Amor-phew-phallus

Foul and fragrant

Pushing our noses into the depths of a rose flower and breathing in deeply has got to be one of life’s pleasures, but not all flower scents are quite as enjoyable. Floral scents are produced to attract pollinators, and while many insects are attracted to the sweet floral scents we are familiar with in our gardens, others are attracted to more unpleasant smells. Some insects are attracted to flesh or dung on which to feed or breed, and these can be tricked into being pollinators by flowers that mimic these unpleasant odours.

Perhaps the most renowned plants for producing foul odours are members of the genus Amorphophallus. There are more than 200 species, occurring throughout Africa and Asia, but the most famous is Amorphophallus titanum (titan arum or corpse flower). When the titan arum flowered at Kew in 1996 for the first time in decades, crowds flocked to the Gardens to experience the sight and smell of this giant bloom. It is now a regular bloomer in the Princess of Wales Conservatory.

Periodic pongers

The rotting smell from the titan arum is mainly produced on a single evening, with just a faint whiff the following evening. Therefore inquisitive titan arum sniffers who visit the plant only during the day may be disappointed.

Many species of Amorphophallus behave in the same way, and trying to predict the evening on which they will perform has complicated attempts to analyse their odours. Our research into the chemical composition of 92 species of Amorphophallus has taken us over 20 years to complete, including countless evenings 'babysitting' glasshouse plants waiting for a pong that often never materialised on the expected evening. The results were recently published in the scientific journal Phytochemistry.

Size does not matter

When it comes to stench capability, the size of an Amorphophallus inflorescence doesn’t always matter. Some species with smaller inflorescences easily out stink the titan arum. Amorphophallus bulbifer, for example, produces a gaseous sulphurous odour that is nauseating. Once sniffed, the smell will unfortunately stay with you for hours.

I smell a (dead) rat

In some species this ‘gaseous’ stench is modified to smell more like rotting meat or other decaying matter. These species tend to have dark inflorescences, making them actually look like decaying flesh – another way they attract insect pollinators. Dimethyl disulphide and dimethyl trisulphide were often found to be the main components of these foul odours.

Fruity, cheesy and fishy...

Once we started to sniff around a number of Amorphophallus species, what struck us was not so much the pugnacious odours but the enormous variety of smells. Not all were foul. Two groups of species had fruity and anise odours, several species had cheesy odours and some had a banana-like quality due to isoamyl acetate ('banana oil'). Trimethylamine was a prominent component of species that had a smell of fried fish, while the odour of A. sylvaticus was almost pure 2,6-nonadien-1-ol, a compound with a cucumber odour.

A couple of species smelt of freshly grated carrots while A. manta smelt of cocoa/chocolate. A few species even had the sweet fragrance of a typical flower.

Pollinator relationships

Comparing odours, either with how species are related to one another or their distribution, revealed some interesting findings. Some groups of species producing similar odours were closely related. However, we also found examples of groups of species occurring in particular regions having similar odours and these were not always closely related, and an instance of two closely related species with overlapping distributions having very different odours. The pressure of attracting pollinators has clearly influenced odour evolution, and in different ways.

Investigating the pollinator relationships thrown up by this study would probably take another 20 years to complete. Identifying the true pollinators, as opposed to casual visitors, would require careful field observations, with much time being spent at close quarters to these malodourous plants. So maybe that is for others to research! It is time for us to have a breath of fresh air.

References

Geoffrey Kite and Wilbert Hetterscheid published their work earlier in 2017 in the journal Phytochemistry:

Kite, G.C. & Hetterscheid, W.L.A. (2017). Phylogenetic trends in the evolution of odours in Amorphophallus.

Phytochemistry 142: 126–142. Available online

Further reading

Kite, G.C. & Hetterscheid, W.L.A. (1997). Inflorescence odours in Amorphophallus and Pseudodracontium (Araceae). Phytochemistry 46: 71‒75. Available online

Kite, G.C., Hetterscheid, W.L.A., Lewis, M., Boyce, P.C., Ollerton, J., Cocklin, E., Diaz, A. & Simmonds, M.S.J. (1998). Inflorescence odours and pollinators of Arum and Amorphophallus (Araceae). In Owens, S. & Rudall. P. eds. Reproductive Biology in Systematics, Conservation and Economic Botany. Royal Botanic Gardens, Kew pp. 295‒315.