Every fall, green leaves turn shades of yellow, orange, red and purple. It may look like the trees are putting on one last show before winter, but what we are really seeing is plant chemistry at work.During spring and summer, leaves are dominated by chlorophyll, the green pigment essential for photosynthesis. Chlorophyll absorbs light energy that plants use to turn carbon dioxide and water into sugars. There are other pigments in the leaf all along, but the abundance of chlorophyll masks
many of their colors.
As autumn approaches, shorter days and cooler temperatures signal deciduous trees that winter is coming. The tree begins preparing to shed its leaves. Chlorophyll production slows and eventually stops, while existing chlorophyll breaks down.
As the green fades, pigments that were there all summer finally become visible.
Carotenoids produce yellow, gold and orange colors. These pigments help plants capture light and protect leaf tissues from excess light during the growing season. They are responsible for the yellow of aspens and the golden-orange colors seen in many other trees. Carotenoids are also what give carrots their orange color.Red and purple fall colors work a little differentlyAnthocyanins produce reds, purples and crimson shades. Unlike carote
noids, which are already present in the leaf, many anthocyanins are produced in late summer and fall. As the tree prepares to close off the connection between the leaf and branch, sugars can become trapped in the leaf. Bright, sunny days encourage sugar production, while cool—not freezing—nights slow the movement of those sugars out of the leaf. Under the right conditions, anthocyanin production increases.
That helps explain why fall color can be spectacular one year and less impressive the next.
Weather matters. A succession of sunny days and cool nights can favor brilliant reds. Drought, an early hard freeze, prolonged warm weather or storms that strip leaves from trees can shorten or dull the display
Eventually, the tree forms an abscission layer where the leaf stem joins the branch. This layer gradually cuts off the movement of water and nutrients. The leaf falls, leaving the tree better prepared to conserve resources through winter.Fall color is beautiful—but it is also a pretty impressive chemistry lesson happening right over our heads.
https://www.fs.usda.gov/visit/fall-colors/joy-and-science

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