Formation of iron plaque on root surface of Kandelia obovata and its role in uptake and translocation of lead, zinc and copper

In recent years, with the rapid development of urbanization and industrialization, a large number of heavy metal pollutants (Pb, Zn, Cu, etc.) entered the mangrove ecosystem and presented the characteristics of compound pollution, which seriously affected the health of the mangrove ecosystem. Iron plaque (IP) formed on the plant root surface plays an essential role in plant tolerance to heavy metal stress. A pot experiment was conducted to investigate the formation of IP under combined Pb, Zn and Cu stress, and its role in Pb, Zn and Cu uptake and translocation within the seedlings of mangrove Kandelia obovata. Results showed that under the combined stress, IP influenced the absorption and transportation of heavy metals in K. obovata: The concentrations of Pb, Zn and Cu fixed inIP were Zn (153.84 ~ 249.75 μg·g ^-1 d.wt), Pb (7.34 ~ 131.16 μg g^-1 d.wt), and Cu (22.45 ~ 55.06 μg·g^-1 d.wt), respectively. The heavy metal enrichment factors of IP are MEF_IP-Zn (0.70~0.86) > MEF_IP-Cu (0.10~0.19) > MEF_IP-Pb (0.01~0.16). The average molar ratio of Pb, Zn and Cu fixed by IP was about 1:14:3. Under combined heavy metal stress, IP of K. obovata had interaction effects on heavy metal accumulations. As for heavy metals, the enrichment concentration or enrichment ability of IP was affected by species, concentrations, composite types and concentration ratios of heavy metals and their interaction effects. The accumulation of Pb in IP promotes the absorption and bioaccumulation of Pb in shoot part. The accumulation of Zn in IP reached a saturated/stabilized state, and IP had a certain hindrance to Zn absorption. IP could promote Cu absorption, and Zn in IP may promotes the absorption and the transfer of Cu to the aerial part of K. obovata.