What is Photoperiodism?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 16 October 2019
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Photoperiodism is a biological response to light levels which occurs in many organisms around the world. Both plants and animals demonstrate this trait to varying degrees, and knowledge of how organisms respond to changes in light levels can be used to manipulate those organisms to produce particular desired outcomes. Researchers are also interested in photoperiodism and its potential applications.

Among plants, photoperiodism can alert a plant to the change of the seasons, which may trigger a variety of responses. For example, when the days are getting longer, the tree might start to produce buds and blooms, since it would sense that spring is on the way. As days shorten, the plant would start to become dormant, producing seeds for next year and taking steps to ensure that it would be ready for winter. Photoperiodism can also play a role in the setting of fruit and the movement of plants which adjust their position to take advantage of available light.

Knowledge of photoperiodism in plants has allowed botanists to trick plants grown in greenhouses by manipulating light levels to trick the plants. This technique is used to make blooms available year round for the floriculture industry, for example, or to force plants to produce seeds which can be distributed for sale. Researchers may also play with light levels in the process of researching variations between plant species, with the goal of learning more about these species and how they live in the natural environment.


This trait is also present in many animals. Animals with coats of fur often grow additional fur in the winter and shed that fur in the summer, responding to changing light levels to start this process. Photoperiodism can also trigger nesting, estrus, and other events in the lives of animals. It may also be related to responses to other environmental cues, such as temperature, rainfall, and so forth, with the animal's body responding to a constellation of indicators that the season is shifting.

While humans may think that they are exempt from photoperiodism, some evidence seems to suggest that humans actually do respond to changing light levels. Seasonal affective disorder (SAD), a common psychological condition, is directly tied to changes in available light and day length. This condition can cause severe emotional distress which can vary from depression to mania, suggesting that many humans are more closely tied to the change of seasons than they might admit.


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Post 4

I have a friend who suffers from seasonal affective disorder, and she gets dangerously depressed during the winter months. She has to take antidepressants for the whole season just to function.

When she heard that the powers that be were considering doing away with daylight savings time and keeping the time the same year-round, she got so happy. She wanted the extra hour of daylight in the afternoon so desperately. She was crushed when the idea fell apart and everyone continued to spring forward and fall back an hour.

She has a couple of days off work each week, and she does spend time outside. However, it is often too cold for her to really enjoy being outdoors, and she ends up back in the house before the sunlight has had a chance to really lift her mood.

Post 3

I experience seasonal affective disorder during the short days of winter. I attribute it to being cooped up in an office during the daylight hours. I think that if I could be outdoors in the early afternoon, the depression would go away.

I have heard that using a certain type of light at your desk can help improve your mood if you are experiencing SAD. The bright light it emits supposedly tricks you into thinking you are getting sunlight.

I should probably try it, though I’m pretty sure I need to get out and experience nature to feel truly fulfilled. I would be okay with the sun setting at 4:30 p.m. if I could get off work at 2:00!

Post 2

I see photoperiodism in my dog with the changing seasons. During the summer, his coat becomes all mottled, and he looks diseased. The first time it happened, I took him to the vet, because I was afraid he had mange.

She told me that his fur was about to go through a major shedding, and it was perfectly normal for his coat to look this way. She recommended that I brush his fur to encourage the process along and make him more attractive.

In the winter, he gets a bit bushier. His coat becomes darker and solid, and it shows no signs of discoloration.

Post 1

I find it fascinating that plants actually lean toward the sun. I have seen several plants that change their growth patterns and develop a permanent slant in the direction of the sunlight.

Leaning plants can look a little strange, but you can manipulate them into straightening up by blocking the light from all angles except directly above. I did this by planting shorter shrubs around my taller slanting rosebushes, and they straightened up rather quickly.

Another option is to plant your flowers or bushes in areas that receive full sunlight all day. They won’t need to reach in any certain direction, because light will shine on them at all times.

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