Nickel hyperaccumulation by the species of the genus Odontarrhena from North Macedonia

Ultramafic soils in North Macedonia are characterised by elevated nickel (Ni) concentrations and provide highly selective environments for metal-tolerant plant species. The genus Odontarrhena (Brassicaceae) includes several Ni hyperaccumulators; however information on accumulation patterns and soil–plant relationships in the region was lacking. To address this, this study assessed Ni hyperaccumulation potential, organ-specific partitioning, and soil–plant relationships in seven Odontarrhena species of the O. muralis group across ultramafic sites in North Macedonia. Plant and soil samples were collected from 10 regions and elemental concentrations of Ni and selected trace elements were quantified. Multivariate analyses (RDA) and linear mixed-effects models were used to disentangle the influence of soil chemistry, species identity, plant organs, and spatial structure on plant elemental composition. All investigated species occurring on ultramafic soils showed clear Ni hyperaccumulation, with leaf concentrations exceeding 25,000 mg/kg in O. skopjensis and O. chalcidica from the Radusha serpentine outcrop. Nickel accumulation followed a consistent organ-specific pattern (root < stem < branch < fruit < leaf), indicating preferential allocation to aboveground tissues. While soil Ni contributed significantly to multivariate patterns of elemental composition, plant organ and species were the strongest predictors of plant Ni concentrations, and soil–plant relationships varied among species. These findings highlight the importance of intrinsic physiological regulation and tissue-specific sequestration in shaping Ni hyperaccumulation, reinforcing the role of Odontarrhena species as key functional components of ultramafic ecosystems in the Balkans.