Multiple heavy metals affect root response, iron plaque formation, and metal bioaccumulation of Kandelia obovata

Multiple heavy metal pollution in mangrove wetlands is serious. Kandelia obovata seedlings were cultured in pots in which lead (Pb), zinc (Zn) and copper (Cu) were added separately and in combinations. The results showed that heavy metal stress improved the rate of root oxygen leakage, enhanced root activity, and reduced root porosity. The root under single heavy metal stress was impacted by the addition of other heavy metals, demonstrating antagonistic or synergistic effects. Iron plaque (IP) formation was improved under single Zn or Cu stress, and inhibited in binary stress of Pb + Cu. The adsorptions of IP on heavy metals in combined stress (Pb, 62-116 μg g^-1; Zn, 194-207 μg g^-1; Cu, 35-52 μg g^-1) were higher than that in single stress (Pb, 18 μg g^-1; Zn, 163 μg g^-1; Cu, 22 μg g^-1). K. obovata accumulated higher levels of heavy metals in root (Pb, 7-200 μg g^-1; Cu, 4-78 μg g^-1), compared with IP (Pb, 18-116 μg g^-1; Cu, 22-52 μg g^-1), stem (Pb, 3-7 μg g^-1; Cu, 9-17 μg g^-1), and leaf (Pb, 2-4 μg g^-1; Cu, 4-7 μg g^-1). Correlation analysis showed that single and binary stresses affected K. obovata, with more significant effect of trinary stress. Regression path analysis showed that multiple heavy metal stress firstly affected root, then indirectly contributed to IP formation, as well as heavy metal in IP and root; at last, heavy metal in IP directly contributed to heavy metal bioaccumulations in root.