colours of autumn

quimica das coisas

 

We naturally associate some colours with the four seasons: the colour of flowers in spring, brown in winter, blue skies in summer. But today we talk about the chemistry of the colours of autumn.

In autumn, leaves abandon green and great us with a beautiful palette of yellows and browns, joined by orange, red and purple. And chemistry explains why!

The green of leaves is due to chlorophyll pigments. The chlorophyll molecules absorb sunlight in the region of red and blue, and therefore the light reflected by leaves lacks these two tones and we see it just as green.

The chlorophyll molecule is not stable and must be synthesized continually by plants, which requires heat and sunlight. With autumn arrives cold weather and weak light days. And what is the consequence of this?

In deciduous plants, production of chlorophyll stops and the green tone fades, allowing us to see other pigments also present in the leaves.

One of these pigments is carotene which absorbs light in the blue and blue-green and then reflects it as yellow. Carotene pigments are much more stable than chlorophyll. When chlorophyll begins to disappear from the leaves, carotenoids give them a golden yellow colour.

In the third group of pigments present in the leaves we find anthocyanins, which absorb light from blue to bright green. So, the light reflected by leaves containing anthocyanins appears to be red!

Anthocyanins result of a reaction with sugars in plant’s cells. The progressive accumulation of sugar leads to the synthesis of anthocyanins in late summer, creating the reddish hues of autumn leaves.

Oh, and one more curiosity: this group of pigments is also responsible for the colour of roses, black grapes and the so-called red fruits!

In fact, the autumn colours palette depends a lot on the weather. Low temperatures and week sunlight destroy chlorophyll.  At the same time they promote the formation of anthocyanins, which in turn also depend on the concentration of sugars in the leaves, which increases the dry weather.

In other words, the brighter colors of autumn appear in dry and sunny days followed by cold nights!

And so we showed you how the beauty of autumn is also the beauty of chemistry!

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