The flower color blue in the spotlight

The liverwort is an example of the comparatively few flowering plants that we perceive as blue. (Photo: Anke Jentsch)

They are often found in the garden, balcony boxes and in the bouquet – because people have a pronounced preference for blue flowers. But as researchers are now making clear, this color is actually comparatively rare in wild plants. According to them, why this is so probably has to do with the great effort involved in producing blue dyes, but also with the fact that, unlike pollinator insects, we have limited color perception.

The color blue has a special meaning for humans, the international research team clarifies in the context of the study, first of all using many examples. Accordingly, it is at the forefront of favorite colors worldwide and has played an important role in art and culture for thousands of years. The flower color blue is therefore also of great importance in ornamental plants – for example, it is a symbol of loyalty and romance. With this in mind, the scientists have now devoted a study to the blue flowers. To this end, they have compiled a large number of findings on the color blue in the world of flowering plants and related them to one another.

Flowers that appear blue to us are rather rare

Due to the strong presence among the ornamental plants and possibly due to our special attention to violets, liverworts and co, the impression can arise that blue is a particularly common flower color. But as the researchers’ research in the world’s largest databases of plant properties shows, this is not the case: only the flowers of around seven percent of flowering plants appear blue to humans. However, it is first necessary to clarify what this color assignment actually means: What we call “blue” is our impression of short-wave frequency ranges within the framework of human perception.

As the researchers emphasize, plants do not produce their flower colors in order to appeal to us. On the other hand, they are intended to attract the attention of animals that ensure pollen transmission and thus seed formation. It is known that pollinators such as insects or special bird and bat species are sensitive to different color spectra than humans – this also applies to the blue areas. “It can be assumed that the color perception of pollinating organisms has significantly influenced the development of flower colors in the course of evolution. This is why the question of how the flowers are perceived by their respective pollinators and which interactions are triggered is important, ”says co-author Anke Jentsch from the University of Bayreuth.

As the researchers explain, in addition to the photoreceptors for red and green light, the human eye also contains a receptor for the short-wave, blue frequencies. But the range of perception in this area is limited. For example, bees can perceive many more bluish frequencies – this also includes color patterns in the ultraviolet range. As a result, the insects perceive many more flowers as “blue” than we do. “Bees see the splendor of the colors of flowering plants very differently than other pollinator groups or than we humans. They are particularly attracted to flowers that shine in the short-wave frequency range, ”says Jentsch.

Blue dyes are “expensive”

This attraction of the colors from the blue color spectrum, however, raises the question of why only comparatively few plant species have blue tones in the range that we can recognize. As the researchers explain, this has probably to do with the comparatively complex production of the flower pigments, which we perceive as blue. In the chemical process required for this, six different coloring substances and six corresponding molecules are involved, which together with metal ions form special ring structures. Apparently, only a comparatively few plant species can afford the expense of producing these complex blue dyes.

As the researchers explain, this only seems to be worthwhile if the plants have to assert themselves in tough competition for pollinators. Because of the “expensive blue tones”, they obviously catch your eye a little more intensely. That is why flowers that appear blue are often found in particularly species-rich meadows – especially in the mountains, the scientists explain. Because there are only a few pollinator insects at higher altitudes due to the rather unfavorable living conditions. There, blue flowers are an important unique selling point: In competition with other species in their immediate vicinity, they are particularly noticeable, so that pollinators are attracted even from a greater distance.

As the researchers finally emphasize, this background also has to do with a special threat to blue flowering wild plants: the intensification of agriculture and further human interventions can further reduce the already low proportion of blue flowering plants. “There are numerous indications that the expansion of agricultural areas, the use of artificial fertilizers, frequent mowing and intensive pasture management are at the expense of species-rich vegetation. There is therefore a risk that the blue flowers in particular will disappear from the landscape, ”says co-author Justyna Giejsztowt from the University of Bayreuth.

Source: University of Bayreuth, specialist article: Frontiers in Plant Science, doi: 10.3389 / fpls.2020.618203

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