Lady Autumn, Queen of the Harvest,
I have seen You in the setting Sun
with Your long auburn tresses
blowing in the cool air that surrounds You.
Your crown of golden leaves is jeweled
with amber, amethyst, and rubies.
Your long, flowing purple robe stretches across the horizon.
In Your hands You hold the ripened fruits...
The purple and orange lingers
and glows like burning embers.
Then all colors fade into the twilight.
Lady Autumn, You are here at last...
– Deirdre Akins

Few people can fail to be moved by the splendour of trees cloaked in their autumn finery. This occurs in the temperate zones of the world when the arrival of cool weather transforms the landscape. At this time, broadleaved trees lose their leaves to conserve water throughout the winter months. The first signs of this come as the leaves change colour from predominantly green to a dazzling array of reds, oranges, yellows and even purples.

From early October it seems that woodlands and forests become ablaze with colour. But how and why does this happen?

Chlorophyll

The green colour of plants, present in stems and leaves, is due to chlorophyll, which is present in sub-cellular structures known as chloroplasts. Chlorophyll is essential for photosynthesis, which is the process by which plants use light energy from the sun to synthesise carbohydrates from carbon dioxide and water. The carbohydrates are essentially the plants' food, or energy source required for growth, reproduction and all other life processes. During this process oxygen is given off as a by-product. The water is absorbed predominantly through the roots from the soil, while the carbon dioxide enters through the leaves:

6CO₂ + 6H₂O + Energy from the Sun ---> C₆H₁₂O₆ + 6O₂

As the light levels begin to fall in early autumn, so the synthesis of chlorophyll by the plant also begins to fall and the autumn leaf begins to lose its greenness. The colours of autumn are due to a combination of new pigments made during the autumn, and pigments that were already present but which were concealed by the dark green chlorophyll.

Senescence

As the level of chlorophyll falls, so less energy is produced. There comes a point at which the leaf requires more energy to maintain than it creates through photosynthesis, and so it begins to die off and is eventually shed. These processes are known as senescence (or ageing), and abscission, respectively.

Senescence is a progressive phenomenon in living things that leads eventually to death. Autumn leaves are examples of senescent organs. The ability of deciduous trees to lose their leaves during autumn¹, is an adaptation to prevent them from desiccating during the winter, when the roots are unable to absorb water from the frozen earth.

Abscission

Abscission, or leaf shedding is a very complex process and involves several plant hormones². Where the leaf's stalk (petiole) is attached to the twig there is a tiny band of cells, which begin to loosen and dry out. Between these cells and the twig, a layer of cork-like cells begins to form, and the vessels (xylem and phloem) bringing sap to and from the leaf become blocked by a gummy substance, thus depriving the leaf of water. Before this happens, the contents of the leaves' cells have already begun to break down chemically, and the soluble products are absorbed back into the tree.

The cells beyond the corky layer degenerate, and the leaf is held onto the stem only by the vessels. Even a gust of wind would be sufficient to sever this link and the dried up leaf eventually falls, leaving scar tissue behind on the stem, which prevents ingress of insects, bacteria and fungi which could cause disease. However, as it will be two weeks or so until the leaves all fall any remaining green chlorophyll is destroyed by the sun's rays, leaving behind the characteristic yellow to purple autumnal hues.

Carotenoids

Outstanding in the array of autumnal colours are the yellows, oranges and reds which are due to carotenoid pigments, which include carotenes and xanthophylls. The orange colour of carrots is due to carotenoid compounds and, indeed, the word 'carotenoid' is derived from the Latin, carota, meaning 'carrot'. Carotenoids are also responsible for the yellow colour of butter and the yellow/orange colour of egg yolks. Sugar-maple leaves possess the orange and chrome hues of carotenoids, whilst hazel and birch leaves have the bright yellow of pure carotene.

Chemically, the carotenoids are polyenes³, and carotenes themselves are hydrocarbons⁴. The xanthophylls are oxygenated derivatives of carotenoids.

The carotenoids assist with, but are not essential to, photosynthesis. They absorb light energy and pass it on to chlorophyll through an electron transport chain.

Anthocyanins

Well, we have explained the origins of the yellow, orange and red hues of autumn as being due to carotenoid compounds, but what about the crimson of red maple, the deep reds of beech, sumac and red oak, or the plum colours of ash?

The brightest reds and purples of autumn are due to anthocyanins, a group of water-soluble pigments occurring in solution in the aqueous cell sap in the vacuoles in flowers, fruits, stems and leaves. They are responsible for the large variety of colours in flowers; the reds, violets and blues. As far as leaves are concerned, the anthocyanins only become evident once the corky layer has completely cut off the flow of sap from the leaf. The leaf continues for a while to make carbohydrate in the form of sugar (glucose). This becomes trapped in the leaf and, if the weather remains cool and bright, these sugars are transformed into anthocyanins. Anthocyanins are amphoteric, meaning that they have dual acid and basic character. If the sap is acidic, the anthocyanins turn red; if neutral to alkaline, they turn blue or purple.

It is thought that the different colours of fallen leaves may reflect their living function. Thus brown colours are due to tannins and lignins, which are known to be protective against herbivorous predators. These also inhibit the breakdown of leaf litter after abscission, and so autumn leaf colour can be a good predictor of decomposition rates.

An Interesting Experiment

It is possible to visualise that leaves possess other pigments than chlorophyll the whole year around by means of paper chromatography⁵.

• Carefully crush some green leaves or grass in a pestle and mortar with about 5cm³ of propanone⁶.
• Cut a small rectangle of filter paper (say 4cm x 10cm), and draw a faint pencil line horizontally along the short edge, about 1 cm from the bottom.
• Take one drop ⁷ of the green solution from the mortar and place it at a position on the pencil line, say 1 cm from the left hand edge. This will allow room to add solutions obtained from different coloured leaves, or leaves from different plants. Dry it with a hair dryer. Add further spots on top, drying each time, if necessary to increase colour intensity.
• Put just enough solvent (propanone or nail varnish remover) into a beaker to reach about half-way to the pencil line on your filter paper, and place the filter paper in the beaker so that the pencil line is at the bottom. (It is important that the propanone solvent doesn't reach as far as to where you've spotted on your leaf extract).
• Allow the propanone solvent to creep as far as possible towards the top of the paper.
• Remove the paper from the beaker and dry with a hair dryer.
• Observe and record (draw) the final appearance of the paper. (This is called a 'chromatogram').
• There is space on the same filter paper to compare extracts from leaves of different colours, leaves from different plants, etc.

If you have access to chemicals (eg a school laboratory) then you may find that the separation can be improved if solvents other than propanone are used.

In many parts of the world, most notably the north eastern section of North America, the arrival of cool weather in the autumn transforms the landscape. Trees lose their leaves to conserve energy through the winter months. The first sign of this comes as the leaves change colour. Without the chlorophyll that gives the leaves their green hue, other colours emerge: bright reds, oranges and yellows. In October it seems that forests become ablaze with colour. The inhabitants of this area have turned this natural phenomenon into a thriving industry, hosting tourists who come specifically to see the foliage as it changes. Though this colour is a sign that the leaves are dying - ultimately they turn brown and fall off - it brings a touch of beauty to the cool weather - a beauty that belies the sombre realisation that all things must pass. No-one who has grown up with this seasonal fireworks display can ever forget it. It becomes written on the soul.
– An h2g2 Researcher