Tuesday, September 27, 2011

Do Insects "Read" the Coloured Leaves of Autumn?

Photo courtesy: beautiful garden

Scientists are still debating this question - so, for now at least, the answer is maybe. Here's what is known: in the fall, chlorophyll, the key chemical in photosynthesis, breaks down in leaves. This reduces the amount of green pigment in a leaf, allowing other pigments to become visible. These pigments are anthocyanins (which are responsible for pink, red and purple colours) and carotnoids (which bring out yellows and oranges). Together, they provide the beautiful fall colours we have come to anticipate each year.

But while we know "how" leaves change colours, controversy still surrounds the "why". There are two leading theories. One, known as co-evolution, argues that leaves turn red to indicate their plant could have higher levels of toxins that would be harmful to insects looking for winter homes, food or places to lay eggs. Thus, trees with brightly-coloured leaves reduce their parasite load, while insects find better hosts for the winter season.

The second theory is that anthocyanins act as sunscreens to prevent damage to leaves from excess light in the fall. During this season, leaves are pumping nutrients into the trees. But since their chlorophyll is breaking down, they don't have the same capacity to harness energy from the sunlight, which can build up in the leaves and cause damage. Anthocyanins block some of this light, protecting the leaves at this stage of their lifecycle.

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