The Effect And Mechanism Of Exogenous Indole-3-acetic Acid On Rhizoremediation Of PAHs Contaminated Soils

With the society development,a large of toxic and harmful substances are discharged into soil,causing a tremendous degrees of damage for the soil environment.One of them,the pollution of PAHs is a global problem and the pollution in China is also a urgent environmental problem which need to be solved.According to the survey report,released by National Environmental Protection department and the Ministry of Land and resources on April 17th in 2014,it shows that the PAHs concnetration in soil exceed nearly 1.4%than the ruled safety concentration.Therefore,the study and explore how to accelerate the removal of PAHs from soil become a research hotspot.At present,the ways to dissipate the PAHs from soil include physical,chemical and biological methods.Phytoremediation is a kind of biological method,and the advantages are simple operation,low cost,beautifying the environment and so on.Thus,the reseachers pay more attention to phytoremediation and widely use it to the PAHs remove from soil.But forPAHs,itself has strong hydrophobicity and high octanol partition coefficient,which make it be diffcult for plant to direct uptake them from the soils.In consideration of these merits and drawbacks,the researchers use some auxiliary means to enhance the effect of phytoremediation,such as the utilization of exogenous microbe,the surfactants and so on.But these measures have certain limitations,such as the surviving competition between the new microbes and the indigenous microorganisms;surfactants are generally synthetic chemicals and its decomposition products may cause secondary pollution in soil.Indole acetic acid?IAA?,a plant hormone,is ubiquitous in nature,and its main function is to promote the increase the biomass of plant root and shoot.The high root biomass can broaden the contact area between plant and soil,which has the important influence on the promotion of rhizosphere microorganisms activity.And the sitmulated activities have the positive effects on the removal of PAHs from soil.However,the effect of IAA on the removal of PAHs in soil and the specific mechanisms are not very clear.Therefore,the single and compound pollution in artificial or natural pollution soil were selectedand a series of treatments are setted.This study was mainly concentrated on:?1?the effect of adding exogenous IAA on the fluoranthene removal from thesoil and the possible mechanism;?2?study on the phenanthrene removal and the mechanism affected by the different doses of IAA;?3?the effect of IAA on the removal of PAHs and the possible mechanism were investigated in the natural pollution soil;?4?the effect of IAA on plant absorption PAHs and the pathway of uptake were studied in the hydroponics sysytem.Through the above experimental settings,the studies try to reveal the mechanism of exogenous IAA on the PAHs removal in soil by plant and systematically and comprehensively provide a theoretical basis for the bioremediation of PAHs contaminated soil with plant hormones.The mian results are as following:?1?This study is about to investigate the different doses of IAA(0,0.24 and 2.4 mg kg^-1)and the ryegrass on the fluoranthene removal from the artificial contaminated soil.The addition of IAA can increase the ryegrass biomass and 2.4 mg kg^-1 IAA can significantly promote the fluoranthene removal from soil.And fluoranthene uptake by ryegrass and removal rate of fluoranthene in soil was significantly correlated.IAA promotes soil fluoranthene removal may be rely on the promotion of ryegrass absorption,the increase translocation of fluoranthene from root to shoot and the stimulation fungi in soil.?2?As is known,fluoranthene is a four rings PAH and the former trial has demonstrated that the IAA application could enhance it removal with the ryegrass presented.But whether the IAA application can affect the low ring PAHs removal is not clear.Therefore,the phenanthrene,a three ring PAH,was selected as the representative PAH to study the effect and the possible mechanism with the combined of 0,1,5,10 mg kg^-1 IAA and ryegrass.The results show that IAA can increase the biomass of plants and significantly promote the dissipation of phenanthrene from soil.The maximum removal rate of phenanthrene and the maximum uptake of ryegrass both appeared inthe treatmentwith 10 mg kg^-1 IAA.IAA addition can significantly influence the change of soil microbial biomass carbon and community structure.The number of PAH-degrading bacteria was also stimulated.?3?The former trailshave carried out that IAA application could enhance the PAHs dissipation from the artificial contaminated soil,but whether the IAA application have the same effect on the PAHs removal from the natural contaminated soil is not clear.Therefore,the nature PAHs contaminated plant site in Wuxi city was selected as the tested soil.The ryegrass and alfalfa were planted in the soil with 0,1,5 and 10mg kg^-1 IAA.The results showed that the addition of IAA can not only promote the removal of PAHs in soil planted ryegrass,but also the PAHs dissipation in soil planting alfalfa was also enhanced.The two plants could both enhance the total PAHs dissipation than the unplanted soil,but the addition of IAA did not make the total PAHs disappeared obviously.For the different number ring PAHs removal has been significant affected.Planting alfalfa with IAA can significantly promote the removal of the four and six rings PAH.While the shoot mainly absorb three and four rings PAH,the root absorb different number rings PAHs.IAA can significantly increased microbial biomass carbon content in alfalfa planting soil at two months;the soil respiration in the ryegrass and alfalfa treatments were both promoted;the number of PAH-degrading bacteria in soil had obvious promotion with the IAA present.?4?The former study confirmed that the addition of IAA can promote the absorption of PAHs by plants,but how theIAA concretely influence the plant absorption patyway still need further studied.With different treatments,the effect of IAA on the plant active and positive pathways were studied.The results show that the IAA application can promote the PAHs absorption by plant.The promotion effect mayrely on the increase of the concentration of IAA,which can enhance the plant transpiration leading to the higher root absorption of PAHs in solution.At the same time,the addition of IAA can enhance the peroxidase activity in plants,which can help to clear the oxygen free radical in plantto ensure the normal growth of plants.The addition of IAA can affect the relatice expression of the detoxification genes,such as GPX,pAPX and Hsp70 genes,in plant leaves and roots.The related gene expressionchange affected by IAA maybe one of the reasons for absorption of plants to PAHs.On the other hand,the IAA application can change type and quantity of the low molecular weight organic acid secreted by ryegrass root.In conclusion,the addition of IAA can promote the increase of plant biomass,enzymes and genes in plant body detoxification function having the effect of promoting the expression,and the absorption of PAHs by plant transpiration are affected;the addition of IAA indirectly promote soil microbial number and amount of community structure changes,especially for the amount of degradation bacteria of polycyclic aromatic hydrocarbons in soil stimulated,these are to promote the remediation of soils contaminated with PAHs plants provide favorable conditions.Although the addition of IAA,the plants absorb the amount of PAHs in soil is still low,but the plant hormone PAHs contaminated soil provides a theoretical basis.