Photoperiodicity: Short-day, Long-day and Day-Neutral Plants

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  • 0:05 Photoperiodicity
  • 2:48 Short-Day Plants
  • 3:26 Long-Day Plants
  • 3:54 Day-Neutral Plants
  • 4:10 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

Ever wonder why some plants will bloom in the spring but others in the summer? The length of daylight can influence when a flower will bloom. We will look at how the amount of sunlight regulates when plants produce flowers.


You may have heard about circadian rhythms and the 24-hour sleep/wake cycle in humans and animals. Interestingly enough, plants also have a daily cycle that is influenced by the amount of sunlight and darkness.

Photoperiodism is a physical response to photoperiods. A photoperiod is the length of day versus the length of night. When talking about the circadian rhythm in people, we generally think of getting sleepy when the sun goes down and then waking again when the sun comes up. In plants, it isn't necessarily that they sleep and wake. However, the amount of daylight influences other aspects of plants.

The most commonly influenced response in plants when it comes to photoperiods is flowering. Plants that flower only under certain day-length conditions are considered to be photoperiodic. There are three classifications of photoperiodic plants: short-day, long-day, and day-neutral. Before we look at these three classifications, let's first identify what is meant by a critical length. In regards to photoperiodicity, the critical length is the length of daylight a plant requires in order to produce flowers. We will look at the three plant classifications regarding critical length in just a minute, but let's first look at some general aspects of this critical day-length.

Some plants require just one day fitting in with the critical length requirements in order to flower. Others need the critical length requirements to be met for weeks before they will flower. Additionally, the critical length can be influenced by the temperature and location. For example, the same plant may require 12 hours of sunlight when it is in a more northern location but only 10 hours of sunlight when it is in a more southern location.

While plants don't have eyes to watch the amount of sunlight, they do have specialized cells. These photoreceptors are able to monitor the amount of light and help the plant determine the correct time to flower.

Even though we've been talking about the amount of sunlight and how that regulates when some flowers will bloom, plants are actually more sensitive to the length of darkness. Plants use photoreceptors to monitor light, so when a period of darkness is interrupted, the photoreceptors sense this change in the environment. Scientists have been able to determine that if the dark period is interrupted - even for just a minute - the plant will not bloom even if the critical length for sunlight has been met. However, if there is an interruption in the light period - for example, if it's dark for a minute - the plant will continue to flower according to its regular requirements for the critical length of sunlight. This information is very useful for growers. Growers can regulate when plants will flower by regulating or interrupting the consistency of the dark period.

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