Nickel hyperaccumulation by Streptanthus polygaloides is associated with herbivory tolerance

Metal hyperaccumulation can increase plant resistance to herbivory, but tolerance as an herbivore defense has been little investigated. This study explored the interaction between Ni hyperaccumulation and herbivory tolerance using Streptanthus polygaloides. Plants were grown in one of two potting soil Ni treatments: Ni-amended (800 µg g⁻¹ added Ni) or unamended (0 µg g⁻¹ added Ni). One-month-old plants were arbitrarily assigned one of four levels of artificial herbivory damage applied to the leaves. Response variables included aboveground dry biomass and Ni concentration, total leaf number, final plant height, and total number of flowers, and were analyzed by two-way ANOVA. We found no consistent soil Ni effect, artificial herbivory effect, or interaction between soil Ni and artificial herbivory treatments for total leaf number or final plant height. However, there was a soil Ni effect for biomass, flower production, and Ni concentrations: all were greater for plants grown in Ni-amended soil when compared to plants grown in unamended soil. There was also a significant interaction between treatments for number of flowers produced (reflecting plant fitness) as well as for aboveground biomass. Plants of S. polygaloides receiving greater damage produced significantly more flowers, and had greater biomass, when grown in Ni-amended soil than plants in unamended soil. We conclude that Ni hyperaccumulation is associated with herbivory tolerance of S. polygaloides, increasing plant fitness when herbivory damage is severe. Herbivory tolerance provides an additional dimension to elemental defenses that can supplement the defense trait of herbivory resistance.