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Friday, 15 February 2013

The chemical allure of plants


We have all been drawn in by that scent carried on a spring breeze – something sweet or fruity, maybe even spicy or with a hint of citrus. If we’re lucky enough, we might even find the source and bury our noses among the petals in order to fill our head with the aroma. We, and ancient cultures before us, have been besotted by the chemical allure of plants.

Recently, I wrote about the ‘Scent of winter’ in the Botanic Garden and how winter blooming plants tend to be incredibly fragrant in order to attract pollinators at this time of year. I immediately wanted to dive into the science behind floral fragrances, but quickly learned that this was a discussion all on its own...perhaps even a tome.  

The scents associated with plants are produced by a mixture of chemical compounds known as volatile organic compounds (VOCs). They are described as volatile because they have a low boiling point, which means they are gases at room-temperature. It is these VOCs that are extracted as essential oils for aromatherapy or medicinal purposes, and that are carried on the wind in order to attract pollinators from long distances.

No two scents are the same


Due to the incredible variety of compounds and small differences in the relative abundances of these compounds, no two scents are exactly the same. In fact, insects are able to use these individualised scents to discriminate between individual flowers of a single species. This can be very important for pollinators as they can distinguish flowers that might offer a greater reward by having more nectar than others.

Scent is particularly important for plants that are pollinated by night-feeding animals like moths or bats, and it is also clearly important in winter when there are fewer pollinators about. Generally speaking, sweet smelling flowers tend to attract bees and flies, while strong musty, spicy or fruity odours attract beetles.


Specialised chemical cocktails


However, the chemical allure of plants is far more complex than just a sweet smell wafting on the breeze. Some species have become so incredibly specialised with their chemical cocktails, that they mimic the natural chemical signals of animals – pheromones - in order to attract pollinators.

One of the most successful groups of mimics is the orchids and within this group, perhaps the best known for their trickery are the bee orchids (Orphys). Not only have the flowers evolved petals that look like a female bee or wasp resting on the flower, the petals also give off an enticing scent that mimics the female’s pheromones. The male bees are lured in by the smell and “mate” with their deceptive petals, with nothing more to show for it than a dusting of pollen. The male will then carry the pollen to the next flower that tricks him.

Another orchid, Dendrobium sinense has a scent that mimics the honey bee alarm pheromone, which attracts hunting hornets as pollinators.


It’s not always about attracting pollinators...


Of course, the scent given off by plants isn’t always about attracting pollinators. Some plants use scent to attract predators and parasitoids of herbivorous insects. When the leaves of these plants are under attack by the herbivores, they will produce more VOCs, which not only bring in the predatory recruits, they also signal neighbouring plants to take action. For example, Lima beans (Phaseolus lunatus) will begin to secrete extrafloral nectar if a neighbouring Lima plant is under attack. This amplifies the call for help and attracts even more natural predators into the battle zone.


...and it’s not always a nice smell


I spent nearly 20 years on the west coast of Canada, the home of Lysichiton americanus. Anyone who has encountered the fragrance of this plant while walking in the wet coastal forests, knows where it gets its common name, skunk cabbage. The distinctive odour might be offensive to humans but for scavenging flies and beetles, which pollinate the plant, it signals the smell of something rotting and definitely worth investigation.

So, while I don’t recommend burying your nose in the flower of a skunk cabbage, it is definitely worth taking a moment to stop and smell the roses...just be sure a bee isn’t in there!

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