Plants make scents: Latitudinal and intraspecific variation in plant volatile organic chemical emission

Background/Question/Methods

Plant volatile organic chemicals (VOCs) are broadly important in ecological interactions and in biosphere-atmosphere dynamics. For VOC emission, a rigorous empirical quantification of intraspecific variation among plant populations has not been undertaken; moreover, additional work should address the integration of this indirect defense into broader theories of plant defense. Our experiment addressed both issues by gauging among-population intraspecific variation for VOC emission and testing whether general patterns for latitudinal variation in plant defense hold true for VOC emission. We investigated latitudinal variation in emission by measuring constitutive and herbivore-induced VOC emission from Asclepias syriaca (common milkweed) plants grown in a common garden. A body of evidence indicates that plant defenses tend to be stronger at low latitudes; we therefore hypothesized that emitted VOC concentration and magnitude of volatile induction would be greater at low latitudes. Seeds were collected from 22 A. syriaca populations (about 3 genotypes per population) across a latitudinal range of 11^o in eastern North America; plants were grown in the common garden for two years. We collected aboveground VOCs on paired control and treatment plants (N=166 plants). Samples were analyzed using a gas chromatograph-mass spectrometer (GCMS); chemicals within VOC blends were quantified against an internal standard.

Results/Conclusions

Across latitude, monarch damage caused total average VOC concentration emitted by plants to increase by more than 100%; herbivory also induced changes in VOC blend. The emitted VOC blend varied among plant populations; however, total VOC concentration per g plant biomass neither differed among populations nor displayed any latitudinal pattern. Given that A. syriaca has a broad geographic distribution across which plant defenses are predicted to vary, it is surprising that the populations were different only with regard to VOC blend. Available evidence suggests that plants at low latitudes tend to be better defended and less palatable to herbivores; the absence of a latitudinal pattern in VOC emission also is an unexpected result. We conclude that A. syriaca responds to herbivore damage in part by altering its VOC profile; however, geographic variation in VOC emission by A. syriaca should be explored further.