Plant science

During a process called acclimation, trees and shrubs prepare for winter and its low temperatures by ceasing growth and becoming dormant.

This process actually starts in late summer in response to decreasing day length and is rarely noticed by our human eyes.

But from September through November, as day length continues to decrease and temperatures drop, we can’t help but notice and appreciate the process of acclimation as deciduous trees and shrubs burst into vivid hues of red, orange, and yellow. These colors signal that trees and shrubs are reducing their photosynthesis levels and moving nutrients from leaves to stems and roots that serve as storage locations in winter. The variety of fall colors we see are created by the interaction of four pigments chlorophyll, carotenoids, anthocyanins, and tannins and variations in temperature, light levels, and water supply.

Each of these pigments is uniquely effective at absorbing particular wavelengths of light. The foliage colors we see are dependent on the wavelengths not absorbed by pigments which are instead reflected to our eyes.

Leaves are energy factories for plants. They are packed with organelles called chloroplasts containing the pigment chlorophyll which absorbs and harvests the energy of sunlight.

Through the process of photosynthesis, this light energy is used to convert carbon dioxide and water into energy rich carbohydrates that plants then use and store for their growth processes. Chlorophyll absorbs violet, blue, red and orange wavelengths very effectively but does a poor job of absorbing green wavelengths. Instead green light is reflected and leaves appear green to our human eyes. Chlorophyll is a short-lived molecule that is easily degraded by bright sunlight.

As it degrades during the growing season, new chlorophyll is constantly produced until autumn when shortened day length and decreasing temperatures signal plants to slow the production of chlorophyll.

Carotenoids are accessory pigments found in plants. Like chlorophyll, they are also bound to membranes in chloroplasts and are present throughout the growing season. Carotenoids are also found in chromoplasts, organelles similar to chloroplasts except that they lack chlorophyll.

They absorb blue and blue-green wavelengths and pass this harvested light energy on to chlorophyll for use in photosynthesis. Carotenoids are much more stable than chlorophyll and have another important role in plants. They help to protect against excessive light and by doing this, extend the life of the more fragile chlorophyll pigment. Chlorophyll and carotenoids working together during the growing season remove violet, blue, red, and orange wavelengths of light, leaving bright green as the reflected color that our eyes see in leaves. But in autumn when chlorophyll production stops and only carotenoids are left, yellow, orange, red, and some green wavelengths are reflected and the human eye sees the yellow and orange colors of willows, honeylocusts, lindens, birches, aspen, maples, elms, ginkgos, and hickories.

Anthocyanins are a third group of pigments found in some, but not all ,trees and shrubs. Contrary to chlorophyll and carotenoids, they are water soluble and are found in vacuoles, which are organelles. Depending on the type of anthocyanin and the pH of the sap in vacuoles, they reflect and appear pink, red, scarlet, and purple in flowers, fruit, and autumn leaves.

When anthocyanins and carotenoids are both present in leaves, we are gifted with brilliant orange fall color. Anthocyanins also absorb light in the ultraviolet spectrum so they serve as a sunscreen to protect plant cells from UV damage. In most trees and shrubs, anthocyanins are produced in autumn when cool temperatures promote the conversion of starch in leaves to sugars. These higher sugar levels then react with proteins to form anthocyanins.

As anthocyanin levels increase and chlorophyll production slows in fall, we see brilliant reds and oranges in red, sugar and Amur maples, sumacs, juneberries, and burning bush. This process is light driven so leaves in full sunlight will be more intensely colored than those growing in shade. This is why plants growing in full sun will turn brilliant red in fall while plants growing in shade are less brilliant.

A fourth group of pigments that impact fall color is the tannins. Tannins are found in vacuoles and in cell walls and play an important role in plants’ defense mechanisms against pathogens, insect feeding, wildlife browsing, and environmental stresses. Like carotenoids, they are always present but are only visible when chlorophyll levels decrease. Tannins are responsible for the brown fall color seen in oaks and other species. They also mute the brilliance of carotenoids and anthocyanin pigments, resulting in the deep and rich burgundies, copper, and golds we see during autumn on oaks.

And imagine, all of this happens without us lifting a finger – Mother Nature is awesome!

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