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Poinsettias are one of winter's most popular flowers. They are beloved around the world as icons of the holiday season, and this is largely due to the fact that they come into their vibrant red bracts (modified leaves) only during the winter. But why is that? How can a plant, without a traditional nervous system to pick up on environmental changes, tell that winter has arrived? Well, the answer lies in the same thing a plant uses for sustenance: light.
Unless you live directly on the equator, the length of the day will vary depending on the time of year. The days will typically be longer in summer and shorter in winter. This causes the amount of light to vary from season to season, a trend that plants have learned to pick up on. They will experience certain physiological changes--like blooming or changing color--only when the light levels indicate a favorable season. This is known as photoperiodism. Poinsettias are short-day photoperiodic, meaning they will begin to exhibit their festive colors only when they receive less light like they would in the winter time. The magic ratio seems to be 14 hours of darkness and between six and eight hours of intense sun. Clever gardeners have used this trend to prompt their poinsettias to flower right on time for the holidays, regardless of the environmental conditions where they live. Covering the poinsettias for the prescribed 14 hours a day starting in October is just about all it takes to see a poinsettia flower through December.
There are a few different theories for how photoperiodism works in plants, and it is likely that no one theory will encapsulate the mechanism at work in all photoperiodic plants. The current dominant theory is called the external coincidence model. The name is a bit of a misnomer; it's not a coincidence in the sense that photoperiodism is caused by random chance, but, rather, the light levels must coincide with certain biological processes within the plant. Much like us, plants have a circadian rhythm. This twenty-four hour cycle governs certain processes in the body, allowing all living things to respond to changes in the day cycle by producing what is necessary during each phase. Certain products (often called "factors") are correlated with changes in physiology, like flowering. Scientists have found that these factors are produced all year long on a circadian cycle in many plant species, but they aren't always in effect because they are often broken down too fast to cause change. Only when the presence or absence of light corresponds to the time these factors are produced in the circadian cycle can they reach their true potential. Instead of being broken down, they are preserved and allowed to cause change in the plant.
Another popular theory that has found evidence in certain plant species, including the poinsettia, is the phytochrome hourglass theory. This theory relies on certain structures in plants known as phytochromes, as its name suggests. Phytochromes are light-absorbing molecules that change shape in response to certain wavelengths of light, as well as periods of darkness. During the day, when light is constantly being absorbed, phytochromes are found in a form known as P_fr (P sub fr). After the sun sets, the darkness makes the phytochromes regress into their P_r (P sub r) form. The ratio of the two phytochrome forms reflects the time since daylight, and therefore the length of the day. The phytochrome hourglass theory asserts that plants use this ratio to tell how long the day is. When the length of the day is right for certain actions, like blooming, the plant will respond in kind.
Photoperiodism isn't always as cut-and-dry as biological processes like these; temperature and other environmental factors can also contribute to whether a plant engages in certain behaviors. In some cases, a proper photoperiod might not even be the determining factor in whether or not a plant blooms--it just might hasten the blooming process.
While light-dependent organisms like plants make obvious candidates for photoperiodicity, they aren't the only organisms that have a physiological response to the length of the day. Some animals that breed seasonally, like sheep, also change their behavior based on the quantity of light they receive. The amount of light received in a day is sensed by the sheep's retinas, which send signals to the brain. These signals regulate melatonin levels, which, in turn, affects many other bodily processes, including breeding. Research is still ongoing to determine whether or not similar processes govern seasonal breeding in other, lesser studied organisms. Even us humans can be influenced by photoperiodism! Some people experience a condition known as Seasonal Affective Disorder (SAD). Commonly, those with SAD will experience depressive symptoms in the fall and winter months, when the light levels are decreased. Using artificial light (a process known as light therapy) is known to help many people with SAD, indicating that they--and perhaps all humans--are affected by the amount of light experienced in a day. Photoperiodism is a fascinating phenomenon with far reaches that we haven't completely explored--a growing field of STEM research that you probably wouldn't expect!
References & Further Reading
This article from Khan Academy explores two light-related phenomena in plants: phototropism (the movement of plants towards or away from light) and photoperiodism. It offers great, easy-to-understand explanations of both processes, and its breakdown of the two leading photoperiodism theories is particularly clear. Make sure to click on the blue links throughout the article to learn even more!
This short article offers a quick summary of photoperiodism as it relates to poinsettias, including a guide to using photoperiodism to encourage poinsettias to bloom over the holidays!
This article from South Dakota State University goes further into detail on the mechanisms of photoperiodism, particularly the way it relates to poinsettias. It includes a lot of fascinating details without diving too deep into academic lingo. An enjoyable and informative read for those of you keen to learn more--and it includes a guide to keeping any poinsettias you have alive for the next holiday season!
This academic paper dives into detail on the photoperiodic mechanisms behind seasonal breeding in sheep and other animals. It might be a bit dense for some, but animal and biology enthusiasts will find a wealth of fascinating information here!
If your interest was piqued by our example of human photoperiodism, this brief descriptive article from the Mayo Clinic is a great place to start learning more.
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