A test of elemental defence against slugs by Ni in hyperaccumulator and non-hyperaccumulator Streptanthus species

Tissues of most plant species contain < 10 mu g Ni g-1 but Ni hyperaccumulators contain more than 1000 mu g Ni g-1 . Hyperaccumulated Ni can defend plants from some herbivores but the defensive role of lesser Ni concentrations is little explored. We raised five species of Streptanthus (Brassicaceae) native to ultramafic soils, one of which (S. polygaloides) is a Ni hyperaccumulator whereas the others are simply Ni-tolerant, on Ni-amended and unamended green-house soils to create plants differing in Ni concentrations. On high-Ni soil, leaves of the hyperaccumulator contained 3800 mu g Ni g-1 whereas leaves of non-hyperaccumulator species contained 41-64 mu g Ni g-1. Plants of all species grown on low-Ni soils had < 14 mu g Ni g-1. Slugs (Limax maximus) were fed plant material in no-choice tests over a 50-day period and survival and mass changes were recorded. All slugs fed high-Ni leaves of the hyperaccumulator species died within 21 d. Slugs fed high-Ni leaves of the other species did not differ significantly in survival or mass change from those fed low-Ni leaves. In choice tests, slugs (Lehmannia valentiana) offered both high- and low-Ni S. polygaloides leaves did little damage to high-Ni leaves. We conclude that hyperacumulated Ni can defend S. polygaloides from slug herbivory via both toxicity and deterrence, but these defensive effects do not extend to Streptanthus species containing < 70 mu g Ni g-1.