Why do leaves change colour in autumn?

The green pigment chlorophyll, which absorbs red and blue light, is not a very stable compound and bright sunlight causes it to break down. So, to maintain the amount of chlorophyll in their leaves, plants continuously have to make or synthesize it. The production of chlorophyll in plants requires sunlight and warm temperatures, so it is continuously broken down and re-made in the leaves of plants and trees during the spring and summer months.

Autumn leaves

Another pigment found in the leaves of many plants is carotene, which absorbs blue-green and blue light, and the light reflected from it appears yellow. The energy of the light absorbed by carotene is transferred to the chlorophyll to be used in photosynthesis. When carotene and chlorophyll occur in the same leaf together, they remove red, blue-green and blue light from the sunlight that falls on the leaf and, therefore, the light reflected by the leaf then appears a very bright green.

The shortening days and cooler nights of autumn trigger changes in the tree. At the point where the stem of the leaf is attached to the branch, a special layer of cells develops and gradually severs the tissues that support the leaf. Because the nutrient flow is interrupted, the production of chlorophyll in the leaf declines, and the green colour of the leaf fades. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar.

Carotene is a much more stable compound than chlorophyll and persists in leaves even when all the chlorophyll has disappeared, so the remaining carotene causes the fading leaf to appear yellow.

A third class of pigments that occur in leaves are the anthocyanins, which absorb blue, blue-green and green light. Therefore, the light reflected by leaves containing anthocyanins appears red or purple, depending on the acidity of their sap. Anthocyanin pigments are responsible for the red skin of ripe apples and the purple skin of ripe grapes. Anthocyanins are formed by a reaction between sugars and certain proteins in cell sap and this reaction does not occur until the concentration of sugar in the sap is quite high. The reaction also requires light which is why apples often appear red on one side and green on the other. The red side was in the sun and the green side was in shade. In some trees, as the concentration of sugar in the leaf increases, the sugar reacts to form anthocyanins, and these pigments cause the yellowing leaves to turn red.

The range and intensity of autumn colours is greatly influenced by the weather. Low temperatures destroy chlorophyll, but if they stay above freezing, they promote the formation of anthocyanins. Bright sunshine also destroys chlorophyll and enhances anthocyanin production. Dry weather, by increasing sugar concentration in sap, also increases the amount of anthocyanin. So the brightest autumn colours are produced when dry, sunny days are followed by cool, dry nights.

Emperor Moth capterpillar foraging leaves

Many trees and shrubs lose their leaves when they are still very colourful. Some plants retain a great deal of their foliage through much of the winter, but the leaves do not retain their colour for long. Like chlorophyll, the other pigments eventually break down in light or when they are frozen. The only pigments that remain are tannins (bitter compounds that are produced in plants to deter foraging animals and insects and help them resist decay), which are brown. Hence the leaves that remain on deciduous trees in the winter are always brown.

Other items that may be of interest:

• Hedgerows
• Photosynthesis and transpiration (made easy)
• Lichens
• Song of the trees
• Twelve tree truths

Fun learning .. Introduction Resources Seaquest for Kid's Free pictures Activities Pond Pack Postbag Diary Fox Club Volunteers Kids stuff Publications Tap Routes SEARCH . . . . . . . . .