The Mechanism Of Cd And Pb Accumulation In Roots Of Athyrium Wadii(Hook.) Grown In Cd And Pb Co-Contaminated Soils

The co-contamination of Cd and Pb is becoming a much severer issue that greatly threats ecological environment and human health in mining areas.Phytostabilization,a low-cost,highly efficient,and environmentally friendly technology,shows great practicality in the remediation of heavy metal-contaminated soils in mining areas.The mining ecotype（ME）of Athyrium wardii（Hook.）,a perennial fern found in a Pb-Zn mine,has been reported to be an ideal candidate for the phytostabilization of Cd-or Pb-contaminated soils.However,few researches are available on the phytostabilization of Cd and Pb co-contaminated soils.Therefore,this study is processed to investigate the mechanisms of Cd and Pb accumulation in roots of the ME when exposed to the co-contamination of Cd and Pb by pot experiments.A summary of the primary results are as follows.1.The combined effect of Cd and Pb enhanced Cd and Pb accumulation in roots of the ME.Cd and Pb accumulation in roots of the ME enhanced with increasing soil Cd or Pb concentrations when exposed to the co-contamination of high Cd(5-50 mg kg^-1)and Pb(200-800 mg kg^-1).Cd accumulation in roots of the ME was up to 1.81 mg plant^-1 when exposed to 25 mg kg^-1 Cd and 600 mg kg^-1 Pb simultaneously,which increased by 118%compared with that exposed to 25 mg kg^-1 Cd.Pb accumulation in roots of the ME reached281 mg kg^-1 when exposed to 25 mg kg^-1 Cd and 800 mg kg^-1 Pb simultaneously,which increased by 32.1%compared with that exposed to 800 mg kg^-1 Pb.The translocation factors of Cd and Pb of the ME were lower than 1 when exposed to Cd and Pb simultaneously.The available Cd and Pb in the rhizosphere soils of the ME after harvest decreased compared with that before transplantation and were less than those of the NME.These results indicate that the ME showed greater potential for the phytostabilization of Cd and Pb co-contaminated soils,especially for Pb.2.The combined exposure of Cd and Pb induced more exudation of organic acids and amino acids by the roots of the ME compared with the exposure of single Cd or Pb.Oxalic acid and cysteine accounted for 42.2%-59.4%and 11.7%-20.1%of the total amounts of organic and amino acids,respectively,and played a key role in the mobilization of Cd and Pb in the rhizosphere soils of the ME.The amounts of dissolved organic matter（DOM）,especially for the acid fractions and hydrophilic fractions,in the rhizosphere soils increased by 59.0%and 53.5%when exposed to Cd and Pb simultaneously,compared with those exposed to single Cd or Pb.There were more O-H,C-O,N-H and C-H,assigned to carboxylic groups,phenolic groups,hydroxyl groups,and/or amino groups,present in DOM from the rhizosphere of ME when exposed to Cd and Pb simultaneously.As a result,the ME showed greater ability to solubilize Cd and Pb by root exudates and DOM in the rhizosphere soils when exposed to Cd and Pb simultaneously than that exposed to single Cd or Pb and also than the NME.This is thereby considered to be one of the key processes for enhancing Cd and Pb uptake by the ME when exposed to Cd and Pb simultaneously.3.The ME showed greater epidermal thickness,exodermis thickness and endodermis thickness when exposed to Cd and Pb simultaneously,compared with those exposed to single Cd or Pb.The ME presented greater lignification and suberification in roots when exposed to Cd and Pb simultaneously,compared with those exposed to single Cd or Pb and also the NME.The ME showed greater lignification in roots,especially for epidermis and exodermis,followed by endodermis,vascular parenchyma cells,and cortical cells,when exposed to Cd and Pb simultaneously.Similarly,the ME also presented greater suberification in roots,especially for xylem parenchyma cells and endodermis,when exposed to Cd and Pb simultaneously.The root anatomical changes of the ME lead to a great change for Cd and Pb distribution in root cross-sections when exposed to Cd and Pb simultaneously.In roots of the ME,the highest concentrations of Cd and Pb were observed in exodermis,cortex,and endodermis,and some Cd and Pb were observed in epidermis and central cylinder,when exposed to Cd and Pb simultaneously.The transportation of Cd and Pb to central cylinder was greatly inhibited by the cortical tissue（including exodermis,cortex,and endodermis）,which plays a major role in Cd and Pb retention in roots of the ME when exposed to Cd and Pb simultaneously.4.Cell wall was the main binding site for Cd and Pb in roots of the ME.The ME showed more Cd and Pb accumulation in root cell walls when exposed to Cd and Pb simultaneously,compared with those exposed to single Cd or Pb as well as the NME,suggesting some modifications for cell walls.The uronic acid contents of pectin and hemicellulose 1（HC1）in root cell walls of the ME increased significantly when exposed to Cd and Pb simultaneously,resulting in more Cd and Pb bound to pectin and HC1.In particular,pectin was found to be the predominant binding site for Cd and Pb.Greater pectin methylesterase activity along with a lower degree of methylesterification were observed in the cell walls of the ME when exposed to Cd and Pb simultaneously.Furthermore,the ME presented more O–H from hydroxyl groups,carboxylic groups and phenolic groups,more N–H from amines,more C–OH,C–O–C,and C–C from pectin,and/or more aromatic structure in root cell walls when exposed to Cd and Pb simultaneously.These changes of root cell wall properties of the ME lead to enhanced cell wall binding ability in response to the combined exposure of Cd and Pb,thus could be considered a key process for enhanced Cd and Pb accumulation in roots of the ME when exposed to Cd and Pb simultaneously.