth)
• Nutrients — essential minerals (K⁺, Mg²⁺, etc.)
• O₂ — for respiration (energy)
• Temperature — plants require a range (20–40°C); enzymes functional below 40°C
• Gravity — affects directional growth
• Hormones — plant hormones control growth

Meristem = Group of actively dividing cells in plants responsible for growth.

• Differentiation — meristematic cells lose ability to divide → become permanent cells with specific functions (e.g. xylem vessels, phloem, stomata)
• Dedifferentiation — permanent cells regain ability to divide → become secondary meristem (e.g. cork cambium forms from cortex cells; vascular cambium formed in some areas)
• Redifferentiation — dedifferentiated cells differentiate again → form new permanent tissues (e.g. cork from cork cambium)
• Development = Growth + Differentiation + Senescence

Plasticity = Ability of plants to change their morphological appearance (phenotype) in response to different environmental conditions without changing genotype.

• Etiolation — new seedlings grown in absence of light show white/pale colour; different behaviour with/without light
• Heterophylly — same plant produces two types of leaves depending on environment:
  • Environmental heterophylly — Ranunculus (buttercup): air leaves vs water leaves
  • Developmental heterophylly — Cotton, Larkspur

• At every division, only ONE daughter cell divides; other differentiates
• Length increases by a constant amount each time interval
• Equation: Lt = L₀ + rt (Lt = final length, L₀ = initial length, r = growth rate, t = time)
• Graph: Linear (straight line)
• Absolute growth = constant; Relative growth = decreasing over time
• Example: root elongation

• Both daughter cells continue to divide (exponential)
• Initial slow phase → rapid exponential phase → stationary phase (resources limited)
• Equation: W₁ = W₀ × eʳᵗ (W₁ = final weight, W₀ = initial weight, r = relative growth rate)
• Graph: Sigmoid (S-shaped) curve — most plant tissues show this
• Phases of S-curve: Lag phase (slow) → Exponential phase (fast) → Stationary phase (resources limit)
• Absolute growth = increasing in geometric growth; Relative growth = constant

Plant hormones (PGRs) = Non-nutrient chemicals produced by plants in trace amounts; act as intercellular messengers to regulate growth, development, and responses.

• Promoters: Auxin, Gibberellin, Cytokinin, Ethylene (in some functions)
• Inhibitors: ABA (Abscisic acid), Ethylene (in some functions)
• Chemical types: Indole compound (Auxin), Terpene (GA), Adenine derivative/Purine (Cytokinin), Gaseous (Ethylene), Carotenoid derivative (ABA)

• Discovered accidentally by Charles Darwin & Francis Darwin (working on phototropism in coleoptile)
• Named and first isolated by F.W. Went (collected in agar block from coleoptile tip)
• First isolated from human urine by Kögl & Smith — IAA (Indole Acetic Acid)
• First natural auxin = IAA (Indole Acetic Acid)
• Other natural: IBA (Indole Butyric Acid)
• Synthetic auxins: NAA (Naphthalene Acetic Acid), 2,4-D (2,4-dichlorophenoxyacetic acid)
• Precursor: Tryptophan (amino acid)
• Site of synthesis: Shoot apical tip, young leaves, developing seeds

• Cell elongation — promotes elongation of cells (loosens cell wall via acidification mechanism → cellulose microfibrils loosen → water enters → cell elongates)
• Apical dominance — high auxin at apical bud suppresses axillary bud growth → promotes apical growth
• Phototropism — unequal distribution of auxin causes bending toward light
• Geotropism — root: +ve geotropism; stem: −ve geotropism
• Rooting — promotes adventitious root formation in stem cuttings (used in horticulture)
• Parthenocarpy — fruit development without fertilization (spray auxin → fruit forms without pollination)
• Prevents leaf/fruit abscission — high auxin in leaves prevents early leaf fall; when level falls, abscission occurs
• Weedicide — 2,4-D kills dicot weeds (monocots like grass are relatively resistant); used in grass lawns
• Promotes xylem differentiation
• Promotes flowering in pineapple

• Discovered from a disease in Japan called "Bakanae disease" (foolish seedling disease) in rice plants — symptoms: plant height much more than normal, sterile plants
• Caused by fungus Gibberella fujikuroi
• More than 100 types of gibberellins known (GA₁, GA₂, GA₃...) — first discovered: GA₃ (Gibberellic acid)
• Chemical nature: Terpene (Diterpenoid)
• Site of synthesis: Fungi and higher plants (young leaves, seeds, root and shoot tips)
• Precursor: Mevalonic acid (Terpenoid)
• Bioassay: Dwarf pea/maize test

• Overcome genetic dwarfism — spray GA on dwarf plant → normal height (phenotype changed, not genotype)
• Elongation of internodes — increases length of axis of grapes stalk (more space between grapes → better development)
• Bolting — plants with rosette habit (short internode, like cabbage) → spray GA → rapid internodal elongation → flowering before time
• Delays senescence — keeps leaves/fruits fresh longer (used in fruit industry)
• Increases fruit size — in pear, apple (seedless grapes using GA)
• Breaks seed and bud dormancy
• Hastens juvenile phase in conifers (cone-bearing plants mature early)
• Promotes flowering in Long Day Plants
• Promotes male flowers in monoecious plants
• Promotes germination of barley seeds: GA → stimulates aleurone layer → secretes α-amylase → starch converted to maltose (sugar) → germination

• First cytokinin isolated = Kinetin — from animal source (herring sperm DNA, autoclave degradation)
• First cytokinin from plant source = Zeatin — isolated from corn (Zea mays) and unripe maize seeds
• Also found in: Coconut milk, young leaves, root tips
• Chemical nature: Adenine derivative (Purine ring with nitrogen base)
• Precursor: Amino purine
• Bioassay: Tobacco pith / Tobaco callus bioassay — cytokinin promotes division of pith cells in presence of auxin
• Site of synthesis: Actively dividing cells — cambium, root tip, young leaf, developing seeds

• Promotes cell division — primary function; works along with auxin for cell division in tissue culture
• Delays senescence (ageing) of leaves — Richmond-Lang effect — cytokinin retards degradation of proteins and chlorophyll in leaves
• Promotes chloroplast division — chloroplast has its own DNA; cytokinin promotes its division
• Breaks apical dominance — promotes lateral branch growth (antagonistic to auxin on this)
• Promotes adventitious shoot formation
• Promotes nutrient mobilization — moves nutrients toward areas of high cytokinin

• Only gaseous plant hormone
• Discovery: volatile substance produced from ripened fruits that promoted ripening in nearby banana — linked to observation that ripened banana causes nearby banana to ripen
• Natural: Ethylene gas; Synthetic: Ethephon (Ethrel) — liquid that releases ethylene slowly
• Chemical precursor: Methionine (amino acid) → ACC → Ethylene
• Bioassay: Triple response in etiolated pea seedlings — (1) inhibition of elongation, (2) swelling of axis, (3) horizontal growth

• Promotes fruit ripening — most important function; climacteric fruits (mango, banana, apple) show a respiratory climacteric during ripening
• Types of ripening: Climacteric (respiratory rate increases then falls — mango, banana, apple, tomato) vs Non-climacteric (respiratory rate doesn't increase — citrus, grapes)
• Promotes abscission of leaves, flowers, and fruits
• Promotes root growth and adventitious root formation (in new roots and stem cuttings)
• Promotes flowering in pineapple (and mango)
• Breaks seed and bud dormancy
• Synchronizes fruit maturation
• Triple response in seedlings: inhibit elongation + radial swelling + horizontal growth
• Promotes female flowers in plants

Photoperiodism = Response of plants to the duration of light and darkness (day length) to initiate flowering.

• Day length = Quantity of light (not quality)
• Key parameter: Duration of darkness (night length) — NOT day length — actually determines flowering
• Light is sensed by the leaves (not meristems); signal moves to meristem as Florigen hormone
• Florigen hormone = Florigen (actual identity: FT protein)

Vernalization = Promotion of flowering in plants by exposure to prolonged cold temperature (0–4°C), preventing early (precocious) flowering out of season.

• Quantitative requirement: depends on number of cold days (weeks)
• Prevents precocious (too early) reproductive development before proper season
• Site of perception: Actively dividing meristem (not leaves)
• Hormone involved: Vernalin (hypothetical; actual = FLC/FT gene products)

• Annual plants — few months growth → flower → die (monocarpic): Wheat, Rice, Dak
• Biennial plants — require vernalization in 2nd year to flower: Cabbage, Radish, Sugar beet (vegetative in 1st year → overwintering cold → reproductive in 2nd year)
• Perennial plants — flower year after year; some need vernalization
• Wheat — winter wheat is sown in autumn → overwintering cold → flowers in spring (Growth stops in winter → resumes with cold effect)
• Rosette habit plants (like cabbage) — short internodes in 1st year; after vernalization → bolting (rapid internode elongation) → flowering

• Devernalization — effect of vernalization can be cancelled by exposure to high temperature after vernalization (before flower induction is complete)
• Vernalization ≠ photoperiodism — both are separate requirements; many plants need BOTH vernalization AND correct photoperiod to flower
• Vernalization + Photoperiodism → Florigen formation in leaves → transported to meristem → flowering

Seed Dormancy = Condition where a seed is NOT able to germinate even under favourable conditions (water, temperature, O₂).

• Reasons for dormancy:
  • Very hard seed coat — prevents water/O₂ entry
  • Embryo development not completed
  • Presence of germination inhibitors (ABA, phenolic acid, plant ascorbic acid)
  • Abscisic acid (ABA) present in seed

• Scarification — breaking/scratching hard seed coat (mechanical or acid treatment)
• Exposure to cold temperature (stratification)
• Light treatment
• Removal of inhibitors by washing
• Application of GA — breaks dormancy and promotes germination (antagonistic to ABA)
• Application of Ethylene — breaks dormancy
• Seed coat removal