Plant Records: The plant that coordinates its colours for the occasion

It's thought that flowers have colours to attract visitors, but it seems that's not always the case.

Name: Daisies
Scientific name: Name: Asteraceae
Known for: Loving you, or loving you not.
Record broken: Best dressed plant

Why are flowers so colourful? To attract pollinators, but it’s not just pollinators that are looking for flowers. Jurene Kemp and Allan Ellis studied the daisies of Namaqualand, South Africa. These flowers bloom en masse for a few days during the Spring. When they do, they’re a target for a voracious hunter, the tortoise. Kemp and Ellis decided to see if daisies had adaptations to hide from a predator that they couldn’t outrun.

Namaqualand is a region of southern Africa, taking in a lot of the west of South Africa and reaching into Namibia. It’s a semi-desert, but in the winter it gets rain, leading to a mass bloom of daisies. For a brief moment, the plants have a small window of opportunity to build flowers, exchange pollen and then seed so the species can survive for another year.

“Most of these Asteraceae species are self‐incompatible and reliant on successful seed set (dependent on pollination and florivory rates) in order to persist across years,” write Kemp and Ellis. “Various studies have shown that the visual phenotype of upper petal surfaces of Asteraceae species in this region is under selection from pollinatorsmainly bee flies, and flower colour in these daisies is not phylogenetically conserved. Pollinators are not active when it is cold, and activity usually occurs between 10 a.m. and 4 p.m. Individual daisy capitula last for multiple days, often up to a week (Kemp, unpublished data), which increases the probability of individual flowerheads being eaten before seedset.”

The armoured killer. Photo: Jurene Kemp.

And this is what happens. The mass bloom is also an all-you-can-eat buffet for herbivores. Ungulates and reptiles such as tortoises rush to grab what they can.

The daisies need to protect the flowers. They could pump them full of poisons to deter herbivores, but that’s not a clever idea. “”When plants defend their flowers chemically, the pollination interactions can be negatively influenced. Our study shows a novel way in which flowers can avoid herbivores, without compromising pollination interactions,” said Dr. Jurene Kemp, lead author of the study in a press statement.

Closed and open Arcotheca calendula. Photo: Jurene Kemp.

“These flowers can potentially circumvent the conflict of attracting both pollinators and herbivores by producing attractive colours on the surfaces that are exposed to pollinators (when flowers are open) and cryptic colours that are exposed when herbivores are active (when flowers are closed).”

Kemp and Ellis tested a few ideas to see if this was what was happening. For a start, closing the flowers should make the flowerheads less visible than the non-closing flowers. You’d also expect quite a contrast between the upper and lower petal surfaces. This should ultimately mean that closed flowers are less visible than open flowers.

The scientists examined the spectrum of light reflected by the flower petals on both sides to see how different they were and if closing flowers had a greater colour change than the always open flowers. They also presented opened and closed flowers to tortoises, Chersina angulata, to see what they preferred.

Sure enough, the botanists found that open flowers were indeed attractive to tortoises. “Our experiments with herbivores confirm this, clearly showing that exposure of conspicuous upper petal surfaces results in detection and targeted consumption of flowers, while exposure of the lower petal surface results in flowers being undetectable by tortoise herbivores, reducing flower herbivory,” write Kemp and Ellis in their paper. When the flowers closed, they were effectively camouflaged from their enemies.

This result leaves a puzzle. If some daisies can do this, why don’t all daisies do this? Why should some choose to remain open? It’s a mystery and one fuelled by the observation that the tortoises avoid eating the continuously open flowers. “One interesting question would be to test whether non-cryptic flowers have chemical defences, and whether these chemical defences are absent in the cryptic flowers,” said Dr Kemp.