Metal accumulation and rhizosphere characteristics of Noccaea rotundifolia ssp. cepaeifolia

This work aimed to investigate the metal accumulation characteristics as well as biogeochemical changes in the rhizosphere and root foraging strategies of this plant species. Previous reports suggested that Noccaea rotundifolia ssp. cepaeifolia is a Zn, Cd and Pb hyperaccumulator. We used hydroponic, rhizobox and split-pot experiments for studying metal accumulation and related rhizosphere processes. Although this species accumulated up to 1250 mg Pb kg^-1 and 27,000 mg Zn kg^-1 in shoots, translocation factors <1 do not meet the hyperaccumulation criteria. Substantial increases in Ca(NO₃)₂-extractable metals in the N. rotundifolia rhizosphere of a metal-spiked soil can be explained by proton release from N. rotundifolia roots to maintain the charge balance during excessive metal uptake; this was not observed for the non-spiked, moderately contaminated control soil. Specific rhizosphere mechanisms targeting the alleviation of metal toxicity in N. rotundifolia rhizosphere were not detected. Generally, N. rotundifolia had larger total root and shoot mass in soils with heterogeneous distribution of Zn and Pb relative to homogeneous treatments, associated with less root mass placed in metal-enriched patches. However, the avoidance strategy was not reflected by low shoot metal concentrations. Metal accumulation rates and translocation factors do not meet the criteria for hyperaccumulation. Changes of pH and DOC in N. rotundifolia rhizosphere were apparently not involved in targeted immobilisation or detoxification of Pb, Zn and Cd. Avoidance of metal-rich patches in soil is a major tolerance strategy of N. rotundifolia.

Keywords: Hydroponic; Hyperaccumulation; Rhizobox; Root foraging.