Do non-green leaves (eg copper on a copper beech, yellow on some leylandii) offer an advantage or disadvantage in energy absorption compared with green leaves?
Leaves appear the colour they do because of the pigments they contain. Most leaves are green in colour – thanks to a pigment called chlorophyll. When sunlight – a mix of light rays of the full range of colours – hits a green leaf, light that’s red and blue in colour is absorbed by the chlorophyll, but the green light isn’t absorbed. Instead, it bounces off – which is why we ‘see’ the leaf as green.
Some leaves don’t just contain chlorophyll, though. A pigment called xanthophyll reflects light that’s yellow in colour (and so absorbs more green light than chlorophyll); another pigment called anthocyanin makes gives copper beech leaves their purple or copper colour.
It’s not always obvious that a leaf contains more than one pigment: chlorophyll, for example, ‘masks’ the presence of xanthophyll, so we see a leaf as green even if it actually contains xanthophyll that’s bouncing off yellow light too.
If leaves contain more than one type of pigment (for example, chlorophyll and xanthophyll) then they will be able to absorb light of a broader range of colours. And the broader the range of colours absorbed, the more energy the leaf will be able to absorb. (Light rays of different colours contain different amounts of energy; the more colours absorbed, the more energy absorbed too.)
So, a leaf that contains both chlorophyll and xanthophyll will be more energy-efficient than one containing just chlorophyll or xanthophyll alone.
Coming back to the original question, though, what about a green leaf versus a yellow one? Well, green light has more energy than yellow light. (As you work your way through the colours of the rainbow, from red to violet, the energy of the light increases.) So, a plant that’s reflecting higher-energy green light and absorbing lower-energy yellow would (in theory) be getting less energy than a plant that’s reflecting lower-energy yellow light and absorbing higher-energy green.
So why don’t we see more yellow leaves? No one is precisely sure why plants have made green their colour of choice. Maybe because absorbing lots of energy around can actually be damaging. (That’s why we wear sun screen. If we don’t, we get sunburn – the energy in sunlight damages our cells.) It may be that green leaves strike a happy balance – a balance between a plant getting enough energy to survive, but not so much that it will be damaged.
Asked by Michael C via email
Answered by Jon Crowe, Molecular Guru