Does combined heavy metal stress enhance iron plaque formation and heavy metal bioaccumulation in Kandelia obovata?

Iron plaque (IP) has significant impact on rhizosphere environment, and affects the uptake and translocation of heavy metals in plants. However, information on IP formation and heavy metal bioaccumulation for mangrove plants under combined heavy metal stress is currently limited. Based on a 120-day pot experiment, this study explored the temporal changes of IP formation on root surface of Kandelia obovata, and heavy metal bioaccumulations under combined heavy metal stress of Cu, Pb and Zn. The results showed that IP formations on root tip, middle and basal roots were improved with combined heavy metal stress and treatment time (root tip: 0.49−0.61 mg g⁻¹ to 2.55–4.77 mg g⁻¹; middle root: 0.27−0.31 mg g⁻¹ to 3.01–4.22 mg g⁻¹; basal root: 0.16−0.23 mg g⁻¹ to 2.24–4.10 mg g⁻¹). IP formation was accompanied with improved depositions of Cu and Pb on root surface, regardless of root tip, middle and basal roots. IP and roots deposited large amounts of Cu (24.97–69.52 μg g^-1, 9.39–154.75 μg g⁻¹), Pb (8.35–84.53 μg g^-1, 12.29–191.49 μg g^-1) and Zn (85.71–139.19 μg g^-1, 51.07–195.65 μg g⁻¹), inhibiting their translocations to aboveground leaves. Based on subcellular distribution and chemical forms of heavy metals in roots, the improved depositions of phosphate-integrated Cu (6.77 %–25.13 %), oxalate-integrated Pb (34.89 %–57.88 %), as well as pectate- and protein-integrated Zn (14.22 %–35.41 %) were detected in root cell vacuoles. In conclusion, the combined heavy metal stress enhanced IP formation and heavy metals deposition (especially for Cu and Pb) on root surface, and improved their vacuolar sequestration and chelation in roots of K. obovata.