Plants are constantly conversing with the world around them;
however instead of words they use a prolific array of chemical signals. Perfumes and
the pungent flavors in spices are all chemical compounds that have evolved to attract
pollinators or ward of insects from eating them. In our research, we try to
decipher why plants produce different floral
scents and in particular we are
interested in how interactions with insects have shaped the different scents
produced by plants.
We are a travelling circus of field biologists comprised of
a postdoc (or two) and 2 to 3 research technicians. We live and work out of a
research truck and travel throughout the western US to collect data on the
floral scent and ecology of 16 species of evening primrose (family Onagraceae).
What brings us to the beautiful Tejon Ranch is the enigmatic Oenothera californica avita –the California evening primrose.
Our days at Tejon start at dusk, when the sunlight is almost
gone. This is when buds of O. californica
avita begin to open to reveal sweet scented, pale-white flowers. In a
frenzy, we run from plant to plant to identify all buds which are likely to split.
Before the flowers have opened and released their scent we enclose them within
a plastic oven bags. We then attach a vacuum pump that will pull the scent away
from the flower and into a filter that will trap the volatile scent compounds.
Photo credit: K. Skogen
Many floral scents have primarily evolved to attract pollinators. Scent is frequently coupled with sugary nectar
so that pollinators associated the scent with a reward. The plant is rewarded in
return by having its pollen dispersed to other mates– a mutualism that
facilitates sexual reproduction in over 90% of all flowering plants.
So who are these visitors attracted to the scent of these evening
primrose flowers? At dusk, after the scent apparatus is setup and turned on, we
each find a patch of flowers to sit down next to and watch who visits. Looking at the flowers’ long floral tube and
pale white petals, we suspect that the most likely visitors hawkmoths. Hawkmoths are large moths in
the family Sphingidae that hover like hummingbirds and have long tongues
adapted for collecting nectar from long-tubed flowers. Hawkmoths are large moths that make an
attractive food source for birds, hence they like to fly in the relative safety
of dusk. For this reason many hawkmoth
pollinated plants will typically open in the evening when moths are most active,
hence the reason for the “evening” in the name evening primrose.
As we sit and wait, as if perfectly
orchestrated, within five minutes of the flowers opening, we see 5 or more
hawkmoths fly in and systematically visit every single flower in our view.
Photo credit: K. Skogen
After watching hawkmoths for an hour, we go back to our
flowers to collect the filters and flowers and head back to camp. Late into the
night, we elute the volatile compounds from the filters into little vials that
will be shipped to Cornell University for analysis. This collection method
allows us to examine each individual component that makes up the complex scent
profile of the evening primrose scent. Using this information we can see how
scent composition varies by species, by population and even within plants of
one population.
While we predominantly think of floral scent for its role as
a pollinator attractant, the signal can be picked up by others. Much like a birdsong can attract mates and predators,
floral scent that is meant to invite pollinators can also signal to potential herbivores
that a tasty plant is close. We think
that scent might vary based not only on which pollinators are in the area, but
also whether or not herbivores are
present to eavesdrop on the flower’s invitation to pollinators.
In the Colorado evening primrose, Oenothera harringtonii, a few of
populations produce scent without a compound called linalool. Remarkably, the presence of linalool was found to be correlated
with the presence of a tiny caterpillar in the genus Mompha that was eating
the seeds. These caterpillars feed
inside developing fruit and reduce the reproductive output of these plants.
Thus populations historically associated with high numbers of these moths might
be under selection to reduce their
advertisement to pollinators to reduce herbivory.
However Mompha
isn’t the only problem faced by these evening
primroses plants. An interesting dilemma for these plants is that one of its
best pollinators, a hawkmoth called Hyles lineata, also uses these
plants to raise its young. The Hyles
lineata caterpillars can chomp on the buds, flowers and fruits leaving
behind a significant trail of damage!
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Photo credit: R. Overson
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The morning after scent collection
we take an inventory of all herbivores on O.
californica avita. We look for Hyles
lineata eggs and caterpillars and check
buds and fruit for Mompha. This year
we found no herbivores on these plants. Is this population truly free from
herbivory? Or could 2015 have been a bad year for Mompha and Hyles
caterpillars in Tejon? Our methods only give us a snapshot into the ecology of O. californica avita at Tejon and we can
only speculate on the absence of herbivory until we learn more.
After two nights of data
collection, we head out on our next adventure – to find a new population, in a
new landscape, to get a glimpse into the ecology and evolution of another
evening primrose!
ABOUT THE AUTHOR
I am a biologist interested in the
ecology and evolution of chemically-mediated
plant-insect interactions. I am currently a post-doctoral
researcher at the
Chicago Botanic Garden in Glencoe,
IL working with
Krissa
Skogen and
Jeremie
Fant on floral scent-mediated diversification of plants in the evening
primrose family. I graduated with a PhD in entomology in Dr.
May Berenbaum's lab where I worked on the ecology
and evolution of wild parsnips and parsnip webworms in New Zealand.
