The following article was written for WONK! magazine and has been published in their most recent issue which you can find here. WONK! is a brilliant magazine for teens, linking fashion and science. I’ve had the honour to write for every issue so far; thank you Hannah for continuing to use my chemistry articles.
Shorter days, crisp sunny mornings, cosy jumpers, hot chocolates and pumpkin spice, all signs that Autumn is back. Personally, it’s my favourite time of year. I’m a sucker for a jumper and an early night with a cup of tea, but it’s the colours that come with Autumn months which always give me that warm feeling. The reds, oranges and yellows that accompany the crunch of fallen leaves are very satisfying, and Instagram worthy. However, what is it that causes the colour changes we see and why do plants lose their leaves?
Pigments give leaves their colour, absorbing some wavelengths of light and reflecting others. The colours we see are the reflected wavelengths. Chlorophyll is the pigment which gives leaves their green colour through spring and summer months, reflecting green light and absorbing all other visible light. The plant uses this absorbed light as energy during photosynthesis. However, as days get shorter there is less time for plants to conduct their day-to-day processes. To save energy, plants stop producing chlorophyll and through the autumn months, leaves turn yellow as chlorophyll is broken down. The yellow colour comes from another molecule found in leaves (xanthophylls) which reflects yellow light and is also responsible for giving sunflowers their characteristic colour.
Whilst we enjoy these shades and the autumnal ombre, yellow leaves are a problem for plants. The yellow colour attracts parasites which can damage plants. As a result, plants have developed a clever chemical defence mechanism, using two other chemicals to make red and orange leaves, masking the yellow ones! Carotenoids are in leaves all year round, but anthocyanins are specially made by plants in autumn months, masking leaves from parasites and acting to protect the leaves from sunlight once the chlorophyll has been broken down. These chemicals are also responsible for the intense orange colour of carrots and pumpkins.
What does this mean for trees and why do they eventually lose their leaves? Trees which lose their leaves are called deciduous and are found in abundance in colder climates, as it takes too much energy to keep their leaves during colder months. Interestingly, trees lose their leaves due to hormones, and no, not like those which cause puberty and periods, but tree hormones. Hormones are just molecules which transport chemical messages in both plants and animals. The decrease in chlorophyll acts as a signal to release a series of hormones. Auxins are a family of chemicals, important for the delivery of nutrients, helping the tree/plant grow. When chlorophyll levels drop, so do auxin levels, triggering the leaf dropping process (called abscission). Ethylene is the other important chemical involved and is produced in the dark, hence there is an abundance in autumn and winter months, key to leaf dropping. Finally, as chemical levels decrease and increase just enough, nutrients have been withheld from the leaves causing them to begin breaking down. To make the final cut, plants use enzymes called glycosidases. Enzymes are nature’s way of making a process faster, and glycosidases work to break down the cellulose in the plant cell wall so a leaf can fall from its branch or stem.
Whilst we enjoy the beautiful autumn ombre, crunching leaves and sipping a pumpkin spiced latte, remember how much effort those trees are going to, in order to create such scenes.