While hiking the trails during the growing season, many people enjoy the brilliant colors of pigments in nature displayed by plants and flowers. Most of the colors that we see in plants are created by pigments. Each of these pigments has a different molecular structure which absorbs and reflects the color wavelengths in white light differently. The reflected colors are the ones we can see. A good example of this is the rainbow that appears when sunlight hits a prism or water droplets.
Green Chlorophyll and Photosynthesis
The most familiar pigment is chlorophyll, responsible for photosynthesis. This pigment takes in light from the sun, absorbing red and blue wavelengths and reflecting a green color. It then uses the sun’s energy to create sugar and oxygen from carbon dioxide and water. The sugars created are used within the plant to grow other parts, including the flowers and their pigments. Like an artist mixing paints on their palette, the wide range of colors seen in the flowers are created from a mixture of these pigments.
The Warm Colored Pigments
Another common group of pigments are the carotenoids, which absorb blue to blue green wavelengths and reflect yellow, orange, and red. These pigments are found in pollen and the reproductive parts of a flower such as bright orange jewelweed (Impatiens capensis). These warm colors signal to pollinators that there is nectar available in the flower. In addition, carotenoids are found in leaves and are used to help dissipate extra light energy as heat, protecting the leaf during photosynthesis. In fall, once the green chlorophyll pigment is no longer synthesized, leaves turn yellow, and we can see the carotenoids.
There are two groups of pigments that have a similar color range and share similar functions but are not found together in the same plants. These two groups are the betalains and the anthocyanins. Betalains can be broken down into two subgroups, the betaxanthins that reflect yellow to orange, and the betacyanin that reflect red to violet. This group can be found in the northern pitcher plant (Sarracenia purpurea) which is found in bogs in Wisconsin, beets, and Swiss chard. The anthocyanins are part of the flavonoids group and reflect red, blue, and purple colors.
Pollination and Seed Dispersal
Both of these pigments welcome pollinators to the flowers, as well as provide color to fruits so animals will find them attractive and help disperse the seeds within. They also act as a form of sunscreen for the plant. However, though their functions are similar, the red color of the betalain in a beet is different that the red color of the anthocyanin in a radish because the different hues within these pigments can be affected by the temperature and the acidity in the plant. In hydrangeas, changing the pH of the soil alters the colors of the flowers they produce. Basic soil creates more pink flowers, and acidic soil creates more blue flowers.
The Flavonoids
Besides anthocyanins, there are other pigments in the flavonoids group, including chalcones and aurones. These two pigments are also known as anthochlor pigments which reflect yellow and UV wavelengths, seen in cosmos (Cosmos sp.) which are found in many gardens. While people can see the yellow coloration, insects that can see UV light will see pigmentation leading them to a flower’s nectar and pollen.
Having this range of pigments is beneficial to plants for attracting insects to help with pollination. However, because some insects also damage plants, they have another type of pigment to help aid in their defenses. Tannins, which are also part of the flavonoid group, are found in the flowers and bark of witch hazel (Hamamelis virginiana). They often reflect a brown color and are bitter to taste. This pigment not only protects the plant from insects, but it also protects it from fungi and bacteria.
Pigments have been used for thousands of years by people to color natural fibers like wool and linen, to tan cow hides into leather, and more recently to be used as coloration for food products. As we enjoy the colors during the growing season, we can appreciate what pigments do for plants in addition to providing their pretty hues.
