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Autumn science: The chemistry of nature’s changing colour

September 24, 2018

 

With back-to-school chaos, cloudy skies and crisp mornings, autumn is well and truly on its way. For a few weeks every year, we get to see the colour changing magic of the season, as plants and trees transform into warm reds, oranges and yellows before the leaves drop for one final time before winter.

With the arrival of spooky season comes rustic colours glowing across parks and gardens, leaves falling from trees and bright, brisk sunny days (if we’re lucky, but we’re not holding out much hope).

So, what is the science behind the most photogenic season of the year?

In short, it comes down to one thing – chlorophyll.

What is chlorophyll?

Chlorophyll is a chemical found in plants, algae, and cyanobacteria.

The green photosynthetic pigment is responsible for trapping sunlight, helping plants absorb energy from the sun, making it vital to the growth and survival of a plant.

What happens to leaves as the weather changes?

As the days get colder and darker and the weather deteriorates, there’s less sunlight for plants to snack on. Some plants stop making chlorophyll and break it down into smaller, colourless compounds. With less chlorophyll, they start showing other pigments such as carotenoids, which are yellow and orange.

Carotenoids are always present in leaves, and the absence of chlorophyll allows the glowing warm tones to finally show through, meaning that yellows will look the same year after year and are not dependent on weather conditions.

There are other pigments at play too, such as anthocyanins, which are only produced in the autumn by certain trees under certain conditions, when sugars become trapped in the leaves. Anthocyanins produce vibrant shades of red, purple and pink, and can be found in fruits such as cranberries, cherries and red apples.

What makes the leaves fall?

Every year, abcission (the fancy word for a leaf drop) starts its annual tradition. The name comes from the abcission layer, which is a layer of cells between a leaf stem and stalk. When autumn arrives, hormones in trees begin to change, with auxin slowing in production. Auxin keeps the abcission layer connected, meaning the leaves stay attached to the trees and without it, leaves begin to break away.

Without enough auxin, the leaves begin to shed, falling with the weather or eventually due to its own weight). When a tree sheds its leaves, it uses less energy for survival, keeping itself alive during the colder months.

The colour change also happens before the leaf drop for the same reason. It takes a lot of energy to make chlorophyll and moving it out of the plant before shedding saves energy.

So, if trees change with the weather, how do Christmas trees hold their colour each year? Evergreen trees such as birch, spruces, cedars and firs keep their green colour and needles throughout the year, due to the heavy wax that coats the trees needles, preventing them from freezing.

To see what’s coming up this autumn at Techniquest, please go here.