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.
Let's now look at the different categories of photoperiodic plants. The first classification we will look at is short-day plants. These are plants that require a light period shorter than the critical length. For example, if a plant has a critical length of 10 hours, it will only bloom if there is less than 10 hours of sunlight per day.
Short-day plants tend to flower in the early spring or fall. This makes sense with the requirement of a shorter light period than the critical length, because there is not as much daylight during the spring and fall when compared to the summer. Examples of short-day plants include poinsettias, strawberries, and primroses.
Let's next look at long-day plants. These are plants that require a light period longer than the critical length. For example, if a plant has a critical length of 10 hours, it will only bloom if there are more than 10 hours of sunlight per day.
Long-day plants tend to flower in the summer. Again, this makes sense with the light requirements because there is more sunlight in the summer than in the other seasons. Examples of long-day plants include spinach, lettuce, and henbane.
The last group of plants is actually not affected by the length of daylight. These are considered to be day-neutral plants because they do not have a critical length. These plants will flower regardless of the amount of sunlight per day and include cucumbers, sunflowers, and rice.
Much like many animals, plants can be influenced by the 24-hour light/dark cycle of each day. In plants, we call this photoperiodicity because some plants are affected by the period of light during the day. Plants sense light with photoreceptors, which can determine if the critical length of light has been met.
We looked at three types of plants: short-day, long-day, and day-neutral. Short-day plants bloom when there is less than a critical length of sunlight and are those plants that you see flower in the spring and fall. Long-day plants bloom when there is more than a critical length of sunlight and are those plants that you see flower in the summer. Day-neutral plants are not affected by the length of the sunlight and may bloom throughout the spring, summer and fall.
After watching this lesson, you should be able to:
- Define photoperiodicity and photoreceptors
- Classify the three types of photoperiodic plants