Population declines of understory plants in temperate forests are linked to invasion of the herbaceous biennial, garlic mustard (Alliaria petiolata). Garlic mustard produces mycorrhizal toxins that disrupt belowground symbiotic interactions of native plants. Carbon-based phenolics known as flavonoids are implicated in this allelochemical response, but the phenological regulation of flavonoid production is unclear.  Since phenolic levels commonly vary with tissue and developmental status, we were interested in determining which part of garlic mustard’s life cycle, the juvenile rosettes or the reproductive adults, produced the bulk of these soil toxins. In addition, we wished to determine whether chemicals were stored in roots or shoots prior to secretion into the soil. Plants from both phases of the life cycle were collected from invaded forests in Virginia, separated into roots and shoots, and analyzed for flavonoids using LC-MS.  

Results/Conclusions

Flavonoids were three times more concentrated in juveniles compared to reproductive adults. Since juveniles often grow in high density clusters as half-sibs, with little migration away from their parents, this suggests that flavonoid concentrations are likely elevated in understory soils invaded by juveniles.  When comparing roots and shoots, flavonoids were more concentrated in shoots, an unexpected finding given their roles as soil allelochemicals.  It is possible that garlic mustard flavonoids are synthesized in leaves and transported to roots prior to secretion into the soil. Constraints on garlic mustard allelochemical production will be discussed in reference to genetic regulation of flavonoid metabolism.