Photoperiodism, vernalization and flowering control

1. Why Does a Plant Not Need a Calendar?

Let us imagine a situation. We are in a field, a forest, or a steppe. We see that some plant has flowered. We can say: “Today is May 15.” But the plant itself does not know that today is May 15. It has neither a calendar, nor a clock, nor access to the Internet.

However, most plants flower with remarkable precision year after year. Wheat flowers in midsummer, chrysanthemums in autumn, and winter rye, if sown in spring, may not flower at all. How is this possible?

The plant solves a non‑trivial problem: how, in constantly changing environmental conditions, to select the only, optimal moment for switching to reproduction.

A mistake here is too costly. If it flowers too early, frost will strike and the generative organs will die. If it flowers too late, the seeds will not have time to ripen before cold weather or drought sets in. Therefore, nature has created mechanisms that allow the plant to orient itself not by absolute dates but by reliable signals that indicate the change of seasons.

Such a signal cannot be simply temperature: in one year warmth may come early, in another late. It cannot be simply rainfall: that is too unstable a factor.

The only absolutely reliable, annually recurring signal in temperate latitudes is day length (Garner & Allard, 1920; Horie, 1994).

Day length depends exclusively on geographic latitude and the current day of the year. It does not change from year to year: June 1 is always longer than October 1. For the plant, this is like a “calendar of nature” that never lies.

That is precisely why the ability to respond to the day‑night ratio – photoperiodism – is one of the key evolutionary adaptations (Roberts & Summerfield, 1987). Note an important nuance: in the tropics, where day length scarcely changes, this mechanism is less relevant, and species that flower upon reaching a certain size (the autonomous pathway) are more common there. But in our latitudes, photoperiodism becomes the chief “conductor” of ontogenesis.

Moreover, the plant does not measure day length as such. As we shall see in the next section, in fact the plant measures the length of the continuous night (Taiz et al., 2023). It records the moment darkness falls and the moment dawn breaks. An error in measurement of even 15–20 minutes can disrupt flowering (Salisbury, 1963). That is why a short break in the night (night break) can completely alter the plant’s developmental programme.

Thus, to the main question “How does the plant tell time?” we get the first, fundamental answer:

The plant tells time by the duration of the dark period of the day. This signal is absolutely reliable, but to read it a special “measuring device” is needed. That device is phytochrome.

It is to phytochrome that we shall turn in the next part of the lecture.

2. How Does a Plant Measure Day Length?

So, the plant does not know the calendar, but it knows exactly how long the night lasted. The secret of its astonishing accuracy lies in the fact that plants have a unique molecular sensor that works like a biological switch. That sensor is phytochrome (Schopfer & Brennicke, 2016; Hopkins & Hüner, 2009).

But it is important to understand right away: phytochrome is not just “a pigment that sees light.” It is a sensor protein that exists in two interconvertible forms. One form is active, the other inactive. The transition between them occurs under the action of light of a strictly defined wavelength.

The two forms of phytochrome: Pr and Pfr

In darkness, phytochrome is synthesised in the inactive form called Pr (from red). This form absorbs light in the red region of the spectrum with a maximum at about 660 nm. Upon exposure to red light, the Pr molecule quickly rearranges and turns into the active form – Pfr (from far‑red). Pfr, in turn, absorbs light in the far‑red region with a maximum at about 730 nm, and under its influence it reverts to Pr (Schopfer & Brennicke, 2016).

This reaction is very fast and reversible:

Pr (inactive) ⇌ Pfr (active)

Red light (660 nm) → activation

Far‑red light (730 nm) → inactivation

It is this remarkable photoreversibility that allows the plant to read the light signal. During the dark period, Pfr gradually breaks down or converts to Pr (this process is called dark reversion). The longer the night, the less active Pfr remains by morning (Taiz et al., 2023). The plant “senses” that the night was long if the Pfr level has fallen below a certain threshold.

But why does the plant measure night rather than day?

The reason is that Pfr is the form that is formed only in the light. If the night is short, Pfr does not have time to decay, and the level of the active sensor remains high. If the night is long, Pfr degrades and its concentration falls. Thus, by measuring the amount of Pfr at dawn, the plant can determine how long the night was.

This mechanism is easily illustrated by the classic night‑break experiment. If a short flash of red light (660 nm) is given in the middle of a long night, a rapid conversion of Pr to Pfr occurs, which is perceived by the plant as an “interruption of the night” and shifts the signal to “short night”. And if far‑red light (730 nm) is given immediately after the red, the effect is cancelled – the plant again “sees” a long night (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

Thus, phytochrome is not just a pigment but a molecular clock that switches developmental programmes depending on the ratio of red to far‑red light in the environment. Moreover, in nature under a plant canopy the ratio of red to far‑red changes, and this also serves as a signal for the shade‑avoidance response, but we will discuss that separately.

An important clarification: cryptochromes and other sensors

We must not forget that besides phytochrome, which operates mainly in the red region, plants have other sensors that perceive blue light – cryptochromes and phototropins (Schopfer & Brennicke, 2016). They also participate in photoperiodic reactions, especially in processes related to circadian rhythms and the stabilisation of key flowering regulators such as the CONSTANS (CO) protein (Taiz et al., 2023). The interaction of all these systems ensures high accuracy and reliability in reading seasonal information.

So, we know how the plant measures night length. Now the next logical question arises: if all plants have this sensor, why do some flower in summer and others in autumn? That depends on how the Pfr signal is interpreted in different species. We shall consider the answer to this question in the next chapter.

3. Why Do Different Plants Flower in Different Seasons?

Thus, phytochrome gives the plant a quantitative estimate of night length: the longer the night, the less active Pfr remains by morning. But then the most interesting part begins. Depending on the genetic programme of the species, a low level of Pfr can be perceived either as a signal “time to flower” or as a signal “not yet”.

This division is the basis of the classical classification of plants by photoperiodic response (Taiz et al., 2023; Horie, 1994).

Short‑Day Plants (SDP)

These are plants that flower only when the night length exceeds some critical value. Since night becomes longer in autumn, such plants flower in late summer or autumn. In terms of phytochrome: for them, a low Pfr level (long night) is the permissive signal for flowering. If the night is too short (in summer), Pfr does not decay to the threshold level, and flowering is blocked.

Classic examples: soybean (Glycine max), chrysanthemum (Chrysanthemum), rice (Oryza sativa), tobacco “Maryland Mammoth” (Garner & Allard, 1920; Horie, 1994). For them, the critical day length may be, for example, 12–14 hours. If the day is longer than this threshold, they vegetate; if shorter, they initiate flowering.

Long‑Day Plants (LDP)

These plants, on the contrary, require the night to be shorter than some critical value. They flower in spring or early summer, when days are long and nights short. For them, a high Pfr level (short night) serves as a stimulus for flowering. If the night is too long (in winter or early spring), Pfr degrades and flowering is not triggered.

Examples: wheat (Triticum aestivum), barley (Hordeum vulgare), spinach (Spinacia oleracea), radish (Horie, 1994; Connor et al., 2011). In wheat, the critical day length may be about 10–14 hours, but the exact value varies greatly among different cultivars.

Day‑Neutral Plants (DNP)

There is also a third group that pays no attention to photoperiod. Such plants flower upon reaching a certain age, size, or accumulated biomass. For them, day‑length signals are irrelevant; their flowering is controlled by autonomous pathways or gibberellins (Connor et al., 2011; Taiz et al., 2023). Examples include tomato, sunflower, maize (many modern varieties). Evolutionarily, they often originate from the tropics, where seasonal day‑length changes are minimal, or from places with unpredictable moisture, where it is more important to “seize the moment” than to wait for a specific season.

Important subtleties: critical day length and facultative responses

When speaking of critical day length, we must clearly understand that this value is not absolute for all plants. Different species and even cultivars have their own. For example, for one soybean cultivar the critical day may be 14 hours, while for another it may be 13.5 (Horie, 1994). That is why soybean varieties are divided into maturity groups: some are adapted to northern latitudes (where summer days are longer), others to southern ones.

Furthermore, the photoperiodic response can be obligate (qualitative) or facultative (quantitative) (Connor et al., 2011; Schopfer & Brennicke, 2016).

• Obligate – the plant never flowers without the required photoperiod. Example: the short‑day cocklebur (Xanthium).
• Facultative – the plant will flower even under a non‑optimal photoperiod, but significantly later. Example: the long‑day Arabidopsis thaliana. In this case, photoperiod accelerates or delays flowering but is not an absolute prohibition.

Ecological meaning: why is this evolutionarily advantageous?

Now the most important point: why did nature create such different strategies? The answer lies in adaptation to specific growing conditions.

• Long‑day plants are typical inhabitants of temperate and high latitudes. They flower in spring or early summer, using the long day to maximise vegetative mass accumulation and seed formation before autumn cold. They also often have a requirement for vernalization (cold) to avoid flowering too early when there is a risk of frost (Connor et al., 2011).
• Short‑day plants often come from tropics and subtropics. They flower in autumn, when days shorten. This allows them to avoid flowering in the height of summer heat and drought, and to complete the reproductive cycle by the start of the dry season (Roberts & Summerfield, 1987; Horie, 1994). For example, soybean originates from East Asia, where shortening days signal the approach of autumn and a wetter period.

Finally, some species exhibit more complex patterns: for example, long‑short‑day or short‑long‑day plants that require a strict sequence of photoperiods for flowering (Hopkins & Hüner, 2009). But these are special cases that do not negate the main principle.

So, we see that one and the same signal (Pfr level) can lead to directly opposite outcomes in different taxonomic groups. This is a vivid example of how evolution uses one molecular mechanism in different “interpretations”.

However, a natural question arises: if the signal is perceived in the leaf, and the flower is formed at the shoot apex, how is the information transmitted from the leaf to the apex? This question leads us to the discovery of florigen – and that will be the topic of the next chapter.

4. Why Does the Leaf Control Flowering?

At first glance, it seems logical that the decision to flower should be made right where the flower appears – in the shoot apical meristem. After all, it is there that floral primordia are laid down, and it is there that morphogenetic changes occur. However, classic experiments of the 20th century showed that this is a profound misconception.

An amazing fact: the photoperiodic signal is perceived not by the apex, but by the mature leaf (Hopkins & Hüner, 2009; Taiz et al., 2023).

How was this proven? The most convincing experiments were carried out by the Russian physiologist Mikhail Kh. Chailakhyan (M. Chailakhyan) and his followers. They used plants in which different parts could be exposed separately – for example, by covering only the shoot tip or only the leaves with light‑proof caps.

Here is the classic experimental scheme (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016):

1. If a short‑day plant (e.g., Perilla or Xanthium) is given a dark cap only on the shoot apex while the leaves remain in long days, flowering does not occur. The apex is under short days (in darkness), but that produces no effect.

2. If, on the other hand, the shoot apex is left in long days (in light), while one or several leaves are subjected to short days, the plant flowers. The leaf “senses” the short day and sends a signal that causes the apex to switch to flowering, despite the apex itself being in non‑inductive conditions.

These experiments convincingly demonstrated: the leaf is the organ of photoperiod perception. It is in the leaf that the night length is measured with the help of phytochrome, and it is in the leaf that some signalling factor is produced and transmitted to the apex.

Proof of the existence of “florigen” – a transmissible signal

Further experiments showed that this signal is not just a local change in the leaf, but a substance that can be transmitted from plant to plant via grafting (Hopkins & Hüner, 2009; Taiz et al., 2023). If an induced leaf (which has received the right photoperiod) is grafted onto a non‑induced plant, the second plant also flowers, even if it itself did not receive the required photoperiod.

Moreover, it turned out that this signal is universal: it can be transferred between different species and even between plants of different photoperiodic groups. For example, grafting a long‑day plant onto a short‑day plant can cause the latter to flower if the donor was induced by long days, and vice versa (Taiz et al., 2023). This indicates that the basis of photoperiodic induction is a single chemical factor, which in 1937 M. Kh. Chailakhyan named “florigen” (Schopfer & Brennicke, 2016; Medvedev, 2012).

Modern understanding: florigen is the FT protein

For decades, the nature of florigen remained a mystery. Only in the 2000s, with the help of molecular‑genetic methods on the model plant Arabidopsis thaliana, was it discovered that the key component of florigen is a protein encoded by the FLOWERING LOCUS T (FT) gene (Taiz et al., 2023; Medvedev, 2012).

The FT gene is expressed precisely in the companion cells of the phloem in the mature leaf in response to an inductive photoperiod. Its expression is activated by the transcription factor CONSTANS (CO), which, as we already mentioned, accumulates in the leaf at a certain day length (under long days in long‑day plants, or under short days in short‑day plants, depending on the species) (Taiz et al., 2023). The FT protein is synthesised in the leaf, then loaded into the phloem and translocated through the vascular system to the shoot apical meristem. This transport occurs via the phloem, which explains why the signal moves at a rate comparable to assimilate flow (Schopfer & Brennicke, 2016).

In the apex, the FT protein interacts with another protein – the transcription factor FD (FLOWERING LOCUS D) – and this complex activates floral meristem identity genes such as LEAFY (LFY) and APETALA1 (AP1) (Taiz et al., 2023; Medvedev, 2012). From that moment, the apex irreversibly switches from the vegetative programme to the generative one – flowering evocation occurs.

Why is such a complex scheme evolutionarily beneficial?

This “decentralised” control gives the plant a huge advantage. The leaf is the organ that directly contacts the environment and can most accurately assess external conditions (light level, spectral composition, presence of competitors). Moreover, different leaves may give signals of different strength, and the phloem integrates these signals and transmits them to the apex. Thus, the decision to flower is made not in an isolated apex, but on the basis of cumulative information coming from the entire leaf surface. This makes the system more flexible and reliable (Horie, 1994; Connor et al., 2011).

So, we now know that:

• The leaf is the organ of photoperiod perception.
• The phloem is the transport route for the signal.
• The FT protein (florigen) is the key mobile link that transmits information from the leaf to the apex.
• The apex is the executive organ where the flower‑formation programme is initiated.

It would seem that the picture is clear. But there is another crucial factor that can either permit or inhibit the response to photoperiod – that is temperature. Many plants, especially in temperate climates, need a prolonged cold exposure to “allow” themselves to flower in response to long days. This process is called vernalization, and it is no less an important regulator than photoperiodism. We will discuss it in the next chapter.

5. Why Does Cold Help Flowering?

Imagine winter wheat. It is sown in autumn. Seedlings emerge, plants vegetate, and then they overwinter as rosettes or small bushes. In spring, they quickly resume growth and flower. If the same wheat is sown in spring, it will not flower at all or will flower with a great delay, failing to yield a crop. Why? Because for this plant to switch to flowering, it must experience a prolonged period of low positive temperatures. This process is called vernalization (from Latin vernum – spring) (Taiz et al., 2023; Schopfer & Brennicke, 2016; Connor et al., 2011).

Vernalization is a physiological process in which a prolonged exposure to cold (usually from 0 to +10 °C, optimum 1–7 °C) makes the plant competent to flower. Without this cold treatment, the plant remains vegetative, even if the photoperiod is ideal for it (Hopkins & Hüner, 2009; Medvedev, 2012).

Ecological meaning: why is cold so important?

The main task of vernalization is to prevent premature flowering in autumn. If winter cereals flowered in autumn after a short period of warmth, they would perish from winter frost without setting seed. Cold serves as a kind of “password”: the plant will not start forming flowers until it is sure that winter has passed. In addition, vernalization allows the plant to clearly distinguish spring from autumn in those periods of the year when day length is the same (e.g., spring equinox and autumn equinox). Cold unequivocally points to winter, and therefore the subsequent warming is spring, not autumn (Taiz et al., 2023).

How and where is cold perceived?

Vernalization is an active metabolic process that requires energy and respiration. It does not act on dry seeds; cold must act on germinating seeds (imbibed) or on actively growing vegetative parts – primarily on meristems (shoot apical meristems, young leaves) (Schopfer & Brennicke, 2016; Connor et al., 2011). Successful vernalization requires not only a certain temperature but also sufficient time – from several weeks to several months. The effectiveness increases with longer cold exposure until saturation is reached (Taiz et al., 2023).

Importantly, vernalization is not always irreversible. If after the cold period the plant is placed in warmth (above 25–30 °C), the process can be devernalized, i.e., cancelled. However, the longer the cold treatment lasted, the more stable the vernalised state becomes (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Molecular mechanisms: epigenetics and repression of inhibitor genes

At the molecular level, vernalization is one of the most striking examples of epigenetic regulation in plants. Studies of this process in the model plant Arabidopsis thaliana led to the discovery of a key flowering repressor gene – FLOWERING LOCUS C (FLC) (Taiz et al., 2023; Medvedev, 2012).

FLC encodes a repressor protein (a MADS‑domain transcription factor) that suppresses the expression of FT in leaves and SOC1 in the apex. As long as FLC is actively expressed, the plant cannot respond to an inductive photoperiod, even if the day is long (Taiz et al., 2023). During vernalization, the following events occur:

1. Prolonged cold activates the expression of genes involved in chromatin remodelling (e.g., VIN3).

2. Chromatin in the FLC locus is modified: histones undergo methylation (in particular, trimethylation of lysine 27 and 9), which places the FLC gene into a heterochromatic, transcriptionally inactive state.

3. The FLC gene is silenced for the remainder of the plant’s life cycle. This state is inherited by cells during division, but is not transmitted to the progeny seeds – in embryogenesis the epigenetic mark is reset, and the offspring again need vernalization (Taiz et al., 2023; Schopfer & Brennicke, 2016).

After FLC suppression, the repression of FT and SOC1 is lifted, and the plant acquires the ability to respond to the photoperiodic signal. Thus, vernalization is a release from blockage, not a direct induction of flowering.

Vernalization in cereals: differences from Arabidopsis

In the most important agricultural crops – wheat, barley, rye – the molecular mechanism of vernalization is different. There, the key genes are VRN1, VRN2, VRN3 (Taiz et al., 2023; Medvedev, 2012; Slafer et al., 2015).

• VRN1 – a gene encoding a transcription factor that is activated by cold. It plays a dual role as both a flowering repressor and a promoter (depending on interactions).
• VRN2 – a repressor that suppresses the expression of VRN3 (an FT analogue) under long days. Cold suppresses VRN2, allowing VRN1 to stimulate VRN3.
• VRN3 – the FT analogue, florigen, which is synthesised in leaves and moves to the apex.

In cereals, vernalization is also associated with epigenetic changes, but the target is not FLC (which they lack) but the promoters of VRN1 and VRN2. As a result, cold alters the expression of these genes, leading to a “permission” to flower when long days arrive in spring (Taiz et al., 2023; Slafer et al., 2015).

Interaction of vernalization and photoperiodism

In most plants that require vernalization, it is not a sufficient condition for flowering. Usually, after the completion of vernalization, the plant also needs an inductive photoperiod (usually long days). Thus, winter wheat must first undergo vernalization in winter, and then in spring, when days become long, FT expression is triggered (via the photoperiodic pathway), and flowering occurs. This dual requirement makes the system especially reliable: the plant will not flower accidentally in autumn (if it happens to be warm) nor will it delay into summer without cold. Thus, temperature and photoperiod work in tandem, forming an integrated system of seasonal flowering control (Connor et al., 2011; Slafer et al., 2015).

So, we have obtained a complete picture: day length (via phytochrome and CO) and temperature (via vernalization and epigenetics) – two independent yet coordinated calendars that allow the plant to choose the ideal time for flowering. It remains to answer the final question: why is all this important for agriculture? We will consider that in the concluding chapter.

6. Why Is This Important for Agriculture?

So, we know that flowering is not a random event but a strictly regulated process dependent on day length and temperature history. For agriculture, this knowledge has a direct, and sometimes decisive, significance. Here are the main practical conclusions that follow from the study of photoperiodism and vernalization.

1. Selection of a cultivar for a specific geographical zone – the basis of adaptation

This is perhaps the most important point. Different regions of the world have different day lengths during the growing season. A variety that grows well in the south (where days are shorter), if moved to the north (where summer days are long), may not flower at all if it is an obligate short‑day plant.

• Classic example – soybean (Glycine max). This crop is divided into maturity groups (from 000 to X) that are linked to photoperiodic sensitivity. Varieties from northern groups (00, 0, I) flower under longer days, while southern ones (VI, VII, VIII) under shorter days. If a farmer in Canada sows a southern soybean variety, under the long northern day the plant will vegetate all season, flower too late, and fail to yield a crop. Conversely, a northern variety in the south will flower too early, produce dwarf plants, and give a meagre yield (Connor et al., 2011; Horie, 1994). Therefore, breeders specifically create varieties with the required critical day length for each zone.

• Spring and winter types of cereals. For wheat, rye, and barley, the vernalization requirement is crucial (Slafer et al., 2015). Winter cultivars will not flower without winter cold. If winter wheat is sown in spring in the Non‑Black Earth region, it will produce only vegetative mass and go into winter without forming ears. Spring varieties, on the other hand, do not require cold, but their yield is often lower than that of winter varieties. The choice between spring and winter forms is a conscious decision by the agronomist, based on knowledge of the climate and variety characteristics.

2. Forecasting flowering and harvest times

Knowing the photoperiodic and temperature response of a cultivar, one can calculate with high accuracy when flowering and then maturation will occur. This is critical for planning:

• Harvest timing: so that combine harvesters are ready on time.
• Irrigation: watering must be most efficient precisely in the critical phases (flowering, grain filling).
• Fertiliser application: fertilisation should be given at the phase of maximum nutrient uptake (e.g., nitrogen during intensive growth).

For this purpose, phenological models have been developed that, based on photoperiod and temperature (in terms of thermal units – growing degree‑days), allow predicting the date of flowering. For example, models for rice (Horie, 1994) or wheat (Angus et al., 1981) give an error within 3–5 days, which is a great help for the agronomist.

3. Controlling flowering in protected cultivation (greenhouses, conservatories)

Knowledge of photoperiodism allows year‑round control of flowering in ornamental crops.

• Short‑day plants (chrysanthemums, poinsettias) can be made to flower at any time of year by artificially creating short days (shading for 12–14 hours) (Taiz et al., 2023; Connor et al., 2011).
• Long‑day plants (lettuce, spinach) – conversely, can be supplemented with light to accelerate flowering (if seed production is needed) or, on the contrary, to avoid bolting (if the commercial product is leaf mass).

Moreover, the night break (red light) is actively used in commercial floriculture to suppress chrysanthemum flowering when it is desired to keep them vegetative (Hopkins & Hüner, 2009). This is an example of how a fundamental scientific fact becomes a technological tool.

4. Breeding for photoperiod and vernalization neutrality

Modern agriculture strives to create varieties with broad ecological plasticity. Varieties that are neutral to day length or do not require vernalization (e.g., many modern cultivars of maize, tomato, sunflower) can be grown over a wide range of latitudes. This simplifies logistics and reduces risks.

However, this approach also has a downside: varieties with a clear photoperiodic requirement are often better adapted to stresses in their particular region. The breeder must choose between “universality” and “specialisation” – and this decision is based on physiological understanding.

5. Weed control

Knowledge of the photoperiodism of weed species helps to predict their flowering and seed set, and hence to plan the timing of mechanical or chemical weed control. If a weed is short‑day, its flowering can be suppressed or stimulated (depending on the goal) by changing agronomic practices, such as shading or adjusting the sowing date of the main crop.

6. Planning sowing dates in risky farming zones

In regions with short summers (Northern Europe, Siberia, Canada), it is important that the crop manages to flower and mature before frost. Using models that take into account photoperiod and vernalization, one can determine the optimal sowing window to fit within the growing season. For example, for spring wheat in Canada, a shift in sowing by 5–7 days can change yield by 20–30% (Slafer et al., 2015).

Conclusion: from theory to practice

Thus, we see that the mechanisms of flowering control are not abstract molecular subtleties. They are tools that allow us to:

• Choose the right variety for the field.
• Predict crop development.
• Control flowering in greenhouses.
• Improve the precision of agronomic practices.

Photoperiodism and vernalization are the “anchors” that link the plant’s genetic potential to actual environmental conditions. By understanding them, the agronomist ceases to be a passive observer and becomes an active manager of the plant organism.

That is the main value of plant physiology for practical agriculture: it provides the keys to controlling the life cycle, and thus to stable and high yields.

Final conclusion of the lecture:

Flowering is the result of the integration of two reliable “calendars” – the photoperiodic one (measuring night by phytochrome) and the temperature one (vernalization). The signals are perceived by the leaf, converted into the mobile FT protein (florigen), and transmitted to the apex, where the genetic programme for flower formation is launched. This knowledge underpins modern variety science, agronomy, and biotechnology.

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