Nickelophilic root foraging by the nickel hyperaccumulator, Streptanthus polygaloides subsp. undulatus (Brassicaceae)

Root foraging may allow hyperaccumulator plants to enhance element accumulation. This study compared root proliferation of two annual serpentine endemics: Streptanthus polygaloides (Ni hyperaccumulator) and Streptanthus insignis (nonhyperaccumulator). In a greenhouse experiment, pots were divided by a sealed partition, Ni-amended soil (800 mg kg ⁻¹) in one half, unamended soil in the other. Seeds were germinated over the partition, allowing roots to explore both soils. After 5 months, roots from each side of each pot were harvested, washed, dried, and weighed. Streptanthus polygaloides root biomass was significantly (twofold) greater in Ni-amended soil whereas S. insignis root biomass was similar in the two soils. In a lab experiment, seedlings were grown in vertical agar-filled petri dishes to determine if Ni affected seedling root growth. Seedlings were placed on either side of a central filter paper strip soaked in either NiCl₂ solution or deionized water. Growth direction of the primary root (toward, away, neutral) and lateral root numbers and lengths were recorded. For seedlings, primary root direction and lateral root numbers/lengths were significantly increased toward Ni-soaked filter paper only for S. polygaloides. We conclude that S. polygaloides exhibited positive root foraging responses. These may enhance Ni uptake and we suggest the term “nickelophilic root foraging” be applied to this behavior.