How do leaves know when to fall off trees

As autumn approaches and the leaf ages a hormone called abscisic acid builds up at the base of the leaf stalk. The hormone encourages the formation of a layer of cells, the abscission layer, which severs the tissues that support the leaf. Simultaneously the tree seals the cut, eventually the leaf is blown off or falls due to its own weight and leaves behind a leaf sear.

Q. How does abscisic acid work

A. Rather than being a growth hormone, abscisic acid puts a stop to growth, promoting dormancy.

Q. Why do leaves fall in the autumn

A. As an energy saving measure. Winters are dry times for trees roots, frosts and snow harden the ground, preventing water from percolating though to the depths where roots lie. Not only do leaves need a lot of water they also provide large surfaces from which it readily evaporates.

Additionally since the Great British winter is traditionally rather on the grey side, leaves, whose main job is to photosynthesise light, are not needed.

Q. Why do leaves change colour before falling

A. Leaves are green in the spring and summer months because they spend their time manufacturing food for the tree using chlorophyll, which is green. Chlorophyll is a remarkable chemical that absorbs the energy from sunlight and uses it to transform carbon dioxide and water into sugars and starch.

There are other pigments within the leaf, like carotenes (orange) and xanthophylls (yellow) but this are masked by the large amounts of chlorophyll.

However when autumn comes around and the amount of available sunlight is reduced the leaves stop their food manufacturing. This causes the chlorophyll to break down, leaving the orange and yellows to show through.

Have you got a question about a plant's coping mechanism for surviving winter

by Lisa Cardy