What happens to the yellow pigments in a leaf during the summer months?

Question

What happens to the yellow pigments in a leaf during the summer months?

Answer 1

During summer, green chlorophyll is produced. Chlorophyll is a pigment that is found only in green plants, and is the plant's food factory. It allows plants to chemically combine carbon dioxide in the air with water from the ground to make glucose, a kind of sugar, a basic building block and energy storage substance for life on Earth. Sunlight provides the energy for this manufacture. Cellulose, which is the primary ingredient in wood, is made from long chains of different kinds of sugars. Chlorophyll is green, so it absorbs all colors except green, which it reflects. Chlorophyll has a metal atom, magnesium, in the middle of its molecule. This molecule is oddly similar in chemical structure to another important life pigment: hemoglobin in warm-blooded animal, which differs largely in having an iron atom in its center.

Two other pigments in plants, carotenes (which are orange) and xanthophylls (which are yellow), absorb the wasted green light, and are thought to help chlorophyll by capturing and saving energy from light that chlorophyll reflects away. Carotene is well known as an important antioxidant in human diet. These two additional pigments are held in the leaf in fat layers, and are also found in carrots and daffodils.

Plants not only manufacture sugar with chlorophyll, they also use oxygen, just like animals, to carry on normal processes of life. Two waste products of metabolism are carbon dioxide, which can be re-used to make sugars by chlorophyll using the energy of sunlight, and tannins, which remain as brown pigments in the leaf. During summer, the orange, yellow and brown pigments are all camouflaged by the intense green color of chlorophyll.

The shortening of day length in September is the major signal to begin arrangements for over-wintering, but cold air is also important. Hormones in trees with deciduous leaves start a chemical reaction that forces nitrogen compounds from the leaves back down into the roots for storage during the winter season. After this happens, a membrane, called the abscission layer, forms at the base of the leaf-stalk. This cuts across the tiny nutrient-carrying tubes that connect the leaf with other living parts of the tree: the green inner bark and the roots. These tubes carry water and minerals from the roots to the leaves, and also return sugar manufactured in the leaves to the rest of the tree. Deprived of water by the strangle of the abscission layer, the leaf begins to die. Sugar and waste products begin to accumulate in the leaf. Before it falls, however, the leaf will give us all the colors of autumn.

Chlorophyll is not a very stable chemical compound and is broken down by bright sunlight. As the leaf begins to die in the fall, the plant can no longer replace its chlorophyll. Carotenes and xanthophylls, however, are more resistant to sunlight than chlorophyll, and they remain bright in the leaf, providing its yellow and orange fall colors. The yellow and orange colors were always there alongside the green color, but are only seen when the deep green fades. You see this happen when new bananas lose their green color to reveal the yellow that was always there.

Lots of sunlight in fall makes what remaining chlorophyll there is produce lots of sugar, but cool air temperature limits life processes in the leaf, so sugars that are produced are not used up. The concentration of sugar in the leaf cell sap is increased if the weather is also dry. Various members of a group of red pigments called anthocyanins are produced in the fall when the amount of sugar increases. Like sugar, anthocyanin (a general term for this group) is soluble in the leaf cells' vacuole (a bubble of water in each cell). Anthocyanin absorbs all colors except red, which it reflects and gives the leaf you see its red color. Sunny days also make life processes go faster in the leaf, and more carbon dioxide is produced as a result. This carbon dioxide is not used up because the amount of chlorophyll is getting less and less. The carbon dioxide dissolves in the cell sap and makes it more acid. The more acid the cell sap is, the more anthocyanin is produced. So sunny, cold but not freezing, dry weather makes leaves more red in fall. You can see the influence of sunlight on anthocyanin production by red color found on only one side of some apples. The purple color of grapes is one kind of anthocyanin. Some anthocyanin pigments may act like litmus paper, producing red if the cell sap is acidic (as in Apples) and blue (as in Grapes) if the sap is alkaline. This is the reason you sometimes find blue colors on some autumn leaves.

In spring, life-giving nitrogen is returned, from where it had been stored all winter in the roots, back to the buds through the rising sap. New green leaves grow from the buds to begin the cycle of the seasons all over again.

Answer 2

Carotenes and xanthophylls are always present in the leaves of plants.  During the summer, the abundant chlorophyll masks the presence of the yellows, oranges, and reds that are also present.  These other pigments are called accessory pigments and also absorb light energy used in photosynthesis.