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Photoperiodism

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Photoperiodism

Have you ever wondered how plants know when it is time to produce flowers or lose leaves? Imagine what would happen if a plant produced flowers when its pollinators were not around. Many plants respond to the changing seasons by detecting the time of year based on the length of days. This ability is called photoperiodism.

Here, we will discuss the definition, importance and examples of photoperiodism in plants.

Photoperiodism in plants

Plants respond to light in various ways including:

  • Photomorphogenesis: the growth and development of plants in response to light by which plants can take full advantage of available light and space.

  • Phototropism: the ability of plants to grow towards or away from the direction of light.

  • Photoperiodism: the ability of plants to determine the time of day and year by detecting wavelengths of sunlight.

The prefix "photo-" means light. So when you encounter words like photoperiodism and photosynthesis, you know it has something to do with light. For example, photo + period = using light to determine a time of day or year. Or photo + synthesis: using light energy to synthesize nutrients.

Photoperiod definition

How do plants determine the time of day and year? A change in the length of a day is the environmental cue that tells a plant what time of the year it is. The length of day (or the number of hours of exposure to daylight) is referred to as the photoperiod.

How does photoperiodism work?

A plant's ability to detect light in its environment is critical to its competitiveness and survival. Plants can detect and respond to blue, red, and far-red wavelengths of light through photoreceptors. These photoreceptors are made up of chromoproteins, each of which consist of a protein and a light-absorbing pigment called chromophore.

Phytochromes are chromoproteins that detect red and far-red light. Phytochromes are classified into two types:

  • Pr (phytochrome red) which can absorb red light.

  • Pfr (phytochrome far-red) which can absorb far-red light.

When Pr absorbs red light, it transforms into Pfr, and when Pfr absorbs far-red light, it transforms back into Pr. The absorption of red or far-red light changes the structure of the chromophore, which influences the conformation and activity of the phytochrome protein to which it is linked.

Put simply, red light initiates phytochrome activity whereas far-red light inhibits it. The two types of phytochrome, known collectively as the phytochrome system, function as a "biological switch".

Note that unfiltered, full sunshine includes significantly more red light than far-red light. This means that at dawn, phytochromes in a leaf would convert to the active Pfr and stay in that state until dusk. When there is no more sunlight, the Pfr gradually reverts back to Pr form, so when the night is lengthy, as it is in winter, the Pfr form completely reverts. If the night is short, as it is in the summer, a significant amount of Pfr may remain by the time the sun rises.

A plant can determine the length of the day/night cycle by detecting the Pr/Pfr ratio at dawn. Furthermore, by retaining this information for several days, a plant can compare the length of the night before with several other preceding nights.

  • If it detects shorter nights, the plant determines that spring is approaching.

  • If it detects longer nights, the plant determines that autumn is approaching.

Detecting the length of day or night along with temperature and water availability enables plants to determine the time of year and modify their physiology accordingly.

Responses to photoperiod include:

  • Flowering

  • Seed germination

  • Bulb dormancy

  • Color change (due to pigments called anthocyanins) in leaves.

Photoperiod effect

Plants are affected by the length of day or night in varying ways. Here we will talk about descriptors you might encounter alongside photoperiodism: long-day, short-day, and day-neutral plants.

We will then talk about night length and how this is a more useful approach to understanding photoperiodism.

Short-day plants

Researchers began to understand how plants detect seasons by observing a mutant variety of tobacco called the Maryland Mammoth which grew tall but was not able to produce flowers during summer. It finally bloomed in December, but under controlled conditions.

So what conditions caused this variety to bloom? Researchers experimented with different variables including temperature, moisture, and mineral nutrition. They found that the Maryland Mammoth only flowered if the photoperiod was less than 14 hours.

Because the Maryland Mammoth required a photoperiod shorter than critical length in order to flower, it is considered a short-day plant. Other examples of short-day plants include chrysanthemums and some varieties of soybeans. Short-day plants tend to flower in late summer, fall, or winter when the days are shorter.

Mutant varieties of plants are those with random genetic variations that result in new and potentially useful traits.

Long-day plants

In contrast to short-day plants, long-day plants flower only when the photoperiod is longer than critical length. These tend to flower in late spring or early summer. Examples of long-day plants include radishes and irises.

Day-neutral plants

Not all plants use the phytochrome system in the aforementioned ways. Unlike short-day and long-day plants, day-neutral plants are not affected by daylength. Regardless of photoperiod, these plants flower when they reach a certain level of maturity. Examples of day-neutral plants include tomatoes, rice, and dandelions.

Critical night length

Researchers used to believe that the amount of daylight a plant received determined whether or not it produced flowers. Experiments suggested something else: turns out, the length of darkness that a plant was exposed to proved to have a more significant effect than the length of day.

By observing cockleburs–a “short-day” plant that flowers only when the photoperiod is less than 16 hours (and the night length is at least 8 hours long)– researchers found that flowering is controlled by night length rather than day length. Their observations can be summed up as follows:

  • If the day length is interrupted by a brief exposure to darkness, the cocklebur will flower.

  • If the night length is interrupted by a brief exposure to light, the cocklebur will not flower.

They noted that cocklebur is not actually responding to day length, rather, it is responding to the length of continuous darkness. The same was observed in other “short-day” plants.

This means that short-day plants can be more accurately described as long-night plants, whereas long-day plants can be described as short-night plants. Note that long-day and short-day plants are not set apart by an absolute night length. Instead they are distinguished based on whether they set a maximum or minimum night length:

  • A long-day (or, short-night) plant has a maximum number of hours of darkness.

    • Night length cannot exceed a specific number of hours.

  • A short-day (or, long-night) plant has a minimum number of hours of darkness.

    • Night length cannot go below a specific number of hours.

It is important to note that rather than an absolute night length, the actual number of hours of darkness is specific to each plant species.

While they could be misleading, the terms short-day and long-day just happened to be more well-established in the world of Botany. The effect of photoperiod on the flowering is shown in Figure 1 below.

Photoperiodism helps plants detect seasonal changes. This is important because it enables plants to adapt to these changes, increasing their chances of survival and reproduction.

Seasonal flowering as a response to change in day or night length helps plants adapt to their local environment. For example, some plants flower when their pollinators are most active to increase reproductive success.

Plants may also reproduce during springtime so that their offspring have enough time to develop before facing the harsh conditions of winter, which increases their survival rate.

Ground-level woodland plants may also flower in early spring so that they are able to complete seed production before the leaf canopy fully emerges and limits the available light.

Besides flowering, plant responses to photoperiod also include bud dormancy. For example, when certain trees and perennial plant species in northern latitudes detect shortening day length, they respond by inducing cold hardiness and bud dormancy as preparation for the incoming freezing winter temperatures. Desert plants like liverwort, on the other hand, use longer days as a signal to go into dormant state so they have a higher chance of survival during the arid summer months.

Bud dormancy is a mechanism by which plant meristems (regions in the plant where cells are actively dividing) become mainly dormant--although physiological processes do not completely stop--in order to survive harsh environmental conditions.

Photoperiodism - Key takeaways

  • Photoperiodism refers to the ability of plants to determine the time of day and year by detecting wavelengths of sunlight.
  • A change in the length of a day is the environmental cue that tells a plant what time of the year it is. The length of day (or the number of hours of exposure to daylight) is referred to as photoperiod.
  • Phytochromes are chromoproteins that detect red and far-red light. Phytochromes are classified into two types: Pr (phytochrome red) which can absorb red light and Pfr (phytochrome far-red) which can absorb far-red light.
  • A plant can determine the length of the day/night cycle by detecting the Pr/Pfr ratio at dawn.
  • Detecting the length of day or night along with temperature and water availability enables plants to determine the time of year and modify their physiology accordingly.

References

  1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
  2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  3. Pokorny, Kym. “What Are Short-Day and Long-Day Plants?” Life at OSU, today.oregonstate.edu, 29 Jan. 2022, https://today.oregonstate.edu/news/what-are-short-day-and-long-day-plants.
  4. Green, James L. “Photoperiod - A Grower Management Tool For Controlling Plant Growth and Development.” Ornamentals Northwest Archives, vol. 8, no. 3, July-August-September 1984, pp. 19–22.
  5. Jackson, Stephen D. “Plant Responses to Photoperiod.” New Phytologist, 2008, pp. 517–31, https://doi.org/10.1111/j.1469-8137.2008.02681.x.
  6. “Bud Dormancy - an Overview | ScienceDirect Topics.” Bud Dormancy - an Overview | ScienceDirect Topics, www.sciencedirect.com, https://www.sciencedirect.com/topics/medicine-and-dentistry/bud-dormancy. Accessed 25 Aug. 2022.
  7. “Mutation Breeding | IAEA.” Mutation Breeding | IAEA, www.iaea.org, https://www.iaea.org/topics/mutation-breeding. Accessed 25 Aug. 2022.

Frequently Asked Questions about Photoperiodism

Photoperiodism refers to the ability of plants to determine the time of day and year by detecting wavelengths of sunlight

Phototropism refers to the ability of plants to grow towards or away from the direction of light while photosynthesis refers to the process of absorbing light energy to produce nutrients. 

Photoperiodism is responsible for detecting the time of day or year so that plants can modify their physiology accordingly.

Final Photoperiodism Quiz

Question

What is photoperiodism?

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Answer

Photoperiodism refers to the ability of plants to determine the time of day and year by detecting wavelengths of sunlight

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Question

What is the environmental cue that tells a plant what time of the year it is?

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Answer

A change in the length of a day

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Question

The length of day or the number of hours of exposure to daylight is referred to as ___.

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Answer

Photoperiod

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Question

_______ are chromoproteins that detect red and far-red light. 

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Answer

Phytochromes

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Question

What are the two types of phytochromes?

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Answer

Phytochrome red

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Question

Phytochrome red ___ red wavelengths of light.

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Answer

absorbs

Show question

Question

Explain how a plant determines the length of day using phytochromes. 

Show answer

Answer

Pr can absorb red light and Pfr can absorb far-red light. When Pr absorbs red light, it transforms into Pfr, and when Pfr absorbs far-red light, it transforms back into Pr. Unfiltered, full sunshine includes significantly more red light than far-red light so at dawn, phytochromes convert to the active Pfr and stay in that state until dusk. A plant can determine the length of the day/night cycle by detecting the Pr/Pfr ratio at dawn.


Show question

Question

Why is photoperiodism important for plants?

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Answer

Detecting the length of day or night along with temperature and water availability enables plants to determine the time of year and modify their physiology accordingly. 

Show question

Question

What happens to the chromophore when it absorbs red or far-red light?

Show answer

Answer

When Pr absorbs red light, it transforms into Pfr, and when Pfr absorbs far-red light, it transforms back into Pr. The absorption of red or far-red light changes the structure of the chromophore, which influences the conformation and activity of the phytochrome protein to which it is linked. 

Show question

Question

Which of the following are true?

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Answer

Red light initiates phytochrome activity

Show question

Question

Explain how the phytochrome system acts as a "biological switch".

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Answer

The two types of phytochrome, known collectively as the phytochrome system, function as a "biological switch". When Pr absorbs red light, it transforms into Pfr, and when Pfr absorbs far-red light, it transforms back into Pr. In its active Pfr form, phytochrome activity is initiated. On the other hand, in its inactive Pr state, activity is inhibited.

Show question

Question

Unfiltered, full sunshine includes significantly ____ . 

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Answer

more red light than far-red light

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Question

What happens to phytochromes when there is no sunlight?

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Answer

When there is no more sunlight, the Pfr gradually reverts back to Pr form.

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Question

Explain the concept of critical night length.

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Answer

The concept of critical night length means that the length of uninterrupted darkness that a plant was exposed has a more significant effect than the length of day in terms of flowering.

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Question

long-day (or, short-night) plant has a _____ number of hours of darkness. 

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Answer

Maximum

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Question

How does seasonal flowering help a plant survive and reproduce?

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Answer

Seasonal flowering as a response to change in day or night length helps plants adapt to their local environment. For example, some plants flower when their pollinators are most active to increase reproductive success. 

 

Plants may also reproduce during springtime so that their offspring have enough time to develop before facing the harsh conditions of winter, which increases their survival rate. 

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