Gardeners and florists use flowers in a range of colours to create their beautiful designs. Whether it’s restful blues and creams, or brash and energising oranges and reds, colour plays a major role in the pleasure we get from flowers. But those beautifully coloured petals aren’t really there for our enjoyment. They have evolved as a signal to pollinators – the plant’s way of hanging a sign out saying ‘eat here’.
Many flowering plants need help, more often than not in the form of an insect, to transfer pollen from one flower to another- that essential step for the production of seed, and the survival of the species. Through time these plants have evolved various ways of attracting the attention of potential pollinators. They have developed showy, coloured petals and enticing scents to draw in bees, butterflies and birds, and sugary nectar to encourage them to linger and pick up sticky pollen. And this relationship between pollinator and plant has been a driving force in the evolution of flower colour. A study in Australia found that, among native plants, the most common flower colours are those most easily seen by bees. These important pollinators have colour vision that’s different to ours – their photoreceptors pick up colours in ultraviolet, blue and green, whereas ours are tuned in to red, blue and green. The Australian research analysed the colours of hundreds of flowers and compared them with the colour range visible to bees. They found that the flowers matched this part of the light spectrum, suggesting that the plants were adapting their flowers to become more attractive to the bees. This reflects the importance of bees as pollinators, not only for wildflowers but for food crops too. The research team also looked at whether there was a similar correlation between flower colour and the colour perception of two other pollinators – birds and butterflies, but no similar fit was found. Bees, it seems, are key when it comes to flower evolution.
There could be another important feature of the bee/flower relationship that is driving colour evolution. Once a plant has managed to get a pollinator to visit its flowers, it needs a strategy to encourage that same pollinator to then visit flowers on other plants of the same species. Otherwise all that precious pollen is going to be wasted. Honey bees and bumblebees are known to show ‘flower constancy’ – they tend to visit flowers of the same type while the food rewards remain good. This works well for the bees because it optimises their foraging trips, and for the plant because it increases the chances of pollen picked up at one flower being transferred to another of the same species. Having flowers that are easily distinguishable through colour make it easier for the bees to learn which are worth visiting.
Of course. more recently our love of flowers in the garden and home has led to a modern form of evolution which has nothing to do with pollinator preferences. Plant breeders are developing new varieties of garden and cut flowers in response to demand from growers. By artificially transferring pollen between carefully chosen plants, they can select for desirable traits such as colour, size or vase life. Thanks to these breeding programmes we have a whole colour palette of flowers to choose from. But even this human controlled evolution takes decades from the first cross being made to a new variety of the plant being produced and marketed. There is now potential to shorten this time scale considerably by using cutting edge gene editing techniques. In the last couple of years, scientists have been able to alter the DNA of one plant and change its flower colour completely. A team in Japan worked with the Japanese morning glory, whose trumpet-shaped flowers are usually purple or pink. They were able to cut out the DNA sequence which codes for an enzyme that gives the flower its colour, resulting in plants with pure white blooms. This research work was experimental, and not a new plant breeding project. But as genetic modification techniques become cheaper and easier to use, it may be possible to reduce thousands of years of evolution to a single generation. Who knows what the flowers of the future could look like.