Broad diversity in monoterpene-sesquiterpene balance across wild sunflowers: implications of leaf and floral volatiles for biotic interactions

• Premise of the study: As plant lineages diversify across environmental gradients, species are predicted to encounter divergent biotic pressures. This study investigates the evolution of volatile secondary metabolism across Helianthus genus.  
• Methods: Leaves and petals of 40 species of wild Helianthus were analyzed via gas chromatography-mass-spectrometry to describe volatile secondary metabolite profiles.
• Key results: Across all species, 500 compounds were identified including 40% sesquiterpenes, 18% monoterpenes, 3% diterpenes, 4% fatty acid derivatives, and 35% other compounds such as phenolics and small organic compounds. Qualitatively, annuals and species from more arid western climates had leaf compositions with a higher proportion of total monoterpenes, while erect perennials and species from more mesic eastern habitats contained a higher proportion of total sesquiterpenes. Among species, mass-based leaf monoterpene and sesquiterpene abundance were identified as largely orthogonal axes of variation by principal components analysis. Leaf and petal profiles were not strongly correlated.
• Conclusions: Wide diversity in volatile metabolism was observed among wild Helianthus, indicating value of this genus as a model system, and a rich genetic resource. The independence of leaf and petal volatile profiles indicates a low level of phenotypic integration between vegetative and reproductive structures, implying vegetative defense and reproductive defense or pollinator attraction functions mediated by terpenoid profiles in these two organs can evolve without major tradeoffs. The major biosynthetic pathways manufacturing major terpenoids identified in wild Helianthus have been well described to date, providing a road map to deeper inquiry into the drivers of this diversity.