# Plant Growth and Development (NEET Biology Class 11): The Five Plant Growth Regulators Source: https://tracknation.in/story/plant-growth-and-development-neet-biology-class-11-the-five-plant-growth-regulators (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Plant Growth and Development, mined from NCERT: growth (its definition, why plant growth is indeterminate, the three phases in a table, arithmetic vs geometric growth and the sigmoid curve, absolute vs relative growth rate), differentiation/dedifferentiation/redifferentiation, plasticity and heterophylly, a table of how each of the five PGRs was discovered, and a complete reference table of the physiological effects and named agricultural applications of auxins, gibberellins, cytok ## 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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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 **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.