Photoperiodism — Revision Notes

⚡ 30-Second Revision

Photoperiodism: — Plant response to day/night length.
SDP (Short-Day Plant): — Flowers when dark period > critical duration (e.g., Xanthium, Chrysanthemum). Red light flash in dark inhibits flowering.
LDP (Long-Day Plant): — Flowers when dark period < critical duration (e.g., Spinach, Wheat). Red light flash in dark promotes flowering.
DNP (Day-Neutral Plant): — Flowering independent of photoperiod (e.g., Tomato, Corn).
Perception Site: — Leaves.
Photoreceptor: — Phytochrome ($P_r leftrightarrow P_{fr}$).
$P_r$: — Red light absorbing (inactive).
$P_{fr}$: — Far-red light absorbing (active). Red light converts $P_r \to P_{fr}$. Far-red light converts $P_{fr} \to P_r$. Dark converts $P_{fr} \to P_r$.
Florigen: — Hypothetical flowering hormone (FT protein) transported from leaves to meristem.

2-Minute Revision

Photoperiodism is how plants use the relative lengths of day and night to time developmental events, primarily flowering. The crucial factor is often the *uninterrupted dark period*. Plants are classified into Short-Day Plants (SDP), Long-Day Plants (LDP), and Day-Neutral Plants (DNP).

SDPs, like Chrysanthemum, need a dark period longer than a critical duration. If this long dark period is interrupted by a brief red light flash, flowering is inhibited because the active phytochrome ($P_{fr}$) is reformed, effectively shortening the perceived night.

LDPs, such as Spinach, require a dark period shorter than a critical duration. A red light flash during the dark period would promote flowering by further shortening the effective dark period, maintaining $P_{fr}$ levels.

DNPs, like Tomato, flower irrespective of day length. The leaves perceive the photoperiodic signal via phytochrome, which exists as $P_r$ (inactive) and $P_{fr}$ (active). $P_{fr}$ is formed in red light and slowly reverts to $P_r$ in darkness. The signal is then transmitted to the shoot apex by florigen (FT protein) to induce flowering.

5-Minute Revision

Photoperiodism is the plant's ability to respond to the duration of light and dark periods, a vital adaptation for synchronizing flowering and other developmental processes with seasonal changes. The key to understanding this phenomenon lies in the *uninterrupted dark period* and the photoreceptor phytochrome.

Plant Classification:

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Short-Day Plants (SDP): — Flower when the uninterrupted dark period is *longer* than a critical duration. Examples: Xanthium, Chrysanthemum, Poinsettia. A brief red light flash during the dark period inhibits flowering.
2
Long-Day Plants (LDP): — Flower when the uninterrupted dark period is *shorter* than a critical duration (or day length is longer than critical). Examples: Spinach, Radish, Wheat. A brief red light flash during the dark period promotes flowering.
3
Day-Neutral Plants (DNP): — Flower irrespective of photoperiod. Examples: Tomato, Corn, Cucumber.

Mechanism of Perception:

The leaves are the site of photoperiod perception.
Phytochrome — is the primary photoreceptor, existing in two interconvertible forms:

* **$P_r$ (phytochrome red):** Absorbs red light (660 nm), physiologically inactive. * **$P_{fr}$ (phytochrome far-red):** Absorbs far-red light (730 nm), physiologically active.

Interconversion: — Red light converts $P_r \to P_{fr}$. Far-red light converts $P_{fr} \to P_r$. In darkness, $P_{fr}$ slowly reverts to $P_r$ (dark reversion) and is also degraded.
Timing: — The ratio of $P_r$ to $P_{fr}$ and the duration of $P_{fr}$ presence (or its decay) during the night acts as the plant's timer.

* SDPs: Require low $P_{fr}$ at dawn (long dark period allows $P_{fr}$ to convert to $P_r$). Red light flash during dark prevents this, inhibiting flowering. * LDPs: Require high $P_{fr}$ at dawn (short dark period means $P_{fr}$ doesn't fully convert). Red light flash during dark maintains $P_{fr}$, promoting flowering.

Signal Transmission:

A hypothetical hormone, florigen, is produced in the leaves and transported via the phloem to the shoot apical meristem.
Molecular studies identify Flowering Locus T (FT) protein as a key component of florigen, which interacts with FD protein at the meristem to activate flowering genes.

Example Scenario: A Xanthium (SDP) plant is given 16 hours of light and 8 hours of dark. It will not flower because the dark period is too short. If it is given 8 hours of light and 16 hours of dark, it will flower. If this 16-hour dark period is interrupted by a 5-minute red light flash, it will not flower, demonstrating the critical role of uninterrupted darkness for SDPs.

Prelims Revision Notes

1
Definition: — Photoperiodism is the physiological response of plants to the relative lengths of day and night, primarily regulating flowering.
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Key Discovery: — Garner and Allard (1920) discovered photoperiodism using 'Maryland Mammoth' tobacco.
3
Critical Factor: — The *uninterrupted dark period* is generally more critical than the light period for flowering.
4
Plant Classification:

* Short-Day Plants (SDP): Flower when the uninterrupted dark period is *longer* than a critical duration. Examples: Xanthium (cocklebur), Chrysanthemum, Poinsettia, Rice, Maryland Mammoth Tobacco.

* Long-Day Plants (LDP): Flower when the uninterrupted dark period is *shorter* than a critical duration (or day length is longer than critical). Examples: Spinach, Radish, Wheat, Barley, Clover.

* Day-Neutral Plants (DNP): Flower irrespective of photoperiod. Examples: Tomato, Corn, Cucumber, Cotton, Sunflower.

1
Critical Photoperiod: — A species-specific threshold. For SDPs, day length < critical; for LDPs, day length > critical.
2
Site of Perception: — Leaves contain the photoreceptors.
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Photoreceptor: — Phytochrome.

* Exists in two forms: $P_r$ (red-light absorbing, inactive) and $P_{fr}$ (far-red-light absorbing, active). * **$P_r xrightarrow{\text{Red Light}} P_{fr}$ * $P_{fr} xrightarrow{\text{Far-red Light}} P_r$ * $P_{fr} xrightarrow{\text{Darkness (slowly)}} P_r$** (Dark reversion)

1
Role of Phytochrome in Flowering:

* SDPs: Require low $P_{fr}$ at dawn. Long dark period allows $P_{fr}$ to convert to $P_r$. Red light flash during dark converts $P_r \to P_{fr}$, inhibiting flowering. Far-red light after red light can reverse inhibition. * LDPs: Require high $P_{fr}$ at dawn. Short dark period means $P_{fr}$ persists. Red light flash during dark maintains $P_{fr}$ or shortens effective dark period, promoting flowering.

1
Florigen: — Hypothetical flowering hormone produced in leaves, transported via phloem to shoot apical meristem.

* Molecularly, Flowering Locus T (FT) protein is a key component of florigen, interacting with FD protein to activate flowering genes.

1
Other Photoperiodic Responses: — Dormancy, tuberization, bud break, leaf abscission.

Vyyuha Quick Recall

Short Day Plants Love Outrageously Nice Gardens (SDP Love LONG nights). Long Day Plants Hate Outrageously Nice Gardens (LDP Hate LONG nights).

For Phytochrome: Red Reacts to Red ($P_r$ absorbs Red). Far-Red For Far-Red ($P_{fr}$ absorbs Far-Red). Pr is Passive, Pfr is Powerful (active).