Making scents of flower power
29 August 2008
Scientists have sniffed out how flowers secrete smells to get the most from pollinators. They might seem fragrant to us, but it turns out that plants mix smelly toxins into their bouquets to repulse as well as attract animal assistants. Antenna picks up the scent...
Image: Danny Kessler
This research was published in the journal Science on 29 August 2008.
Flowers have evolved bright colours, fragrant smells and sweet nectar to entice animals to pollinate their blooms. But while flower colour has been well studied, until recently the importance of odour has wafted past researchers.
The sweet aroma of tobacco flowers draws in hawk moths for nighttime nectar.
Image: Danny Kessler
Scientists chose to nose around this aromatic area with the help of wild tobacco plants. The trumpet-like flowers of this notorious weed don't smell much to us, but the blooms are irresistible to hummingbirds and hawk moths, which fuel their flight with the plants' energy-rich nectar.
Manipulating a tobacco bloom.
Image: Danny Kessler
Using genetic modification, the scientists created two distinct tobacco plants: one unable to produce benzyl acetone, a jasmine-like fragrance that's believed to attract pollinators; and one unable to produce nicotine, a well-known toxin that acts as a deterrent. These chemicals are the two most pungent odours the tobacco plant produces.
Scientists grew the plants in Utah's Great Basin Desert to see what pollinators would make of the new blooms.
Image: Andre Kessler
Video monitoring revealed the flowers without benzyl acetone to be far less enticing to hawk moths and hummingbirds. The flowers were much less likely to be pollinated.
'But when we got rid of the nicotine the hummingbirds and the hawk moths just drank the flowers dry - you could literally see them gulping the nectar down.' Ian Baldwin, plant expert, Max Planck Institute, Germany
With pollinators hogging the nectar, fewer were able to visit. This had serious consequences - pollination was much less likely.
Flowers also suffered from petal-chomping pests. 'We saw carpenter bees rob the plant and moth larvae feeding inside. This has only ever been seen in the wild after we knocked out the repulsive nicotine toxin.' Ian Baldwin
Carpenter bees sneakily drink nectar from the base of the flower, avoiding a dusting of pollen.
Image: Danny Kessler
So why would tobacco plants want to deter pollinators?
'There are costs associated with advertising flowers. Some insects that are drawn to them are not well behaved - they drink too much nectar or steal it without pollinating them. It's therefore very important that plants can move them on and that's what the nicotine's for.' Ian Baldwin
Ian Baldwin, plant expert.
Image: Danny Kessler
Do other types of plant send mixed messages too?
'This study clearly shows that it's not good enough for a plant to simply be attractive to pollinators. You have to be able to show them the door as well! It seems reasonable to me to expect other plants to work like this - we just need to go out and look.' Robert Raguso, pollination expert
Robert Raguso, pollination expert, Cornell University, USA.
Image: Robert Raguso