'Perfumed' parasitic plant lures in pollinating mammals

Emma Brennand
Earth News reporter

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Elephant shrews have a nose for it

The unique "perfume" of a rare parasitic plant attracts small mammals, a new study has revealed.

The plant, Cytinus visseri, has specialised flowers adapted for pollen dispersal by ground dwelling-mammals.

But the plant has to lure in these mammal-pollinators and it uses a unique combination of chemical scents to do so.

This study is the first to identify the plant's floral aroma and its effect on foraging mammals.

Professor Steven Johnson and his research team from the University of KwaZulu-Natal, South Africa, have been studying the Cytinaceae parasitic family of plants for six years.

The flowers emit a strong, unusual plastic-like odor Professor Steven Johnson, University of KwaZulu-Natal, South Africa

This family of South African plants do not produce chlorophyll; instead they obtain it by burying themselves into a host scrub plant, usually of the Asteraceae family.

Only the dark maroon flowers and the resulting fruit of C. visseri protrude from the dense canopy of the host scrub.

In collaboration with colleagues from the South African National Biodiversity Institute, South Africa, the University of Calgary, Canada and the University of Bayreuth, Germany, Professor Johnson set about capturing and identifying the chemical scents emitted by C. visseri.

Over five years, the researchers mapped the populations of male and female plants in the largest known C. visseri population situated in the Long Tom Pass in Mpumalanga Province, South Africa. Worldwide only a few hundred plants of this species are known.

Volatile molecules emitted from the plant's flowers and nectar were collected by the team and identified using gas chromatography mass spectrometry (GC-MS).

"The flowers of C. visseri emit a strong, unusual plastic-like odour," explains Professor Johnson.

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"The scent comprised over 30 compounds, especially ketones, fatty-acid derivatives, mono- and sesquiterpenoids."

The three most abundant scent chemicals were: 1-hexen-3-one in flower and nectar samples, 3-hexanone in flower samples and ethyl butyrate in male flowers.

To test the impact of the chemical scent on the movement of striped field mice, the team scented one leg of a y-shaped maze.

When the mice entered the maze they were drawn to the scented pathway that contained 3-hexanone, either on its own or when mixed with 1-hexe-3-one.

However, when the researchers tested 1-hexen-3-one alone the mice were repelled, consistently opting for the scent-free route.

Professor Johnson suggests that this shows that just one compound, 3-hexanone, is the main attractant, whilst the second one may be a product of the plant's metabolic pathway or serve a function that is yet to be identified.

"It turns out that 3-hexanone has also been found in some bat-pollinated flowers, so it may be a general mammal attractant," Professor Johnson told BBC News.

"Perhaps because it signifies rich food sources or plays a role in communication among mammals."

Animal attraction

Like mice, humans find 3-hexanone to have a pleasant smell. The molecule is routinely used in artificial flavouring to produce a sweet fruity grape-like flavour.

In contrast, flowers which aim to attract bats tend to be characterised by sulphur compounds, which typically smell of rotten eggs to humans.

Scent cues are particularly important to plants pollinated by small ground-dwelling mammals as they are usually visited at night when visual cues are less effective.