The researchers found that flowers such as this bottlebrush hakea (Hakea francisiana) contain up to 10 milligrams of cyanide per gram
(Source: Professor Byron Lamont)
The finding challenges conventional thought that flowers evolved as a way for plants to attract birds and animals that help them cross-pollinate.
In the latest edition of New Phytologist, Professor Byron Lamont and his colleagues suggest the animals that may eat the plant also play a role in its floral development.
"Flowers are not just an evolutionary response to pollinators, but also to herbivores," says the director of the Centre for Ecosystem Diversity and Dynamics at Curtin University of Technology in Western Australia.
Spines and cyanide
Lamont says the team studied 51 species of Hakea, and found they could be easily divided into two groupings.
Insect-pollinated species have predominantly tiny, white flowers surrounded by spiky, dense foliage, which they suggest stops animals such as emus and cockatoos from eating the flower.
Bird-pollinated species instead have soft open leaves and bright, easily accessible, usually red, flowers with room for birds to land on stems.
This makes the plant vulnerable to being eaten by emus and cockatoos.
Lamont and colleagues travelled to the heathlands north and south of Perth to collect samples of Hakea.
They macerated the flowers on-site and then used an enzyme and a strip of paper that was sensitive to cyanide to test for its presence.
He says they found that plants with red flowers contain 10 milligrams of cyanide per gram, enough to make an animal sick.
Co-author Dr Mick Hanley, of the University of Plymouth, says animals that eat the red Hakea flowers may learn to associate the colour with the bitter taste produced by the cyanide.
"The colour red acts as a warning to large vertebrate herbivores like emus, parrots and kangaroos that the flower contains distasteful or even poisonous cyanogenic compounds," he says.
"It seems that Western Australian plants have not only developed a remarkable defence against would-be flower predators, but that they also clearly advertise the fact."
Lamont, who holds a personal chair in plant ecology, believes the poison came first in the plant's evolution.
"Bird pollination came after plants were already having to protect themselves against herbivores," he says.
Plants would have responded to the arrival of honey-eating birds about 50 million years ago to encourage the more-efficient pollinator.
"[But] by changing their morphology and shape they became more vulnerable to animals such as emus and cockatoos," he says.
Lamont accepts much of the paper's theories have not been truly tested.
"We haven't really shown the spiny leaves stop emus and cockatoos and we haven't really shown the cyanide makes the herbivores sick," he says.
Proving these points is the next step in the research.
This will include making moulds of animal heads, which when pushed into the foliage will detect whether the animal is prickled or deterred by the spikes.
Source: BBC News