The leaves are falling and the days are growing shorter. What does this mean to us, and what might that mean to the bigger world that lives around us? Do you live where Daylight Saving Time is recognized? As I originated in Saskatchewan, these human clock shifts have always been challenging, so I am very happy if I do not have to adjust my human clocks twice a year.
This got me thinking about the greater environment that surrounds us and what effect, if any, this might have on our trees, shrubs, migratory patterns and more. These whimsical thoughts quickly disappeared as reasoning prevailed. Plants do not read human clocks; they instead use an internal circadian rhythm driven by actual sunlight, temperature shifts and natural day length (photoperiods) rather than time zones or man-made clock adjustments.
Interestingly enough, plants have a biological clock that runs on a 24-hour cycle using light-sensing proteins to co-ordinate their activities like photosynthesis and metabolism. As we all live at least for the school year in the Northern Hemisphere, the amount of physical sunlight a plant might receive depends on the season and the position of the sun and never on what our human clocks might be telling us.
Photoperiodism in plants is when plants use the length of light and darkness to regulate flowering and other processes. Photoperiodism acts without any input from the ambient temperature or the weather. It is strictly a light/dark kind of thing. However, photoperiodism is dependent on the ability of the plant to sense light. Plants have photoreceptors or cells that contain a light-absorbing pigment. When triggered, these cells will alter specific proteins within the plant, which cause changes in hormone production, gene expression and in growth. What this means in simpler terms is that photoperiodism is a response of plants to light exposure, which then causes change within the plant.
Some plants are labelled as short-day plants and these plants will bloom when the length of daylight drops below a critical threshold, typically in the late summer or autumn. This fall, we enjoyed a great example of this with the fall-blooming chrysanthemums that are so popular in autumn. It used to be difficult to see any fall-blooming mums (chrysanthemums) show colour before our temperatures dropped below freezing. Thus, the “Morden” mums were developed at the research station in Morden, Man., to have a trigger to bloom that was related to light but also to temperature, thus causing this change in the proteins within the plant prior to when freeze-up happened in Saskatchewan. A couple of other common indoor plants that are considered short-day plants include the Christmas cactus and the poinsettia.
Alternatively, long-day plants will flower only when the amount of daylight lasts longer than their critical threshold, which is often earlier in the growing season when our day length becomes longer. Common plants that are considered long-day plants include lettuce, spinach and radishes. What this means is that there is a protein change within these plants when the daylight becomes shorter. So…next time you see your favourite salad greens bolting to flowers, it may be due to more than just the temperature!
The complexity of what happens within the plants around us is endless, and to me extremely captivating. However, I also recognize that not everyone is this fascinated by what we cannot see but may be happening all around us. On that note, may you pause for a moment and enjoy the natural world around us and may you easily adjust and flow with all of the changes in your lives.
Hanbidge is the Lead Horticulturist with Orchid Horticulture. Find us at www.orchidhort.com; by email at growyourfuture@gmail.com, on Facebook @orchidhort and on Instagram at #orchidhort.
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