Why do flowers have so many colors?

There are more than 300,000 species of animal-pollinated flowering plants. Each one of them produces flowers of a unique color in nature, which creates an immense and at the same time fascinating color range. Producing striking coloring makes the flowers stand out against the green background of their leaves, and this allows them to successfully attract pollinators. Let’s see how.

Floral pigments, a great evolutionary invention

Humans do well by imitating what happens in nature. While painters use natural or synthetic pigments to capture their ideas on a white canvas, plants produce them themselves to color their flowers.

The floral color usually resides in the petals, which are nothing more than layers of cells that accumulate pigments. One of the most interesting aspects of these coloring substances is that even in small amounts they have a great capacity to absorb part of the visible light coming from the sun.

Thus, when sunlight hits the petals, a part is absorbed by the pigments and another part is reflected. For example, we perceive the red color of the petals of a poppy because they only reflect the red part of the visible light and absorb the rest.

In addition to green chlorophylls, which are relatively rare in flowers, there are three main groups of floral pigments:

  • The carotenoidsresponsible for the orange color of carrots, normally give yellow tones, as happens in daffodils, sunflowers, marigolds or acacias.

  • The betalains they provide the characteristic red stain of beets. These pigments are found only in the cactus family and other related ones, where they produce yellow, pink or reddish colors.

  • The flavonoids They constitute, without a doubt, the most frequent group of pigments and the one that provides the greatest range of colors to flowers. Among them are anthocyanins, the queens of pigments, since the extensive color palette they produce ranges from aquamarine blue to purple black, passing through violet, pink, orange or red tones.

The color of the flowers of the anagalis, Lysimachia monelliis produced by the accumulation of an orange pigment, anthocyanins.
edward narbona, Author provided

Other flavonoids, such as flavonols and flavones, are colorless to humans, since they absorb ultraviolet light and we cannot perceive it. However, the main groups of pollinators –mainly bees, flies, butterflies or birds– have visual systems that allow them to see this type of light.

In flowers, these compounds are part of the floral guides. Although they are generally invisible to the naked eye, they are useful for pollinators to locate nectar or other floral rewards.

A flower can harbor one or several types of pigments. When it contains more than one, they can accumulate in the same area, producing a new color as a result of mixing. In addition, the coloring substances can be located in different parts of the flower and generate extremely complex color patterns, as is the case with many orchids.

file 20221222 23 8jc0t4.jpg?ixlib=rb 1.1
The flowers of the mirror orchid of Venus, Ophrys speculum, mimic the shape and color of a female wasp. These complex color patterns are produced by the combination of various types of pigments, such as anthocyanins, carotenoids, and chlorophylls. Also, the underside produces a specular-looking glow.
edward narbona, Author provided

How to produce many colors

Although only these large groups of pigments exist, the diversity of colors they generate can be immense, since their basic structure can undergo almost infinite molecular modifications. For example, more than 600 types of carotenoids and anthocyanins that vary in their chemical structure have been documented.

Additionally, anthocyanins can molecularly bind with metals or other flavonoids and produce intense colors, such as the blue of the cornflower flower. This hue, so rare in wildflowers, could have evolved to be very showy and attract bees.

As if that were not enough, other processes derived from the reflection of light on the petals can increase the range of colors. For example, in the California poppy, the cells on the petal surface are ridged, which enhances its yellow-orange color and produces a characteristic silky sheen.

file 20221222 22 mdjwl6.png?ixlib=rb 1.1
california poppy, Eschscholzia californicacontains yellow and orange carotenoids and a silky sheen on the entire surface of the petals.
edward narbona, Author provided

Even more surprising is the case of the Venus mirror orchid, in which the cells in the central area are so flat that they generate a very powerful white glow, creating a specular appearance.

A tournament for attention

In their evolution, flowering plants have developed mechanisms to favor pollination between the same individuals of a species and thus avoid the deposition of foreign pollen. A specific color may be a mechanism that promotes this fidelity of pollinators to make visits between plants of the same species.

On the other hand, in plant communities there are times when many species flourish at the same time. In general, they share the same pollinators, which produces a competition between the plants to attract them.

As a result of this rivalry, plants have developed flower colors that are more different from each other than would be expected by chance. This has been found in grasslands in areas as diverse as Australia, North America or Northern Europe.

However, not everything is known about the evolution of flower color. In a recent study it has been shown that anthocyanins, the most frequent pigments in them, are not the most striking for pollinators. Therefore, there must be other ecological or evolutionary factors that are affecting the production of floral pigments.

Flowers and their colors still hide great secrets from us. Discovering them is undoubtedly a great challenge that we must assume to better understand the relationship of plants with the environment, even more so in the current context of biodiversity loss.

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