Introduction to Growth and Development
Growth is one of the most fundamental features of all living organisms. In plants, growth is closely tied to development, which involves cell division, enlargement, differentiation, and morphogenesis. Together, growth and development transform a single fertilised egg (zygote) into a complete adult plant with all its tissues, organs, and reproductive structures.
Definition of Growth
Growth is an irreversible permanent increase in size, mass, or volume of a cell, organ, or whole organism. It results from cell division and cell enlargement.
Key features of plant growth:
- Indeterminate / open growth: plants keep growing throughout their life. New organs (leaves, branches, roots) keep forming even in old plants.
- Localised growth: growth is confined to specific regions called meristems (root tip, shoot tip, intercalary meristem, lateral cambium).
- Diffuse vs apical: in plants, growth is mostly apical (at tips). In animals, it is mostly diffuse.
- Quantifiable: can be measured as increase in length, area, mass, or cell number.
- Distinct from development: growth = quantitative; development = qualitative (cells become specialised, organs form).
Phases of Growth
Plant growth at root and shoot tips happens in three sequential phases (also called zones in roots). The phases overlap but are distinct enough to identify under a microscope.
1. Meristematic Phase
- Location: at the very tip (root apex / shoot apex). Root apex is covered by a root cap.
- Cell state: cells are small, isodiametric (cube-shaped), with thin cell walls, dense protoplasm, prominent nuclei, and few or no vacuoles.
- Activity: rapid mitosis. New cells are continuously produced.
- Function: source of all new cells.
2. Phase of Elongation
- Location: just behind the meristematic phase.
- Cell state: cells are larger, elongated, with a large central vacuole. Cell walls become slightly thicker.
- Activity: cells stop dividing and start elongating rapidly by taking up water in the central vacuole.
- Function: most of the actual length increase happens here. Pushes the meristematic tip deeper into the soil (root) or upward (shoot).
3. Phase of Maturation
- Location: above the elongation phase (further from the tip).
- Cell state: cells reach final size and differentiate into specific cell types.
- Activity: differentiation. Tracheary elements (vessels, tracheids) lose protoplasm; root hairs form on epidermis; vascular bundles become functional.
- Function: the zone of root hairs and functional vascular tissue.
NEET trap: which phase has root hairs?
Root hairs appear in the maturation phase, NOT in the meristematic or elongation phases. They are extensions of mature epidermal cells.
Phases of growth: meristematic, elongation, maturation
Plant growth happens in three distinct zones at the root or shoot tip. Click each zone to see cell state, examples, and how the zones overlap.
Meristematic Zone
Position: At the very tip (root apex / shoot apex)
Cells divide rapidly. They are small, thin-walled, with dense protoplasm and prominent nuclei. This is the source of all new cells.
Cell state:
• Small, isodiametric (cube-shaped)
• Thin primary walls
• Dense cytoplasm with prominent nucleus
• High mitotic activity
• No vacuoles or very small vacuoles
Where to find it:
• Root apical meristem (covered by root cap)
• Shoot apical meristem (apex of stem)
• Cells with active mitosis
NEET key facts
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Three growth phases: meristematic → elongation → maturation. (Also called the three zones in roots.)
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Meristematic phase: cells DIVIDE actively. Small cells, dense protoplasm, no vacuoles.
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Elongation phase: cells STRETCH (take up water in central vacuole). Most of the actual length increase happens here.
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Maturation phase: cells DIFFERENTIATE into specific tissues (vessels, sieve tubes, root hairs).
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Root cap PROTECTS the meristematic zone (covers the root tip).
Try this
- Click "Meristematic": notice the SMALL, BOX-SHAPED cells at the very tip. This is where mitosis is happening.
- Click "Elongation": notice the cells become RECTANGULAR (stretched). This is where roots actually push deeper into soil.
- Click "Maturation": notice the ROOT HAIRS appearing. This zone is also where xylem and phloem mature into functional tissue.
Growth Rates: Arithmetic and Geometric
The rate of growth (the increase in size per unit time) can take two main mathematical forms in plants.
Arithmetic Growth
- After each mitotic division, only ONE of the two daughter cells continues to divide. The other matures and exits the cell cycle.
- Rate of growth is constant.
- Graph: linear (straight line).
- Formula: Lt = L0 + r·t where L0 = initial length, r = growth rate, t = time.
- Example: a root growing at a constant rate.
Geometric Growth
- After each mitotic division, BOTH daughter cells continue to divide.
- Rate of growth is proportional to the current size.
- Graph: exponential (J-shaped, accelerating).
- Formula: W1 = W0 · e^(r·t) where W0 = initial size, r = growth rate.
- Example: log phase of population or organ growth, bacterial culture.
Absolute vs Relative Growth Rate
- Absolute growth rate (AGR): total increase per unit time. Just dL/dt.
- Relative growth rate (RGR): increase per unit current size per unit time. (1/L) · dL/dt.
- RGR is fairer when comparing organs of different starting sizes.
- Example: a 5 mm structure that grows to 10 mm has the same RGR as a 50 mm structure growing to 100 mm (both doubled), even though the absolute growth differs.
Growth rate calculator: arithmetic vs geometric
Compare the two main growth patterns. In arithmetic growth, only one daughter cell divides further. In geometric (exponential) growth, both divide. Adjust initial size, rate, and time to see how they differ.
Initial size (L0): 10
Growth rate (r): 2
Time (t): 10
Final size at t = 10
30.00
L = 10 + 2 × 10 = 30.00
Arithmetic vs Geometric: side by side
Arithmetic
• Only ONE daughter cell divides further
• Constant rate of growth
• Linear (straight-line) graph
• Formula: Lt = L0 + r·t
• Example: root growing at constant rate
Geometric / Exponential
• BOTH daughter cells divide further
• Rate ∝ current size
• Exponential (J-shaped) graph
• Formula: W1 = W0 · e^(r·t)
• Example: log phase of growth, bacterial culture
NEET key facts
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Arithmetic growth: only ONE daughter cell continues to divide; rate is constant; linear curve.
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Geometric growth: BOTH daughter cells continue to divide; rate is proportional to current size; exponential curve.
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Arithmetic formula: Lt = L0 + r·t. Geometric formula: W1 = W0 · e^(r·t).
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Absolute growth rate: per unit time. Relative growth rate: per unit size per unit time (allows fair comparison of small and big organs).
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Real organisms show geometric growth in log phase (resources abundant) and arithmetic / stationary as they reach limits.
Try this
- Set initial = 10, rate = 2, time = 10. In arithmetic mode: 10 + 2×10 = 30. Toggle to geometric: 10 × e^(2) ≈ 73. Same starting size, much bigger result with geometric.
- Notice how arithmetic gives a straight line and geometric gives a J-shape. That J-shape is the signature of exponential growth.
- In real plants, the early "log" phase of the sigmoid curve is geometric. As resources run out, growth slows and becomes more arithmetic, then plateaus.
Sigmoid (S-shaped) Growth Curve
When growth is plotted against time for a typical organ, organism, or population, the result is an S-shaped curve called the sigmoid growth curve. It has three phases.
- Lag phase: slow initial growth. Cells preparing to divide. Curve has low slope.
- Log / exponential phase: rapid growth. Both daughter cells dividing (geometric growth). Curve is steepest here.
- Stationary phase: growth plateaus. Resources become limiting. Cell division balanced by maturation. Curve flattens.
Why is the curve sigmoidal?
The lag phase is slow because cells need time to set up division machinery. The log phase is rapid because resources are abundant and both daughter cells divide. The stationary phase is slow because resources run out, space is limiting, and many cells exit the cell cycle. The S-shape captures all three.
Sigmoid (S-shaped) growth curve explorer
Drag the time slider to scrub through the three phases of growth. Watch the marker move along the S-curve and the active phase highlight.
Time on growth curve: 50 (0 to 100)
Log / Exponential phase
When: Middle of growth
Rapid growth as both daughter cells of every division also divide. Resources are abundant. The growth rate is at its maximum.
Characteristics:
• Maximum rate of growth
• Both daughter cells divide (geometric / exponential)
• Steepest slope of the S-curve
• Doubling time is shortest in this phase
Why: Resources (water, nutrients, light) are abundant. No competition or limitation yet. Cells double in number rapidly.
NEET key facts
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Sigmoid curve = S-shaped curve = typical growth curve. Three phases: lag, log (exponential), stationary.
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Lag phase: slow initial growth, cells preparing.
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Log phase: rapid growth, geometric / exponential, both daughter cells divide.
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Stationary phase: growth slows due to limited resources, cells reach equilibrium.
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The exponential phase represents geometric growth; lag and stationary represent natural slowing.
Try this
- Drag the slider from 0 to 100. Watch the marker move along the S-curve and the highlighted phase change. Notice the steepness peaks during log phase.
- Set the slider to t = 50 (middle of log phase): this is when growth is fastest. Set to t = 90 (stationary): notice the curve has flattened.
- The S-curve is the classic representation of any organism's growth. Bacteria, plant organs, populations, even tumour cells follow this pattern.
Differentiation, Dedifferentiation, Redifferentiation
These three closely related but distinct processes describe how plant cells change their state from dividing to specialised and back. They reflect plant cell totipotency: every plant cell carries the genetic information to make a whole new plant.
Differentiation
- Meristematic cells (which divide actively) develop into permanent specialised cells.
- They lose the capacity to divide and gain specific functions.
- Examples: meristem → tracheary elements (vessels, tracheids; lose protoplasm), sieve tubes (lose nucleus), sclerenchyma (thick walls), parenchyma (thin walls), cork (suberised).
Dedifferentiation
- Mature differentiated cells, which had stopped dividing, regain their dividing capacity.
- Examples:
- Cork cambium forms from cortical cells.
- Interfascicular cambium forms from medullary ray cells.
- Callus is produced from any plant tissue in tissue culture.
- Wound healing cells around the damaged area.
- Demonstrates plant cell totipotency.
Redifferentiation
- Dedifferentiated cells (which had regained dividing capacity) once again lose this capacity and mature into specialised cells.
- Examples: secondary xylem and secondary phloem are produced by the dedifferentiated vascular cambium, which redifferentiates into vessels, sieve tubes, etc. Cork cambium produces cork cells which are redifferentiated.
Differentiation, dedifferentiation, redifferentiation
Three closely related but distinct processes. Click each to see the cell state changes, examples, and how they show plant cell totipotency.
Differentiation
Meristematic → Mature specialised
Meristematic cells (which divide actively) develop into permanent specialised cells. They lose the capacity to divide and gain specific functions. For example, meristematic cells become tracheary elements (vessels, tracheids), losing protoplasm to function as efficient water conductors.
Cell state: Cell becomes mature, specialised, often non-dividing
Examples:
• Meristem → Vessels and tracheids (water conduction; cells lose protoplasm)
• Meristem → Sieve tubes (food conduction; remain alive but lose nucleus)
• Meristem → Sclerenchyma (mechanical strength; thick lignified walls)
• Meristem → Parenchyma (storage; living cells with thin walls)
• Meristem → Cork cells (protective; suberised walls, lose protoplasm)
Plant cell totipotency
Plant cells are totipotent: every cell carries the genetic information to make a whole new plant. Dedifferentiation is the process by which this totipotency is realised. Tissue culture relies on this: a small piece of any plant tissue can be made to dedifferentiate into callus and then redifferentiate into a complete plant.
NEET key facts
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Differentiation: meristematic cells → mature specialised cells (e.g., vessels, tracheids, sclerenchyma).
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Dedifferentiation: mature cells regain dividing capacity (cork cambium from cortex, callus in tissue culture).
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Redifferentiation: dedifferentiated cells specialise again (secondary xylem/phloem from cambium).
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Plant cells are TOTIPOTENT (any cell can make a new plant). Dedifferentiation demonstrates this.
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Dead conducting cells (vessels, tracheids) lose their protoplasm during differentiation - this is irreversible.
Try this
- Click "Differentiation": meristem cells specialise (one-way; vessels lose protoplasm and become dead).
- Click "Dedifferentiation": cork cambium forms from cortical cells (a mature cell BECOMES dividing again). This is what tissue culture uses.
- Click "Redifferentiation": vascular cambium produces new xylem and phloem cells. Their daughter cells specialise again - that is redifferentiation.
Development and Plasticity
Development is the sum total of growth + differentiation. It is the orderly progression from seed to mature plant with all its tissues and organs.
- Development includes germination, vegetative growth, flowering, fruit set, senescence, and abscission.
- Plant development is highly plastic: the same plant can produce different forms in different conditions.
- Example: heterophylly in coriander / cotton: aquatic and aerial leaves of the same plant look different. The plant adjusts to the environment.
- Buttercup (Ranunculus) leaves: simple in air, deeply dissected underwater.
- Plant development is governed by intrinsic factors (hormones, genetic) and extrinsic factors (light, temperature, water, nutrients).
Plasticity
The ability of a plant to alter its development based on the environment is called plasticity or phenotypic plasticity. It is one reason why the same plant can show different forms (heterophylly) in air versus water, in shade versus sun, etc.
Plant Growth Regulators (PGRs): Overview
Plant growth regulators (PGRs) are small organic chemicals produced in tiny amounts by plants. They control most aspects of growth, differentiation, and response to environmental cues. There are FIVE major PGRs for NEET.
- Growth-promoting: auxin, gibberellin, cytokinin (called the PGAs - promote growth, antagonised by ABA)
- Growth-inhibiting: abscisic acid (ABA - the stress hormone)
- Gaseous: ethylene (the only gas; the ripening hormone)
How PGRs Differ from Animal Hormones
- Plants do not have specific endocrine glands. PGRs are produced in many tissues (root tips, leaves, fruits, etc.).
- PGRs are not delivered by a circulatory system; they spread by polar transport, diffusion, or through the phloem / xylem.
- The same PGR can have different effects in different tissues (context-dependent).
- The ratio of PGRs (e.g., auxin / cytokinin) often matters more than the absolute amount.
Auxin (IAA)
Auxin was the FIRST plant hormone to be discovered. The discovery story (Darwin → Boysen-Jensen → Paal → Went) is itself a NEET favourite.
Discovery
- Charles & Francis Darwin (1880): coleoptile of canary grass bends towards light; tip senses, base bends.
- Boysen-Jensen (1913): showed signal is a chemical (passes through gelatin, blocked by mica).
- Paal (1919): asymmetric tip = bending in dark; signal can be unevenly distributed.
- F.W. Went (1928): isolated the substance using agar block. Named it auxin (from Greek "auxein" = to grow). Used the Avena coleoptile bend test.
- Kogl (1934): identified the chemical as indole-3-acetic acid (IAA).
Natural and Synthetic Auxins
- Natural: IAA (indole-3-acetic acid; the main natural auxin), IBA (indole butyric acid).
- Synthetic: 2,4-D (2,4-dichlorophenoxyacetic acid; a herbicide), NAA (naphthalene acetic acid; for rooting).
Effects of Auxin
- Cell elongation (especially in coleoptiles and stems).
- Apical dominance: auxin from the shoot tip suppresses lateral bud growth (gives a single tall stem).
- Phototropism and geotropism: uneven distribution of auxin causes bending.
- Adventitious root initiation in stem cuttings (used in horticulture).
- Parthenocarpy: seedless fruit development (used commercially in tomato).
- Xylem differentiation.
- Prevents fruit and leaf drop (sprayed before harvest).
Commercial Applications
- 2,4-D as broadleaf weedicide: kills dicots, spares monocots like wheat / rice. The classic NEET application.
- NAA / IBA for rooting: stem cuttings dipped in NAA paste develop adventitious roots.
- IAA for parthenocarpy: seedless tomatoes.
- Promotes flowering in pineapples (atypical).
- Apical dominance utilised in tea plantations: nipping the apex makes the plant bushier (more leaves).
Phototropism: classical experiments leading to auxin discovery
Click through the 6 famous experiments that led to the discovery of auxin. From Darwin (1880) to Went (1928), each experiment narrowed down the source, nature, and identity of the bending signal.
Charles & Francis Darwin (1880)
Setup:
Coleoptile of canary grass exposed to one-sided light. Tip intact.
Observation:
Coleoptile bends towards the light.
Conclusion:
Coleoptile bends towards light (positive phototropism). The tip is the photoreceptive region.
The discovery of auxin: timeline
- 1880 - Darwin: Coleoptile bends to light; tip senses, base bends.
- 1913 - Boysen-Jensen: Signal is chemical (passes through gelatin, blocked by mica).
- 1919 - Paal: Asymmetric tip placement causes bending in dark - signal can be unevenly distributed.
- 1928 - F.W. Went: Isolates the chemical using agar block. Names it AUXIN. Quantifies it via the "Avena coleoptile bend test."
- 1934 - Kogl: Identifies the chemical as INDOLE-3-ACETIC ACID (IAA).
NEET key facts
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Darwin (1880): coleoptile of canary grass / Avena bends to light. Tip senses, base bends.
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Boysen-Jensen (1913): chemical signal (passes through gelatin, blocked by mica).
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Paal (1919): asymmetric tip = asymmetric distribution = bending even in DARK.
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F.W. Went (1928): isolated the chemical (named it AUXIN). Used Avena coleoptile bend test.
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IAA (indole-3-acetic acid) was later identified as the natural auxin.
Try this
- Click through the 6 experiments in order: Darwin → Darwin-cap → Darwin-base → Boysen-Jensen → Paal → Went. See how each narrowed down the answer.
- Boysen-Jensen used GELATIN (chemical can pass) vs MICA (chemical cannot pass). The contrast proved the signal is chemical.
- F.W. Went isolated the chemical using AGAR (chemical diffuses out of tip into agar). He coined the name AUXIN. NEET often asks who isolated auxin: F.W. Went, 1928.
Gibberellins (GA)
Gibberellins were discovered from a fungal disease in rice. Today over 100 GAs are known; GA3 (gibberellic acid) is the most studied.
Discovery
- Eiichi Kurosawa (Japan, 1926): studied "foolish seedling" / bakanae disease in rice. Infected plants grew abnormally tall and weak. The fungus was Gibberella fujikuroi (now Fusarium fujikuroi).
- The fungal substance causing the abnormal growth was named gibberellin.
- Today, gibberellins are produced in healthy plants too (in young leaves, roots, developing seeds).
Effects of Gibberellin
- Bolting: sudden internode elongation just before flowering in rosette plants. Examples: cabbage, beetroot, lettuce.
- Stem and internode elongation (reverses dwarfism in pea).
- Breaks seed dormancy: induces alpha-amylase synthesis in barley aleurone for starch breakdown.
- Breaks bud dormancy in deciduous trees.
- Promotes flowering in long-day plants under non-inductive (short-day) conditions.
- Promotes maleness in flowers (more male flowers in cucurbits).
- Increases fruit size (commercial use in grapes).
Commercial Applications
- Sugarcane: GA3 increases the length of internodes; sugar yield can rise by up to 20 tonnes per acre. Major commercial use.
- Brewing industry: GA accelerates malting in barley (alpha-amylase induction increases sugar release for fermentation).
- Grape industry: GA3 spray makes seedless grapes (Thompson, Sultana) larger.
- Hastens flowering and fruit setting in apples and pears.
Cytokinins
Cytokinins are derivatives of adenine. They were named for their ability to induce cytokinesis (cell division) in tissue culture.
Discovery
- Skoog and Miller (1955): autoclaved herring sperm DNA could induce cytokinesis in tobacco pith tissue cultures. They named the active compound kinetin.
- Letham: identified zeatin in coconut milk and corn kernels.
Effects of Cytokinin
- Cell division (cytokinesis) in tissue culture.
- Delays senescence (Richmond-Lang effect): detached leaves treated with cytokinin stay green for much longer. Stabilises chlorophyll, RNA, and protein.
- Counteracts apical dominance: high cytokinin / auxin ratio promotes lateral bud growth.
- Helps mineral mobilisation from older to younger parts of the plant.
- Promotes chloroplast development (chlorophyll synthesis).
- Breaks seed dormancy in some species.
Commercial Applications
- Tissue culture: cytokinin / auxin ratio determines what the explant produces. High cytokinin / low auxin → shoots; low cytokinin / high auxin → roots; equal ratio → callus (undifferentiated mass).
- Delays senescence in cut flowers and leafy vegetables (longer shelf life).
- Increases yield in some crops.
Abscisic Acid (ABA)
Abscisic acid is the only major growth-INHIBITING hormone among the five PGRs. It is also called the stress hormone.
Discovery
- Originally given two names independently by different researchers:
- Wareing isolated it as dormin (a bud dormancy inducer).
- Addicott isolated it as abscisin II (an abscission accelerator).
- Both substances were later found to be the same; renamed abscisic acid (ABA).
Effects of ABA
- Stomatal closure during water stress: ABA causes K+ efflux from guard cells, making them flaccid. Stoma closes, water loss is reduced. THIS IS THE KEY ROLE.
- Promotes seed dormancy: high ABA in seeds keeps them dormant.
- Promotes bud dormancy in deciduous trees (helps survive winter).
- Inhibits seed germination (antagonist of GA).
- Promotes leaf and fruit abscission.
- Inhibits stem and root elongation.
- Antagonises growth-promoting hormones.
NEET trap: ABA does NOT cause abscission directly
Despite the name "abscisic acid," ABA is NOT the main cause of abscission; ethylene is. ABA's main role is the stress response (stomatal closure, dormancy). The historical name is misleading.
Ethylene
Ethylene (C₂H₄) is the only gaseous plant hormone. Because it is a gas, it spreads easily through plant tissues and even between adjacent fruits.
Discovery
- Detected first in coal gas (which contains ethylene).
- Cousins (1910): noticed that oranges shipped with bananas ripened the bananas. The mediator was ethylene gas.
- Today, ethylene is known to be produced naturally by ripening fruits, ageing leaves, and damaged tissues.
Effects of Ethylene
- Fruit ripening (especially climacteric fruits: banana, apple, mango, tomato). Causes softening, sweetening, colour change, aroma.
- Promotes leaf and fruit abscission.
- Promotes senescence (ageing).
- Breaks seed and bud dormancy in some species.
- Promotes femaleness in cucurbits (more female flowers, more fruits).
- Promotes flowering in pineapples (atypical; in most plants ethylene inhibits flowering).
- Triple response in seedlings: shorter, thicker stem, horizontal growth. Helps seedlings push through soil.
Commercial Applications
- Ethephon (2-chloroethylphosphonic acid): a liquid that, when sprayed, slowly releases ethylene. Used to ripen tomatoes, mangoes, apples uniformly.
- Pineapple flowering: ethephon synchronises fruit-set in pineapples (improves harvest).
- Femaleness in cucurbits: ethylene increases female flower number, increasing yield.
- Hastens fruit drop for easier harvest (cherries, rubber).
- Climacteric vs non-climacteric: climacteric fruits show a burst of ethylene at the start of ripening (banana, apple, mango); non-climacteric do not (citrus, grapes, strawberry).
Plant growth regulators (PGRs) explorer
Click any of the 5 PGRs (auxin, gibberellin, cytokinin, ABA, ethylene) to explore its discoverer, source, effects, applications, and NEET traps.
Auxin (IAA)
Type: Growth-promoting
Discovery
By: Charles Darwin & Francis Darwin (1880, phototropism); F.W. Went (1928, isolation)
From: Coleoptile tips of Avena (oats)
Natural forms
IAA (indole-3-acetic acid), IBA (indole butyric acid)
Biological effects
• Cell elongation (especially in coleoptiles and stems)
• Apical dominance (suppresses lateral buds)
• Phototropism and geotropism (uneven distribution causes bending)
• Initiation of adventitious roots in stem cuttings
• Promotes parthenocarpy (seedless fruit development)
• Xylem differentiation
Agricultural / Commercial uses
• 2,4-D as broadleaf weedicide (kills dicots, spares monocots like wheat)
• NAA / IBA for rooting in stem cuttings
• Prevent fruit and leaf drop (sprayed before harvest)
• Induce parthenocarpy in tomato
• Promotes flowering in pineapples
NEET FAVOURITE FACTS
• Auxin was the FIRST plant hormone to be discovered.
• Apical dominance is the classic auxin effect; cytokinin counteracts it.
• 2,4-D is a synthetic auxin and a popular weedicide.
Quick reference: discoverers and sources
Auxin
Charles Darwin & Francis Darwin
Coleoptile tips of Avena (oats)
Gibberellin
Eiichi Kurosawa
Gibberella fujikuroi (a fungus that causes "bakanae" / foolish seedling in rice)
Cytokinin
Skoog and Miller
Autoclaved herring sperm DNA (kinetin); coconut milk and corn kernels (zeatin)
Abscisic Acid
Wareing
Cotton bolls, sycamore leaves
Ethylene
H.H. Cousins
Detected first in coal gas; produced naturally by ripening fruits
Try this
- Click each hormone in turn. Notice the discoverer-source-effect-application patterns. NEET asks all four for each PGR.
- Key trick: Auxin (IAA) - apical dominance + 2,4-D weedicide. Gibberellin - bolting + alpha-amylase. Cytokinin - cell division + delays senescence.
- ABA = stress hormone (stomatal closure, dormancy). Ethylene = gaseous ripening hormone. Memorise these by their FIRST KEY WORD.
Hormone matching: effect → hormone
For each effect, click the hormone that causes it. Check your answers when done.
Match 0 / 16 done
Alpha-amylase synthesis in barley
Phototropism (uneven distribution)
Rooting in stem cuttings
Delayed senescence (Richmond-Lang effect)
Flowering in pineapples
Lateral bud growth (counters apical dominance)
Parthenocarpy
Bolting in rosette plants
Stomatal closure during drought
Climacteric fruit ripening
Seed dormancy
Ethephon (slow release in fruits)
Apical dominance
Cell division in tissue culture
Sugarcane yield boost (20 t/acre)
Stress hormone (drought, salinity)
Try this
- Memory anchors: AUXIN-apical / phototropism / 2,4-D weedicide / parthenocarpy. GIBBERELLIN-bolting / amylase / sugarcane.
- CYTOKININ-cell division / Richmond-Lang (delays senescence) / lateral buds. ABA-stress (stomatal closure) / dormancy.
- ETHYLENE-ripening (climacteric) / pineapple flowering / ethephon. These are the highest-frequency NEET items.
Photoperiodism
Photoperiodism is the response of plants to the relative length of day and night (the photoperiod). It governs many developmental processes, but most importantly flowering.
Three Categories of Plants
- Long-day plants (LDP): flower when day length is LONGER than a critical value. Bloom in summer. Examples: wheat, barley, oats, radish, spinach, henbane.
- Short-day plants (SDP): flower when day length is SHORTER than a critical value. Bloom in autumn / early winter. Examples: rice, soybean, cotton, chrysanthemum, tobacco, Xanthium, coffee.
- Day-neutral plants (DNP): not affected by day length. Flower based on age. Examples: tomato, cucumber, sunflower, maize.
It is actually the DARK PERIOD that matters
Although called "photoperiodism" or "day length response," experiments show that it is the LENGTH OF THE DARK PERIOD that the plant actually measures. A flash of light in the middle of the night can DISRUPT flowering in SDPs (because it interrupts the long dark period required).
Where the Photoperiod is Sensed
- Photoperiod is sensed by the LEAVES, not the apex.
- The pigment phytochrome in leaves perceives light / dark.
- Once the right photoperiod is sensed, leaves produce a flowering signal called florigen (now identified as the FT protein in Arabidopsis).
- Florigen travels from leaves to the shoot apex via phloem and converts the apex from vegetative to flowering.
- Even ONE leaf exposed to the right photoperiod can induce flowering in some plants.
Photoperiodism: classify plants by photoperiod
Three categories of flowering plants based on day-length response: long-day, short-day, day-neutral. Click each category to see typical examples, then test yourself with the quiz.
Long-Day Plants (LDP)
Flower when day length is LONGER than a critical value (typically 12-14+ hours).
Day length > critical period → flowering
Bloom in summer (long days). Native to temperate regions.
Examples (7):
Where is the photoperiod sensed?
The LEAF is the photoperiodic sensor. The pigment phytochrome in leaves perceives the dark/light period.
Once the right photoperiod is sensed, the leaf produces a flowering signal called florigen (now identified as the FT protein in Arabidopsis). Florigen travels from the leaves to the shoot apex via the phloem and converts the apex from vegetative to flowering.
NEET trap: even ONE leaf exposed to the right photoperiod is enough to induce flowering.
NEET key facts
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LDP (long-day): wheat, spinach, radish, barley, henbane (Hyoscyamus). Flower in summer.
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SDP (short-day): rice, soybean, cotton, chrysanthemum, tobacco, Xanthium. Flower in autumn / early winter.
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DNP (day-neutral): tomato, cucumber, sunflower, maize. Flower based on age, not day length.
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Photoperiod is sensed by LEAVES (via phytochrome). Florigen / FT protein travels from leaves to shoot apex.
!
It is actually the LENGTH OF THE DARK PERIOD that matters, not the day length. (A flash of light in the middle of the night can disrupt SDP flowering.)
Try this
- Memory trick for SDP: rice (R), soybean (S), cotton (C), chrysanthemum (C), tobacco (T), Xanthium (X). RSC-CTX = "Rice-Soy-Cotton-Chrysanthemum-Tobacco-Xanthium" all bloom in short days.
- Memory trick for LDP: wheat, spinach, radish - the staples. They bloom in summer (long days).
- Take the quiz with 10 plants. Common confusion: cotton (SDP, not DNP), sunflower (DNP, not LDP), spinach (LDP, not DNP).
Vernalisation
Vernalisation is the cold treatment required by some plants to induce flowering. Without this cold period, the plant remains vegetative and does not flower.
Plants Requiring Vernalisation
- Winter wheat, rye, barley: sown in autumn, exposed to winter cold, flower in spring.
- Biennials: cabbage, carrot, beetroot, sugar beet, foxglove. Year 1 = vegetative growth (rosette); winter cold; Year 2 = flowering.
Cold Treatment Conditions
- Temperature: typically 1 to 7 degrees Celsius.
- Duration: several weeks (the longer, the more reliable the response).
- Can be applied to seeds or seedlings (for off-season cultivation).
Where Vernalisation is Sensed
- Sensed at the shoot apical meristem (SAM), NOT leaves.
- Cold treatment causes epigenetic silencing of a flowering-repressor gene (FLC, FLOWERING LOCUS C in Arabidopsis).
- The effect is remembered through cell divisions (long-lasting).
NEET trap: vernalisation vs photoperiodism
Vernalisation = cold treatment, sensed at SAM. Photoperiodism = day length, sensed at leaves. Different cues, different sensors. Some plants need BOTH (e.g., winter wheat needs cold + long days).
Vernalisation: cold treatment for flowering
Some plants require a cold period to flower. Toggle between plants and watch the flowering outcome change with or without the cold treatment.
Cold treatment given?
✓ Plant flowers
Reason: Cold treatment was given (vernalisation requirement met). Plant senses winter has passed and triggers flowering.
Real life: If sown in spring (skipping winter), winter wheat stays vegetative and does NOT flower - the cold trigger is missing.
About Winter wheat
Sown in autumn (October-November). Seedlings experience winter cold. Vernalisation requirement is met. They flower in spring and ripen by summer.
Examples: Winter wheat (Triticum) varieties grown in temperate regions
Where is vernalisation sensed?
Vernalisation is sensed by the shoot apical meristem (SAM), NOT by leaves. (Photoperiodism, in contrast, is sensed by leaves.)
The cold treatment causes epigenetic silencing of a flowering-repressor gene (FLC, FLOWERING LOCUS C in Arabidopsis). Once silenced, the plant can flower when day length and temperature are right. The effect is remembered through cell divisions.
NEET key facts
!
Vernalisation = COLD TREATMENT (1-7 degrees Celsius) required to induce flowering.
!
Plants requiring vernalisation: winter wheat, rye, barley (sown autumn, flower spring); biennials (cabbage, carrot, beetroot, sugar beet).
!
Without cold treatment, these plants stay vegetative and do NOT flower.
!
Vernalisation is sensed at the SHOOT APICAL MERISTEM (NOT leaves). Photoperiod is sensed by leaves.
!
Vernalisation can be artificially supplied to seeds for off-season cultivation.
!
NEET trap: vernalisation = cold; photoperiodism = day length. These are SEPARATE flowering controls.
Try this
- Choose "Winter wheat" + toggle cold OFF: notice the plant does NOT flower. This is what would happen if winter wheat was grown in tropical conditions.
- Choose "Biennial" (cabbage, carrot): you NEED a winter to trigger flowering in year 2. This is why biennials produce vegetables in year 1 and flowers in year 2.
- Switch to "Spring wheat" or "Rice": neither requires cold. Both can be grown without a winter trigger.
Seed Dormancy
Seed dormancy is a state where viable seeds do NOT germinate even when conditions look favourable. This is a survival adaptation to time germination with the right season or location.
Causes of Dormancy
- Impermeable seed coat: hard testa prevents water and oxygen from reaching the embryo. Examples: many legumes (Acacia), lotus seeds (can stay dormant for centuries).
- Chemical inhibitors: high ABA in seeds prevents germination. Examples: ABA in tomato fruit pulp, cyanogenic glycosides in apples.
- Immature embryo: some seeds are shed before the embryo is fully developed. Need after-ripening time.
- Unfavourable physiological state: the seed is intact but its internal physiology blocks germination. Broken by specific cues like cold or light.
How to Break Dormancy
- Scarification: mechanically (sandpaper) or chemically (concentrated sulphuric acid) damaging the seed coat. For impermeable coats.
- Stratification: exposing seeds to cold, moist conditions for several weeks. Mimics winter. Breaks physiological dormancy.
- Gibberellin (GA) treatment: antagonises ABA, promotes alpha-amylase synthesis. Breaks dormancy in many species.
- Leaching / washing: removes water-soluble inhibitors from the seed coat. (Why desert seeds need rain.)
The GA / ABA antagonism
ABA promotes dormancy. GA breaks dormancy. The ratio of GA to ABA determines whether a seed germinates or stays dormant. This antagonism is the central regulator of germination.
Seed dormancy: causes and ways to break it
Seeds often refuse to germinate even when conditions look favourable. Pick a cause of dormancy, then pick a treatment, and see whether the treatment will actually work for that cause.
1. Pick a CAUSE of dormancy:
Impermeable seed coat
Hard, water-impermeable seed coat (testa) prevents water and oxygen from reaching the embryo. The embryo cannot germinate even with favourable conditions.
Examples:
• Hard-coated legumes (Acacia)
• Lotus seeds (can stay dormant for centuries)
• Many wild grasses
2. Pick a METHOD to break dormancy:
Scarification
Mechanically or chemically damaging the seed coat to allow water and oxygen entry. Methods: rubbing with sandpaper, treatment with concentrated sulphuric acid, hot water soaking.
✓ Will work!
Scarification addresses the impermeable seed coat problem. The seed will germinate.
Cause-method matching reference
Impermeable seed coat
→
Scarification
Chemical inhibitors
→
Stratification
, Gibberellin (GA) treatment
, Leaching / washing
Immature embryo
→
Unfavourable physiological state
→
Stratification
, Gibberellin (GA) treatment
NEET key facts
!
Seed dormancy: a state where viable seeds do NOT germinate even in favourable conditions.
!
Causes: impermeable seed coat, chemical inhibitors (ABA), immature embryo, unfavourable physiological state.
!
Methods to break dormancy: scarification (for hard coats), stratification (cold-moist for chilling requirement), gibberellin (antagonises ABA), leaching (washes out inhibitors).
!
ABA promotes dormancy. GA breaks dormancy. The GA / ABA ratio decides germination.
!
Lotus seeds have shown viability after 1300+ years (impermeable coat enables long dormancy).
Try this
- Pick "Impermeable seed coat" + "Scarification": will work! Sandpaper / acid etches the coat to let water in.
- Pick "Impermeable seed coat" + "Gibberellin": will NOT work! GA cannot enter through an impermeable coat.
- Pick "Chemical inhibitors" + "Leaching" or "GA": both work. Washing removes ABA, and GA antagonises whatever ABA is left.
Worked NEET Problems
NEET-style problem · PGR Effects
Question
Solution
NEET-style problem · Photoperiodism
Question
Solution
NEET-style problem · Hormone Discovery
Question
Solution
NEET-style problem · Vernalisation
Question
Solution
NEET-style problem · Tissue Culture
Question
Solution
Cheat Sheet
Growth Definition
Irreversible permanent increase in size, mass, or volume of cell, organ, or organism.
Growth Phases
Meristematic (division) → Elongation (stretch) → Maturation (differentiation)
Growth Rates
Arithmetic: Lt = L0 + r·t (linear) | Geometric: W1 = W0 · e^(r·t) (exponential)
Sigmoid Curve
S-shape: Lag → Log (exponential) → Stationary. Universal growth pattern.
Auxin (IAA)
F.W. Went 1928 | Apical dominance | 2,4-D weedicide | Phototropism | Parthenocarpy
Gibberellin (GA3)
Kurosawa | Gibberella fujikuroi | Bolting | Alpha-amylase | Sugarcane (+20 t/acre)
Cytokinin
Skoog & Miller (1955) | Herring sperm DNA, coconut milk | Cell division | Richmond-Lang
ABA
Stress hormone | Stomatal closure | Seed dormancy | Antagonist of GA | Originally dormin/abscisin II
Ethylene (C2H4)
Gaseous hormone | Climacteric ripening | Pineapple flowering | Ethephon | Triple response
Photoperiodism
LDP: wheat, spinach, radish | SDP: rice, soybean, cotton, Xanthium | DNP: tomato, sunflower, maize | Sensed by leaves
Vernalisation
Cold treatment to induce flowering | Winter wheat, biennials | Sensed at SAM, NOT leaves
Seed Dormancy
Causes: hard coat, ABA, immature embryo. Break: scarification, stratification, GA, leaching. ABA / GA ratio is key.
Frequently asked questions
How often does Plant Growth and Development appear in NEET?
Plant Growth and Development is a Medium Weightage chapter with 3 to 5 questions in most NEET exams. Questions focus on the five plant growth regulators (auxin, gibberellin, cytokinin, ABA, ethylene), their discoverers, biological effects, agricultural applications, photoperiodism (short-day, long-day, day-neutral plants and the role of phytochrome), vernalisation, and the sigmoid growth curve. Knowing the discoverer-effect-application combinations for each hormone is essential for a strong NEET score.
What is the difference between arithmetic and geometric growth?
In arithmetic growth, only one of the daughter cells continues to divide; the other matures and exits the cell cycle. The rate of growth is constant. The graph of growth vs time is a straight line. Formula: Lt = L0 + rt, where L0 is initial length, r is growth rate, and t is time. In geometric (or exponential) growth, both daughter cells continue to divide. The rate of growth is proportional to the current size. The graph is exponential (J-shaped). Formula: W1 = W0 e^(rt). All organisms show geometric growth in the early stages (when nutrients are abundant) and shift to arithmetic-like growth as they hit limits.
What is the sigmoid growth curve?
The sigmoid growth curve is the S-shaped curve that represents the typical growth pattern of organisms (and individual organs). It has three phases: (1) Lag phase - slow initial growth as cells prepare to divide, (2) Log / exponential phase - rapid growth as both daughter cells divide and resources are abundant, and (3) Stationary phase - growth plateaus as resources run out and cells reach equilibrium. This curve is sometimes also called the typical growth curve. The exponential phase represents geometric growth; the lag and stationary phases represent the natural slowing as conditions become limiting.
What is the difference between differentiation, dedifferentiation, and redifferentiation?
Differentiation: meristematic cells (which divide actively) mature into permanent specialised cells. For example, meristematic cells become tracheary elements (vessels, tracheids); they lose their protoplasm to function as efficient water conductors. Dedifferentiation: mature, differentiated, non-dividing cells regain the capacity to divide. Examples: cork cambium forms from cortical cells; interfascicular cambium forms from medullary ray cells; callus forms from any tissue in tissue culture. This shows plant cell totipotency. Redifferentiation: dedifferentiated cells, which can divide, lose this capacity again and mature into specialised cells. Example: secondary xylem and phloem are produced by the dedifferentiated cambium and they redifferentiate into specific cell types.
What is the difference between long-day plants (LDP) and short-day plants (SDP)?
Long-day plants (LDP) flower when the day length is LONGER than a certain critical value. They flower in the long days of summer. Examples: wheat, barley, oats, radish, spinach. Short-day plants (SDP) flower when the day length is SHORTER than a certain critical value. They flower in the short days of autumn or early winter. Examples: rice, soybean, chrysanthemum, cotton, tobacco, Xanthium. Day-neutral plants (DNP) are not affected by day length. Examples: tomato, cucumber, sunflower, maize. The critical factor is actually the length of the dark period, not the light period. The leaves perceive the photoperiod and produce a flowering signal (florigen, now identified as the FT protein in Arabidopsis) that travels to the shoot apex to induce flowering.
What is vernalisation and why is it important?
Vernalisation is the requirement of low-temperature exposure (cold treatment) to induce flowering in some plants. Without this cold period, the plant remains vegetative and does not flower. It is found in winter varieties of wheat, barley, rye (sown in autumn, exposed to winter cold, flower in spring), and biennials such as cabbage, carrot, beetroot. The cold treatment can be artificially provided to seeds or seedlings to allow off-season cultivation. Vernalisation is sensed by the shoot apex and the cold response involves epigenetic changes (such as silencing of FLC gene in Arabidopsis). Vernalisation prevents premature flowering before winter; it ensures flowering occurs in optimal seasonal conditions.
Which plant hormone causes fruit ripening, and how is it used commercially?
Ethylene (also called the ripening hormone) is a gaseous plant hormone that triggers fruit ripening, particularly in climacteric fruits (such as banana, apple, mango, tomato). It causes softening, sweetening, colour change, and aroma development. Commercial use: (1) Ethephon (2-chloroethylphosphonic acid) is sprayed on fruits because it releases ethylene slowly when absorbed; (2) Calcium carbide pieces release acetylene (which mimics ethylene) and are sometimes used to ripen mangoes off the tree (this practice is illegal in India because of the impurities); (3) Ethylene gas is used in ripening chambers to ripen mangoes, bananas, and tomatoes uniformly after harvest; (4) Smoke from burning leaves was traditionally used in Indonesia to ripen mangoes (the smoke contains ethylene). Ethylene also promotes flowering in pineapples, induces abscission of leaves and fruits, and breaks seed and bud dormancy.
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Respiration in Plants
Sexual Reproduction in Flowering Plants
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