Phytoremediation of cadmium (Cd)-contaminated soils using (hyper)accumulating plants is considered to be a promising technique, especially for moderately-polluted soils. However, most plants used for Cd phytoremediation are glycophytes and hence not suitable to remediate contaminated saline soils. Australian native halophyte Carpobrotus rossii produces high biomass, and has been shown to tolerate a mixture of heavy metals and to accumulate Cd. However, limited information is available on the effect of salinity on Cd phytoextraction and associated mechanism in this species. This study examined the effect of salinity on Cd accumulation, translocation, and speciation in C. rossii. Plants were grown in nutrient solution with different types and levels of salts in the presence of Cd. Plant growth and Cd uptake were measured, and Cd speciation analyzed using synchrotron-based X-ray absorption spectroscopy.

In the first experiment, increasing NaCl concentration from 0 to 100 mM in nutrient solution without Cd did not significantly affect plant growth. At 15 μM Cd, NaCl addition increased plant relative growth rate by up to 82%, which had resulted from decreases in Cd root uptake and root-to-shoot translocation and hence shoot Cd accumulation. Increasing NaCl addition decreased the concentration of phytochelatins, while the concentration of phytochelatins correlated positively with Cd concentrations in plants. The responses of organic acids and amino acids in plants to the addition of Cd and NaCl were inconsistent.

The second experiment confirmed that salt addition decreased shoot Cd accumulation due to decreased root-to-shoot translocation, irrespective of salt type (25 mM NaNO₃, 12.5 mM Na₂SO₄ or 25 mM NaCl). The decrease in Cd translocation was not due to changes in Cd speciation within the plant tissues after 10- to 20-d treatment. After 10-d treatment, 61-94% Cd was bound to S-containing ligands (Cd-S) in plant, while the Cd in the xylem sap existed as free Cd²⁺ or complexes with carboxyl groups (Cd-OH). However, when exposed to Cd for ≤ 24 h, 70% of the Cd in the roots was present as Cd-OH. Furthermore, salt addition increased the proportion of Cd-S but decreased that of Cd-OH in the roots within 24 h, which was not due to changes in glutathione and phytochelatin concentrations in plant tissues.

In conclusions, salinity could decrease shoot Cd accumulation by decreasing Cd root-to-shoot translocation even under constant Cd²⁺ activities in the growing media. The decreased Cd translocation had partly resulted from the rapid formation of less-mobile Cd-S complexes within the root. Peptides and organic acids, particularly phytochelatins, played an important role in plant Cd tolerance and accumulation although the changes of those metabolites were not the main reason for the decreased Cd accumulation in C. rossii.