What is the primary factor that plants measure in photoperiodism: day length or night length?

While the term 'photoperiodism' suggests day length, experimental evidence, particularly with Short-Day Plants (SDPs), clearly indicates that the duration of the *uninterrupted dark period* is the most critical factor. Interrupting the dark period with even a brief flash of light can significantly alter the flowering response, whereas interrupting the light period usually has little effect. This highlights the plant's ability to 'measure' the length of continuous darkness.

How do plants 'measure' the length of day and night?

Plants measure day and night length using specialized photoreceptors, primarily phytochromes. Phytochrome exists in two interconvertible forms: $P_r$ (red-light absorbing, inactive) and $P_{fr}$ (far-red-light absorbing, active). Red light converts $P_r$ to $P_{fr}$, while far-red light converts $P_{fr}$ to $P_r$. During darkness, $P_{fr}$ slowly reverts to $P_r$. The ratio of these forms, and the time it takes for $P_{fr}$ to decay during the night, acts as the plant's biological clock to gauge the duration of light and darkness.

What is florigen, and where is it produced?

Florigen is a hypothetical, hormone-like substance that is believed to be produced in the leaves in response to the appropriate photoperiodic stimulus. Once produced, it is transported through the phloem to the shoot apical meristem, where it induces the initiation of flowering. Although florigen itself has not been isolated, molecular studies have identified a protein called Flowering Locus T (FT) as a key component of this mobile flowering signal, acting as the 'florigen' signal.

Can a Short-Day Plant (SDP) be induced to flower in summer?

Yes, an SDP can be induced to flower in summer (when days are naturally long and nights are short) by artificially providing it with long, uninterrupted dark periods. This can be achieved by covering the plants with opaque material for a portion of the day, effectively creating 'short days' or, more accurately, 'long nights'. This horticultural practice is commonly used to produce flowers like chrysanthemums out of their natural season.

What is the significance of the critical photoperiod?

The critical photoperiod is a specific threshold of day length (or night length) unique to each plant species, which determines its flowering response. It's not an absolute value but a reference point. For Short-Day Plants, flowering occurs when the day length is *less than* their critical photoperiod. For Long-Day Plants, flowering occurs when the day length is *greater than* their critical photoperiod. This critical value allows plants to precisely time their flowering to specific seasons.

Photoperiodism

Biology

NEET UG

Version 1Updated 22 Mar 2026

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Photoperiodism refers to the physiological response of plants to the relative lengths of day and night, particularly the duration of the dark period. This environmental cue is critical for regulating various developmental processes, most notably flowering, but also dormancy, tuberization, and bud break. Plants perceive the photoperiodic signal primarily through specialized photoreceptors, such as …

Quick Summary

Photoperiodism is the physiological response of plants to the relative lengths of day and night, primarily influencing flowering. Plants are categorized into Short-Day Plants (SDP), Long-Day Plants (LDP), and Day-Neutral Plants (DNP).

SDPs flower when the uninterrupted dark period exceeds a critical length (e.g., Chrysanthemum), while LDPs flower when the uninterrupted dark period is shorter than a critical length (e.g., Spinach). DNPs flower independently of photoperiod (e.

g., Tomato). The perception of photoperiod occurs in the leaves, mainly through phytochrome, a photoreversible pigment existing as $P_r$ (inactive) and $P_{fr}$ (active). $P_{fr}$ accumulates in light and slowly reverts to $P_r$ in darkness.

The ratio and duration of $P_{fr}$ presence dictate the flowering response. A hypothetical hormone, florigen (now understood to involve FT protein), transmits the signal from leaves to the apical meristems, inducing flower formation.

Understanding photoperiodism is crucial for agricultural practices like controlled flowering and crop timing.

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Key Concepts

Phytochrome Interconversion and its Role

Phytochrome is a crucial photoreceptor existing in two forms: $P_r$ (red-light absorbing) and $P_{fr}$ …

Critical Dark Period vs. Critical Photoperiod

While the term 'photoperiodism' implies light duration, it is the *uninterrupted dark period* that is often…

Florigen and its Transport

Florigen is the elusive signaling molecule produced in the leaves under appropriate photoperiodic conditions,…

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.

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).