Flowers create blue 'halo' to attract bees

Scientists at Cambridge University have discovered plants are great illusionists. Plants use a trick of the light to give their flowers a blue halo, which might make them more attractive to bees. Pollinating insects, like bees, see blue-coloured light very well, but blue pigments are very hard for plants to make, which is why blue flowers are quite rare. But rather than go to the expense of producing blue coloured chemicals in their petals, many plant species have instead evolved nano-scale wrinkles on the surfaces of their flowers. These wrinkles scatter the blue colours in sunlight, making the flower look blue - and potentially more attractive - to a bee. Beverley Glover, the director of the Botanic Gardens in Cambridge and one of the authors of the new study. Beverley explained to Georgia Mills how they discovered these "invisible" blue halos. 

Beverley - They’re not always invisible to us. If the pigment that the flower has underneath the blue halo is a dark purply black colour, we can sometimes get a hint of it and it was looking at a flower like that  that first gave us the clue. Then we worked out how to measure the blue halo, so you can record this scattered light in the blue and ultraviolet coming off the flower of a quite wide angular range. Then we used the collection of plants at the Cambridge Botanic Gardens and just waded our way through them measuring flowers that we thought looked possible and had the right sort of ridges on their surface. And discovered that, actually, a surprising number and quite widely dispersed across the flowering plant family tree are doing the same trick.

Georgia - How do we know this is to attract the bees?

Beverley - We don’t know yet that that’s what it is for, but it’s an obvious hypothesis, an obvious idea because that’s what flowers are all about. They’re all about attracting animals to pollinate them, and the petals particularly are really just there as an advertising too. So we did some experiments where we made artificial surfaces that could make the same blue halo, and we asked bumble bees in our lab setup whether they could see the blue halo even if we couldn’t, and they certainly could. Then we just used that flowers and asked how long it took them to find them? And we found that having the blue halo made the flowers stand out much more to the bees and they were much quicker finding them. So that all adds up to the idea that probably they’re involved in attracting the bees, but we’d like to get out there in the wild and test that there too.

Georgia - The blue makes it more easy for the bee to find it, which helps them when they’re foraging, and then helps the flower because they’re more likely to get pollinated?

Beverley - Exactly. Flying bees are having quite a hard time energetically and anything that makes it easier for them to find flowers, get hold of the sugar quickly, and move on to the next one has to be a good thing. And of course, anything that’s a good thing for the bee and is going to make the flower more attractive, is food for the lower too.

Georgia - You mentioned this trait was found in quite a lot of plant species. Is it that one plant had this great idea many years ago and they all evolved from it?

Beverley - No, so that was the surprising thing. When I say quite a lot, it's not very common, most flowers don’t have it. There are, obviously, hundreds of thousands of flowering plant species, but what we found was that there are flowers in all of the major groups of the flowering plant family that have it widely dispersed families. What that tells us is that it probably evolved convergently multiple times. It’s not there is the oldest flowering plants that we still have alive today, but it’s there is scattered species in all the other groups, so we think it’s probably repeatedly or convergently evolving.

Georgia - Wow. So all of these different lines of plants have separately worked out, I guess, this trick to make themselves look blue when they’re not actually blue?

Beverley - Yeah. And I think that’s probably not surprising because they really have to do is work out how to produce a ridged effect on their surface out of their cuticle. All plants have a cuticle that protects them from drying out and so probably it’s not that surprising that more than one plant’s come up with the idea of folding that cuticle up if that give you a really good blue effect.

Georgia - If quite a lot of them have managed to do it and it’s supposedly an advantage, why do we think they all don’t do it?

Beverley - Well, that’s one of the great mysteries of evolution, isn’t it? I could really use wings some days. I think about flowers as all trying to stand out against a green background. They’re all shouting at the pollinators ‘look at me, I’m over here, come and pollinate me,’ and there are hundreds of thousands of different ways of doing that. You can do it by being blue, or being sparkly, or by being yellow, or by having patterns on you, by smelling nice, and I guess some of them have come up with the blue halo idea, but others have got other ideas too.

Georgia - Is it really that hard to make blue pigment that the plants have had to invent this other way of looking blue?

Beverley - Yes, it really is. It’s surprising isn’t it? If you think about cut flowers that you might buy for the house or garden flowers, it’s very hard to get a true blue. You come up with something like cornflower or Himalayan mountain poppy, they’re aren’t many species that do it. And that’s because the main pigment that plants use for colour is a molecule of anthocyanin, and anthocyanin comes in a sort of red, pink, purple range.

To make it look blue, you first of all have to add extra hydroxyl groups to the molecule and not many plants have the right enzyme to do that, and even then gives you a kind of purply colour. To make it properly blue, you need to put it at an alkaline ph, so you need to change the ph of the vacuole in the cell where the pigments sitting to get that true blue effect, and that’s really difficult to do. That involves pumping ions across the cell membrane and not many plants have come up with the ability to do it which, of course, is why you can’t blue blue roses in the garden centre.

Georgia - Of course, unless they’re ones that have been dyed.

Beverley - Absolutely.

Georgia - So what’s next; are you going to try and find out if it really does confer this advantage in the wild?

Beverley - We’d like to do that. But to do that I need a line of flowers that doesn’t make it, that’s otherwise identical to a line of flowers that does make it. I don’t want to compare two different species, one with and one without, because that’ll have lots of other confounding effects involved. So, we’re working on, at the moment, the really exciting question of how you develop this structure? How the flower manages to control the patterning of it’s surface cuticle in a way that you get these ridges, and you get this colour effect? And, of course, in the process of working that out we’re likely to generate mutants that don’t do it because we’re using a genetic approach. So once we’ve got a nice mutant that doesn’t do it then certainly, we’ll be out there in the field seeing what happens.