Science

The Chemistry of Autumn Colours

As the days draw in and the temperature falls as we enter into autumn, deciduous trees collectively shed billions of tonnes of leaves in preparation for winter. Leaves that were green all through spring and summer turn spectacular shades of red, orange and yellow. The colour change is due to leaf pigmentation transformations. The green colour typical of leaves in summer is due to the compound chlorophyll (C₅₅H₇₀MgN₄₀₆).

This is a fairly unstable compound and in bright sunlight slowly decomposes. Hence plants continually have to synthesise chlorophyll to maintain their levels. As chlorophyll requires warmth and sunlight to be produced, in summer plants are in a constant cycle of producing it to replenish that which has decomposed. Chlorophyll absorbs the red and blue wavelengths of light and hence leaves appear green, as this is the wavelength that is not absorbed. Chlorophyll, which is not water-soluble, exists attached to disc-like structures called chloroplasts inside leaf cells. Photosynthesis, which is the process of producing chemical energy from visible electromagnetic radiation (light energy), occurs at these chloroplast sites. It is this energy that plants use to convert carbon dioxide (CO₂) and water (H₂O) into oxygen (O₂) and carbohydrates (C_x(H₂O)_y).

This reaction is endothermic (absorbs energy) with the reaction product, carbohydrate storing the adsorbed energy. By processing carbohydrates plants derive the ability for growth and reproduction.

Chlorophyll is not a very stable compound: bright sunlight causes it to decompose. To maintain the amount of chlorophyll in their leaves, plants continuously synthesise it. The synthesis of chlorophyll in plants requires sunlight and warm temperatures. Therefore, during summer chlorophyll is continuously broken down and regenerated in the leaves of trees.

Another pigment found in the leaves of many plants is carotene (C₄₀H₃₆). Carotene absorbs blue-green and blue light, thus making carotene appear yellow. Carotene is contained in the chloroplasts of many plants. When carotene and chlorophyll occur in the same leaf, together they remove red, blue-green, and blue wavelengths of light that falls on the leaf. The light reflected by the leaf appears green. Carotene functions as an accessory absorber: the energy of the light absorbed by carotene is transferred to chlorophyll, which uses the energy in photosynthesis. Carotene is a much more stable compound than chlorophyll, persisting in leaves even when chlorophyll has disappeared. When chlorophyll disappears from a leaf, the remaining carotene causes the leaf to appear yellow.

A third type of pigment that occurs in leaves are the anthocyanins. Anthocyanins absorb blue, blue-green, and green light. Therefore, the light reflected by leaves containing anthocyanins appears red. Unlike chlorophyll and carotene, anthocyanins are not attached to cell membranes, but are dissolved in the cell sap. The colour produced by these pigments is sensitive to the pH of the cell sap. If the sap is quite acidic, the pigments impart a bright red colour; if the sap is less acidic, its colour is more purple. Anthocyanin pigments are responsible for the red skin of ripe apples and the purple of ripe grapes. Anthocyanins are formed by a reaction between sugars and certain proteins in cell sap. This reaction does not occur until the concentration of sugar in the sap is quite high. The reaction also requires light. This 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.

During summer, the leaves of trees are factories producing sugar from carbon dioxide and water by the action of light on chlorophyll. Water and nutrients flow from the roots, through the branches, and into the leaves. The sugars produced by photosynthesis flow from the leaves to other parts of the tree, where some of the chemical energy is used for growth and some is stored. The shortening days and cooler nights of autumn trigger changes in the tree. One of these changes is the growth of a corky membrane between the branch and the leaf stem. This membrane interferes with the flow of nutrients into the leaf. Because the nutrient flow is interrupted, the production of chlorophyll in the leaf declines and the green colour of the leaf fades. If the leaf contains carotene, it will change from green to bright yellow as the chlorophyll disappears. In some trees, as the concentration of sugar in the leaf increases, the sugar reacts to form anthocyanins. 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, and if they stay above freezing, 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.