Photoperiodism
Photoperiodism refers to the physiological response of plants to changes in the length of day and night (i.e., the duration of light and dark periods). This adaptation helps plants synchronize their growth, development, and reproduction with seasonal environmental changes, such as preparing for flowering in favorable conditions like spring or autumn.
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| Xanthium (cockleber) |
Historical Discovery
- Key Researchers: In 1920, W.W. Garner and H.A. Allard first studied photoperiodism using tobacco plants (Nicotiana tabacum).
- Initial Observation: Tobacco flowers only after exposure to a series of short days (long nights). Naturally, this occurs in autumn, but it can be artificially induced by simulating short days in a lab.
- Explanation: This demonstrated that day length acts as an environmental cue for flowering, not just temperature or other factors.
Classification of Plants Based on Photoperiodic Response
Plants are categorized into three main types based on how day length influences flowering. (Note: Later research showed that the night length is actually the critical factor, not the day length. Short-day plants are essentially "long-night plants," and vice versa.)
Classification of Plants Based on Photoperiodic Response
Plants are grouped into 3 main types based on how day length (or really night length) affects flowering.
Quick Note: Research shows night length is the real key factor – not days! So, Short-Day Plants = "Long-Night Plants" (need long dark for flowers), and vice versa.
1. Short-Day Plants (SDPs) – Long-Night Lovers
- How It Works: Flowering starts when nights > critical length (e.g., >8.5 hours for cocklebur).
- Key Trick: Brief light in long night? Flowering stops (inhibited).
- Critical Dark Period: Must be longer than threshold (e.g., cocklebur: 8.5 h; tobacco: 10-11 h).
- Examples: Cocklebur (Xanthium), chrysanthemum, soybean, tobacco, strawberry.
- Easy Tip: Think "Short Day = Long Dark Party" for flowers to bloom!
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| Henbane |
2. Long-Day Plants (LDPs) – Short-Night Lovers
- How It Works: Flowering starts when nights < critical length (e.g., <13 hours for henbane).
- Key Trick: Brief light in long night? Flowering starts (promoted, even in short days).
- Critical Dark Period: Must be shorter than threshold (e.g., henbane: 13 h).
- Examples: Henbane (Hyoscyamus niger), snapdragon, cabbage, spring wheat, spring barley.
- Easy Tip: "Long Day = Short Dark Quickie" – lights up flowers fast!
3. Day-Neutral Plants (DNPs) – No-Rules Plants
- How It Works: Flowers anytime – day or night length doesn't matter.
- Key Trick: Photoperiod has zero effect; other factors (like age) control.
- Critical Dark Period: Independent (no fixed rule).
- Examples: Cucumber, tomato, garden pea, maize, cotton.
- Easy Tip: "Day Neutral = Don't Need Rules" – blooms on its own schedule!
https://www.rhs.org.uk/plants/293619/solanum-lycopersicum/details
Key Term: Critical Length
- What?: The exact night duration threshold that triggers or blocks flowering.
- Why?: Acts as a plant's built-in "timer" to match reproduction with best seasons (e.g., pollinators ready).
- Easy Tip: Like a "darkness alarm clock" – hit the time, flowers wake up!
Revise Hack: SDP (Long Night = Yes Flowers), LDP (Short Night = Yes), DNP (Anytime = Yes). Draw a quick chart for memory!
Mechanism of Flower Induction
The process involves photoreceptors and internal "clocks" that measure light/dark periods, leading to hormone production for flowering.
Role of Phytochrome
- Phytochrome: A blue-green photoreceptor protein in plants that detects red and far-red light. It exists in two interconvertible forms:
- P660 (Pr): The inactive (quiescent) form. Absorbs red light (660 nm wavelength) and converts to the active form.
- P730 (Pfr): The active form. Absorbs far-red light (730 nm wavelength) and reverts to P660.
- Daily Cycle:
- Daylight (red light-rich): Converts P660 → P730 (accumulation of active form).
- Night (darkness): Slowly converts P730 → P660 (depletion of active form).
- Biological Clock: The rate of P730 breakdown in darkness acts as a "timer" for night length. This helps the plant "measure" how long it's been dark.
Light's Effect on Flowering
- In Short-Day Plants (SDPs): Red light (promoting P730 formation) inhibits flowering if it interrupts the long night. P730 buildup signals "night is too short."
- In Long-Day Plants (LDPs): Red light promotes flowering by maintaining P730 levels during short nights.
- Hypothesis (Photoperiodic Timekeeper): P730 acts as a regulator:
- In SDPs: High P730 at night end inhibits flowering unless the night is long enough to deplete it fully.
- In LDPs: Residual P730 at dawn promotes flowering in short nights.
- Modern Understanding: Phytochrome interconversion is important but not the sole controller. Other factors include:
- Direct light/dark duration.
- Internal circadian rhythms (the plant's ~24-hour "biological clock").
- Florigen Hormone: Once the photoperiodic signal is received (via the biological clock), leaves produce florigen (a mobile flowering hormone). Florigen travels through the phloem (vascular tissue) to shoot apical meristems (floral buds), triggering flower formation.
- Explanation: Florigen is not a single molecule but a complex signal (involving proteins like FT – Flowering Locus T). It's produced in leaves under the right photoperiod and acts like a "messenger" for systemic flowering.
Additional Notes
- Photoperiodism ensures plants flower when pollinators are active and conditions favor seed survival.
- It promotes genetic diversity by synchronizing flowering across a population for cross-pollination.
7.12 Vernalization
Vernalization is the promotion of flowering in certain plants by prolonged exposure to low (chilling) temperatures. It "vernalizes" (from Latin vernalis meaning "of spring") the plant, preparing it for reproduction after winter.
Key Differences from Photoperiodism
- Site of Perception: Unlike photoperiodism (perceived by leaves), vernalization is sensed by the shoot apical meristem (growing tip of the stem) or the embryo in seeds.
- Stimulus: Low temperature (not light/dark cycles).
- Purpose: Helps biennials (2-year cycle) and some perennials overwinter and flower in the next season.
Requirements and Process
- Temperature and Duration: Optimal at ~4°C for 4 days to 3 months (plant-specific).
- Too short: No effect.
- Too cold (<0°C): Can damage tissues.
- Types of Plants Affected:
- Absolute Requirement: Essential for flowering (e.g., winter wheat).
- Promotive: Speeds up or enhances flowering but not mandatory.
- Hormone Involved: Induces production of vernalin (a hypothetical hormone, possibly gibberellins or related signals).
- Explanation: Vernalin** is produced in the meristem during chilling. It epigenetically modifies genes (e.g., via histone changes) to "remember" the cold exposure, activating flowering genes later under warm conditions.
- Mechanism: Cold stabilizes flowering repressors (like FLC gene in Arabidopsis), silencing them so flowering can proceed in spring.
Ecological Role
- Synchronization: Aligns reproduction with post-winter growth periods, avoiding energy waste in harsh conditions.
- Cross-Pollination: Ensures population-level flowering timing for genetic variability, similar to photoperiodism.
- Examples:
- Biennials: Carrot, cabbage (flower after 1st-year chilling).
- Perennials: Tulips (bulb chilling mimics winter).
Integration with Photoperiodism
Both mechanisms work together:
- Photoperiodism cues seasonal timing.
- Vernalization adds a "winter counter" for temperate plants. This combo ensures flowering only when both cold exposure and correct day length align (e.g., spring equinox).
✅ SECTION A – MCQs (With Answers and Explanations)
Topic: Photoperiodism & Vernalisation
1. Photoperiodism is the response of plants to:
A) Water availability
B) Temperature changes
C) Length of day and night
D) Soil minerals
Answer: C
Explanation: Photoperiodism is the response of plants to day length and night duration, which affects flowering and other growth processes.
2. Photoperiodism was first studied by:
A) Mendel and Darwin
B) Julius von Sachs
C) Garner and Allard
D) Watson and Crick
Answer: C
Explanation: In 1920, Garner and Allard discovered photoperiodism while working on tobacco and soybean.
3. Short-day plants are also known as:
A) Short-night plants
B) Long-night plants
C) Neutral plants
D) Summer plants
Answer: B
Explanation: Short-day plants flower when nights are long, so they are actually long-night plants.
4. Which of the following is a short-day plant (SDP)?
A) Wheat
B) Henbane
C) Tobacco
D) Cabbage
Answer: C
Explanation: Tobacco and soybean are classic short-day plants that need long nights to flower.
5. Long-day plants flower when:
A) Night is long
B) Dark period is shorter than a critical length
C) Light period is short
D) Day and night are equal
Answer: B
Explanation: Long-day plants (LDPs) flower when nights are short and days are long.
6. Which of the following is a long-day plant (LDP)?
A) Rice
B) Cucumber
C) Tomato
D) Henbane
Answer: D
Explanation: Henbane (Hyoscyamus niger), wheat, and barley are long-day plants.
7. Day-neutral plants:
A) Do not flower
B) Flower only in summer
C) Are unaffected by day length
D) Need long nights
Answer: C
Explanation: Day-neutral plants (DNPs) flower irrespective of photoperiod.
8. Identify a day-neutral plant:
A) Chrysanthemum
B) Soybean
C) Cabbage
D) Tomato
Answer: D
Explanation: Tomato, maize, cotton, and cucumber are day-neutral plants.
9. Most critical factor in photoperiodism is:
A) Length of light period
B) Length of dark period
C) CO₂ concentration
D) Temperature
Answer: B
Explanation: Length of night is more important than day length in flowering response.
10. Phytochrome exists in how many forms?
A) 1
B) 2
C) 3
D) 4
Answer: B
Explanation: Phytochrome has two forms: P660 (inactive) and P730 (active).
11. Which form of phytochrome absorbs red light (660 nm)?
A) P300
B) P500
C) P660
D) P730
Answer: C
Explanation: P660 absorbs red light (660 nm) and gets converted to P730.
12. Which form of phytochrome absorbs far-red light (730 nm)?
A) P330
B) P500
C) P660
D) P730
Answer: D
Explanation: P730 absorbs far-red light (730 nm) and converts back to P660.
13. In nature, P660 is converted to P730 during:
A) Darkness
B) Daylight
C) High temperature
D) Low temperature
Answer: B
Explanation: Sunlight contains red light which converts P660 → P730.
14. Florigen is produced in:
A) Roots
B) Stem
C) Leaves
D) Flower buds
Answer: C
Explanation: Florigen hormone is produced in leaves and transported to floral buds via phloem.
15. Vernalisation is:
A) Response to light
B) Response to gravity
C) Flowering due to low temperature
D) Growth due to water
Answer: C
Explanation: Vernalisation is flower induction by exposure to low temperature.
16. Vernalisation is mostly seen in:
A) Annual plants
B) Biennial plants
C) Hydrophytes
D) Day-neutral plants
Answer: B
Explanation: Biennial plants like carrot, sugar beet need chilling treatment to flower.
17. The chemical responsible for vernalisation is called:
A) Auxin
B) Florigen
C) Cytokinin
D) Vernalin
Answer: D
Explanation: Vernalin is the hypothetical hormone that induces vernalisation.
18. Optimum temperature for vernalisation is about:
A) 37°C
B) 25°C
C) 4°C
D) –10°C
Answer: C
Explanation: Vernalisation usually occurs at low temperatures around 4°C.
19. Vernalisation stimulus is perceived by:
A) Leaves
B) Flowers
C) Shoot apex
D) Roots
Answer: C
Explanation: Low temperature is detected by shoot apex or embryo, not by leaves.
20. Photoperiodism and vernalisation help in:
A) Storing food
B) Seed dormancy
C) Synchronising flowering with seasons
D) Photosynthesis
Answer: C
Explanation: Both photoperiodism and vernalisation ensure flowering at the right season for reproductive success.
✅ SECTION B – Short Questions (with Answers)
Topic: Photoperiodism & Vernalisation
1. What is photoperiodism?
Answer: Photoperiodism is the response of plants to the relative lengths of day and night, which affects processes like flowering.
2. Who discovered photoperiodism and when?
Answer: Garner and Allard discovered photoperiodism in 1920 while studying tobacco and soybean plants.
3. What is critical photoperiod?
Answer: Critical photoperiod is the minimum duration of light or dark period required by a plant to induce flowering.
4. Name three types of plants based on photoperiodic response.
Answer:
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Short-day plants (SDPs)
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Long-day plants (LDPs)
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Day-neutral plants (DNPs)
5. Give two examples of short-day plants.
Answer: Tobacco and soybean are examples of short-day plants.
6. Why are short-day plants called long-night plants?
Answer: Because the duration of darkness is more important than light for flowering, so they flower only when nights are long.
7. What is phytochrome?
Answer: Phytochrome is a light-sensitive pigment that helps plants to measure photoperiod and control flowering.
8. Name the two forms of phytochrome.
Answer:
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P660 (inactive form)
-
P730 (active form)
9. What is florigen?
Answer: Florigen is a hypothetical flowering hormone produced in leaves that moves to buds to induce flowering.
10. What is vernalisation?
Answer: Vernalisation is the acceleration of flowering in plants by exposure to low temperature.
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