Does a petunia of any other name smell as sweet? Unraveling the complexity of floral scents
Amy L. Parachnowitsch
Plant Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden 75236. email: firstname.lastname@example.org
As the perfume industry and anyone who has ever dabbed on scent before a date are keenly aware, smells can help attract mates. Flowers use such volatile blends, which we, in turn, often use in our perfumes, to attract the pollinators that do their mating for them. However, much like other floral characters (Strauss and Whittall 2006), there is a growing appreciation that agents other than pollinators may drive the function and evolution of floral scents.
In Ecology Letters, Kessler et al. (2012) examine the possibility that some components of volatile blends might function to deter herbivores, rather than attract pollinators. The problem many plants may face with advertising floral rewards to pollinators with scent or really any showy trait, is that plants might inadvertently attract herbivores (e.g. Theis and Adler 2011). Thus, floral traits may represent a compromise between attracting mutualistic and antagonistic visitors. The conflict in scent production might be particularly important for night flowering plants that cannot rely on visual cues to attract pollinators. Kessler et al’s study builds on previous work that has shown volatiles can function to reduce herbivory (e.g. Kessler et al. 2008, Junker and Blüthgen 2010, Galen et al. 2011). Here, the authors take advantage of genetically transformed night flowering petunia (Petunia x hybrida) that were gene-silenced at different points in the shikimate and phenylpropaniod scent pathways. Using five different lines, they were able expose plants with different floral scent blends to herbivory and compare their performance. What makes their approach particularly powerful is that the genetically transformed plants were exposed to field conditions to assess the ecological impacts of the blends. Kessler et al. focused on florivore interactions because florivores are particularly likely to be sensitive to floral scents (florivores eat flowers), and are likely to exert selection on floral traits due to their more direct impact on fitness than leaf herbivores (McCall and Irwin 2006).
The authors found three petunia floral scents that were important in plant-florivore interactions. Methyl benzoate, an important attractant of petunia pollinators, was a host location cue for generalist florivores. Therefore, methyl benzoate production is likely under conflicting selection by mutualists and antagonists – a hypothesis that could be tested in wild populations of native petunias. Unfortunately, this study did not examine pollinator behaviour, so it is unknown whether pollinators would also avoid lines with reduced methyl benzoate. For two other compounds, isoeugenol and benzyl benzoate, increased emission acted as feeding deterrents for generalist florivores. In an elegant manipulation, the authors paired field and feeding trial results with a volatile-addition experiment. Adding the feeding deterrents to the volatile headspace of lines that lacked these compounds rescued the deterrent effect on florivores, confirming the function of these volatiles. Again, pollinator response is unknown. However, this set of experiments was able to definitively determine the functional role of these three compounds in generalist florivore interactions. These plants are not native to the habitat where they were tested, it is thus possible that interactions in the native range and for native plants could differ from what was observed here.
Nonetheless, these results suggest that complex floral volatile blends are likely the result of selection by multiple agents and function to both attract pollinators and repel herbivores.
It is an exciting time to be studying floral scents. As analyses of these traits becomes more accessible, we are gaining a more holistic understanding of floral scents and their role in plant-insect interactions (e.g. Schiestl 2010). A complete picture of the role of scent in floral diversity will come from studying floral phenotypes in an integrated fashion as well as studying the multiple agents that exert selection on floral traits. Combining mechanistic studies such as the example discussed here with studies of naturally occurring plant phenotypes (e.g. Galen et al. 2011, Parachnowitsch et al. 2012) will strengthen our understanding of function and evolution of scent in flowers.
Galen, C., R. Kaczorowski, S. L. Todd, J. Geib, and R. A. Raguso. 2011. Dosage dependent impacts of a floral volatile compound on pollinators, larcenists, and the potential for floral evolution in the alpine skypilot Polemonium viscosum. American Naturalist 177:258-272.
Junker, R. R. and N. Blüthgen. 2010. Floral scents repel facultative flower visitors, but attract obligate ones. Annals of Botany 105:777-782.
Kessler, D., C. Diezel, D. G. Clark, T. A. Colquhoun, and I. T. Baldwin. 2012. Petunia flowers solve the defence/apparency dilemma of pollinator attraction by deploying complex floral blends. Ecology Letters.
Kessler, D., K. Gase, and I. T. Baldwin. 2008. Field experiments with transformed plants reveal the sense of floral scents. Science 321:1200-1202.
McCall, A. C. and R. E. Irwin. 2006. Florivory: The intersection of pollination and herbivory. Ecology Letters 9:1351-1365.
Parachnowitsch, A. L., R. A. Raguso, and A. Kessler. 2012. Phenotypic selection to increase floral scent emission, but not flower size or colour in bee-pollinated Penstemon digitalis. New Phytologist 195:667-675.
Schiestl, F. P. 2010. The evolution of floral scent and insect chemical communication. Ecology Letters 13:643-656.
Strauss, S. Y. and J. B. Whittall. 2006. Non-pollinator agents of selection on floral traits. In: LD Harder and SC H. Barrett, editors. Ecology and Evolution of Flowers. Oxford University Press, Oxford, UK. pp. 120–138.
Theis, N. and L. S. Adler. 2011. Advertising to the enemy: enhanced floral fragrance increases beetle attraction and reduces plant reproduction. Ecology 93:430-435.