Question: Why do leaves absorb all colour apart from green? Why can't they use green in photosynthesis? Why are some leaves purple and some leaves have marks on their leaves? How are they different? Why do leaves turn brown when they die?

Susan Cartwright answered on 16 Jun 2015:

Basically, photosynthesis consists of several different reactions chained together, all involving a large molecule called chlorophyll. It uses light to excite electrons to higher energy levels in order to drive two key stages of this reaction. Because of quantum mechanics, only certain wavelengths of light carry the right energy to excite these electrons: in chlorophyll, blue and red light are absorbed, but green isn’t. The complex series of reactions involved in photosynthesis in plants evolved from simpler reaction series occurring in bacteria, so the fact that it’s blue and red that are absorbed and not green is probably more or less an evolutionary accident.

The red pigments that produce purple leaves, e.g. in the copper beech, are anthocyanins, which are also responsible for the red colour of some tree leaves in autumn. In the case of autumn leaf colours, it is possible that producing the red colour protects other reactions going on inside the leaf from UV radiation, and allows the leaf to reabsorb the nutrients – especially nitrogen – from the leaf it is about to drop. Plants that have red leaves all the year round usually have a mutation which causes this pigment to be produced all year instead of just in autumn. It is not helpful to the plant, because it makes photosynthesis less efficient, but it is pretty, so it has been artificially selected for by gardeners. The same is true of green-and-white variegated plants: parts of their leaves don’t produce chlorophyll at all. Again, not good for the plant, but pretty, and therefore selected by gardeners. (Given a chance, many variegated plants revert back to all green.)

Leaves turn brown when they die because the chlorophyll breaks down and most of the useful chemicals are reabsorbed back into the parent plant, leaving only the basic structural material, which is brown. Many trees that live in the temperate zones (mid-latitudes, like the UK) find that it is not efficient to try to harvest the weak winter sunlight, so they drop their leaves in winter and regrow them in summer. In contrast, most tropical plants are evergreen (they keep their leaves all year because the sun is always high in the sky) and so are most trees in arctic or subarctic regions (they need all the sunlight they can get!).