[tizzicat06]

tizzicat06 asked the Naked Scientists:
   
I know that trees need chlorophyll to convert carbon dioxide into oxygen through photosynthesis, but how do red-leaved trees do this?

Jane, from Witham.

What do you think?

[chris]

Hi Jane

you're right; as a rule, plants contain chlorophyll, which enables them to photosynthesise, a process through which energy from sunlight is used to drive chemical reactions resulting in the conversation of carbon dioxide and water to glucose and oxygen.

But the colour of the plant foliage is not necessarily an indication of the presence - and relative concentration - of chlorophyll, or the ability of the leaf to photosynthesise.

In fact, leaf colour is determined by multiple molecules, all of which can have different colours.

For instance, the reason leaves look green is because chlorophyll rejects green light whilst absorbing (and therefore utilising the energy within) red and blue lights. And because the red and blue light wavelengths are relatively more absorbed than green, this makes the leaf look green.

But, if the leaf also produces a high concentration of, say, an anthocyanin, this increases the amount of blue light that is absorbed (beyond what the cholorophyll requires), making the leaf look relatively red. You'll notice that such "red" leaves are actually quite a dark red, because very little light is actually being reflected back; most wavelengths are actually being absorbed, and used.

Interestingly, the first "plants" on Earth were actually aquatic and were probably reddish - as are many seaweeds (macroalgae) today, because water strongly absorbs red light and therefore there was no cost to the plant in rejecting red light because there was very little of it in the water.

Chris