In situ chemical imaging shows differential element mobilization in the rhizosphere of the Ni hyperaccumulator Odontarrhena chalcidica

Nickel (Ni) hyperaccumulating plants can accumulate Ni in their aboveground biomass at mass fractions exceeding 1000 μg g⁻¹ (dry weight). However, the processes controlling the acquisition of soil-borne Ni by hyperaccumulators remain poorly understood, particularly in relation to root-induced changes of rhizosphere chemistry. Using in situ, high-resolution chemical imaging via planar optodes and diffusive gradients in thin-films (DGT) combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), this study investigated localized changes in the spatial distribution of pH along with Ni, iron (Fe), zinc (Zn), and phosphorus (P) availability in the rhizosphere of the Ni hyperaccumulator Odontarrhena chalcidica grown on two ultramafic soils differing in pH (5.9 vs. 6.5) as well as total (552 vs. 1465 mg kg⁻¹) and extractable (42 vs. 158 mg kg⁻¹) Ni. Significant rhizosphere alkalinization of up to 1.5 pH units was observed at the immediate root surface in both soils, indicating root exudation of alkaline compounds. While Ni, Fe, and P fluxes were generally depleted around roots, increased Ni fluxes were observed only at root tips in the lower-Ni soil, highlighting a distinct biogeochemical niche for Ni mobilization. In contrast, increased Zn fluxes were observed consistently, irrespective of the soil or root type, revealing a previously unrecognized process for enhanced Zn availability in the rhizosphere. These findings suggest that O. chalcidica employs a highly selective rhizosphere modification strategy, combining pH shifts with element-specific mobilization mechanisms to control trace element availability and plant uptake.