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Why leaves turn yellow in autumn?

p1Recently we’ve discussed why leaves are green. Now the summer is over and soon the trees will change colors. Do you know why leaves turn yellow in autumn? This article is devoted to color change of green plants with change of seasons.

Deciduous trees in the temperate zones change their colors in autumn. Being green in summer, they turn orange, yellow, purple, red or even brown. This transformation is triggered by day shortening in September. The photoreceptor phytochromereacts to the change in the length of the day. When the days are shortening, plants green pigment chlorophyll starts to break down.  Chlorophyll destruction unmasks accessory pigments, e.g. carotenoids that have yellow-orange colors.

The cause of initial degradation of chlorophylls is plants requirement of nitrogen, which is a part of the chlorophyll molecule. Plants reuse nitrogen from chlorophyll because reserves of this essential macro element are depleted in autumn. On the other hand, carotenoids are composed almost entirely of carbohydrates. They do not contain elements that are in short supply in autumn. At the same time, these accessory pigments continue to protect leaves against photooxidative damage after chlorophyll breakdown. Thus, plants that do not accumulate other accessory pigments besides of carotenoids have autumn leaves painted in orange and yellow colors.

autumn-leaves colors

Red-purple coloration of leaves is determined by another pigments anthocyanins. Anthocyaninsare newly synthesized by plants in autumn, contrary to carotenoids which preexist in chloroplasts along with chlorophylls. The most probable meaning of such synthesize is photoprotection of leaves from photo-oxidative damage, risk of which is especially high in autumn. Consequently, functional leaves enable a better resorption of nutrients, especially nitrogen and phosphorus.Also, the coevolution hypothesis is worth mentioning, according to which autumn coloration is a signal for insects that migrate to the trees in autumn.Alteration of green leaves colormight deter insects from occupying the soon-to-be-shed leaves. In such a waytrees can defend themselves from insects. 

tree four seasons


Brown coloration of the leaves usually indicates the deathof their cells. Besides, combined presence of anthocyanins and chlorophylls can also result in brown color of the leaves.

Recently we received the following question:

hi! i wonder why leaves stop being green at autumn? i mean how do they know and why color are so different?

In this section we did our best to answer it and discussed related concepts. Have your own interesting how-it-works questions? The next article may be devoted to one of them, so don’t hesitate to ask!

autumn leaves