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Why Leaves Go Red And Fall Off Trees

Why Leaves Go Red And Fall Off Trees

Q: Why leaves go red and fall off trees

A: Leaves are the lungs and food factories of a plant, needing carbon dioxide, water and sunlight to make oxygen and glucose. As days become shorter and colder, indicating less sunlight and water in the coming months, trees begin to break down the chlorophyll in their leaves and distribute the nutrients from the leaves to the trunk and roots, where they can be used to help the tree last through the winter. The loss of chlorophyll unmasks the yellow carotenoids that were already in the leaves – they reside in chloroplasts and give color to vegetables such as carrots and sweet potatoes.

The low temperatures, coupled with sunlight, result in the generation of the pigment anthocynanin, which makes leaves bright red. Anthocyanins give red and purple colors (depending on the pH) to plants such as blueberries, raspberries, eggplant (aubergine) and red cabbage. There are many theories as to why trees use up energy generating anthocyanin in their leaves just before losing them: anthocyanin is a powerful antioxidant that may protect the tree against the harsh conditions of bright light and extreme cold, red may attract pollinating birds, red may disguise the leaves from herbivores that can’t see red wavelengths, or possibly increase the temperature of the leaves by absorbing more sunlight. It has even been suggested that the redness of a tree’s leaves indicates how hard they are trying to retrieve nutrients from their leaves: trees in nutrient-poor soils tend to have bright red leaves, whereas trees that grow in soil packed with goodness remain yellow.

A chemical signal is then released by the tree, and small ‘abscission’ cells form where the leaf stem meets the branch. These cells act as scissors, to cut the leaf from the tree: they form a thin ridge and push the leaf away from the branch, but by bit. The wind finishes the job, releasing these dangling leaves from their parent tree.

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