Why leaves fall off trees is discovered

The reason that leaves fall off trees has been discovered.

Deciduous - Latin for 'falling' - trees, use an elaborate cellular mechanism to part company from their leaves, which act as "solar cells" in the summer but become superfluous in the darker winter months.

At the base of each leaf is a special layer called the abscission zone. When the time comes in autumn to shed a leaf, cells in this layer begin to swell, slowing the transport of nutrients between the tree and leaf.

Once the abscission zone has been blocked, a tear line forms and moves downwards, until eventually the leaf is blown away or falls off. A protective layer seals the wound, preventing water evaporating and bugs getting in.

The discovery into how trees take on their winter aspect follows a study explaining the bright colours of autumn foliage,

Leaves naturally turn yellow as the green chlorophyl that generates energy from sunlight is drained from them but the colour attracts aphids. To conceal themselves from the invading insects, some species inject a bright red pigment.

Other, seasonal applications, are also possible. "At least in concept, one should be able to alter pines trees to create a "non drop Xmas tree", says Prof John Walker of the University of Missouri, who reports the work today with colleagues in the Proceedings of the National Academy of Sciences.

But Prof Walker and his colleagues are the first to identify a pathway of genes involved in the process of abscission in Arabidopsis by using a combination of molecular genetics and imagine techniques.

Another study recently revealed the reason why leaves turn red in autumn has been explained - it helps trees fight aphids. Production of red pigments helps to conceal the yellow colour that is attractive to the sap sucking insects, says a new theory set out by Dr Thomas Döring of Imperial College London.

As for why some trees stay yellow, Dr Döring said it was a balance of the cost of insect attack and the cost of making the red pigment. "If insect attack is generally high, causing higher costs than the costs for the production of red anthocyanins in autumn, trees would benefit from being red. If the costs entailed by the insects is lower, than you can afford to stay yellow.