Biology · BotanyBOT 08
Plant Growth & Hormones
Auxins, gibberellins, cytokinins, ABA, ethylene — plant growth regulators and sigmoid growth curve
Concept Core
Essential theory — everything NCERT tests on Plant Growth & Hormones
GROWTH CURVE & GROWTH RATES
Sigmoid growth curve: Lag → Log (exponential) → Stationary phases.
Arithmetic growth: L = L₀ + rt (one cell stays meristematic).
Geometric/exponential: W₁ = W₀ · ert (both daughter cells divide).
RGR = (W₁−W₀)/(W₀·t). Meristematic cells have highest RGR.
AUXINS
Produced in shoot apex (IAA). Promotes cell elongation via H⁺ pump (acid growth). Apical dominance: high auxin suppresses lateral buds. Polar basipetal transport in shoot.
Synthetic: 2,4-D (dicot weedicide), NAA (fruit set), IBA (root initiation). Low [auxin] → promotes root; higher → promotes shoot; even higher → inhibits all.
GIBBERELLINS
GA promotes internode elongation (bolting of rosette plants). Stimulates α-amylase in aleurone layer of barley seeds → starch breakdown → germination. Breaks dormancy. Promotes male flowers. Induces parthenocarpy in some plants.
CYTOKININS
Promotes cell division (cytokinesis). Synthesised in root apex, transported up. Delays senescence (Richmond-Lang effect — stays green). Tissue culture: high cytokinin/auxin ratio → shoot. High auxin/cytokinin → root. Equal → callus.
ABA & ETHYLENE
ABA: stress hormone. Promotes seed dormancy, stomatal closure, abscission. Inhibits growth.
Ethylene: gaseous PGR. Triple response in etiolated seedlings (swelling + horizontal + reduced elongation). Promotes fruit ripening (climacteric), abscission, senescence, epinasty. Bioassay = triple response.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Hormone Roles
Auxin: elongation, apical dominance
Gibberellin: bolting, α-amylase, dormancy break
Cytokinin: cell division, delay senescence
ABA: dormancy, stress, stomata close
Ethylene: ripening, abscission, triple response
Synthetic PGRs
2,4-D: herbicide (auxin mimic)
NAA: fruit set (auxin)
IBA: root initiation (auxin)
Ethephon: releases ethylene commercially
Growth Formulae
Arithmetic: L = L₀ + rt
Geometric: W₁ = W₀·e^rt
RGR = ΔW/(W₀·t)
Absolute GR = dW/dt
Worked Examples
NEET-style questions solved step-by-step
EASYTriple response in etiolated seedlings is caused by:▾
Triple response in etiolated seedlings is caused by:
Ethylene. Classic bioassay for ethylene. Response = (1) swelling of stem, (2) horizontal growth, (3) reduced elongation. Occurs in dark-grown (etiolated) seedlings.
MEDIUMGA₃ is applied to a rosette plant. What happens?▾
GA₃ is applied to a rosette plant. What happens?
Rosette plants have compressed internodes. GA₃ causes bolting — rapid elongation of internodes. The plant grows tall rapidly, often flowers early. GA overcomes genetic dwarfism.
HARDIn tissue culture, a high auxin:cytokinin ratio promotes:▾
In tissue culture, a high auxin:cytokinin ratio promotes:
Skoog & Miller: High auxin/low cytokinin → root formation. High cytokinin/low auxin → shoot. Equal ratio → undifferentiated callus growth.
Mistake DNA
Common NEET traps for this chapter
⚠ ABA vs Ethylene for abscission
Both promote abscission but via different mechanisms. ABA is the main stress/dormancy signal. Ethylene specifically triggers the abscission zone senescence.
✓ Fix: ABA = drought/stress. Ethylene = senescence/ripening/abscission zone activation
⚠ Auxin concentration biphasic effect
Auxin does NOT always promote growth. At high concentrations, auxin inhibits (apical dominance, herbicide effect of 2,4-D).
✓ Fix: Draw the concentration-response curve — bell-shaped
⚠ Gibberellin always breaks dormancy
GA breaks dormancy in most seeds but some seeds (light-requiring) also need phytochrome. GA stimulates α-amylase specifically in cereal aleurone.
✓ Fix: Barley aleurone + GA → α-amylase is the classic NEET example
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. Ethylene triple response, GA bolting/α-amylase, cytokinin Richmond-Lang effect, 2,4-D weedicide, tissue culture auxin:cytokinin ratio.
Must Know
2,4-D = herbicide. GA = bolting + α-amylase. ABA = stress hormone. Ethylene = gaseous PGR, triple response bioassay. Cytokinin delays senescence.
Strategy
Build a 5-column table per hormone: source, main effect, commercial use, bioassay, discovery scientist. Covers ~90% of NEET questions.
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