Elemental defense of nickel hyperaccumulator seeds against a generalist insect granivore

Abstract

Hyperaccumulation has been proposed as an elemental defense against herbivores; however, few studies have examined seed defense. This study included two annual Streptanthus species (Brassicaceae) from California serpentine soils: a non-hyperaccumulator (S. insignis) and three populations (representing Y, P, and U morphs) of a Ni hyperaccumulator (S. polygaloides). Adults of the generalist seed herbivore Tribolium confusum (Coleoptera: Tenebrionidae) were fed either whole or cut seeds, survival was recorded for 7 weeks, and Ni concentrations of both beetles and seeds were determined using ICP-OES. Survival analysis showed significantly more rapid mortality for beetles consuming S. polygaloides seeds compared to those consuming S. insignis. Mortality of beetles fed whole S. polygaloides seeds was more rapid than those fed cut S. polygaloides seeds. Seeds of the S. polygaloides populations contained approximately 300 µg Ni g⁻¹ whereas S. insignis contained approximately 5 µg Ni g⁻¹. Beetles fed whole S. polygaloides seeds contained more than 2.5-fold greater Ni concentrations than those fed cut seeds (approximately 60 and 25 µg Ni g⁻¹, respectively), whereas beetles fed either cut or whole S. insignis seeds contained < 0.3 µg Ni g⁻¹. An artificial diet study, using Ni-amended cornmeal, confirmed that diet Ni concentrations greater than 240 µg Ni g⁻¹ were toxic to T. confusum. We conclude that Ni in S. polygaloides seeds can act as an elemental defense against seed herbivores even at 300 µg Ni g⁻¹, a level below the 1000 µg Ni g⁻¹ hyperaccumulation threshold concentration.