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Do you know why some plants flower only during a particular season? Why do we get certain fruits only during a specific time of the year? It is because every plant needs a certain duration of light to flower and form fruits. Plants usually get a longer duration of sunlight during the summer season, which decreases significantly during winter. As a result, some plants only flower in summer, while some fruits only ripen in winter.

Animals, similar to plants, are also found to respond to light, though it may work differently for them.

What is Photoperiodism

‘Photo’ means ‘light’ and ‘period’ means ‘time-length’, so, in biology, the relative period of day and night is called ‘photoperiod’. Photoperiodism is thus defined as the developmental response in plants and animals to the length of day and night.

The phenomenon of photoperiodism was first discovered by Garner and Allard while experimenting with growing Biloxi soybeans and Maryland mammoth tobacco.

Photoperiodism in Plants

Plants can sense changes in the length of day and night, and flower only when they get the right duration of light and darkness. This sense is achieved by using a light-sensitive protein called phytochrome. This protein also has a role in sensing seasonal changes in the environment.

How Photoperiodism Affect Plant Growth

As we know, every plant needs a certain amount of light to perform its basic functions such as food production, flowering, and fruiting. Without these activities, plants will not be able to grow and reproduce. The length of daylight above/below which a plant cannot flower is called its critical photoperiod. Different plant species have different critical photoperiods depending upon the environmental condition where they grow.

Types of Plants Based on Their Photoperiod Requirements

Based on the critical duration of photoperiod, plants can be classified mainly into three main groups:

1) Long Day Plants (LDP): These require a longer duration of light of about 14-16 hours to flower. They do not flower when exposed to a longer period of darkness, and thus are also known as short night plants. These plants will flower when there is an increase in the duration of light, or if there is a brief exposure of light in the middle of the dark period. LDP plants usually grow in places that have long daylight hours, and they flower during late spring or early summer as the daylight hours are longest in these seasons.

Examples– Spinach, radish, and sugar beet.

2) Short Day Plants (SDP): These prefer the opposite light conditions of the LDP plants, requiring a shorter duration of light of about 8-10 hours and a continuous period of about 14-16 hours of darkness to flower. In SDP plants, flowering is prevented if the dark period is interrupted in any way, no matter how briefly. These plants are usually found in places where the length of the day is short. Recently, it has been found that getting an uninterrupted period of darkness is more crucial for an SDP plant to flower than getting the 8-hour daylight.

Examples– Sunflower, rice, and soybeans.

There are some plants that follow certain features of both long and short day plants to flower. For example, long short day plants, such as goethe and night jasmine, require exposure to a more extended daylight during their early growth phase. On the other hand, short long day plants, such as winter rye, candytuft, and certain varieties of wheat, require a longer period of darkness during their early growth phase.

3) Day Neutral Plants (DNP): These are plants whose flowering is not affected by the length of the photoperiod or darkness. In other words, they are neutral to the length of day and night.

Examples – Roses, tomato, and pea plants.

Photoperiodism in Animals

Like plants, animals also respond to photoperiods though in an entirely different way. Several biological and behavioral changes in animals, such as the migration of all seasonal birds, an increase in the singing frequency of canaries during summer (when the day is long), as well as the sexual reproductive behavior of seasonal breeders such as red deer and wapiti (they reproduce only when the day in short) — are all examples of how the changing lengths of day and night influence animals and their life cycles.

Importance of Photoperiodism

  • Determines the season when the flowering plants will disperse their seeds for the purpose of reproduction.
  • Plants such as radish and carrot can be kept in the state of their vegetative growth to increase their yield by exposing them to favourable lengths of day and night.
  • Helps in the development of cross-bred plants to make them less sensitive to photoperiod than their parents so they can have a higher yield, be more resistant to pests, or more attractive to pollinators.
  • Induces flowering in some plants growing in temperate climates such as henbane, that require a period of low temperature along with exposure to a certain duration of light prior to their flowering — a process which is known as vernalization.
  • Helps plants to flower throughout the year under greenhouse conditions where they are exposed to artificial light sources or darkness as required by the plant.
  • Helps with artificial breeding in animals such as deer and calves that reproduce only when they are exposed to a certain duration of day and night.
  • Increases the survival rate of certain artificially bred animals, such as some species of deer, under specific weather conditions based on day-night duration.


Q1. What is the difference between phototropism and photoperiodism?

Ans. Phototropism is the growth or movement of plants or any organism in response to light. In contrast, photoperiodism is the developmental response of plants or any organism to the relative length of day and night.

Q2. What is the difference between photoperiodism and vernalization?

Ans. Photoperiodism is the developmental response of plants or any organism to the relative length of day and night. In contrast, vernalization is the process of inducing flowering in plants by exposing them to prolonged cold temperature.

Article was last reviewed on Monday, October 12, 2020

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