Why are roses red and violets blue?

What do flower colors mean in plant language? - Around this time each year, thousands of people will ask Google the meaning of the different rose colors. Red is romance, yellow is friendship, lavender enchantment, and white purity... among many others. But in my opinion, the real question is: what is the meaning of these colors in their native tongue?! What do they mean in plant language? Today, Plants are Chemists explores the answers to these questions.

Figure 1: Three major classes of plant flower pigment molecules. Different plants synthesize different molecules to color their petals. The three major classes are carotenoids, which generally make yellow, orange, and red colors, flavonoids, which can make yellow and red but also purple and bluish colors, and finally betalains, which usually make purple colors. For the most part, plants accumulate these molecules in their flower petals to attract pollinators, or in their fruits to attract mammals or birds.

Flowers are colored because of pigment molecules they create - Just like human skin, the color of flower "skin" comes from pigment molecules present in the epidermis. While you have probably heard of the light-harvesting plant pigment that makes leaves green (chlorophyll), flower color pigments are different, belonging to groups called carotenoids, flavonoids, and betalains. Members of each of these families can give rise to a great diversity of colors (Figure 1).

Plants use flower pigments to attract pollinators - Plants cannot get up and walk around to find mates like mammals, nor are all plants adept at spreading their seeds far and wide on their own. Many plants, including melons, tomatoes, berries, and peppers, rely on insects, animals, or birds to spread their pollen or seeds. To attract these bugs and beasts, plants develop brightly colored flowers and berries. These large, colorful displays proclaim "Here is your nectar reward! Right this way to a nectar reward in exchange for pollination!" or, "Here are the berries! Eat them and disperse my seeds!"

Figure 2: The color-changing flowers of Weigela coraeensis. New flowers of the common garden shrubbery W. coraeensis are white and full of nectar, but after pollination they change their color to pink. Ecologists and botanists have observed that W. coraeensis achieves very high rates of pollination (almost 100% of its flowers are pollinated), while the flowers of its sister species that do not change color only have a 25% pollination rate, perhaps because pollinators less efficiently find unpollinated flowers on the sister species. This has led to the idea that W. coraeensis is deliberately changing flower color to increase pollination efficiency.

Some plants use flower color to speak to insects - Some plant species give flower color additional meaning. For example, prior to pollination, the flowers of the common garden shrubbery Weigela coraeensis are white, but after pollination their color changes to red/purple, and the flowers stop producing nectar [1]. Furthermore, when compared to its sister species Weigela hortensis that does not change flower color, W. coraeensis flowers achieve near 95% pollination, while W. hortensis only around 25% [2]. These observations suggest that W. hortensis uses flower color change as a cue to insects that red/purple flowers do not contain nectar (and that they have already been pollinated) and that the insect should visit the white flowers instead - leading to fewer repeat visits to a single flower and thus leading to a higher percentage of the flowers being pollinated! Cool! It is not clear if white to red/purple color changes are common color cues used by many plant species to alert insects. Seeing as there are some species whose flowers are always red or purple, it is certainly not a universal characteristic among plants. As research in this area continues, it will be interesting to see if other color change patters are discovered that have other meanings or if there are other types of specific cues flower colors give to insects, birds, or animals.

Flower color and humans - If you see flowers today, pause and think about how successful flowering plants have been not only in attracting and signalling insects and birds with their bright colors, but also in attracting human attention. Because of their color chemistry, roses have become a highly successful species via their relationship with humans. They get whole gardens dedicated to them, and their color diversity plays a role in our language. Happy Valentine's Day!

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[1] Suzuki, Miki F., and Kazuharu Ohashi. "How does a floral colour‐changing species differ from its non‐colour‐changing congener?–a comparison of trait combinations and their effects on pollination." Functional ecology 28.3 (2014): 549-560.
[2] Ruxton, Graeme D., and H. Martin Schaefer. "Floral colour change as a potential signal to pollinators." Current Opinion in Plant Biology 32 (2016): 96-100.