Why Do Leaves Change Color?

You must have enjoyed walking down a park, with huge trees giving you shade, or seen your children play around the giant trees in your backyard. Then suddenly, with the arrival of fall, you observe the leaves changing color from a cool green to flamboyant yellow, orange, or red. What got into these gentle giants suddenly, that made them flare up so wild?

Every leaf of a tree is a tiny factory that works day in and day out to produce food, that is, energy. Their colors are due to the presence of 3 very important pigments present in a plant cell. The first pigment is chlorophyll that gives the green color.

Chlorophyll is the most important pigment, as it helps in the production of food in the presence of sunlight. Carotenoid gives leaves the yellow, orange, and brown colors. Carotenoids are also present in fruits and vegetables, like carrots, bananas, corn, etc. The red color occurs due to anthocyanins.

This color pigment is also present in cherries, red apples, strawberries, cranberries, etc. Chlorophyll in the leaves covers the carotenoids, and therefore leaves are green in color during summer.

Chlorophyll pigments are present in majority in an active leaf during summer. It absorbs the red and blue light from the sunlight, and the light reflected by the leaves is diminished into red and blue. This makes the leaves look green in color.

The chlorophyll molecule (C₅₅H₇MgN₄O₆) is a large molecule that is not soluble in the aqueous fluid in the plant cells. They are attached to the chloroplasts, that are disc-like plant cell organelles present inside the cells. The steps of photosynthesis occur in the chloroplasts.

Light energy is converted into chemical energy, that helps in the transformation of carbon dioxide and water, used by plants for oxygen and carbohydrates.
xCO₂ + yH₂O --------> xO₂ + Cx(H₂O)y

This is an endothermic reaction, and the light energy converted to chemical energy is stored in the form of sugar and starch in the leaves. Chlorophyll can easily decompose in bright sunlight, and therefore, plants continuously synthesize chlorophyll to maintain balance. Chlorophyll is synthesized in the presence of sunlight and warm temperature.

Carotenoids present in leaves absorb the blue-green and blue light from the sunlight. This light reflected back by the leaves appears to be yellow in color. Carotenoids (C₄₀H₃₆) is also present in the chloroplasts of many plants.

Leaves appear green when carotenoids and chlorophyll, both are present in the chloroplasts, and reflect the blue-green and blue light of the sunlight. Carotenes absorb light energy and transfer it to form chlorophyll. Carotenoids are more stable compounds than chlorophyll.

Therefore, when chlorophyll is completely depleted, the leaves turn yellow in color.

Anthocyamins absorb blue, blue-green, and green light. The light reflected by the leaves, therefore, appears to be red in color. The anthocyanins are dissolved in the cell sap of the leaves.

The pigments are sensitive to the cell sap pH, and thus, lead to the production of color. If the cell sap is acidic in nature, the color so produced is bright red, and less acidic produces more of a purple shade. Anthocyanins are produced due to the reaction between the sugars and certain proteins in the cell sap, in the presence of light.

Therefore, often, apples are red on one side and a little green on the other, due to the amount of light received by the apple's surface.

The length daytime decreases in fall, and there are many changes in temperature. As mentioned above, chlorophyll pigments are produced in the presence of light. Less chlorophyll is produced in autumn, as the days are shorter.

The rate of decomposition of chlorophyll remains the same, but the rate of production of chlorophyll decreases. This leads to the 'fading off' of the green leaves.

Carotenoids present in the leaves do not decompose as quickly as the chlorophyll molecules, as they are more stable.

Also, carotenoid production does not depend on light. Therefore, the number of carotenoid molecules increases as chlorophyll decreases. This leads to the leaves changing color into yellow or orange, and sometimes, even red. Anthocyanin is another color pigment present in leaves.

If the leaves contain anthocyanin, with the decrease in chlorophyll molecules, the colors developed will be due to this pigment.Thus, leaves changing color to red or purple is due to the high amount of anthocyanin. If carotenoids and anthocyanin, both are present in the leaves, the leaf's color is orange.

If both are absent, other chemical compounds in the leaf like tannins, give it a brownish color. For example, oak tree leaves turning brown. Temperature too plays a major role in leaves changing color. If the days are warm, the colors are bright, as anthocyanin requires the presence of light.

In case the sky is overcast or the light intensity is dull, the leaves will appear to be brown or yellow.

The yellow and red pigments will soon decompose and disintegrate, once the leaves fall on the ground. The dead leaves will be brown in color.

Soon, there will be a huge carpet of dried, brown leaves on the ground, for you to rack into a stack, and the huge, majestic trees will stand barren without their leafy cover.

You must have enjoyed walking down a park, with huge trees giving you shade, or seen your children play around the giant trees in your backyard. Then suddenly, with the arrival of fall, you observe the leaves changing color from a cool green to flamboyant yellow, orange, or red. What got into these gentle giants suddenly, that made them flare up so wild?