🌅 Photoperiodism & Vernalization
Learn photoperiodism and vernalization in plant physiology for CUET Agriculture. Short-day, long-day plants, phytochrome and cold treatment.
Photoperiodism
Definition
Photoperiodism is the physiological response of plants to the relative length of day (light period) and night (dark period) that controls flowering. Plants use day/night duration as a reliable environmental signal to determine the best time to flower and reproduce.
Discovery
- Discovered by Garner and Allard in 1920
- They studied the Maryland Mammoth variety of tobacco (Nicotiana tabacum) which flowered only under short-day conditions — it refused to flower during long summer days in Maryland, USA
- This was the first demonstration that day length controls flowering — a revolutionary finding in plant biology
Classification of Plants Based on Photoperiodism
Plants are classified into 3 main categories based on their flowering response to day length:
(1) Short Day Plants (SDP)
- These plants require a light period shorter than the critical day length to flower — in other words, they need long uninterrupted dark periods
- They flower when days are short and nights are long (typically autumn/winter in temperate regions)
Examples: Tobacco (Nicotiana), soybean, rice (Oryza sativa), chrysanthemum, Xanthium (cocklebur), dahlia, sugarcane, jowar (sorghum), mustard
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Photoperiodism
Definition
Photoperiodism is the physiological response of plants to the relative length of day (light period) and night (dark period) that controls flowering. Plants use day/night duration as a reliable environmental signal to determine the best time to flower and reproduce.
Discovery
- Discovered by Garner and Allard in 1920
- They studied the Maryland Mammoth variety of tobacco (Nicotiana tabacum) which flowered only under short-day conditions — it refused to flower during long summer days in Maryland, USA
- This was the first demonstration that day length controls flowering — a revolutionary finding in plant biology
Classification of Plants Based on Photoperiodism
Plants are classified into 3 main categories based on their flowering response to day length:
(1) Short Day Plants (SDP)
- These plants require a light period shorter than the critical day length to flower — in other words, they need long uninterrupted dark periods
- They flower when days are short and nights are long (typically autumn/winter in temperate regions)
Examples: Tobacco (Nicotiana), soybean, rice (Oryza sativa), chrysanthemum, Xanthium (cocklebur), dahlia, sugarcane, jowar (sorghum), mustard
(2) Long Day Plants (LDP)
- These plants require a light period longer than 12 hours (the critical day length) to flower
- They flower when days are long and nights are short (typically spring/summer)
Examples: Spinach (palak), radish (muli), sugarbeet (chukandar), oat, wheat (gehu), carrot (gajar)
(3) Day Neutral Plants (DNP)
- These plants are not affected by photoperiod — they flower regardless of day length
- Flowering is controlled by other factors such as age, temperature, or nutritional status
Examples: Tomato, cotton, maize/corn, sunflower (surajmukhi), cucumber
IMPORTANT
A critical concept for exams: It is actually the length of the dark period (night) that is more important than the light period. Short-day plants are really "long-night plants" — they need an uninterrupted dark period exceeding a critical length.
Mechanism of Photoperiodism
(1) Florigen Hypothesis
- Proposed by Chailakhyan (Russian scientist)
- He suggested that a hormone called florigen is produced in leaves under the appropriate photoperiod
- Florigen is transported to the shoot apex where it induces flowering
- Florigen has never been isolated in pure form — it remains a hypothetical hormone
- Recent research suggests florigen may be the FT protein (Flowering Locus T), but this is still being studied
(2) Phytochrome System
Proposed by Borthwick and Hendricks. This is the most widely accepted mechanism for how plants detect day length.
Phytochrome is a photoreceptor pigment (blue-green chromoprotein) that exists in two interconvertible forms:
| Form | Abbreviation | Absorbs | Role |
|---|---|---|---|
| Phytochrome Red | Pr (P₆₆₀) | Red light (660 nm) | Inactive form — stable in dark, accumulates during long nights |
| Phytochrome Far-Red | Pfr (P₇₃₀) | Far-red light (730 nm) | Active form — promotes flowering in LDP, inhibits flowering in SDP |
Interconversion:
- Red light (660 nm): Pr → Pfr (converts inactive to active form — happens during daytime)
- Far-red light (730 nm): Pfr → Pr (converts active back to inactive form)
- Darkness: Pfr slowly reverts to Pr (spontaneous conversion in the dark — this is how plants "measure" night length)
How it works in different plant types:
- In Long Day Plants (LDP): Pfr (active form) promotes flowering. Long days provide more red light → more Pfr accumulates → enough Pfr triggers flowering
- In Short Day Plants (SDP): Pfr inhibits flowering. During long dark periods, Pfr slowly reverts to Pr → Pfr levels drop below the inhibitory threshold → flowering is "released" and occurs
- The critical dark period is more important than the light period for SDP — the night must be long enough for sufficient Pfr to convert back to Pr
Key Experimental Evidence:
- A brief flash of red light during the dark period can prevent flowering in SDP — because it converts Pr to Pfr, resetting the "clock" and maintaining inhibition
- A subsequent flash of far-red light can reverse this effect — converting Pfr back to Pr, allowing SDP to flower
- This red/far-red reversibility proved that phytochrome is the photoreceptor responsible for photoperiodism
TIP
Remember: Pfr = the active form. In LDP, Pfr promotes flowering (long days = lots of Pfr = flowering). In SDP, Pfr inhibits flowering (long nights = Pfr degrades = inhibition removed = flowering). The plant "measures" night length by tracking how much Pfr converts back to Pr in darkness.
Vernalization
Definition
Vernalization is the process of promoting flowering by exposing plants (or seeds) to low temperature (cold treatment) for a specific period. Many temperate plants require a period of cold before they can flower — this prevents them from flowering during autumn's short warm spell and ensures they flower in spring after surviving winter.
History
- Term coined by Chouard (French scientist)
- The concept was first studied and popularized by T.D. Lysenko (Russian scientist)
- Lysenko demonstrated that winter wheat could be converted to spring wheat behavior by cold treatment of seeds — a major agricultural breakthrough
Key Facts
| Property | Detail |
|---|---|
| Site of perception | Shoot apex (apical meristem) — the cold stimulus is perceived at the growing tip |
| Temperature required | 0-5°C (low temperature) |
| Duration | Varies by species — from days to weeks |
| Effective stage | Germinating seeds or young seedlings |
| Plants requiring vernalization | Winter wheat, rye, barley, beet, cabbage, carrot |
Process of Vernalization
- Cold exposure: Seeds or young plants are exposed to low temperature (0-5°C) for a specific period
- Vernalin production: Cold treatment induces production of a hypothetical substance called vernalin in the shoot apex — this "remembers" the cold experience
- Florigen activation: After vernalization, when the plant experiences the appropriate photoperiod, florigen is produced → flowering occurs
Sequence: Cold treatment → Vernalization → Vernalin → Photoperiod → Florigen → Flowering
Devernalization
- The reversal of the vernalization effect by exposing vernalized plants/seeds to high temperature
- If vernalized seeds are exposed to high temperature before sowing, the cold treatment effect is nullified
- The plant returns to its non-vernalized state and will not flower until re-vernalized
Gibberellin as Vernalization Substitute
- GA₃ (Gibberellic acid) can substitute for cold treatment in many plants
- GA₃ application can induce flowering in biennials without cold exposure — this is a powerful agricultural tool
- This confirms that vernalization affects the hormonal balance of the plant
NOTE
Vernalization and photoperiodism work together: Vernalization is the "permission" step (has the plant experienced winter?), and photoperiodism is the "timing" step (is it the right season to flower?). Both conditions must be met for flowering in plants that require both.
Seed Dormancy
Definition
Seed dormancy is the condition where viable seeds fail to germinate even when provided with favorable conditions (water, temperature, oxygen). It is a survival strategy — dormancy prevents seeds from germinating at the wrong time (e.g., just before winter).
Types of Dormancy
| Type | Cause |
|---|---|
| Exogenous dormancy | Due to external factors — hard seed coat that is impermeable to water or gases |
| Endogenous dormancy | Due to internal factors — immature embryo, hormonal imbalance, or presence of growth inhibitors like ABA |
| Combined dormancy | Both external and internal factors contribute |
Methods of Breaking Dormancy
| Method | Description |
|---|---|
| Scarification | Physical or chemical breaking/weakening of the hard seed coat — mechanical abrasion, or acid treatment with H₂SO₄ (sulfuric acid) |
| Stratification | Exposure to cold, moist conditions for a specific period — mimics winter conditions; seeds are layered in moist sand and refrigerated |
| GA₃ treatment | Application of gibberellic acid to overcome hormonal inhibition (GA counteracts ABA) |
| Leaching | Washing away chemical inhibitors from the seed coat with running water |
| Light treatment | Some seeds require light (photoblastic seeds) for germination — the phytochrome system is involved |
| Temperature treatment | Alternating or specific temperature exposure can break dormancy |
| Ethylene treatment | Can break dormancy in some seeds by counteracting ABA effects |
Seed Germination
Definition
Germination is the process by which a dormant seed develops into a seedling when favorable conditions are met. It begins with water absorption (imbibition) and ends with the emergence of the radicle (embryonic root).
Conditions Required for Germination
| Condition | Details |
|---|---|
| Water | Essential for imbibition (seed swelling), enzyme activation, and metabolic processes — the first and most critical factor |
| Oxygen | Required for aerobic respiration to provide energy for growth |
| Temperature | Optimal range varies by species; generally 20-30°C |
| Light | Some seeds are photoblastic (require light); others germinate in darkness |
Types of Germination
| Type | Description | Examples |
|---|---|---|
| Epigeal germination | Cotyledons emerge above the soil surface; the hypocotyl elongates, pushing cotyledons up into the air where they often photosynthesize briefly | Bean (Phaseolus), castor, cotton, onion |
| Hypogeal germination | Cotyledons remain below the soil surface; the epicotyl elongates, pushing the plumule (embryonic shoot) above the soil | Pea, maize, rice, wheat, mango, coconut |
TIP
Memory aid: Epigeal = epi means "above" (cotyledons above ground). Hypogeal = hypo means "below" (cotyledons below ground). In epigeal, the hypocotyl elongates (pushing up). In hypogeal, the epicotyl elongates (pushing up).
Phytochrome and Its Role in Plant Responses
Properties of Phytochrome
- It is a chromoprotein — composed of a protein component bound to a chromophore (light-absorbing molecule)
- The chromophore is a linear tetrapyrrole (phytochromobilin) — similar in structure to bile pigments
- Found in the cytoplasm of plant cells
- Present in very small amounts — detected using sensitive spectrophotometry
- Pr form absorbs red light (660 nm) — this is the stable form that accumulates in darkness
- Pfr form absorbs far-red light (730 nm) — this is the biologically active form that triggers physiological responses
Phytochrome-Mediated Responses
Phytochrome controls many aspects of plant development beyond just flowering:
| Response | Type | Details |
|---|---|---|
| Seed germination | Rapid | Red light promotes germination (Pr → Pfr); far-red inhibits |
| Stem elongation | Rapid | Pfr inhibits elongation (prevents etiolation — the spindly growth seen in darkness) |
| Leaf expansion | Slow | Pfr promotes leaf unfolding and expansion |
| Chloroplast development | Slow | Pfr promotes chlorophyll synthesis (greening of leaves) |
| Flowering (LDP) | Slow | Pfr promotes flowering |
| Flowering (SDP) | Slow | Pfr inhibits flowering |
Key Facts for Exam Revision
| Fact | Detail |
|---|---|
| Photoperiodism discovered by | Garner and Allard (1920) |
| Plant studied | Maryland Mammoth tobacco |
| SDP examples | Rice, chrysanthemum, tobacco, soybean, Xanthium |
| LDP examples | Wheat, oat, spinach, radish, sugarbeet |
| DNP examples | Tomato, maize, sunflower, cotton |
| Florigen proposed by | Chailakhyan |
| Phytochrome proposed by | Borthwick and Hendricks |
| Pr absorbs | Red light (660 nm) |
| Pfr absorbs | Far-red light (730 nm) |
| Active phytochrome form | Pfr |
| Vernalization term by | Chouard |
| Vernalization studied by | T.D. Lysenko |
| Vernalization temperature | 0-5°C |
| Site of vernalization perception | Shoot apex |
| Devernalization | High temperature reverses vernalization |
| GA₃ substitutes for | Vernalization (cold treatment) |
| Vernalin | Hypothetical substance produced after cold treatment |
| Scarification | Breaking hard seed coat |
| Stratification | Cold moist treatment for dormancy breaking |
| Epigeal germination | Cotyledons above soil (bean, castor) |
| Hypogeal germination | Cotyledons below soil (pea, maize, rice) |
| Climacteric fruits | Ripen with ethylene after harvest |
| Critical factor in SDP | Night length (dark period), not day length |
| Red light flash in dark | Converts Pr → Pfr (prevents SDP flowering) |
| Far-red flash reversal | Converts Pfr → Pr (allows SDP flowering) |
Practice Questions (from PDF)
Practice Questions with Answers (Click to expand)
- Bolting is stimulated by: — (d) Gibberellin
- Maximum growth occurs in: — (d) Log phase
- Richmond-Lang effect is from: — (c) Cytokinin
- Which is the site of auxin transport? — (d) Basipetal (polar transport from apex downward)
- NAA increases flower number? — (a) NAA actually reduces flower number but increases fruit size
- Which hormone causes fruit drop? — (c) NAA at high concentration
- Natural plant hormone in purest form: — (c) GA₃ (gibberellic acid)
- Which inhibits vernalization? — (d) GA₃ (substitutes for vernalization, bypassing the cold requirement)
- Which promotes vegetative growth suppression? — (a) Cytokinin promotes lateral growth
- Gibberellin promotes in rice seedlings: — Elongation (causes bakanae/foolish seedling disease)
- If auxin concentration increases, water absorption will: — (d) Increase
- Coconut water contains: — (d) Cytokinin
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Photoperiodism definition | Plant flowering response to relative length of day and night |
| Photoperiodism discovered by | Garner and Allard (1920); studied Maryland Mammoth tobacco |
| Short Day Plants (SDP) | Need long uninterrupted dark periods; examples: rice, tobacco, soybean, chrysanthemum, Xanthium, sugarcane |
| Long Day Plants (LDP) | Need light >12 hours; examples: wheat, spinach, radish, sugarbeet, oat, carrot |
| Day Neutral Plants (DNP) | Not affected by photoperiod; examples: tomato, cotton, maize, sunflower, cucumber |
| Critical factor for SDP | Night length (dark period) is more important than light period |
| Florigen hypothesis | By Chailakhyan; hypothetical hormone produced in leaves → transported to shoot apex → induces flowering; never isolated |
| Phytochrome system | By Borthwick and Hendricks; most widely accepted mechanism for photoperiod detection |
| Pr (P₆₆₀) | Absorbs red light (660 nm); inactive form; stable in dark; accumulates during long nights |
| Pfr (P₇₃₀) | Absorbs far-red light (730 nm); active form; promotes flowering in LDP, inhibits in SDP |
| Pr ↔ Pfr conversion | Red light: Pr → Pfr; Far-red light: Pfr → Pr; Darkness: Pfr slowly reverts to Pr |
| LDP flowering mechanism | Long days → more Pfr accumulates → Pfr promotes flowering |
| SDP flowering mechanism | Long nights → Pfr reverts to Pr → inhibition removed → flowering occurs |
| Red light flash in dark | Converts Pr → Pfr → prevents SDP flowering |
| Far-red light reversal | Converts Pfr → Pr → allows SDP flowering |
| Vernalization definition | Promoting flowering by low temperature (0-5°C) exposure |
| Vernalization term by | Chouard; studied by T.D. Lysenko |
| Site of vernalization perception | Shoot apex (apical meristem) |
| Vernalization sequence | Cold → Vernalization → Vernalin → Photoperiod → Florigen → Flowering |
| Devernalization | High temperature reverses vernalization effect |
| GA₃ as vernalization substitute | Gibberellic acid replaces cold treatment in many plants |
| Seed dormancy types | Exogenous (hard seed coat), Endogenous (immature embryo/ABA), Combined (both) |
| Breaking dormancy methods | Scarification (H₂SO₄/abrasion), Stratification (cold moist), GA₃, Leaching, Light, Ethylene |
| Epigeal germination | Cotyledons above soil; hypocotyl elongates; examples: bean, castor, cotton, onion |
| Hypogeal germination | Cotyledons below soil; epicotyl elongates; examples: pea, maize, rice, wheat, mango |
| Germination conditions | Water (imbibition), Oxygen (respiration), Temperature (20-30°C), Light (some seeds) |
| Phytochrome properties | Chromoprotein with linear tetrapyrrole chromophore; found in cytoplasm |
| Phytochrome other roles | Seed germination (red promotes), stem elongation (Pfr inhibits), leaf expansion, chloroplast development |
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