Why do autumn leaves turn colorful?

Whe wanted to hike to the summit of the Altkönig near Frankfurt last Saturday, didn’t need a hiking map, you just lined up in the caravan of brightly colored anoraks and followed the people, who usually trotted up the paths in pairs at a distance of a few meters. It’s lockdown and the sun is shining, so let’s go to the local recreation area of ​​choice while the trees still have a few leaves. And while you are walking along bright orange-red beeches and enjoying the colors of autumn, you can talk about it, because the lockdown monotony sometimes lacks topics of conversation: Why are the leaves colored in the first place? In fact, this question has not yet been fully resolved.

Johanna Kuroczik

Editor in the “Science” section of the Frankfurter Allgemeine Sonntagszeitung.

If the days get shorter and cooler, the leaves are no longer worthwhile for the tree. Before throwing them off, he secures their juice and nitrogen-containing proteins. To do this, the green chlorophyll has to be “defused” because the nitrogen treasures are partly located in the membranes of the photosynthetic apparatus. In a leaf structure that is no longer intact, the chlorophyll, which reacts to light, could use the energy of sunlight to form so-called free radicals, aggressive molecule fragments that damage the tree. Chlorophyll breaks down into a colorless substance, and this is how the carotenoids appear. These yellow pigments also color carrots and serve as sun protection in the leaf, even while it is still green: the strong chlorophyll covers the yellow color in summer.

Winter then surprises with rose gold and pink

With red-purple tree tops, trees from North America stand out in autumn. But the responsible plant pigment anthocyanin is only available in small quantities and is mostly newly formed. Why does the tree waste precious energy on this? “There are many hypotheses,” says chemist Bernhard Kräutler from the University of Innsbruck, who has been working on the breakdown of chlorophyll for decades. One possibility is that the red coloring agents provide additional protection from the sun and catch free radicals. Or they show fighting spirit: anthocyanins could warn insects that this tree is so strong that it can afford a decadent dye production. “This is to prevent the animals from laying their eggs on the plant,” explains Klaus Minol from the plant research department, a project of the Federal Ministry of Education and Research. That is not one hundred percent certain, but a “wonderful theory”. Back in 2008, researcher Thomas Döring at Imperial College London showed that aphids preferred yellow colors. The red could therefore be formed as a camouflage.

Whether red or yellow, in the next few weeks the colorful leaves will turn to brown mud. But even the barest branches shone red and gold last weekend on the descent from the Altkönig in the late afternoon: You can also rely on a color spectacle in winter, with intense sunsets in gold, red and pink. In the cold season, the sun is lower for a longer period of time, where its light travels to us through thicker layers of the atmosphere, so that the air molecules scatter more blue out of the line of sight and thus make the daytime star appear redder. This topic of conversation is then saved for a winter walk.

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