Fall chemistry turns leaves from green to gold

October 18, 2011

By Warren Kagarise

The mercury dips, rain starts to fall more often and leaves metamorphose from a demure green to riotous colors.

Why? The change of color each autumn involves a complex chemical process as trees prepare for the coming winter.

Throughout the growing season, chlorophyll is produced constantly. Then, as autumn starts and nights turn longer, chlorophyll production slows and then stops. The process allows the other colors in a leaf’s palette —brilliant golds, oranges and reds — to appear.

“The other colors that you see when the green pigment goes away — because the chloroplasts that contain chlorophyll sort of die off as things get cold and things are going to go dormant for the year — those other colors are always there, they’re just masked by the green chlorophyll,” city Arborist Alan Haywood said.

Chemistry lesson

The color palette responsible for fiery leaf colors each fall results from a series of pigments contained in the leaf. The chemistry behind the autumn changeover is complex, but understanding some basics about photosynthesis and pigments is important.

Chlorophyll — Chlorophyll, as many folks remember from elementary school science, gives leaves a green hue and absorbs sunlight. The substance is necessary for photosynthesis — the process plants use to turn energy from the sun into food. In the spring and summer, chlorophyll masks the other pigments in a leaf.

Carotenoids — The same chemicals responsible for the distinctive oranges and yellows in bananas and carrots exist inside leaves. Chlorophyll, after cues from day length and temperature, starts to break down in the fall, revealing the carotenoids underneath.

Anthocyanins — The water-soluble substance responsible for the brilliant reds and purples in autumn leaves is produced in sugars in the leaf throughout the fall. Anthocyanins also lend a bright burst to apples and cranberries.

Sources: Issaquah City Arborist Alan Haywood, U.S. Forest Service

The autumn weather conditions influence leaves’ color and intensity, too. The formula for vibrant fall displays: sun-splashed, warm days and cool, crisp nights in succession. The amount of moisture in soil also influences leaf colors.

“When we get sunny days and cold nights — sometimes even frost — that really triggers lots of good color,” Haywood said. “Often, we get our best color when we have that great September weather, that kind of Indian summer-type weather, where you get the warm days and you get the cold, clear nights.”

In the daytime, if the weather is sunny, leaves produce abundant sugars. The combination of sunlight and sugar production causes the leaf to produce anthocyanins — pigments responsible for reddish hues.

The amount of anthocyanins produced influences the color from year to year. The amount of carotenoids — pigments responsible for flame-tinted hues — is more consistent from year to year.

“The pigments — they’re a combination of, they’re already there and then there’s some formation that takes place in relation to the changes that are going on in the plant,” Haywood said.

Why do leaves fall from trees?

In early autumn, in response to the shorter days and less intense sunlight, leaves start a shutdown process. In a gradual process, the veins carrying fluids into and out of the leaf start to close off as a cell layer forms at the base of the leaf. The clogged veins trap sugars in the leaf and promote anthocyanin production. The leaf is ready to fall from the tree after the seal is complete and the connecting tissue is severed.

Source: U.S. Forest Service

Warren Kagarise: 392-6434, ext. 234, or wkagarise@isspress.com. Comment at www.issaquahpress.com.

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