Why autumn leaves turn yellow

Carotenoids are present in leaves the whole year round, but their orange-yellow colors are usually masked by green chlorophyll.

As autumn approaches, shorter days and cooler nights cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments in the cells begin to show through. These are carotenoids and they provide yellow, brown, orange, and the many hues in between.

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