Accumulation Characteristics Of Cd,Pb And The Root Secretion Mechanism Of Low Molecular Organic Acids In Sonchus Asper And Crop Intercropping System

The soil heavy metal pollution of farmland around mining area is serious,which threatens food security seriously.The phytoremediation technology of intercropping hyperaccumulators with crops can facilitate metal uptake by the hyperaccumulator and concurrently reduce metal accumulation in the intercrops,ultimately improving remediation efficiency and reducing the metal contents of the crops.This approach is highly feasible and has great development potential,as it allows to simultaneously control heavy-metal-contaminated soil and harvest agricultural products in line with health standards during standard agricultural production.A new topic of phytoremediation that is worthy of study is the selection of appropriate plant species to maximize heavy metal uptake by the hyperaccumulator and minimize heavy metal accumulation in the intercrops.Low molecular weight organic acids?LMWOAs?of plant root secretion affect the heavy metal ions adsorption-desorption on roots and soil interface and coordination-dissociation reaction,ultimately change the occurrence morphology and biological effectiveness of heavy metals in soil,and affected the absorption and accumulation of heavy metal in plants.There is a closely relation among rhizosphere environment,LMWOAs in plants and accumulation of heavy metals in plants in the intercropping of accumulators and crops under heavy metal contamination soil.However,the specific mechanism remains unclear.Therefore,the local accumulator Sonchus asper?S.asper?founded in Yunnan huize lead-zinc mine area and corn,beans were chosen as the research object.Through pot and hydroponic experiments,the effect of intercropping with S.asper and crop on Cd,Pb accumulation characteristics in plant was studied,and the effect of LMWOAs in plant root secretion on Cd,Pb form in rhizosphere soil were analyzed,the effect of LMWOAs on the Cd,Pb distribution in plant were analyzed.The response mechanism of Cd,Pb absorbtion in plants and LMWOAs secreted by plant root was emphatically discussed under intercropping.The main results were as follows:?1?The subcellular distribution of Cd,Pb content in S.asper,V.faba and Z.mays were followed: the cell walls > soluble components > organelles.Cd,Pb were mainly stored in plant cell walls.Compared with monoculture,?1?Cd and Pb contents of S.asper in cell wall,soluble components and organelles increased by 15.7%³8.6%,significantly,under intercropping,while decreased in Z.mays and the ground part of V.faba.Intercropping increased the ratio of Cd and Pb in the cell wall.?2?The contents of Cd and Pb chemical forms in S.asper were increased significantly,while decreased in Z.mays,and the contents of Cd chemical forms in V.faba decreased significantly.?3?In pot experiment,the Cd and Pb content of S.asper increased significantly by 30.6%¹44%,while that of Z.mays decreased by 23.1% and 24.3%,and Pb content of V.faba decreased by 22.2% with intercropping.The results indicate that uptake and accumulation of Cd and Pb in S.asper under intercropping system could be promoted and content of Cd and Pb in Z.mays and V.faba could be reduced.The system of S.asper intercropping with Z.mays is better than that of S.asper intercropping with V.faba.?2?The dominant organic acids in the shoot,root and root secretion of S.asper,Z.mays and V.faba were oxalic and citric acids,which accounted for 80.3% 99.2% of total organic acids.Intercropping changed the type and content of organic acids secreted by root.In pot culture,intercropping increased lactic acid in roots secretion of S.asper,but reduced the maleic acid and succinic acid,and reduced lactic acid and succinic acid in root secretion of V.faba.Intercropping significantly increased the amounts of total organic acids in the root exudates of S.asper and V.faba.by 9.7%⁴3.3%,and increased by 22.6%¹02.5% of S.asper and Z.mays,compared to monoculture,under hydroponic culture.?3?The organic acids of plant roots secretion can activate the rhizosphere Cd,Pb,and affect the accumulation of Cd,Pb in various parts of plant.In pot culture,the amounts of oxalic acid and citric acid secreted by S.asper roots significantly increased by 23.9%⁷5.4%,while decreased by 18.5 and 18.7% of Z.mays,compared to monoculture.The exchangeable Cd and Pb in rhizosphere soil of S.asper increased significantly by 8.1%²7.8% with intercropping compared to monoculture,while that of Z.mays decreased significantly by 6.2% and 23.4%.Significant positive correlations between citric and oxalic acid contents of the plant root exudates and available rhizosphere soil Cd,Pb,and plant Cd,Pb were observed.Meanwhile,there was a significant positive correlation between available Cd,Pb in rhizosphere soil and plant Cd,Pb.So,oxalic acid and citric acid of root secretion in intercropping changed the content of available Cd,Pb in rhizosphere soil of S.asper and Z.mays,and uptake and accumulation of Cd,Pb in S.asper under intercropping system could be promoted,while the content of Cd,Pb in Z.mays could be reduced.Intercropping influenced the Cd and Pb contents in the plants of S.asper and Z.mays and was closely related to the change in the available Cd and Pb content in the soil,which was mediated by plant roots that secreted oxalic acid and citric acid.In hydroponic culture,there was a good correlation between the oxalic and citric acid of root secretion in S.asper,Z.mays and V.faba with various subcellular Cd,Pb content of plants.This showed that intercropping changed the oxalic and citric acid of plant roots secretion to influence the distribution and storage amount of Cd and Pb in various subcellular components.?4?Under Cd and Pb stress,there was a large difference in shoot and root of S.asper,V.faba and Z.mays in intercropping.Shoot and root organic acid in S.asper,V.faba and Z.may were significant positive correlation with root secretion organic acid,and shoots and roots of 6 kinds of organic acids was significantly positive correlation.Maybe Cd and Pb stress affected plant root secretion of LMWOAS,led to the change of the accumulation and storage of organic acids in plants,further,influenced the distribution of Cd,Pb in plant.The content of oxalic acid in shoot and root of S.asper,V.faba and Z.may were significantly positive correlation with the content of Cd and Pb in plants,and significantly positive correlation with the shoot Cd content in cell walls and root Cd content in vacuole.The results indicated that oxalic acid involved in the accumulation and storage of Cd,Pb in plant,ultimately Cd and Pb were stored in the cell wall and vacuole.?5?The effect of adding exogenous organic acids on the uptake of Cd and Pb by S.asper,and Z.may under intercropping system in pot experiment indicated that exogenous addition of oxalic and citric acid activated rhizosphere soil Cd and Pb,and promoted the absorption of shoot and root Cd,Pb in S.asper.Significantly positive correlations between shoot and root Cd,Pb of S.asper and soil available Cd,Pb contents were observed,with correlation coefficients 0.910?P<0.05?and 0.932?P<0.05?,respectively,in intercropping system of S.asper and Z.may.When S.asper intercropping with V.faba,the exogenous addition of oxalic and citric acid activated rhizosphere soil Cd and Pb,the activation effect was best when the concentration of oxalic acid treatment was 10 mg·kg^-1 and 12 mg·kg^-1 citric acid treatment.Citric acid promoted Cd accumulation of S.asper and V.faba.Oxalic acid promoted Pb accumulation of S.asper,while reduced in V.faba.?6?Hydroponic experiment was carried out to study the effects of carbendazim?diluted 1000,800,500,300,100 times?on the Cd,Pb accumulation of S.asper and V.faba under intercropping system.The results showed that shoot and root Cd,Pb content of S.asper increased by 15.2%³70.4%,and increased by 6.8%²11.6 % in V.faba.The results indicated that when praying suitable concentration of carbendazim,the remediation efficiency of heavy metal was improved,in the meantime,promoted the growth of V.faba.