leaves – Dr. How's Science Wows

It’s funny how Autumn comes around every year and I realise how much I love this time of year…. it’s as though I seem to forget I like it all throughout the other seasons. Of course we have had a particularly nice Autumn this year in the West of Ireland and maybe that has re-enforced my happy memories of the season. The days have been bright and crisp showing off all the beautiful colours in all their glory and splendour.

photo credit: Stellas mom via photopin cc

I grew up in Co. Wicklow surrounded by some beautiful deciduous woods and forests and this Autumn has really brought my childhood memories flooding back. My mother brought us often to the woods as children and we would hunt around for hidden treasures and delights to bring home and turn into some “masterful” collage in homage to the season. There was also the foraging, a distinctive primordial instinct in us all, there is nothing as pleasing as returning home with your bounty… be it blackberries or sweet horse chestnuts- to be turned into jams and tarts or painstakingly peeled of all nasty layers to reveal the divinely sweet, fruity, nutty delight beneath. In fact the joy that came with eating the nut always made it suddenly worth your while to start the arduous task of peeling all over again!

…and I hope that I will never outgrow the delight of running, kicking, shuffling through a crisp new crop of fallen leaves!

As many people know, the lovely green of most leaves is caused by the pigment chlorophyll… green in colour (obviously) and capable of using sunlight to convert water and carbon dioxide into energy (sugar) for the plant. However, when the sunlight hours fade coming into winter so too does the chlorophyll in the leaves of trees, or, to be more precise, the pigment begins to degrade and is not replaced. Once the green colour is gone other pigments that are often present in the leaf come into view… carotenoids are pigments responsible for the yellow/orange colour of leaves, anthocyanins are responsible for the redder colour of leaves and tannins are responsible for the brown colour of leaves. There is, within this pigmented system, a sense of hierarchy, at least in part. But did you ever wonder about the science behind those wonderful colours? I did… why the green suddenly disappears, where does it go and how do the other colours get there in its place? So, if like me, you ever wondered about these things… here is some insight into the why and what of Autumn!

Carotenoids are the pigments responsible for the orange colour of carrots. If carotenoids are present their colour tends to dominate leaving the leaves yellowy and orange.

In the absence of carotenoid, anthocyanin is the dominant pigment. Anthocyanin (the same pigment found in red onions, red grapes, red apples and red cabbage) is a natural pH indicator, meaning that it can change colour depending on the levels of acids or bases/alkali in its environment. In fact one of my favourite experiments that I often do with children is to demonstrate this colour changing using anthocyanin extracted from red cabbage (but that’s a whole other blog in itself). Anyway, at the beginning of Autumn the levels of sugar in the leaves tends to be quite high, increasing the acid levels in the leaves, this strengthens the red colour of Anthocyanin if it is present in the leaves.

At the end of Autumn the leaves die off and the levels of carotenoids and anthocynins die off too, leaving another pigment to dominate… and this is the brown pigment of tannin, the same pigment that give a cup of tea it’s colour!

So there you have it… next time you are crunching through those leaves you may wonder why you are suddenly thinking of carrots and cabbages and cups of tea!!!