Photoperiodism describes the way living things detect and react to the changing lengths of nights and days.
Just as many of us feel more energized in summer and more likely to hibernate in winter, other living things are hardwired to behave certain ways in different seasons. In the world of plants, this ensures the proper timing of flowering, fruit set, building food stores, and preparing for dormancy, making it more likely that a plant will be able to pass on its genetic information.
Many angiosperms (flowering plants) contain photoreceptor proteins that can detect and signal information about the length of each consecutive night. This is done by two different proteins: one absorbs red light and the other absorbs far-red light. These proteins then trigger seed germination, internode elongation (stem length), and flowering. While we call certain plants long-day or short-day varieties, we should technically be calling them short-night and long-night plants, respectively.
Plant responses to hours of darkness are categorized as either obligate (mandatory) or facultative (likely to occur). In most cases, a plant with an obligate photoperiodic response is triggered to flower at a particular length of darkness, while a facultative plant may be more likely to flower within a range of nighttime hours. Temperature, plant age, environmental conditions, and plant health also play roles in the way plants use this accumulated information. In some cases, plants can be obligate at one temperature and facultative at a different temperature. To further complicate things, some plants need long nights followed by short nights to start flowering!
Long-day (short night) plants
Long-day plants are triggered to flower as nights get shorter. This usually means they tend to bloom in late spring and early summer. If a long-day plant is exposed to more than 12 hours of darkness each day, it will not bloom. Lettuce, peas, barley, and wheat are long-day facultative plants, while oats, henbane and carnations are long-day obligates. Spinach (13 hours), potatoes, California poppies, dill (11 hours), asters, and coneflowers are all long-day plants.
Short-day (long night) plants
Short-day plants require long periods of continuous darkness to develop flowers. If these plants are installed near a motion-detector light that goes on and off during the night, they are less likely to flower. Some short-day plants include sorghum, onion, cotton, soybeans, poinsettia (10 hours), chrysanthemum (15 hours), and marijuana.
The flowering of day-neutral plants is not affected by light. Many of these plants evolved close to the equator, where day length changes very little. Tomatoes, corn, cucumbers, roses, and other day-neutral plants respond instead, to changing temperatures, other environmental conditions, or as a function of plant development.
Artificial light and darkness
Commercial nurseries use photoperiodism to ensure that there are plenty of flowers for specific holidays by artificially creating longer and short nights. To lengthen daylight hours, grow lights can be added outside of daylight time, much the way hens are exposed to artificial lighting to keep them laying eggs during winter. To simulate longer nights, plants can be covered, to block sunlight.
In your own garden, you can use photoperiodism to place short-day plants where they will not be exposed to nighttime lights. You can also cover plants that tend to bolt in summer, or that need longer nights to start flowering.
If you want to read more about the science behind photoperiodism, check out Photoperiodism in Plants (Thomas & Vince-Prue, 1997).
An interesting note on photoperiodism: when leaves of one plant have been exposed to the correct amount of light, and are then removed from the parent plant and grafted onto a plant that has not received enough light, flowering is triggered in the new plant. This is because the leaves release plant hormones that initiate flowering after the correct amount of darkness has occurred.
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