The North American Indians greatly respected the land and its fierce animal inhabitants. Deserving the greatest of respect was the bear.
According to Indian folklore, brave hunters in the skies of heaven slew the Great Bear. When it fell in battle, its’ red blood rained down from heaven above, coloring the leaves. As the victorious hunters above cooked their prize, fat from the fires dripped to the earth below, coloring the leaves different shades of yellow.
Great story, huh? Though not as poetic, the scientific reason is fascinating nonetheless.
Why Leaves Change Their Color
Those of us who live in USDA Zone 5 are very fortunate. Even though winter is often nothing more than endless days of drab, overcast gray light and spring bounces from summer-like heat one day to bone chilling cold the next and summer is a miserable battle with drought and tropical humidity, the autumn is always a dancing festival of stunning beauty.
That one gorgeous season makes the frustrating unpredictability of the other nine months minor inconveniences.
How and Why Does This All Happen?
Behind the breath-taking beauty of it all there is a scientific explanation.
Unlike the foliage on evergreen plants, the leaves of deciduous trees and shrubs are thin and ill-suited to withstanding the rigors of winter. So Nature devised a method for the elimination of those leaves and decided to make a party of the whole process.
All those glorious colors you see every fall are also there in the spring and summer, but you can’t see them. Chlorophyll, the green pigment in leaves, is dominate and masks the other colors all spring and summer. Chlorophyll absorbs red and blue light from the sunlight that falls on leaves so the light reflected by the leaves is diminished in red and blue and appears green.
But chlorophyll is not a very stable compound. Bright sunlight causes it to decompose and plants are continuously synthesizing chlorophyll to maintain the proper levels. Because this process requires sunlight and warm temperatures, it occurs non-stop during the summer.
In autumn, as the days become shorter and the temperatures a little cooler, the synthesizing of chlorophyll gradually stops; chlorophyll begins to break down and the other colors gradually appear.
There are numerous reasons accounting for the various colors within the leaves of a tree or shrub. When the days of fall bring warm, cloudy and rainy days there tends to be less red in the leaves. Trees that are protected by shade tend to be yellow, while those exposed to sun are more reddish in color.
What Makes a Leaf Yellow?
Carotene is another pigment found in the leaves of many plants. This pigment absorbs blue-green and blue light to make the reflected light appear yellow. When carotene and chlorophyll occur in the same leaf, together they remove red, blue-green, and blue light from the sunlight falling on the leaf and the leaf appears green.
Carotene functions as an accessory absorber. The energy of the light absorbed by carotene is transferred to chlorophyll, which uses the energy in photosynthesis. Carotene is a much more stable compound than chlorophyll, persisting in leaves even when chlorophyll vanishes. When the chlorophyll is gone the carotene causes the leaf to appear yellow.
Why Are Some Leaves Red?
A third pigment, or class of pigments, that occur in leaves are the anthocyanins. Their reflected light is red because these pigments absorb blue, blue-green, and green light.
Unlike chlorophyll and carotene, anthocyanins are not attached to cell membranes, but are dissolved in the cell sap and the color produced by them is sensitive to the pH of the cell sap. If the sap is quite acidic, the pigments impart a bright red color; if the sap is less acidic, its color is more purple.
Anthocyanin pigments are responsible for the red skin of ripe apples and the purple of ripe grapes. Anthocyanins are formed by a reaction between sugars and certain proteins in cell sap but this reaction doesn’t occur until the concentration of sugar in the sap is quite high. The reaction also requires light. This is why apples often appear red on one side and green on the other; the red side was in the sun and the green side was in shade.
One More Important Influence
So now that we know all we ever wanted to know about chlorophyll, carotene and anthocyanins, there is one more variable in this fall color equation: the weather.
The weather has a profound influence on the range and intensity of autumn colors. Low temperatures destroy chlorophyll, and, if the temperatures stay above freezing, promote the formation of anthocyanins. Bright sunshine also destroys chlorophyll and enhances anthocyanin production.
Dry weather, by increasing sugar concentration in sap, also increases the amount of anthocyanin. So the brightest autumn colors are produced when dry, sunny days are followed by cool, dry nights.
Midwesterner Tom Schneider is a horticulturist, avid gardener and retired arborist. When not gardening he is assisting his wife, Deb, with their business, Windstar Embroidery. Visit Tom and Deb for embroidered gifts and machine embroidery designs.