| Root exudates,as an important intermediate,are critical to substance exchange between plant roots and soils.This kind of root exudates not only has an influence on rhizosphere environment of plants and participates in plant nutrient transport,but also plays a significant role in recovering heavy metal contaminated soil.Once the plant roots are subjected to different levels of environmental stresses,the plants are usually capable of altering the composition of root exudates in order to acclimate the new conditions.Chlorophytum Comosum(C.comosum)belongs to the Chlorophytum genus(liliaceae),which exhibits an obvious tolerance to heavy metal lead.Particularly under Pb stresses,the composition of C.comosum roots exudates is likely to be changed and further in favor of enhancing remediation efficiency of Pb soil pollution.It provides a novel method and thought in terms of Pb soil pollution remediation originated from the composition change of C.comosum roots exudates and its role.Therefore,plant physiological and morphological indexes,and root exudates of C.comosum were investigated under Pb stress at different concentrations,based on hydroponics experiment.Effect of the root exudates on soil enzyme activity and soil microbial species and quantities were analyzed,by adding phytol and cymenein in the heavy metal lead contaminated soil.This study is of particular importance to reveal the tolerance mechanism of C.comosum to lead soil pollution and phytoremediation.The main conclusions are as follows:(1)Hydroponics experiment under lead stress conditions(0,50,100,200 mg/L)showed that increment of fresh weight and root,the plant height,the superoxide dismutase(SOD)activities and peroxidase(POD)activities increased first and then decreased.The root is the main enrichment site of lead,and the ability of transfer lead ions of the C.comosum is weak,according to three different testing periods(15d?30d?60d)in the experiment.At the first stage,both damage degree of the plant and the change of membrane permeability were little.With extending of treatment time,the damage of cell membrane gradually increased,and the change trend of MDA was consistent with that of cell membrane.When treated for 60 days,the growth of the plants were slowed down,and even the poisoning phenomenon such as yellowing and wilting appeared,and the activity of the SOD and POD decreased sharply.Comparing the three time periods,when the intensity of lead stress is at 50mg/L.Best growth of the C.comosum,highest activity of protective enzyme,and best enrichment effect of rootwere appeared at 30 day.(2)The ingredients of root exudates were analyzed according to the extraction method of ethyl excetate extracts by using GC-MS.As a result,the root exudates contained complex constituents,mainly including esters,hydrocarbons and benzences,amounted to 21 compounds with relatively high levels detected at 15 days,and 35 compounds detected at 30 and 60 days.Among the root exudates,the most common species are hydrocarbon compounds.At the same time,phytol was detected with more lead ion enrichment in the plant,while other treatment groups existing cymene were monitored with antibacterial and anti-inflammatory effects.(3)Catalase,sucrase,urease,alkaline phosphatase activity and soil lead availability were determinated,through adding the phytol and cymene in leadcontaminated soil during experiment at two different testing periods(7d,21d).It was found that adding the cymene had a negative effect on soil enzyme activity and the order of the effect on the four soil enzymes was phosphatase > urease >sucrase > catalase.Adding the phytol had a positive effect on the activities of catalase,sucrase and phosphatase,but a negative effect on the activity of urease.The four soil enzymes had different sensitivity to phytol,and the phosphatase was the most sensitive.Besides,the addition of cymene and phytol in the soil can effectively reduce the content of available lead and the activity of lead,and alleviate the toxic effect of lead on soil organisms.When the lead intensity was at 600 mg/kg and cymene intensity was at 100 mg/kg,the available lead was achieved to the lowest content in the soil.(4)In the soil culture experiment,the activity of soil enzymes showed a different changes with the treatment time.With prolonging treatment time,the activity of various soil enzymes became stronger,and the effective state of lead ions was also increased,in the soil with cymene.The result indicated that microorganisms in the soil gradually adapted,depleted or decomposed cymene with the extension of culture time to enhance the soil enzyme activity and promote the conversion efficiency of lead in the soil.The effect of cymene on soil enzymes gradually decreased with increasing the processing time due to the cymene consumed by the microorganisms.In the treatment group of phytol,the activity of sucrase and phosphatase in soil increased with the increase of phytol concentration,and the effective state of lead ions decreased.It indicated that phytol can increase the activities of the soil enzyme and reduce the effective state of the soil in lead.With the increase of time,the repair effect of phytol gradually decreased on lead-contaminated soil.(5)In the soil culture experiment,it was found that the addition of cymene and phytol was better in the short-term recovery of lead-contaminated soil compared with that in the long-term remediation.The number of soil microorganisms were changed with the concentration of cymene.When training for 7 days,the number of bacteria,fungi and actinomycete were decreased.When the concentration of lead was at 0 or 600 mg/kg,the number of bacteria and actinomycete increased first and then decreased and the amounts of fungi showed a rising trend,by adding the phytol.When the concentration of lead was at 1200 mg/kg,the number of bacteria and actinomycetes decreased significantly,and the fungi still showed an upward trend. |
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