Plant Growth and Development (NEET Biology Class 11): The Five Plant Growth Regulators
Growth: Definition, Phases & Rates
📌 NEET priority: Important. 1–2 questions a year, but they are almost always about the five plant growth regulators — who discovered each, and which specific application belongs to which PGR. The table below is the whole chapter.
Growth — definition and why plants are special
Growth is an IRREVERSIBLE permanent increase in size of an organ, its parts, or a single cell, accompanied by metabolism at the expense of energy. (So a piece of wood swelling in water is not growth — it's reversible.)
Plant growth is indeterminate — plants retain unlimited growth capacity for life because of meristems whose cells divide and self-perpetuate. This is the open form of growth. Apical meristems drive primary growth (elongation); lateral meristems (vascular cambium and cork cambium, in dicots and gymnosperms) drive secondary growth (girth).
Growth is measured by fresh/dry weight, length, area, volume or cell number. Two vivid NCERT numbers: a single maize root apical meristem can produce more than 17,500 new cells per hour (growth as cell number), while cells in a watermelon may increase in size up to 350,000 times (growth as cell size).
The three phases of growth
Phase | Location (root tip) | Characteristics |
Meristematic | At the root/shoot apex | Constantly dividing cells, rich in protoplasm, large conspicuous nuclei, thin primary cellulosic walls, abundant plasmodesmata |
Elongation | Just proximal to the meristematic zone | Increased vacuolation, cell enlargement, new cell wall deposition |
Maturation | Further proximal, beyond elongation | Cells attain maximal size, with wall thickening and protoplasmic modification |
Growth rates — arithmetic vs geometric
Arithmetic growth: after mitosis, only ONE daughter cell continues to divide while the other differentiates. Plotting length against time gives a linear curve: Lt = L0 + rt, where r is growth rate per unit time.
Geometric growth gives an exponential (log) phase, and overall growth follows a sigmoid (S-shaped) curve with three phases — lag, log (exponential), and stationary/senescent.
Absolute growth rate = total growth per unit time. Relative growth rate = growth per unit time per unit initial parameter — this is what lets you fairly compare a small and a large organ.
Differentiation, Plasticity & the Discovery of PGRs
Differentiation, dedifferentiation and redifferentiation
Differentiation: cells derived from meristems mature to perform specific functions.
Dedifferentiation: living differentiated cells regain the capacity to divide — e.g. interfascicular cambium and cork cambium forming from fully differentiated parenchyma.
Redifferentiation: those dedifferentiated cells again lose the capacity to divide and mature into specific functions.
Because differentiation in plants is open (a cell's fate depends on its position), development is flexible. Development = growth + differentiation.
Plasticity & heterophylly
Plasticity is the ability of plants to follow different pathways to form different structures in response to environment or life phase. Heterophylly — different leaf shapes on the same plant — is the classic example, seen in larkspur and buttercup.
Discovery of the five PGRs — all accidental
PGR | Discovered by | How |
Auxin | Charles Darwin & Francis Darwin; isolated by F.W. Went | Canary grass coleoptiles bent towards unilateral light (phototropism); the TIP was found to be the source of the transmittable influence. Went isolated auxin from oat coleoptile tips |
Gibberellin | E. Kurosawa (1926) | The 'bakanae' (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi — sterile fungal filtrates reproduced the symptoms |
Cytokinin | F. Skoog and co-workers; kinetin crystallised by Miller et al. (1955) | Tobacco stem callus proliferated only when auxin was supplemented with vascular tissue extract, yeast extract, coconut milk or DNA |
Abscisic acid (ABA) | Three independent groups, mid-1960s | Inhibitor-B, abscission II and dormin were purified separately, then proved to be chemically identical — renamed abscisic acid |
Ethylene | H.H. Cousins (1910) | A volatile substance from ripened oranges hastened ripening of stored unripe bananas |
Chemical nature: auxin = indole compound (IAA); cytokinin = adenine derivative (N⁶-furfurylamino purine, kinetin); ABA = carotenoid derivative; gibberellic acid (GA₃) = a terpene; ethylene (C₂H₄) = a gas. Promoters = auxins, gibberellins, cytokinins. Inhibitor = ABA. Ethylene is largely an inhibitor but fits either group.
The Five Plant Growth Regulators & Why This Matters for NEET
The five PGRs — the complete reference table
PGR | Key physiological effects | Named applications NEET asks |
Auxins (IAA, IBA natural; NAA, 2,4-D synthetic) | Apical dominance (apical bud suppresses lateral buds); controls xylem differentiation; helps cell division | Rooting in stem cuttings; promotes flowering in PINEAPPLE; prevents early fruit/leaf drop but promotes abscission of OLDER leaves/fruits; induces parthenocarpy in TOMATO; 2,4-D as herbicide kills dicot weeds without affecting mature monocots |
Gibberellins (GA₁, GA₂, GA₃… — over 100 known; all acidic) | Increase length of the axis; delay senescence; promote bolting (internode elongation just before flowering) in beet, cabbage and rosette plants | Lengthens GRAPE stalks; elongates and improves shape of APPLE; speeds malting in the BREWING industry; sprayed on SUGARCANE raises yield by up to 20 tonnes per acre; hastens maturity of juvenile conifers for early seed production |
Cytokinins (kinetin — a modified adenine/purine) | Specific effect on cytokinesis; promote nutrient mobilisation; help OVERCOME apical dominance (opposite of auxin) | Delay leaf senescence; used in tissue culture with auxin for callus proliferation |
Ethylene (gas; source compound = ethephon) | Horizontal growth of seedlings, swelling of axis, apical hook formation in dicot seedlings; promotes senescence and abscission; breaks seed and bud dormancy; promotes root growth and root hair formation | Fruit ripening in TOMATO and APPLE (raises respiration — the 'respiratory climactic'); initiates germination in PEANUT seeds and sprouting of POTATO tubers; rapid internode elongation in DEEP WATER RICE; flowering/fruit-set synchronisation in PINEAPPLE and flowering in MANGO; promotes FEMALE flowers in cucumber; thinning of cotton, cherry, walnut |
Abscisic acid (ABA) — the 'stress hormone' | General plant growth INHIBITOR and metabolic inhibitor; inhibits seed germination; stimulates STOMATAL CLOSURE; increases stress tolerance; role in seed development, maturation and dormancy | Induces dormancy so seeds withstand desiccation; acts as an ANTAGONIST to gibberellins in most situations |
Why this matters for NEET
High-value one-liners: growth is irreversible; arithmetic growth = only one daughter cell keeps dividing; auxin causes apical dominance while cytokinin overcomes it; 2,4-D kills dicot weeds only; ethephon is the ethylene source; ABA = stress hormone and closes stomata; Gibberella fujikuroi caused bakanae disease.
Trap: ABA and gibberellins are antagonists — ABA induces dormancy and inhibits germination, GA breaks it. Similarly auxin and cytokinin are antagonistic on apical dominance. Also, ethylene increases respiration during ripening (respiratory climactic) — it does not suppress it. And dedifferentiation is regaining the ability to divide, while redifferentiation is losing it again — students routinely swap these.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Growth is best defined as:
(a) Swelling of a tissue in water
(b) Any increase in size, reversible or not
(c) An irreversible permanent increase in size
(d) Only an increase in cell number
Show answer
Answer: (c) — Growth is an IRREVERSIBLE permanent increase in size — which is why wood swelling in water does not count as growth.
Q2. The open form of growth in plants is possible because of:
(a) Large vacuoles
(b) The absence of centrioles
(c) Thick cell walls
(d) The presence of meristems whose cells divide and self-perpetuate
Show answer
Answer: (d) — Meristems retain the capacity to divide and self-perpetuate, giving plants indeterminate open growth.
Q3. In arithmetic growth, after mitotic division:
(a) The cell dies
(b) Both daughter cells continue to divide
(c) Neither daughter cell divides
(d) Only one daughter cell continues to divide while the other differentiates
Show answer
Answer: (d) — Arithmetic growth means only one daughter cell keeps dividing; the other differentiates and matures, giving a linear curve.
Q4. The mathematical expression for arithmetic growth is:
(a) Lt = r x t^2
(b) Lt = L0 / rt
(c) Lt = L0 + rt
(d) Lt = L0 x rt
Show answer
Answer: (c) — Arithmetic growth follows Lt = L0 + rt, where L0 is length at time zero and r is the elongation per unit time.
Q5. A single maize root apical meristem can produce approximately how many new cells per hour?
(a) 350,000
(b) 175
(c) 17,500
(d) 1,750
Show answer
Answer: (c) — A maize root apical meristem can give rise to more than 17,500 new cells per hour.
Q6. Growth per unit time expressed per unit initial parameter is called:
(a) Relative growth rate
(b) Absolute growth rate
(c) Geometric growth
(d) Arithmetic growth
Show answer
Answer: (a) — Relative growth rate is measured per unit initial parameter, allowing fair comparison between organs of different starting size.
Q7. The formation of interfascicular cambium from fully differentiated parenchyma is an example of:
(a) Redifferentiation
(b) Differentiation
(c) Dedifferentiation
(d) Senescence
Show answer
Answer: (c) — Dedifferentiation is when living differentiated cells regain the capacity to divide, as in interfascicular and cork cambium formation.
Q8. Heterophylly, seen in larkspur and buttercup, is an example of:
(a) Senescence
(b) Apical dominance
(c) Dedifferentiation
(d) Plasticity
Show answer
Answer: (d) — Plasticity is the ability to form different structures in response to environment; heterophylly (different leaf forms) is its classic example.
Q9. The bakanae or 'foolish seedling' disease of rice, which led to the discovery of gibberellins, is caused by:
(a) Xanthomonas oryzae
(b) Puccinia graminis
(c) Agrobacterium tumefaciens
(d) Gibberella fujikuroi
Show answer
Answer: (d) — E. Kurosawa (1926) linked the disease to the fungus Gibberella fujikuroi, whose sterile filtrates reproduced the symptoms.
Q10. Auxin was isolated from tips of oat seedling coleoptiles by:
(a) Charles Darwin
(b) H.H. Cousins
(c) F.W. Went
(d) F. Skoog
Show answer
Answer: (c) — The Darwins showed the coleoptile tip was the source of the influence; F.W. Went isolated auxin from oat coleoptile tips.
Q11. Apical dominance, in which the growing apical bud suppresses lateral bud growth, is caused by:
(a) Abscisic acid
(b) Cytokinin
(c) Auxin
(d) Ethylene
Show answer
Answer: (c) — Auxin causes apical dominance; removal of the shoot tip (decapitation) releases the lateral buds — the basis of tea plantation pruning.
Q12. Which growth regulator helps OVERCOME apical dominance?
(a) Gibberellin
(b) Abscisic acid
(c) Auxin
(d) Cytokinin
Show answer
Answer: (d) — Cytokinins help overcome apical dominance — the opposite effect to auxin.
Q13. 2,4-D is widely used as a herbicide because it:
(a) Only prevents seed germination
(b) Kills dicotyledonous weeds without affecting mature monocots
(c) Kills only monocots
(d) Kills all plants equally
Show answer
Answer: (b) — 2,4-D kills dicot weeds but does not affect mature monocotyledonous plants — hence its use for weed-free lawns.
Q14. Spraying sugarcane with gibberellins can increase yield by up to:
(a) It does not affect yield
(b) 2 tonnes per acre
(c) 20 tonnes per acre
(d) 200 tonnes per acre
Show answer
Answer: (c) — Gibberellins increase sugarcane stem length, raising yield by as much as 20 tonnes per acre.
Q15. The rise in respiration rate during fruit ripening, caused by ethylene, is called:
(a) Photorespiration
(b) Respiratory climactic
(c) Bolting
(d) Oxidative phosphorylation
Show answer
Answer: (b) — Ethylene enhances the respiration rate during ripening — this rise is called the respiratory climactic.
Q16. Abscisic acid is called the stress hormone because it:
(a) Promotes fruit ripening
(b) Promotes rapid growth
(c) Stimulates stomatal closure and increases stress tolerance
(d) Breaks seed dormancy
Show answer
Answer: (c) — ABA stimulates closure of stomata and increases plant tolerance to various stresses — hence the name stress hormone.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q17. Which plant growth regulator is a gas? (NEET PYQ)
(a) Auxin
(b) Gibberellic acid
(c) Abscisic acid
(d) Ethylene
Show answer
Answer: (d) — Ethylene (C2H4) is the gaseous PGR; ethephon is the most widely used compound that releases it.
Q18. Which PGR induces parthenocarpy in tomatoes? (NEET PYQ)
(a) Ethylene
(b) Cytokinin
(c) Auxin
(d) Abscisic acid
Show answer
Answer: (c) — Auxins induce parthenocarpy (fruit development without fertilisation), e.g. in tomatoes.
Q19. Bolting, the elongation of internodes just prior to flowering, is promoted by: (NEET PYQ)
(a) ABA
(b) Cytokinin
(c) Auxin
(d) Gibberellin
Show answer
Answer: (d) — Gibberellins promote bolting in beet, cabbage and other rosette-habit plants.
Q20. Abscisic acid generally acts as an antagonist to: (NEET PYQ)
(a) Gibberellins
(b) Cytokinins
(c) Auxins
(d) Ethylene
Show answer
Answer: (a) — In most situations ABA acts as an antagonist to gibberellins — ABA induces dormancy, GA breaks it.
Q21. Which PGR promotes female flowers in cucumber, increasing yield? (NEET PYQ)
(a) Cytokinin
(b) Gibberellin
(c) Auxin
(d) Ethylene
Show answer
Answer: (d) — Ethylene promotes female flowers in cucumbers, thereby increasing the yield.
Q22. Kinetin, a cytokinin, is a modified form of: (NEET PYQ)
(a) Adenine (a purine)
(b) A terpene
(c) Indole
(d) A carotenoid
Show answer
Answer: (a) — Kinetin is a modified form of adenine, a purine — specifically N6-furfurylamino purine.
Q23. The sigmoid growth curve consists of which three phases? (NEET PYQ)
(a) Lag, log (exponential) and stationary/senescent
(b) Primary, secondary, tertiary
(c) Meristematic, elongation, maturation
(d) Arithmetic, geometric, relative
Show answer
Answer: (a) — The idealised sigmoid growth curve has a lag phase, a log or exponential phase, and a stationary/senescent phase.
Q24. Ethylene promotes rapid internode elongation, keeping leaves above water, in: (NEET PYQ)
(a) Deep water rice plants
(b) Conifers
(c) Desert cacti
(d) Rosette plants
Show answer
Answer: (a) — Ethylene promotes rapid internode/petiole elongation in deep water rice, helping the shoot stay above water.
Active Recall Prompt
Write everything you can recall about Plant Growth and Development, naming each part first: growth (its definition, why plant growth is indeterminate, how it is measured, the three phases, arithmetic vs geometric growth and the sigmoid curve, absolute vs relative growth rate); differentiation, dedifferentiation and redifferentiation; plasticity and heterophylly; the discovery of each of the five PGRs (who, and how); and the physiological effects and named applications of auxins, gibberellins, cytokinins, ethylene and abscisic acid. Begin each fact with its topic and end it with a full stop.