Hyperaccumulation of nickel by Streptanthus polygaloides (Brassicaceae): Implications for elemental defense

Plants that accumulate high concentrations of elements into their tissues have been termed hyperaccumulators. Metal hyperaccumulators contain >100 mug g-1 of Cd, >1,000 mug g-1 of Co, Cu, Cr, Ni, or Pb, or >10,000 mug g-1 of Mn or Zn in their tissues. Several functions of hyperaccumulation have been suggested, but most studies have demonstrated that hyperaccumulation provides resistance against herbivores. These experiments explored the elemental defense hypothesis. I first examined the oviposition choice of a crucifer specialist ( Plutella xylostella) using caging experiments. Female moths were presented a choice of the Ni hyperaccumulating plant, Streptanthus polygaloides, grown on high-Ni soil or grown on low-N soil. I also used P. xylostella larvae to determine the toxicities of Cd, Ni, Pb and Zn, relative to the hyperaccumulation thresholds for each metal. Artificial diet treatments amended with low concentrations of metals were fed to P. xylostella larvae alone and in combination with other metals. I also explored the possible additive effect of low concentrations of Ni and organic plant defense chemicals. Finally, I investigated the defensive potential of hyperaccumulated Ni by S. polygaloides on arthropod herbivores representing various feeding modes by using a series of feeding experiments. Arthropods representing the following feeding modes were used: folivore, rhizovore, vascular tissue feeder (xylem or phloem), and cell-disruptor. My results demonstrate that hyperaccumulated Ni can defend S. polygaloides against a crucifer specialist. More eggs were laid on low-Ni plants and more feeding damage and fewer flowers were found for low-Ni plants. Low concentrations of metals (below the hyperaccumulation threshold) can negatively affect P. xylostella larval survival. When two metals are combined at low concentrations, survival of P. xylostella larvae was significantly decreased compared to each metal alone. Low concentrations of Ni combined with organic defense chemicals also resulted in significantly reduced survival of larvae compared to either Ni or organic chemical treatments alone. My results indicate that the potential defensive benefit of hyperaccumulated Ni by S. polygaloides depends on herbivore feeding mode. Hyperaccumulated Ni negatively affected survival of the folivores, rhizovore, and reduced population growth rate of one cell-disruptor. However, xylem- and phloem-feeding herbivores and one cell-disruptor were unaffected. My results will guide future studies of elemental plant defenses and will be useful in examining possible mechanisms for the evolution of hyperaccumulation.