From its captivating origins to the macroevolutionary changes that produce a breathtaking array of hues, Kellenberger and Glover explore the enchanting world of flower colour diversity
Among the countless natural elements that attract human attention, the colourful universe formed by flowers certainly has a privileged place. Not in vain, the diversity of flower colours and how they are distributed and organised in nature have fascinated scientists for quite a long time. Among these researchers are Dr Roman T. Kellenberger and Dr Beverley J. Glover from the Department of Plant Sciences at the University of Cambridge in the United Kingdom, who are interested in delving into the mechanisms behind different flower characteristics and how these characteristics shape the interaction between flowers and their pollinators. In one of their recent works in Current Biology, these researchers bring us an exciting overview of the kaleidoscopic evolution of colours in flowers: delving into the very roots of its formation, decoding the intricate molecular and biochemical mechanisms behind it, and unravelling the dynamic evolution across distinct temporal scales.
The building blocks of colours
Colour results from the interaction between two equally important components. The first one is the reflection of light wavelengths. You’ve probably heard that white colour is a combination of all colours, and black results from their absence, correct? Well, this is partially true because white surfaces intensely reflect all wavelengths, resulting in the absence of colour. In contrast, reflection from black surfaces is almost negligible, leading to predominant absorption and, thus, the lack of colour. This raises a fundamental question: what determines the colour of a flower? Here comes the second component: the eye of the observer (Figure 1). When we –humans– see a red flower, our trichromatic vision allows us to perceive reflected light at wavelengths between 400 and 700 nm (which represent blue, green, and red, respectively), creating the perception of red. However, to a bee, which has a limited sensitivity to red, that same red flower may appear black. Have you ever imagined seeing from the UV spectrum to red? Well then, lucky are the birds that enjoy this privileged view!
Although we might be able to identify a broad and heterogeneous colour palette in flowers, the truth is that the list of pigments responsible for it is pretty short, including chlorophylls, carotenoids and anthocyanins. The subtle variations in colour shades result from species-specific modifications in the metabolic pathways that produce these pigments. For example, the cyanidin that gives place to red roses might resemble the cyanidin responsible for red dahlias. Still, the structure of each pigment might have little modifications that give place to its differences. Moreover, pigments (and their combinations) can be integrated with other floral characteristics, such as the biophysical properties of petal cells (e.g., velvety surfaces), creating infinite combinations that culminate in the spectacle of colours we see (Figure 2).
How and why did flower colour appear?
Kellenberger and Glover point out that the colour of the petals must have originated in leaf-like structures meant to protect plant reproductive organs. For instance, in gymnosperms, such as the European larch (Larix decidua) and the seagrape (Ephedra distachya), the seed cones display a striking hue due to the accumulation of red pigments in the scales (Figure 3). Therefore, it is possible that the sterile protective organs of the common ancestor of angiosperms and gymnosperms also already had bright colours.
In the first lineages of angiosperms, flowers predominantly displayed light colours, but some species might exhibit more vibrant ones –such as orange, pink, and blue. As the pigments responsible for these colours were already present in plants, albeit with other functions such as light harvesting and stress responses, it is believed that throughout plant evolution, they were harnessed to dye the structures that would give rise to flowers and diversify their colours. You might be asking yourself, why do this? The most likely answer is that these features would make the flowers stand out more amidst the green kaleidoscope of leaves, making them more striking to their pollinators, particularly insects, which have a remarkable visual capacity. This nascent interaction between flowers and their pollinators was one of the precursors to the success of flowering plants on our planet. As a result, floral colour emerges as one of the stars in this evolutionary narrative, along with other cues that might shape such interactions, such as flower morphology and aroma.
How do animals interact with flowers?
To understand the relationship between flower and animal diversity, researchers coined the concept of “pollination syndromes”, which describe the combinations of floral characteristics that have evolved independently in species with the same group of pollinators. Naturally, flower colour is one of the main components of this categorization, with blue and yellow flowers being pollinated by bees, while red and white flowers being visited by birds and moths. However, the concept of pollination syndromes has been the subject of extensive debate and even a recent large-scale analysis indicated that colour is one of the least informative characteristics to identify the pollinator of a given species. This has raised are a lot of doubts about the role of floral colour in plant evolution.
This controversy has led to increasingly detailed studies on the evolution of colour in flowers, even on smaller scales. For example, this research shows that changes in floral colour are more common than in other floral attributes. Mutations in floral pigments do not harm the plant, as they rarely mess with key physiological processes. In this way, new mutations can easily appear, giving rise to the colour variations we know, which then can be subject to evolutionary selections.
Flowers, colours, and people
Leaving behind its biology and evolution, we cannot neglect the intrinsic and ancient relationship between humans and flowers. For a long time, humans have shown interest in flowers and their colours, as evidenced by the artefacts with floral patterns present in several ancient civilisations: from the Minoan Frescoes in Ancient Greece to the Incan and Quechua textiles in South America. This attraction can be attributed, in large part, to the positive emotions that flowers awaken, dating back to the first humans, who associated vibrant colours with either tasty and valuable fruits or poisonous items that should be avoided. Furthermore, throughout history, humans have developed several ways to incorporate flowers into their daily lives, creating floral colours artificially. It is no surprise that flowers and their colours are the subject of some of the most recognisable works of art, such as Van Gogh’s Sunflowers and the Water Lilies of Monet.
However, this colourful and optimistic scenario contrasts with the gloomy reality of biodiversity loss and habitat destruction we currently face, including the worrying reduction in pollinators. Hopefully, future research and knowledge advances to understand the evolution and ecology of floral colour will pave the way to a better understanding of pollinators and effective ways to prevent their loss.
ABOUT THE AUTHOR
Ana Carolina S. Oliveira is a pollination biologist fascinated by understanding the choice of pollinators through the visual signs of flowers, especially how bees interpret the universe of floral colours. Currently, in her PhD, she is trying to understand how floral colour modulates the reproduction and structuring of oil flower communities and the preference of bees in this context. For more information, follow her on Twitter at @CarolSabino06.
SUGGESTED READING
Kellenberger, R. T., Glover, B. J. (2023). The evolution of flower colour. Current Biology. https://doi.org/10.1016/j.cub.2023.01.055
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