A genetically-based latitudinal cline in the emission of herbivore-induced plant volatile organic compounds.

The existence of predictable latitudinal variation in plant defense against herbivores remains controversial. A prevailing view holds that higher levels of plant defense evolve at low latitudes compared to high latitudes as an adaptive plant response to higher herbivore pressure on low-latitude plants. To date, this prediction has not been examined with respect to volatile organic compounds (VOCs) that many plants emit, often thus attracting the natural enemies of herbivores. Here, we compared genetically-based constitutive and herbivore-induced aboveground vegetative VOC emissions from plants originating across a gradient of more than 10° of latitude (>1,500 km). We collected headspace VOCs from Asclepias syriaca (common milkweed) originating from 20 populations across its natural range and grown in a common garden near the range center. Feeding by specialist Danaus plexippus (monarch) larvae induced VOCs, and field environmental conditions (temperature, light, and humidity) also influenced emissions. Monarch damage increased plant VOC concentrations and altered VOC blends. We found that genetically-based induced VOC emissions varied with the latitude of plant population origin, although the pattern followed the reverse of that predicted-induced VOC concentration increased with increasing latitude. This pattern appeared to be driven by a greater induction of sesquiterpenoids at higher latitudes. In contrast, constitutive VOC emission did not vary systematically with latitude, and the induction of green leafy volatiles declined with latitude. Our results do not support the prevailing view that plant defense is greater at lower than at higher latitudes. That the pattern holds only for herbivore-induced VOC emission, and not constitutive emission, suggests that latitudinal variation in VOCs is not a simple adaptive response to climatic factors.