Extending the elemental defense hypothesis: Dietary metal concentrations below hyperaccumulator levels could harm herbivores

Previous work has shown that hyperaccumulator levels of some metals can defend plants against herbivores, but the possibility of defense by metal concentrations at accumulator or normal levels is unexplored. This study tested the hypothesis that metals can defend plants at low concentrations. We determined the relative toxicities of eight metals commonly acquired by plants: Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn. Larvae of the diamondback moth (Plutella xylostella), a representative crucifer specialist, were fed with artificial diet amended with concentrations of metal varying from 2 to 3,000 mu g/g. Different concentration ranges were used for each of the eight metals, and larval survival at 10-14 days was calculated for each concentration. All metals were toxic to diamondback moth larvae at hyperaccumulator levels. All metals, however, were also toxic to larvae at accumulator concentrations, far below those found in hyperaccumulating plants. Five metals (Cd, Mn, Ni, Pb, and Zn) were toxic below accumulator levels, Cd and Pb were toxic near the concentration ranges of normal plants, and Zn was toxic at a concentration within the normal range. Our results indicate that uptake of certain metals may provide a defensive benefit for plants, and that elemental defenses may be effective at concentrations far lower than previously hypothesized. This study implies that elemental defenses are more widespread in plants than previously believed, and that the ecological consequences of even low levels of metal accumulation need to be explored.