Plant Growth and Development — NEET notes

Plant Growth

Plant growth is indeterminate — it continues throughout life because plants retain meristematic tissues. Growth occurs in three phases: cell division, cell elongation, and cell differentiation. Growth rate can be arithmetic (constant addition) or geometric (exponential doubling). When plotted over time, growth follows a sigmoid (S-shaped) curve with lag, log, and stationary phases. Growth can be measured by increase in fresh weight, dry weight, length, area, volume, or cell number.

Geometric growth: W_t = W_0 e^ rt

Characteristics of Plant Growth

Unlike animals, plants grow throughout their lifetime because they retain permanent regions of dividing cells called meristems. This makes plant growth indeterminate and open.

Plant growth is unique in the living world. While most animals stop growing after reaching maturity, plants never stop. A banyan tree keeps adding new branches at 200 years old. This is because plants retain meristematic tissues — pockets of perpetually dividing cells — throughout their lives.

• Indeterminate growth — plants grow throughout their lifespan (no fixed adult size)
• Open form of growth — new organs (leaves, branches, roots) keep appearing; the body plan is never 'complete'
• Growth is restricted to regions with meristematic cells (not the whole body)
• Root apical meristem and shoot apical meristem are the primary growth engines
• Lateral meristems (vascular cambium, cork cambium) add girth — responsible for secondary growth

Three Phases of Growth

Every plant cell passes through three sequential phases: meristematic (division), elongation (expansion), and maturation (differentiation). These correspond to distinct zones in a root tip.

If you look at a root tip under a microscope, you see three clear zones from tip to base. Each zone represents one phase of growth — and every growing cell passes through all three in sequence.

• Phase 1 — Cell Division (Meristematic phase): Cells are small, thin-walled, densely cytoplasmic, with large nuclei. They divide rapidly. Found at the root tip and shoot tip.
• Phase 2 — Cell Elongation: Cells stop dividing and begin to enlarge. Vacuoles form and expand, pushing the cell wall outward. Cell size increases dramatically. This is the zone of elongation (proximal to the meristematic zone).
• Phase 3 — Cell Differentiation (Maturation): Cells acquire their final shape, size, and function. They develop thick walls (if sclerenchyma), lose nuclei (if xylem vessels), or become sieve tubes (if phloem). This is the zone of maturation.
• Differentiation is the process by which a cell from the meristem becomes specialized for a specific function
• Dedifferentiation — mature cells can revert to dividing cells (e.g., interfascicular cambium from medullary rays)
• Redifferentiation — dedifferentiated cells mature again into a new cell type

Arithmetic vs Geometric Growth

In arithmetic growth, one daughter cell divides while the other differentiates — giving a linear increase. In geometric growth, all daughter cells keep dividing — giving exponential increase. Real plant growth shows an initial geometric phase that eventually plateaus.

How fast do plants grow? It depends on whether the new cells keep dividing or stop. This gives us two mathematical models of growth rate.

Arithmetic: L_t = L_0 + rt

Geometric: W_t = W_0 e^ rt

• In L_t = L_0 + rt : L_t = length at time t , L_0 = initial length, r = growth rate per unit time
• In W_t = W_0 e^ rt : W_t = final size, W_0 = initial size, r = relative growth rate, t = time, e = base of natural log
• Geometric growth cannot continue indefinitely — resources become limiting
• Relative growth rate (RGR) — growth per unit time per unit initial parameter: useful for comparing growth of different organs/species

Sigmoid Growth Curve

When growth is plotted against time, most plant organs show an S-shaped (sigmoid) curve with three distinct phases: slow start (lag), rapid growth (log/exponential), and plateau (stationary).

Plot the height of a plant (or dry weight, or cell number) against time, and you almost always get an S-shaped curve. This sigmoid pattern emerges because initial geometric growth eventually hits resource limits and slows down.

• Lag phase — slow initial growth; cells preparing for division, synthesizing enzymes and metabolites
• Log phase (exponential phase) — rapid growth; cells actively dividing, resources are abundant, growth is near-geometric
• Stationary phase (plateau) — growth rate declines and eventually stops; nutrients depleted, space constraints, senescence begins
• The sigmoid curve applies to: individual organs (leaf area), whole plants (height), and even cell cultures
• Absolute growth rate = total growth per unit time (e.g., cm/day) — measured as the slope of the growth curve
• Relative growth rate = growth per unit time per unit of existing growth — more useful for comparing organs of different sizes

Quick recall

A student measures the dry weight of a plant culture at different times and plots a graph. The curve is S-shaped. Which phase shows the steepest slope?

The log (exponential) phase — this is where growth rate is maximum. The slope is steepest in the middle portion of the sigmoid curve.

Conditions for Growth

Plant growth depends on both internal (hormones, genetics) and external (water, temperature, light, nutrients, oxygen) factors. Water is the single most important external requirement.

Growth doesn't happen in a vacuum. A seed might have the genetic potential of a giant sequoia, but without the right environment, it won't even germinate. Both internal and external factors must align.

• Water — most important external factor; provides turgor for cell enlargement, medium for enzymatic reactions, and transport of nutrients
• Oxygen — needed for aerobic respiration that generates ATP for growth processes
• Nutrients — macro (N, P, K, Ca, Mg, S) and micro (Fe, Mn, Zn, Cu, B, Mo, Cl) elements required for building cellular components
• Temperature — optimum range varies by species; growth rate increases with temperature up to an optimum, then drops sharply
• Light — essential for photosynthesis; also acts as a signal (photoperiodism, photomorphogenesis)
• Internal factors — plant growth regulators (auxins, gibberellins, cytokinins, ethylene, ABA) and genetic makeup

Growth Regulators: Auxins & Gibberellins

Auxins (mainly IAA) were the first plant hormones discovered. They promote cell elongation, apical dominance, phototropism, and root initiation. Gibberellins (GAs) promote stem elongation, bolting in rosette plants, seed germination by inducing alpha-amylase in the aleurone layer, and parthenocarpy. Both are growth promoters but act through different mechanisms and target different processes.

Went's Experiment: Agar block + auxin arrow differential growth arrow curvature

Auxin Discovery

The story of auxin discovery spans multiple scientists: Darwin showed the tip perceives light, Boysen-Jensen proved the signal is chemical and diffusible, and Went isolated auxin using agar blocks.

The discovery of auxin is one of the great detective stories in biology. It started with Charles Darwin and his son Francis (1880) observing that grass coleoptiles bend toward light — but ONLY if the tip is intact. Removing or covering the tip abolished the bending.

• Darwin (1880) — coleoptile tip perceives light; bending occurs in the zone below the tip. Concluded: some 'influence' moves from tip to lower region
• Boysen-Jensen (1913) — placed a gelatin block between tip and base → bending still occurred. Placed a mica barrier → bending stopped. Proved the signal is chemical, not electrical, and diffuses through gelatin
• F.W. Went (1928) — placed cut coleoptile tips on agar blocks → auxin diffused into agar → placing the agar block on a decapitated coleoptile caused bending. Named the substance 'auxin' (Greek: auxein = to grow)
• IAA (Indole-3-Acetic Acid) — the most common naturally occurring auxin in plants
• Other natural auxins: IBA (Indole-3-Butyric Acid). Synthetic auxins: 2,4-D, NAA

Auxin Effects

Auxin has diverse physiological effects: promotes cell elongation in stems, inhibits growth at high concentrations in roots, maintains apical dominance, induces phototropism and gravitropism, promotes root initiation, and acts as a herbicide at high concentrations.

Auxin is the Swiss Army knife of plant hormones. It does different things in different tissues and at different concentrations — making it the most versatile growth regulator.

• Cell elongation — auxin loosens cell walls (acid growth hypothesis: auxin activates proton pumps → H⁺ acidifies cell wall → expansins loosen cellulose microfibrils → turgor-driven expansion)
• Apical dominance — high auxin from the apical bud INHIBITS lateral bud growth. Remove the apex → lateral buds sprout (this is why pruning makes bushes bushy!)
• Phototropism — unilateral light causes auxin to redistribute to the shaded side → shaded side elongates more → shoot bends toward light
• Gravitropism — in horizontal roots, auxin accumulates on the lower side. Lower side is inhibited (roots are sensitive to high auxin), upper side grows faster → root bends downward
• Root initiation — auxin (especially IBA and NAA) promotes adventitious root formation — used in horticulture for stem cuttings
• Herbicidal action — synthetic auxins like 2,4-D at high concentration kill dicot weeds selectively (monocots are resistant) — widely used in lawns and cereal fields
• Parthenocarpy — auxin can induce fruit development without fertilization (seedless fruits)

Quick recall

A gardener applies IAA to the cut end of a stem cutting. What is the expected result?

Adventitious root formation at the cut end. Auxin (especially IAA/IBA/NAA) promotes initiation of roots from stem tissue — this is a standard horticultural technique.

Gibberellins Discovery

Gibberellins were discovered from the fungus Gibberella fujikuroi that caused rice seedlings to grow abnormally tall. Over 100 forms are known; GA₃ (gibberellic acid) is the most studied.

Japanese rice farmers noticed some seedlings growing absurdly tall and thin — then falling over and dying. They called it 'bakanae' (foolish seedling disease). In 1926, E. Kurosawa found that the fungus Gibberella fujikuroi (now *Fusarium fujikuroi*) secreted a growth-promoting substance. This was later purified and named gibberellin.

• Discovered from Gibberella fujikuroi fungus — hence the name 'gibberellin'
• E. Kurosawa (1926) — showed the fungal filtrate caused excessive stem elongation in rice
• Over 100 gibberellins are known (GA₁, GA₂, ... GA₁₀₀+) — collectively called GAs
• GA₃ (gibberellic acid) is the most widely studied and commercially used form
• GAs are terpenoid compounds (derived from the mevalonic acid pathway)
• Synthesized mainly in young leaves, root tips, and developing seeds

Gibberellin Effects

Gibberellins promote stem elongation (especially in dwarf varieties), trigger bolting in rosette plants, break seed dormancy, induce alpha-amylase in germinating cereals, and can cause parthenocarpy.

If auxin is the Swiss Army knife, gibberellin is the growth accelerator. Its most dramatic effect is making things longer — stems, internodes, flower stalks. Spray GA₃ on a dwarf plant, and it looks like the tall variety.

• Stem elongation — GAs cause elongation of internodes, especially in genetically dwarf plants (dwarf pea, dwarf maize become tall on GA₃ treatment)
• Bolting — in rosette plants (cabbage, beet), GAs trigger rapid elongation of the condensed stem before flowering. Without GA, these plants stay compact.
• Seed germination — GA₃ stimulates production of α-amylase in the aleurone layer of cereal seeds → breaks down starch into sugars → feeds the embryo
• Parthenocarpy — GAs can induce seedless fruit formation (like auxin, but especially effective in apples, tomatoes)
• Delay of senescence — GAs can keep fruits and leaves green longer (similar to cytokinins)
• Sex expression — GAs promote maleness in some plants (increase staminate flowers in cucurbits)

Quick recall

A rosette plant like cabbage suddenly produces a tall flowering stem. Which hormone is responsible?

Gibberellin — this process is called bolting. GAs cause rapid internode elongation in rosette plants, converting the compact rosette into a tall flowering stalk. This is triggered naturally by long days or cold treatment.

Growth Regulators: Cytokinins & Ethylene

Cytokinins promote cell division (cytokinesis), delay senescence (Richmond-Lang effect), promote shoot differentiation in tissue culture, and counteract apical dominance by stimulating lateral bud growth. Ethylene is a gaseous hormone that promotes fruit ripening, senescence, abscission of leaves/flowers, the triple response in seedlings, and breaks seed/bud dormancy. Together, they represent the yin-yang of plant development: cytokinin keeps things young, ethylene ages them.

High cytokinin:auxin ratio arrow shoot formation

Cytokinins Discovery

Cytokinins were discovered when Miller et al. found that autoclaved herring sperm DNA (kinetin) promoted cell division. The first natural cytokinin, zeatin, was isolated from maize kernels.

In the 1950s, Skoog and Miller were trying to grow tobacco pith tissue in culture. It wouldn't divide. On a hunch, they added herring sperm DNA that had been autoclaved (degraded by heat) — and the cells started dividing furiously! The active molecule was named kinetin (it promoted kinesis = cell division).

• Kinetin — first cytokinin discovered; derived from degraded herring sperm DNA; NOT natural (modified form of adenine)
• Kinetin is a synthetic cytokinin — it does not occur naturally in plants
• Zeatin — first natural cytokinin isolated; found in maize (Zea mays) endosperm (hence the name)
• All natural cytokinins are adenine derivatives (modified purines)
• Synthesized mainly in root tips and transported upward in the xylem
• Also found in areas of rapid cell division: developing seeds, young fruits, root tips

Cytokinin Effects

Cytokinins promote cell division, delay leaf senescence (Richmond-Lang effect), counteract apical dominance, and control differentiation in tissue culture (high CK:auxin ratio = shoots).

Cytokinins are the anti-aging hormones of the plant world. Their primary effect is promoting cell division, but their most dramatic demonstrated effect is keeping detached leaves green for weeks — the famous Richmond-Lang effect.

• Cell division (cytokinesis) — primary effect; cytokinins stimulate mitosis in the presence of auxin
• Delay of senescence (Richmond-Lang effect) — cytokinin-treated leaves stay green much longer than untreated ones; delays chlorophyll degradation and protein breakdown
• Morphogenesis in tissue culture — the auxin:cytokinin ratio determines what forms. High CK:auxin → shoots. High auxin:CK → roots. Equal ratio → callus (undifferentiated mass)
• Counteracts apical dominance — cytokinin applied to lateral buds promotes their growth even in the presence of an intact apical bud (opposes auxin's inhibitory effect)
• Promotes nutrient mobilization — cytokinin-treated leaves act as nutrient sinks, attracting amino acids and sugars from other parts

Quick recall

A scientist wants to induce shoot formation in a callus culture. What should they do?

Increase the cytokinin:auxin ratio in the culture medium. High cytokinin relative to auxin promotes shoot differentiation.

Ethylene

Ethylene (C₂H₄) is the simplest plant hormone and the only gaseous one. It was discovered through observations of premature fruit ripening and leaf abscission near gas-leaking streetlamps.

In the early 1900s, people noticed that trees near gas-leaking streetlamps lost their leaves prematurely. Cousins (1910) observed that oranges stored with bananas ripened faster. The culprit? Ethylene (CH₂=CH₂) — a simple two-carbon gas that turned out to be a powerful plant hormone.

• Simplest organic molecule that functions as a plant hormone — just C_2H_4
• Only gaseous hormone in plants — it can diffuse between cells and even between plants
• Synthesized from methionine via the pathway: Methionine → SAM (S-adenosyl methionine) → ACC (1-aminocyclopropane-1-carboxylic acid) → Ethylene
• Produced by ripening fruits, senescing tissues, and stressed tissues
• Almost all parts of the plant produce ethylene, but ripening fruits produce the most

Ethylene Effects

Ethylene promotes fruit ripening (climacteric fruits), causes the triple response in seedlings, induces abscission of leaves and flowers, breaks dormancy, promotes senescence, and influences sex expression toward femaleness.

Ethylene is the aging hormone of plants. If cytokinins keep things young, ethylene pushes everything toward maturity, senescence, and eventual death. But it's not all destruction — ripening a fruit is ethylene's most commercially important job.

• Fruit ripening — ethylene triggers color change, softening, aroma production, and sugar accumulation. Commercially used to ripen bananas, mangoes, and tomatoes ('ethephon' releases ethylene)
• Triple response (in dark-grown seedlings exposed to ethylene) — (1) inhibition of stem elongation, (2) radial swelling of the stem (thickening), (3) horizontal growth (loss of gravitropism)
• Abscission — ethylene promotes the formation of the abscission layer at the base of leaves, flowers, and fruits, leading to their shedding
• Senescence — accelerates aging and death of leaves, petals, and whole plants
• Breaks seed and bud dormancy — ethylene can substitute for the cold requirement in some species
• Sex expression — promotes femaleness in cucurbits (contrast: GA promotes maleness)
• Internode flooding response — in deep-water rice, ethylene promotes rapid internode elongation to keep leaves above water

Quick recall

What are the three components of the 'triple response' caused by ethylene?

In dark-grown seedlings: (1) Inhibition of stem elongation (shorter), (2) Radial swelling of the stem (thicker), (3) Horizontal growth (loss of normal gravitropic response). This classic response is used to bioassay for ethylene.

Abscisic Acid

Abscisic acid (ABA) is the 'stress hormone' of plants. It inhibits growth, promotes stomatal closure during water stress, induces and maintains seed dormancy, and acts as an antagonist to gibberellins and auxins. Despite its name, ABA's role in abscission is now considered minor compared to ethylene. It plays a crucial role in helping plants survive drought, salt stress, and cold.

ABA arrow Stomata close arrow Water loss

ABA Discovery & Nature

ABA was independently discovered by two groups studying different phenomena — one studying abscission, another studying dormancy. Its name suggests abscission, but its primary role is stress response and dormancy.

ABA has the most misleading name in plant biology. It was called 'abscisic acid' because early researchers thought it promoted leaf abscission (shedding). We now know that ethylene is the primary abscission hormone — ABA's real fame comes from its role as the stress hormone.

• Discovered independently as 'abscisin II' (from cotton bolls) and 'dormin' (from dormant buds) — later unified as ABA
• Chemically a terpenoid (15-carbon compound derived from carotenoids)
• Synthesized in mature leaves, root caps, and stems, especially under stress conditions
• Transported through both xylem and phloem — can move long distances
• Often called 'stress hormone' because its levels spike dramatically under drought, salinity, cold, and wounding
• Acts as a growth inhibitor — antagonistic to auxin, gibberellin, and cytokinin

ABA & Stomatal Closure

Under water stress, ABA accumulates in leaves and triggers rapid stomatal closure by causing K⁺ efflux from guard cells. This reduces transpiration and conserves water — the plant's most important drought survival strategy.

When soil dries out, root cells sense the water deficit and synthesize ABA. This ABA travels up to the leaves via the xylem and acts on guard cells of stomata. Within minutes, stomata close — dramatically reducing water loss through transpiration.

• ABA acts on guard cells → triggers K⁺ ion efflux (potassium leaves the guard cells)
• Loss of K⁺ → loss of solutes → water moves OUT of guard cells by osmosis → guard cells become flaccid → stomata close
• This is the fastest hormonal response in plants — stomatal closure happens within minutes of ABA application
• ABA is sometimes called the 'anti-transpirant' because it reduces transpirational water loss
• Under prolonged drought, ABA also inhibits shoot growth to reduce water demand
• When stress is relieved, ABA is rapidly degraded and stomata reopen

Quick recall

During a drought, which hormone accumulates in leaves and what is its immediate effect on stomata?

ABA (Abscisic Acid) accumulates. It causes K⁺ efflux from guard cells → guard cells lose turgor → stomata close → reduced transpiration.

ABA & Seed Dormancy

ABA induces and maintains seed dormancy by inhibiting germination. It opposes the action of GA₃ which promotes germination. The balance between ABA and GA determines whether a seed germinates or stays dormant.

A seed that germinates in the middle of winter will die. ABA prevents this premature germination by keeping the seed in a state of dormancy — a period of suspended growth where metabolic activity is minimal. The seed 'waits' for the right conditions.

• ABA is present at high levels in mature, dormant seeds — it actively prevents germination
• ABA inhibits the synthesis of hydrolytic enzymes (like α-amylase) that GA induces during germination
• ABA:GA ratio determines seed fate: High ABA:GA → dormancy maintained. High GA:ABA → dormancy broken, germination proceeds
• During seed maturation, ABA levels rise → prevents vivipary (premature germination while still on the parent plant)
• ABA is gradually degraded during imbibition (water uptake) and cold stratification → allows GA to trigger germination
• ABA also inhibits RNA and protein synthesis in dormant seeds

Other ABA Effects

Beyond stomatal closure and dormancy, ABA inhibits overall plant growth, promotes tolerance to various abiotic stresses (cold, salt, heavy metals), and induces genes involved in stress protection.

ABA's philosophy is simple: when times are tough, stop growing and survive. Every effect of ABA — growth inhibition, stomatal closure, dormancy, stress gene activation — serves this one strategy.

• Inhibits shoot growth — reduces stem elongation and leaf expansion during stress (conserves resources)
• Promotes root growth at moderate concentrations during drought — helps the plant reach deeper water sources
• Cold tolerance — ABA induces synthesis of cold-protective proteins and sugars that prevent ice crystal damage
• Salt tolerance — ABA activates ion channels and transporters that exclude toxic Na⁺ from sensitive tissues
• Induces LEA proteins (Late Embryogenesis Abundant) — stress-protective proteins in seeds and vegetative tissues
• Promotes leaf senescence (works WITH ethylene in this case)
• Inhibits fruit ripening — opposes ethylene's ripening effect

Remember the five major plant hormones and their one-word summaries: Auxin = elongation, GA = growth, Cytokinin = division, Ethylene = ripening, ABA = stress. This helps you quickly categorize any NEET question.

Photoperiodism

Photoperiodism is the response of plants to the relative length of day and night (photoperiod) that governs flowering. Plants are classified as short-day plants (SDP), long-day plants (LDP), and day-neutral plants (DNP). The critical factor is actually the dark period, not the light period — a night break with a brief flash of red light can prevent SDP flowering. The phytochrome system (Pr/Pfr interconversion) is the photoreceptor that measures the photoperiod.

P_r Red light (660 nm) P_ fr Far-red light (730 nm) P_r

Photoperiodism Discovery

Photoperiodism was discovered by Garner and Allard (1920) when they observed that a mutant tobacco variety (Maryland Mammoth) only flowered under short days, not under long summer days.

In the early 1920s, Garner and Allard were puzzled by a tobacco mutant called 'Maryland Mammoth' — it grew spectacularly tall in summer but refused to flower. It only flowered in winter or when artificially given short days. They coined the term photoperiodism to describe a plant's response to the relative duration of light and dark periods.

• Photoperiodism — the response of plants to the relative length of day (light period) and night (dark period) that controls flowering
• Discovered by Garner and Allard (1920) using Maryland Mammoth tobacco
• They found that flowering in many species is triggered not by temperature or nutrients, but by day length
• The term comes from: photo (light) + period (duration) + ism (response)
• Led to classification of plants into Short-Day, Long-Day, and Day-Neutral categories

SDP, LDP, and DNP

Short-day plants flower when day length is below a critical photoperiod. Long-day plants flower when day length exceeds the critical photoperiod. Day-neutral plants flower regardless of photoperiod.

Based on their flowering response to day length, plants fall into three neat categories. But here's the catch that NEET loves: it's actually the LENGTH OF THE DARK PERIOD that matters, not the light period. SDPs need a long uninterrupted night, LDPs need a short night.

• Short-Day Plants (SDP) — flower when day length is LESS than the critical photoperiod (= need long, uninterrupted dark period). Examples: rice, tobacco (Maryland Mammoth), chrysanthemum, soybean, *Xanthium*
• Long-Day Plants (LDP) — flower when day length is MORE than the critical photoperiod (= need short dark period). Examples: spinach, radish, wheat, barley, henbane (*Hyoscyamus*)
• Day-Neutral Plants (DNP) — flowering is NOT affected by photoperiod. Examples: tomato, cucumber, cotton, sunflower, maize
• Critical photoperiod — the specific day length threshold that determines whether a plant flowers. It's different for each species
• A SDP and an LDP can have the SAME critical photoperiod (say 12 hours) — SDP flowers below it, LDP flowers above it

Critical Photoperiod & Night Break

Critical photoperiod is the specific threshold day length for flowering. Night break experiments proved that the uninterrupted dark period is what plants actually measure. A flash of red light during the dark period prevents SDP flowering.

The night break experiment is one of the most elegant in plant physiology. Give an SDP a long night (say 16 hours of darkness) — it flowers. Now interrupt that 16-hour night with just a few minutes of red light in the middle — it fails to flower! The night break 'resets the clock.'

• Night break = brief exposure to light (especially red light, 660 nm) during the dark period
• In SDPs: night break prevents flowering (breaks the required long, continuous dark period)
• In LDPs: night break promotes flowering (effectively shortens the dark period below the critical threshold)
• Red light (660 nm) is most effective at causing the night break effect
• Far-red light (730 nm) immediately after red light — reverses the night break effect (SDP flowers again!)
• This red/far-red reversibility proved that phytochrome is the photoreceptor involved

Quick recall

An SDP is given 8 hr light + 16 hr dark but with a 5-min red light flash at hour 8 of darkness. Will it flower? What if far-red light follows the red flash?

With just the red flash — NO, it won't flower (night break inhibits SDP flowering). With far-red light immediately after red — YES, it will flower (far-red reverses the red light effect by converting Pfr back to Pr).

Phytochrome System

Phytochrome is a blue-green pigment protein that exists in two interconvertible forms: Pr (absorbs red light, inactive) and Pfr (absorbs far-red light, active/physiologically active form). It is the photoreceptor for photoperiodism, seed germination, and many other light-mediated responses.

How does a plant 'know' whether it's day or night? The answer is phytochrome — a pigment-protein photoreceptor that exists in two forms and flips between them depending on the light quality.

P_r Red light (660 nm) P_ fr Far-red light (730 nm) P_r

• Pr (Phytochrome red) — absorbs red light (660 nm); biologically INACTIVE form; blue-colored; stable in dark
• Pfr (Phytochrome far-red) — absorbs far-red light (730 nm); biologically ACTIVE form; triggers physiological responses; unstable in dark (slowly reverts to Pr)
• Red light converts Pr → Pfr (activation). Far-red light converts Pfr → Pr (inactivation)
• In sunlight: both red and far-red are present, but red is slightly more effective → Pfr accumulates during the day
• In darkness: Pfr slowly reverts to Pr (dark reversion) → Pr accumulates during the night
• Pfr is the active form that triggers responses like seed germination, inhibits SDP flowering, and promotes LDP flowering

• SDP flowering logic: Need low Pfr (= long dark period for Pfr → Pr dark reversion). Night break adds Pfr → inhibits flowering
• LDP flowering logic: Need high Pfr (= short dark period, so Pfr doesn't fully revert to Pr). Night break maintains Pfr → promotes flowering
• Seed germination: Red light promotes germination (Pfr stimulates it). Far-red inhibits (Pr form doesn't trigger germination). Classic experiment with lettuce seeds.
• Phytochrome also mediates: chloroplast movement, leaf expansion, stem elongation inhibition, and de-etiolation

Vernalization & Seed Dormancy

Vernalization is the promotion of flowering by exposure to prolonged cold treatment. Winter varieties of wheat and rye require vernalization to flower. The stimulus is perceived at the shoot apex and the effect can be reversed by high temperature (devernalization). Seed dormancy is a state of suspended growth caused by hard seed coats, chemical inhibitors (ABA), or immature embryos. It can be broken by scarification, stratification, light, or gibberellin treatment. Vivipary is the germination of seeds while still attached to the parent plant.

Vernalization: Cold (3–5°C for weeks) arrow Competence to flower

Vernalization

Vernalization is the process by which prolonged cold exposure (0–5°C for weeks to months) at the seedling or seed stage makes a plant competent to flower later. Without this cold treatment, winter varieties of wheat, rye, and barley won't flower even under the right photoperiod.

Some plants refuse to flower unless they experience a cold winter first. This is vernalization — the 'requirement for winter.' It ensures that plants don't flower in autumn (when winter would kill the flowers) but instead wait until spring. The term comes from Latin: *vernus* = spring.

• Definition: Promotion of flowering by a period of low temperature (typically 3–5°C for several weeks)
• Discovered by: T.D. Lysenko (1928) in wheat — he found that winter wheat seeds moistened and kept at low temperature could behave like spring wheat
• Chouard (1960) coined the actual term 'vernalization'
• Winter vs Spring varieties: Winter wheat NEEDS vernalization to flower; spring wheat does NOT
• Perceived at: the shoot apical meristem (actively dividing cells) — not leaves, not roots
• Temperature range: 0°C to 5°C for durations ranging from a few days to months depending on species
• Devernalization — exposure to high temperature immediately after cold treatment can reverse the effect

Quick recall

A winter wheat plant is given cold treatment at 4°C for 6 weeks, then immediately placed at 40°C for 2 days, then returned to normal temperature. Will it flower?

No — the high temperature (40°C) immediately after cold treatment causes devernalization, reversing the cold stimulus. The plant would need another round of cold treatment.

Seed Dormancy

Seed dormancy is a survival mechanism where viable seeds do not germinate even under favorable conditions. Causes include hard/impermeable seed coats, chemical inhibitors (ABA), immature embryos, and light/temperature requirements.

A seed that germinates the moment it's formed — in the middle of a scorching summer or a freezing winter — is a dead seed. Dormancy is nature's way of programming seeds to wait for the right season. It's controlled by the balance between growth inhibitors (mainly ABA) and growth promoters (mainly GA).

• Dormancy = a state where a viable seed fails to germinate even under favorable conditions (water, oxygen, temperature, light)
• Causes of dormancy:
• — Hard, impermeable seed coat (prevents water/gas entry): e.g., legumes, lotus
• — Chemical inhibitors (ABA, phenolic compounds, coumarin) in seed coat or endosperm
• — Immature embryo at the time of seed dispersal (needs post-dispersal development)
• — Light requirement (some seeds need light to germinate; phytochrome-mediated)
• — Temperature requirement (need cold stratification or warm period)
• Advantages of dormancy: (1) Prevents germination in unfavorable season, (2) Allows seed dispersal over time, (3) Maintains a soil seed bank for long-term survival

Breaking Dormancy

Seed dormancy can be broken by physical methods (scarification), cold treatment (stratification), chemical treatment (GA₃, nitrates), light exposure (red light via phytochrome), or simply time (after-ripening).

If dormancy is the lock, there are several keys. Nature provides most of them — passage through an animal's gut scratches the seed coat, winter cold degrades ABA, spring rain provides moisture. Humans have learned to mimic these natural processes.

• Scarification — physical/chemical weakening of the seed coat. Physical: rubbing with sandpaper, cracking with a hammer. Chemical: soaking in concentrated H₂SO₄. Natural: passage through animal digestive tract (acid + enzymes soften coat)
• Stratification (cold stratification) — placing moist seeds at low temperature (1–5°C) for weeks. Mimics winter. Reduces ABA levels and increases GA sensitivity
• Gibberellin (GA₃) treatment — GA₃ applied externally can substitute for cold requirement. It induces α-amylase and overcomes ABA's inhibition
• Light — red light triggers germination in light-sensitive seeds (lettuce, tobacco) via Pfr formation. Far-red inhibits it
• After-ripening — simply storing dry seeds for a period allows natural degradation of inhibitors. Many crop seeds need 2–6 months of dry storage
• Leaching — water washes away water-soluble inhibitors from the seed coat (natural rain does this)

Quick recall

A farmer has dormant seeds with very hard seed coats. Which TWO methods would be most effective for breaking dormancy?

(1) Scarification — physically scratch/crack the seed coat to allow water entry. (2) Soaking in concentrated H₂SO₄ (chemical scarification) — acid softens the impermeable coat. For hard seed coat dormancy, physical/chemical methods work better than hormone treatments.

Vivipary

Vivipary is the premature germination of seeds while still inside the fruit and attached to the parent plant. It occurs naturally in mangroves (Rhizophora) and is prevented in most plants by ABA.

In most plants, ABA prevents the seed from germinating while still on the mother plant. But in mangroves like *Rhizophora*, this control is naturally absent — the seed germinates while still attached, sending out a long root (radicle) that drops into the mud below. This is vivipary.

• Vivipary — seed germinates while still attached to the parent plant; the seedling develops within/on the fruit
• Natural vivipary — seen in mangroves (*Rhizophora*, *Avicennia*, *Sonneratia*) as an adaptation to the muddy, tidal environment
• The seedling grows a long, pointed propagule (hypocotyl) that drops dart-like into the soft mud and anchors immediately
• ABA prevents vivipary in most plants — ABA-deficient mutants (like viviparous maize mutants) show premature germination on the cob
• Vivipary is the exception that proves the rule: dormancy (maintained by ABA) is the default; vivipary occurs when ABA signaling fails or is bypassed