| Casuarina equisetifolia and mangroves are the main afforestation species along the coast of Fujian, China. The aim of this study is to reduce heavy metal pollution of the coastal zone, protect and develop the coastal shelterbelt rationally, and give full play to their ecological and economic benefits. By determining the content of heavy metals(Cr, Cu, Zn, Cd, Pb, Hg) of Casuarina equisetifolia in different areas of Fujian and mangroves from Fujian Jiulong River estuary, absorption and reduction of heavy metals by Casuarina equisetifolia and mangroves were analyzed. In order to discuss tolerance mechanism of Hg in Kandelia obovata, Kandelia obovata were cultivated in water and treated with different concentrations of mercury. The influence of different mercury contents on ecophysiological properties, mercury content of Kandelia obovata organs and ultrastructure of mesophyll cells of Kandelia obovata seedlings were studied. Sequential chemical extraction method was also used to study the subcellular distribution and the chemical forms of heavy metals in the leaves. The main results are as follows:1. Soil heavy metal content of Casuarina equisetifolia forest in Dongshan, Huian, Pingtan were lower than background values of soil heavy metal content of Fujian province and Fujian coastal zone. Casuarina equisetifolia shelterbelt showed no soil pollution. Heavy metal content of Casuarina equisetifolia in three areas were all higher than the normal region of plant. The heavy metal content of the root was significantly higher than the other parts, and bioaccumulation coefficient (BCF) of root was also the highest in Casuarina equisetifolia. In addition to the branchlet on Cr in Huian, BCF of Cr, Cu, Zn, Cd were greater than1. There was a significant positive correlation between content of Zn and Cd in Casuarina equisetifolia root. Similarly, BCF of branchlet and root of Dongshan Casuarina equisetifolia on Cu, Zn, Huian on Cr, Zn, branchlet of Pingtan on Cr, Pb, root on Zn were all had a significant positive correlation with stand age.2. According to the existing biomass of Casuarina equisetifolia shelterbelt, existing reserves of Cr, Cu, Zn, Cd, Pb and Hg were calculated. Existing reserves of Cr, Cu, Zn, Cd, Pb and Hg were3.06～3.46kg·hm-2,1.14-1.57kg·hm-2,11.67～12.57kg·hm-2,0.05～0.09kg·hm-2,0.45～1.84kg·hm-2and0～0.0002kg·hm-2. Heavy metals were mainly existed in the root and trunk of Casuarina equisetifolia. 3. There were differences for content of heavy metals in different parts of mangroves. Mangrove root was a main organ for heavy metal accumulating. Ranges of Cr, Cu, Zn, Cd, Pb and Hg content in Kandelia obovata were3.81～12.18μg·g-12.70～7.42μg·g-1,10.14～22.08μg·g-1,0.02-0.26μg·g-1,1.53-2.53μg·g-1and0.66～2.96μg·g-1respectively. In Agiceras corniculatum, they were3.23-8.33μg·g-1,1.66-4.07μg·g-1,9.57-21.36μg·g-1,0.07-0.23μg·g-1.41-3.71μg·g-1,0.998～1.46μg·g-1. In Kandelia obovata community, total reserves of the6kind of heavy metals was5.83kg·hm-2, and the order was Zn (2.82kg·hm-2)>Cr (1.47kg·hm-2)> Cu(0.76kg·hm-2)> Hg(0.40kg·hm-2)>Pb(0.36kg·hm-2)>Cd(0.03kg·hm-2). Most mercury was in the stem of Kandelia obovata, but the5kind of heavy metals were mostly reserved in the root, and followed by the stem. There was only a tiny part of heavy metals in the stems and leaves of Agiceras corniculatum, and total reserves of the6kind of heavy metals was6.74kg·hm-2, Zn(3.07kg-hm-2)>Cr(1.20kg-hm-2)>Cu(0.58kg·hm-2)>Pb (0.53kg·hm-2)> Hg(0.21kg·hm-2)>Cd(0.03kg·hm-2).4. With the increase of stress time concentration, photosynthesis of the Kandelia obovata leaves was significantly inhibited under Hg stress, and the limiting factor may not be stoma. Fv/Fm in the third and fifth days, content of chlorophyll, soluble sugar, proline and malondialdehyde were increased firstly and then decreased with concentration of Hg heading up. There was irreversible damage for chloroplasts and mitochondria in leaf cells of Kandelia obovata, and the damage was more and more obvious.5. The main forms of Hg in leaves of Kandelia obovata were water-extractable, residual, ethanol-extractable and NaCl-extractable forms. Content of Hg existed as water-extractable was the highest, which indicated that water soluble state of organic ligand may played a key role in the subcellular distribution of Hg. Hg in the root was predominated with HCl-extractable, the second is residual. Higher active chemical form of Hg in root was in favor of transfer of Hg to other parts in Kandelia obovata, so that it could reduce Hg accumulation in root. Existence of the lower active chemical form of Hg was an important reason for the storage of Hg in the root, which could reduce damage of other parts. Resistance of Kandelia obovata to mercury was revealed under their interaction.With the increase of Hg stress concentration, Hg content of each part of Kandelia obovata also significantly arised. In the CK group,0.2ppm, lppm stress group, order of total content of Hg was root>leaf>stem; when the stress concentration were 10and50ppm, the content of the heavy metals in Kandelia obovata was ranked from high to low as root>stem>leaf. The results indicated that levees of Kandelia obovata could absorbed a certain amount of Hg from the environment, root was the main part for Hg enrichment of Kandelia obovata. |
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