Wednesday, 20 November 2013

Plant pigments: the science of autumn colours

Here in the UK, temperatures have plummeted, so we're having to wrap up warm. The trees went orange quite late this year though, so it still looks like autumn, though it feels like winter. I think autumn is the most beautiful season - though it comes with a sense of melancholy as the days shorten, nothing rivals the breath-taking display of oranges, golds and reds. And of course it means Christmas is around the corner!

But have you ever wondered why trees shed their leaves, or what's responsible for the stunning colours of autumn?


When groundwater freezes it becomes difficult to draw up enough water to replace the water lost by transpiration, so in autumn, deciduous plants shed their leaves to reduce water loss.

One of the first chemicals in the leaves that's broken down is chlorophyll. Chlorophyll is the green pigment that absorbs light to fuel photosynthesis. The plant absorbs the products of breakdown and stores it for spring. When the chlorophyll breaks down, other photosynthetic pigments are revealed.

Carotenoids are 'accessory pigments' that absorb light energy and pass it to chlorophyll, and also protect chloropyll from being damaged by high energy ultraviolet (UV) radiation from the sun. A famous carotenoid is beta-carotene in carrots, which is good for you because it's an antioxidant, and it can be turned into vitamin A.  Fucoxanthin is what makes some seaweed and algae brown. Anthocyanins also protect plant tissues from UV.
Wavelengths of visible light absorbed by chlorophylls and carotenoids
From LED Grow Lights
Whatever colours a pigment doesn't absorb, it reflects, and the reflected colour is what we see. Chlorophylls absorbs red and blue, so appear green. Carotenoids absorb mostly blue, so appear yellow, orange or red.

The most stunning range of autumn colours is produced with sunny days and cool nights. The sunny days promotes the synthesis of yellow pigments, which makes the colours more intense. Cool nights slows down the transport of sugars from the leaves to the roots, so the extra sugars left in the leaves get turned into red anthocyanins.

3 comments:

Thank you ♥

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