If you’ve been looking at the foliage on your way to class this past week, you’ll have noticed a change in the trees around campus. The various greens are starting to fade, replaced by an array of reds, oranges and yellow. But how do the leaves know when to begin changing?
The leaves changing color and eventually falling is a process called “senescence.” Senescence is the last stage of the plant’s development for the season, during which cells cease operations such as photosynthesis and begin to degrade, eventually dying off. Many plants senesce in the fall, likely to increase odds of survival over the winter.
This doesn’t answer our question, however: How do the leaves know when to begin senescing?
Senescence is the result of a signal cascade, or a series of chemical reactions that occur within an organism as a result of a stimulus or received signal. Other examples would include your body responding to a cut or scrape by initiating the blood clotting process. In this case, the signal for the leaves to cease carbon assimilation (photosynthesis) is the decrease in the length of the days leading up to winter.
As the day length decreases, so does the availability of sunlight for use in photosynthesis. As a result, the trees signal for chlorophyll, the photosynthetic pigment in leaves that makes them green, to begin degrading. This is why leaves change color from the base to the tip; the part of the leaf closest to the tree receives the message first. It is the degradation of chlorophyll that allows yellow leaf pigments to shine through.
But what about the reds and oranges? Instead of producing sugars, the leaves are now focused on shuttling nutrients to the stem of the tree for storage to facilitate survival over the winter and the growth of new leaves in the spring. As this occurs, macromolecules in the leaves themselves are hydrolyzed — broken down by water. This process creates new pigments that lead to a wide range of fall colors.