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Above Image: Fruits and vegetables of varying pigments © leonori, iStockphoto.com

Role of Pigments in Plants

Have you ever wondered where the wonderful colours of plants come from? What makes leaves green and tomatoes red? Pigments are behind the many colours we see in plants.

What Are the Major Pigments Found in Plants?

Three major pigments found in plants are chlorophylls, carotenoids, and flavonoids. Chlorophylls are responsible for the green colours in plants. Yellow, orange, and red colours usually result from the presence of carotenoids. The presence of flavonoids can produce blue and purple hues in addition to red and yellow. In combination, these different plant pigments create the amazing spectrum of colours we observe in the plant kingdom (see Figure 1).

Chlorophylls

Chlorophyll is a molecule that absorbs energy from light to form carbohydrates from carbon dioxide and water (through a process known as photosynthesis). Chlorophyll is produced in the chloroplasts in the photosynthetic tissues. There are two types of chlorophylls found in green plants: chlorophyll a and chlorophyll b. The structure of these molecules is very similar, but differs slightly in the composition of a side chain. To learn more about the role of chlorophyll in photosynthesis, go to Light & Plants.

Chlorophyll a and b both absorb in the blue and red regions of the spectrum but have their peak absorption at different wavelengths (see Figure 2). Chlorophylls do not absorb strongly in the green region (490 to 550 nanometers (nm)). The green wavelength is reflected which is why plant leaves appear as green to us.

Carotenoids

There are two main types of carotenoids: carotenes and xanthophylls. Carotenes are molecules containing hydrogen and carbon atoms. In addition to carbon and hydrogen atoms, xanthophylls also contain oxygen atoms. Carotenoids have two main functions. First, they play a part in photosynthesis by transferring some of the light energy they absorb to chlorophylls. Second, they protect the plant from photodamage by releasing the excess energy in the form of heat. Carotenoids are formed in the plastids (chloroplasts and chromoplasts) of plant cells.

Carotenoids absorb at wavelengths ranging from 400 to 550 nm. Carotenoids primarily absorb in the blue wavelengths but reflect the wavelengths producing the yellow, orange, and red colours. A common carotenoid, beta-carotene, is produced in the chromoplasts of sunflower petals to produce the bright yellow and orange colours we associate with these flowers. Beta-carotene is also responsible for the orange colour in carrots and sweet potatoes. In autumn leaves, carotenoids are revealed when chlorophyll is broken down in response to shorter daylight hours. To learn more about why leaves change colours, see the CurioCity article entitled "Why do oak trees only turn brown in the fall while other tree leaves become more colourful?"

Flavonoids

Flavonoids are mainly visible in flowers and fruits and can be found in the chromoplasts of plant cells. These pigment molecules produce colours which help attract pollinators as well as protect the plants from light stress. The most common type of flavonoid is anthocyanin which is found in cell vacuoles.

Many flavonoids absorb at wavelengths ranging from 250 to 550 nm in the blue-green and ultraviolet regions. They also tend to form patterns on petals only visible to bees. The red colour in roses, apples, cherries, and autumn leaves on trees such as maples, is due to anthocyanins.

Which Pigments Are Found in Tomatoes and Why Are Tomatoes Red?

The red colour of tomato fruit is due mainly to a carotenoid called lycopene. There have been many studies linking lycopene with potential health benefits including reducing the risk of certain cancers. Other studies have shown that cooking tomato fruit helps increase their nutritional value since your body is better able to absorb lycopene from cooked tomatoes.

Lycopene is not the only pigment found in tomato plants. Chlorophylls are obviously present in all green plants and in tomato leaves. When the leaves become old and are ready to fall off, the chlorophylls break down and the yellow carotenoids become visible. This is just like the colour change we can observe in some autumn leaves.

Other than lycopene, other carotenoids present in tomatoes include beta-carotene and xanthophylls. When consumed, these pigments in tomato fruit act as antioxidants to protect your cells. Unlike in the related eggplant and pepper fruits, tomato fruit do not usually produce anthocyanins, but anthocyanins are present in tomato leaf tissue.

Glossary

Anthocyanins

Pigments that belong to the flavonoids group. They give red and blue colour to flowers and fruits.

Antioxidant

A molecule that prevents cell damage by inhibiting oxidation of other molecules.

Beta-carotene

A red-orange pigment that belongs to the carotenoids group.

Carbon dioxide

A gas used by plants for photosynthesis; a gas produced as waste during cellular respiration. During the day plants use more carbon dioxide during photosynthesis than they give off during cellular respiration.

Carbohydrate

One of the main kinds of nutrients in food and a source of energy for plants and animals. Energy comes from the carbohydrates sugar and starch. Sugar is a simple carbohydrate and starch is a complex carbohydrate made from many sugar molecules linked together.

Carotenes

Pigments that belong to the carotenoids group. They give plants their yellow, orange, and red colours.

Carotenoid

A class of pigments produced in plants.

Chromoplast

An organelle that produces and stores pigments other than chlorophyll.

Chlorophyll

A class of pigments produced in plants that give plants their green colour. These include chlorophyll a and b.

Chloroplast

An organelle found in plants and some algae where photosynthesis takes place.

Flavonoid

A group of pigments found in fruits and vegetables. They give plants their red, blue, purple, and yellow colours.

Lycopene

A red pigment that belongs to the carotenoids group which is abundant in tomato fruit.

Photodamage

Damage caused by exposure of plant tissue to excess light.

Pigment

A natural colouring matter in plants and animals.

Plastid

An organelle found in plant cells and algae whose function is to make and store important chemical compounds used by a cell.

Vacuole

An organelle that stores food, nutrients or waste for a cell.

Xanthophyll

Pigments that belong to the carotenoid group. They give some leaves their yellow colour.

References