Effects of Intraspecific Density on Garlic Mustard (Alliaria Petiolata) Sinigrin Concentration

The production of secondary defense chemicals in plants represents a trade-off between defense and the primary functions of growth and reproduction, but the relative allocation to growth versus defense varies across species, types of defenses, ontogeny, and environment. Alliaria petiolata (garlic mustard) is a brassica that produces glucosinolates, a class of constituent secondary metabolites that defend against herbivores and pathogens. Sinigrin, a hydrolyzed product of glucosinolate present in garlic mustard, may aid in its success as an invasive species by disrupting native plant–mycorrhizae mutualisms and decreasing forest species diversity in North America. Here, we measured sinigrin concentration in garlic mustard populations of different field densities and in greenhouse experiments to evaluate the relationship between sinigrin concentration and growth in response to density and varying environmental conditions. We found clear evidence for growth vs. defense tradeoffs in both experimental and field settings, as well as higher levels of defense in more densely growing, smaller individual plants. However, sinigrin levels and tradeoffs were not explained by soil fertility or light, allowing us to conclude that sinigrin expression is not controlled by limitations in the measured abiotic factors. Our findings suggest sinigrin leaf concentration increases at high densities despite the pressures of intraspecific competition that demand allocation to growth.