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.