Variation in cadmium accumulation and speciation within the same population of the hyperaccumulator Noccaea caerulescens grown in a moderately contaminated soil

Background and aims: Phytoextraction is an eco-friendly approach for remediation of heavy metal contaminated soil. The aim is to screen Noccaea caerulescens lines with higher cadmium (Cd) phytoextraction efficiency and investigate differences in Cd species and distribution in the leaves of high and low Cd accumulating lines. Methods: Biomass production and Cd bioaccumulation capacities of 29 Noccaea caerulescens lines, generated through single-seed-descent from a Cd hyperaccumulating calamine population, were assessed in a pot experiment with a moderately Cd contaminated soil (2.1 mg Cd kg^{− 1}). Synchrotron-based techniques were employed to identify and characterize Cd speciation and distribution in Noccaea caerulescens leaves. Results: The largest biomass of Noccaea caerulescens reached 5.0 ± 3.3 g (D. W. pot^{− 1}) after 6 months growth. The Cd concentrations in shoots varied from 85 to 203 mg kg^{− 1}. The most efficient line removed 0.64 mg Cd pot^{− 1} and lowered the total Cd in soil by 30%. Synchrotron-based X-ray absorption spectroscopy showed that the dominant Cd species was Cd-thiol complexes. Cadmium-carboxyl and Cd-phytate/phosphate were present in the leaves of high and low Cd accumulating lines, respectively. Micro X-ray fluorescence microscopy showed cadmium was concentrated in leaf veins. Conclusions: There are wide variations including both biomass production and Cd accumulation capacity among different lines within the same calamine ecotype of Noccaea caerulescens. Cadmium-thiol complexes play the most important role in Cd detoxification in leaves of Noccaea caerulescens grown in moderately Cd contaminated paddy soil. These findings provide a physiological basis for breeding high Cd accumulation varieties of Noccaea caerulescens.