Login Free Test Start Mock

Botany · Plant Growth and Development

Photoperiodism

Photoperiodism — the regulation of flowering by the relative duration of light and dark periods — is one of the most reliable NEET subtopics from Chapter 13. Examiners return to it repeatedly because the biology contains multiple reversals of intuition: plants measure night length, not day length; perception occurs in the leaf, not the shoot apex; and the active phytochrome form (Pfr) has opposite effects in short-day and long-day plants. Two direct PYQs from 2019 and 2021 test exactly these reversal points.

NCERT Grounding

NCERT Class XI Biology, Chapter 13 (Plant Growth and Development), Section 13.4.3 closes with the statement: "Many of the extrinsic factors such as temperature and light, control plant growth and development via PGR. Some of such events could be: vernalisation, flowering, dormancy, seed germination, plant movements, etc." The chapter's Exercise Q.7 explicitly asks students to explain how both a short-day plant and a long-day plant can flower simultaneously at the same location — a question that requires understanding of critical night length. The NIOS Chapter 20 (Section 20.8) supplements this with a concrete operational definition and plant classifications, and Section 20.9 introduces florigen and phytochrome Pr/Pfr interconversion.

"Would a defoliated plant respond to photoperiodic cycle? Why?"

NCERT Class XI, Chapter 13, Exercise Q.9 — anchoring the leaf as site of perception

Definition

Photoperiodism is the response of a plant to the relative duration of the light and dark periods in a 24-hour day. The term was coined by Garner and Allard (1920) after their experiments with Maryland Mammoth tobacco, which failed to flower under long summer days but flowered readily when days were artificially shortened.

A critical clarification underlies all subsequent analysis: despite the name suggesting a response to light, the biological clock within the plant actually measures the length of the uninterrupted dark period (night length). Day length and night length are complementary in a 24-hour cycle, but the plant's mechanism uses darkness as the primary signal, a fact confirmed by dark-period interruption experiments (discussed below).

Three Plant Types

Classification basis: flowering response relative to the critical night length (the threshold duration of uninterrupted darkness for each species).

Short-Day Plants (SDP)

Night > CNL

Night length must exceed Critical Night Length

Condition to flower: long night (short day)

Examples: Chrysanthemum, tobacco, Xanthium, sugarcane, soybean, rice (some varieties)

Season: flower in autumn / winter when nights are long

High NEET frequency

Long-Day Plants (LDP)

Night < CNL

Night length must be less than Critical Night Length

Condition to flower: short night (long day)

Examples: wheat, radish, lettuce, henbane (Hyoscyamus niger), spinach

Season: flower in spring / early summer when nights are short

Moderate NEET frequency

Day-Neutral Plants (DNP)

Any

Flowering independent of photoperiod

Condition to flower: reaches maturity regardless of night/day duration

Examples: tomato, cucumber, sunflower, maize

Season: flower across seasons once vegetative phase is complete

Low NEET frequency

Critical Night Period — What Plants Actually Measure

Every photoperiodically sensitive species has a critical night length (CNL) — a species-specific threshold of continuous darkness. The plant does not directly measure day length; it measures how long the night has been dark without interruption.

Figure 1 Critical Night Period in SDP vs LDP Critical Night Length — SDP vs LDP Flowering Decision CNL Scenario Night Length Short Night LDP flowers SDP no flower Long Night SDP flowers LDP no flower Any Night DNP always flowers Short night Long night Any night

Figure 1. Flowering decisions for SDP, LDP, and DNP relative to the critical night length (CNL, red dashed line). Bars above the CNL line represent nights longer than CNL; bars below represent nights shorter than CNL. SDPs flower only when night exceeds CNL; LDPs flower only when night falls below CNL; DNPs flower regardless of night length.

A pivotal demonstration: if a short-day plant and a long-day plant share the same location and the ambient photoperiod sits exactly at the CNL boundary, both can flower simultaneously. The SDP experiences a night just barely long enough, while the LDP experiences a night just barely short enough — because their CNLs can be set at the same threshold. This is the answer NCERT Chapter 13 Exercise Q.7 demands.

Florigen — The Leaf-to-Apex Signal

Garner and Allard's discovery established that photoperiodism controls flowering, but left unresolved the question of where in the plant the light signal is perceived and how the information reaches the shoot apex where floral organs eventually develop.

Grafting experiments answered the first question decisively. When a photoperiod-exposed leaf from a flowering plant was grafted onto a non-induced plant kept in non-inductive conditions, the recipient plant flowered. This demonstrated that a transmissible signal — hypothetically called florigen — is produced in the leaf under appropriate photoperiod and travels through the phloem to the shoot apex, triggering the transition from vegetative to reproductive growth.

Florigen Pathway — Leaf to Apex

4 steps
  1. Step 1

    Photoperiod Detected

    Phytochrome in leaf mesophyll cells measures night length over successive 24-h cycles.

    Site: Leaf
  2. Step 2

    Florigen Synthesised

    Under inductive photoperiod, the leaf produces the flowering stimulus (florigen).

    Site: Leaf
  3. Step 3

    Phloem Transport

    Florigen moves from leaf through phloem to the shoot apex; defoliation blocks this pathway.

    Pathway: Phloem
  4. Step 4

    Floral Induction

    Florigen arrives at shoot apex meristem; vegetative-to-reproductive transition begins.

    Site: Shoot Apex

Florigen remains a hypothetical hormone — no single compound fitting the classical definition has been isolated. The leading molecular candidate is the protein product of the FLOWERING LOCUS T (FT) gene, which moves from leaf to shoot apex in a manner consistent with grafting data. For NEET purposes, florigen is described as a hypothetical flowering stimulus synthesised in the leaf and transported to the shoot apex.

Phytochrome — The Molecular Light Sensor

The photoreceptor responsible for detecting light/dark cycles in leaves is phytochrome, a blue-green chromoprotein. It exists in two interconvertible forms whose relative concentrations encode information about the light environment.

Phytochrome Interconversion — Pr vs Pfr

Pr (Phytochrome Red)

660 nm

Absorption peak (red light)

  • Inactive form — present in darkness and at dawn
  • Absorbs red light (660 nm) → converts to Pfr
  • Accumulates during long nights as Pfr slowly reverts
  • High Pr ratio signals "long night" to the plant
  • Promotes SDP flowering (indirectly, by not having Pfr)
⇌

Pfr (Phytochrome Far-Red)

730 nm

Absorption peak (far-red light)

  • Active form — present after daytime light exposure
  • Absorbs far-red light (730 nm) → converts back to Pr
  • Also reverts to Pr slowly in darkness (dark reversion)
  • High Pfr ratio signals "short night / long day" to the plant
  • Promotes LDP flowering; inhibits SDP flowering

The key functional outcome: Pfr is the biologically active form. During long days (short nights), Pfr levels remain high at the end of the night because dark reversion has not had enough time to convert much Pfr back to Pr. This sustained Pfr signals "short night" — the condition LDPs need to flower. During long nights, Pfr reverts extensively to Pr, and the low Pfr / high Pr ratio signals "long night" — the condition SDPs need to flower.

660

nm — Red Light

Pr absorbs at 660 nm → converts to Pfr. Far-red light at 730 nm reverses this: Pfr → Pr. The Pr ⇌ Pfr interconversion is the molecular switch plants use to measure photoperiod.

/
730

nm — Far-Red Light

Pfr absorbs at 730 nm → converts to Pr. Pfr also undergoes slow dark reversion to Pr during the night, making night length measurable by the ratio of the two forms.

Dark-Period Interruption — The Defining Experiment

The experiment that proved plants measure night length, not day length, involves interrupting the dark period with a brief pulse of light. If a short-day plant (requiring a long uninterrupted night) is given a long night but with a brief red-light flash in the middle, it fails to flower — even though the total dark duration was sufficient. The interruption breaks the continuous darkness the SDP requires.

The same flash during the night of a long-day plant (which needs only a short night) causes it to flower even when placed in otherwise short-day conditions, because the red-light flash effectively signals "daytime" and prevents a long uninterrupted night from accumulating.

Figure 2 Dark-Period Interruption Experiment Dark-Period Interruption by Brief Red-Light Flash SDP LDP Long Uninterrupted Night FLOWERS R 660 NO FLOWER Long night intact → SDP flowers Night broken by red flash → SDP fails NO FLOWER R 660 FLOWERS Short night only → LDP fails (long night given) Long night + flash → LDP flowers Dark period Red-light flash (660 nm)

Figure 2. Dark-period interruption experiment. A red-light flash (660 nm) mid-night inhibits SDP flowering but promotes LDP flowering, demonstrating that plants measure the duration of uninterrupted darkness, not total day length. The effect of the flash is reversed by an immediate far-red pulse (730 nm), confirming phytochrome mediation.

Critically, the inhibitory effect of a red-light flash on SDP flowering is reversed by an immediate far-red (730 nm) pulse after the red flash. Red converts Pr to Pfr (interrupts night); far-red immediately after reconverts Pfr to Pr (restores "dark" signal). This reversibility is the molecular signature of phytochrome involvement and has been reproduced across many species.

Worked Examples

Worked example 1

A plant of Xanthium (cocklebur, an SDP with CNL = 8.5 hours) is placed in a growth chamber with a 16-hour light / 8-hour dark cycle. Will it flower? What if the dark period is extended to 9 hours?

Solution: Xanthium is a short-day plant with a critical night length of approximately 8.5 hours. Under a 16L/8D cycle, the night is 8 hours — below the CNL of 8.5 h. It will not flower. If the dark period is extended to 9 hours (9 h > 8.5 h CNL), the requirement is met and it will flower. Note: the plant does not care that day length changed from 16 h to 15 h; only the night length relative to the CNL matters.

Worked example 2

A scientist removes all leaves from an otherwise healthy short-day plant and exposes it to inductive photoperiod (long nights) for four weeks. Does it flower?

Solution: No. The leaf is the site of photoperiod perception and the site of florigen synthesis. A defoliated plant has no tissue capable of detecting the photoperiod signal or producing florigen. Without florigen being transported to the shoot apex, floral induction cannot occur despite the correct photoperiod. This is precisely what NCERT Exercise Q.9 probes.

Worked example 3

In an experiment, an SDP in long-night conditions receives a brief flash of red light (660 nm) mid-night, followed immediately by a flash of far-red light (730 nm). Does it flower?

Solution: Yes. The red flash converts Pr → Pfr (interrupting the dark period signal). The immediate far-red flash reverses this: Pfr → Pr (restoring the dark signal). Because Pfr was immediately converted back to Pr, the plant "perceives" an uninterrupted dark period. The SDP flowers as if no light interruption occurred. This reversibility confirms phytochrome as the photoreceptor.

Common Confusion & NEET Traps

SDP vs LDP — Side-by-Side Comparison

Short-Day Plant (SDP)

Long Night

Requirement for flowering

  • Night > critical night length
  • Pfr reverts to Pr during long night → Pr signals flowering
  • Red-light interruption of dark period → blocks flowering
  • Examples: Chrysanthemum, tobacco, Xanthium, soybean, sugarcane
  • Typically autumn/winter flowering plants
VS

Long-Day Plant (LDP)

Short Night

Requirement for flowering

  • Night < critical night length
  • Pfr remains high at end of short night → Pfr signals flowering
  • Red-light interruption of dark period → promotes flowering
  • Examples: wheat, radish, lettuce, henbane, spinach
  • Typically spring/early summer flowering plants

NEET PYQ Snapshot — Photoperiodism

Both confirmed PYQs test the same high-yield concept: site of photoperiod perception. Secure these two marks before moving to phytochrome mechanics.

NEET 2021 · Q.123

The site of perception of light in plants during photoperiodism is:

  1. Leaf
  2. Shoot apex
  3. Root apex
  4. Stem
Answer: (1) Leaf

Why: Phytochrome, the photoreceptor for photoperiodism, is present in leaf mesophyll cells. The leaf detects the photoperiod and synthesises florigen, which is then transported to the shoot apex. Students who confuse perception (leaf) with response (shoot apex) choose option (2) — a trap NEET has set twice in two years.

NEET 2019 · Q.53

What is the site of perception of photoperiod necessary for induction of flowering in plants?

  1. Shoot apex
  2. Auxillary meristem
  3. Root tip
  4. Leaves
Answer: (4) Leaves

Why: Grafting experiments established that a leaf exposed to inductive photoperiod transmits a flowering stimulus to an unexposed shoot apex. Removing all leaves (defoliation) prevents flowering even under inductive photoperiod. The shoot apex is the site of the eventual floral response, not the initial perception.

Concept · High Priority

Which of the following is a short-day plant?

  1. Radish
  2. Wheat
  3. Xanthium
  4. Lettuce
Answer: (3) Xanthium

Why: Xanthium (cocklebur) is the classic SDP used in most photoperiodism experiments. Radish (1), wheat (2), and lettuce (4) are long-day plants. Memorise the SDP list: Chrysanthemum, tobacco, Xanthium, soybean, sugarcane; and the LDP list: wheat, radish, lettuce, henbane.

FAQs — Photoperiodism

Frequently asked conceptual questions from NEET aspirants and chapter exercises.

What is photoperiodism?

Photoperiodism is the response of a plant to the relative duration of the light and dark periods in a 24-hour cycle. Although the word suggests a response to light, plants actually measure the length of the uninterrupted dark period (night length), not the light period, when determining whether conditions are right for flowering.

Which organ perceives the photoperiod stimulus in plants?

The leaf is the site of photoperiod perception in plants. The photoreceptor phytochrome present in leaves detects the light/dark signals. The flowering stimulus (florigen) is then synthesised in the leaf and transported to the shoot apex where flowering is induced. This is a frequently tested NEET concept — the shoot apex is the site of flowering, not perception.

What is the critical night length and how does it differ between SDP and LDP?

The critical night length is the minimum duration of continuous darkness required (in SDPs) or the maximum duration of darkness that can be tolerated (in LDPs) for flowering to occur. Short-day plants flower only when the night length exceeds their critical night length. Long-day plants flower only when the night length is shorter than their critical night length.

What happens when the dark period is interrupted by a brief flash of red light?

A brief red-light flash (660 nm) during the dark period converts Pr to Pfr. This effectively breaks the continuous dark period. Short-day plants, which need a long uninterrupted night, will not flower. Long-day plants, which need only a short night, will flower even though the total night was long, because their critical darkness threshold was interrupted. This experiment proved plants measure night length, not day length.

What is florigen and where is it produced?

Florigen is a hypothetical flowering hormone whose existence is inferred from grafting experiments. Under appropriate photoperiod conditions, it is synthesised in the leaf and then transported through the phloem to the shoot apex, where it triggers the transition from vegetative to floral development. Florigen has not been isolated as a single chemical entity; the leading molecular candidate is the protein encoded by the FLOWERING LOCUS T (FT) gene.

What are phytochromes Pr and Pfr, and which one promotes flowering in LDP?

Phytochrome is a blue-green chromoprotein photoreceptor present in two interconvertible forms. Pr (phytochrome red) absorbs red light at 660 nm and converts to Pfr. Pfr (phytochrome far-red) absorbs far-red light at 730 nm and reverts to Pr; it also reverts slowly in darkness. Pfr is the active form — it promotes flowering in long-day plants and inhibits flowering in short-day plants. During long nights Pfr reverts to Pr, which is why SDPs flower in short days (long nights).

Give two examples each of short-day plants, long-day plants, and day-neutral plants.

Short-day plants (SDP): Chrysanthemum, tobacco, Xanthium, sugarcane, soybean. Long-day plants (LDP): wheat, radish, lettuce, henbane (Hyoscyamus). Day-neutral plants (DNP): tomato, cucumber, sunflower. DNPs flower regardless of photoperiod once they reach maturity.

Related Topics
Plant Growth and Development Back to the full chapter notes hub. Vernalisation Temperature-induced acceleration of flowering — the cold-requirement parallel to photoperiodism. Auxins First plant growth regulator; roles in phototropism, apical dominance, and rooting. Gibberellins Bolting, internode elongation, and seed germination — PGR that can substitute for some photoperiod requirements in LDPs. Abscisic Acid Stress hormone; promotes dormancy — the functional antagonist of flowering promoters. Development and Plasticity Intrinsic vs extrinsic control of development; plasticity in response to environment. Growth — Definition, Phases, Rate Meristematic, elongation, maturation phases and growth rate equations.