| Chinese farmland polluted by cadmium seriously which caused dual loss both in economy andenvironment. Using super accumulation plant to remediate the polluting farmland took manyadvantages because of its low cost, no destroying in the soil ecological environment, no causingsecondary pollution and so on, which draw many researchers’ attention all over the world. In this study,Perilla frutescens, a Chinese medicinal plant cultured for extracting essential oils, was selected as atesting plant to illustrate the basic physiological and molecular mechanism responsing to the stress ofcadmium pollution and how it accumulated the heavy metal. Furthermore, field experiments, whichusing the plant to remediate soil polluted by the heavy metal, were carried out and evaluated by its thephytoremediation effect. The main results are as follows:（1） The inhibition effect on the growth of plant height, stem length and biomass under differentconcentration of cadmium stress were investigated by hydroponic culture experiments.The resultsshowed that under the treatments of cadmium concentration at2mg·L-1,5mg·L-1and10mg·L-1, thebiomass of Perilla frutescens was71.6%,57.6%and34.0%as higher as the control respectively,significant difference was found among the treatments. In addition, spot experiments in the soil, whichtreated by cadmium with the concentration levels at2,5,10,20,50and100mg·kg-1,showed that thebiomass of Perilla frutescens was94.94%,90.28%,81.22%,70.52%,65.51%and54.90%of thecontrol respectively, and significant difference was found among different treatment. The biomass ofPerilla frutescens declined significantly, showing the growth of the plant was inhibited obviously bycadmium pollution.（2） The cadmium absorption characteristics of Perilla frutescens under cadmium stress byhydroponic experiments showed that the cadmium content in Perilla frutescens organs increasingsignificantly with the exogenous added cadmium concentration, and there were significant differencesbetween treatments. The accumulation of cadmium in the plant performed as357.4to505.4μg·plant-1in the root,105.0to226.3μg·plant-1in the aboveparts, and462.4to705.0μg·plant-1in the wholeplant.The content and accumulation of cadmium in roots were significantly higher than the aboveparts.And the bioconcentration factor of cadmium in Perilla frutescens were between51.1and91.9, whichpeaked at5mg·L-1; transportion factor of cadmium increased with cadmium concentration, and thehighest value was0.59. Under the condition of spot experiments using soil treated by cadmium at2,5,10,20,50and100mg·kg-1, soil cadmium concentrations decreased by33.55%,64.26%,47.94%,15.93%,23.47%and33.12%compared to the control, the largest absorption effect was found thetreatment at5mg·kg-1. The accumulation of cadmium in Perilla frutescens increased with theexogenous added cadmium concentration, the absorption amount of cadmium was between107.46μg·plant-1and588.23μg·plant-1. When the cadmium concentration arrived at20mg·kg-1, thecadmium accumulation in the plant increased significantly. The bioconcentration factor of cadmium inPerilla frutescens were between5.88and53.73, which peaked at2mg·kg-1; transportion factor ofcadmium in Perilla frutescens were between0.73and1.43, the highest transportion factor was alsofound in the treatment at2mg·kg-1. （3） The results of soil physical and chemical properties showed that the pH of treated soilincreased slightly, total nitrogen content increased significantly when the cadmium concentrationreached to5mg·kg-1; and the content of total phosphorus decreased significantly, the content ofpotassium increased obviously, after cultivated the plant Perilla frutescens.（4） Physiological response of cadmium stress under hydroponic experiments showed that, theSOD activity was obviously higher than the control when the cadmium concentration rose to5.0mg·L-1and10.0mg·L-1. POD activity in Perilla frutescens was significantly lower than the control,which decreased by18.7%,40.1%and71.7%in order. CAT activity significantly declined with thecadmium treatment concentration increased. MDA contents were1.27,1.59and1.98times as higheras the control, respectively. GSH content decreased significantly in lower cadmium treated Perillafrutescens, and significantly increased when the cadmium concentration up to5.0mg·L-1. The netphotosynthetic rates of Perilla frutescens under cadmium stress were67.7%,41.1%and36.5%ashigher as the control. Under the stress of cadmium,the determination of hormone content in the plantsperformed as that: little influence was found in ZR conten; low concentration cadmium treatmentpromoted while high concentration inhibited ABA content, higher GA and IAA content were foundafter the treatment, and the content peaked at2mg·L-1. It suggested that low concentration cadmiumtreatment can induce the secretion of the hormonehormone with high concentration cadmiumtreatment inhibit the producing of hormonehormone in Perilla frutescens. In addition, the analysis ofdifferential expressed proteins under cadmium stress showed that cadmium stress responsing proteins,such as manganese superoxide dismutase, ascorbic acid peroxidase, adhesion receptor precursor andisoflavone reductase homolog, were all upregulated. the expression of acyl-CoA thioesterase, copinedomain containing protein and NAD （+）-specific glutamate were upregulated at2mg·kg1anddownregulated at5mg·kg1and10mg·kg1cadmium treatment.There were no significant differenceswere found in the content and chemical compositions of the essential oil of Perilla frutescens underdifferent cadmium stress.（5） In the application of Perilla frutescens to remediate cadmium pollution soil in fieldexperiments, the results showed that the cadmium content in the pollution soil, after cultivated Perillafrutescens, decreased with a range from14.21%to79.44%compared to the control. There were noobvious differences were found in the content of soil organic matter, total nitrogen and total potassiumafter the application, the pH declined, the content phosphorus increased significantly and availablepotassium content reduced obviously after the experiments. After the cultivation of Perilla frutescens,the enzyme activities in the soils showed that peroxidase, polyphenol oxidase and urease activity hadno significant change; the catalase and sucrase activity declined, it suggested that thephytoremediation by Perilla frutescens had a positive effect to the alleviation of the increase ofcatalase and sucrase activities; while the increasing of phosphomonoesterase activities significantlywill promote the cycle of soil phosphorus and carbon.（6） The results of T-RFLP analysis showed that, there were22bacteria genera in Perillafrutescens rhizosphere soil which were not found in the control soil. Among those, the beneficialbacteria genera including the Frankia, Azospirillum, Bradyrhizobium and Sinorhizobium, whichengaged in nitrogen fixation and nitrification, relating to soil nitrogen cycle; The thiobacillus,desulfotomaculum and desulfohalobium which involved in soil sulfur cycle; the erythromonas of lightheterotrophic, the anaerobranca which had acid resistance and the eubacterium which coulddecomposed xylanase etc. The special bacteria genera in the pollution soil where existed of beneficialbacteria genera such as rhizobium and mesorhizobium, but also had harmful bacteria genera like Brucella, Acetobacter, Sphingobacterium, Sporomusa, Anaeroplasma and so on. The Biolog eco-plateanalysis on soil microbial functional diversity showed that the AWCD value significant higher thancontrol soil after phytoremediation, which mean that the application of the plant enhanced the activityof microorganism in the soil significantly. Significant difference between phytoremediation soils andcontrol soil was found in the diversity of utilized substrates by soil microbe.（7） The growth of Perilla frutescens wasn’t inhibited obviously in the field experiments. theessential oil content in Perilla frutescens were higher than0.3%, which gave the highest value at0.42%. There were no significant differences between both content and chemical compositions in theessential oil of Perilla frutescens The highest cadmium concentration of Perilla frutescens was4.37mg kg1, while the highest absorb concentration in root was5.79mg·kg-1, the cadmium content in thestem and leaves of the plant were more lower than in the root. Base on higher biomass accumulationin the field experiments, the highest cadmium accumulation in Perilla frutescens reached to1488μg·plant-1.Above all, under the stress of cadmium pollution, the biomass of Perilla frutescens decreasedwith the increase of treated cadmium concentration, but the plant could maintain its growth in anormally way. The plant performed strong cadmium accumulation capacity both in hydroponic andsoil culture conditions. Under the stress of cadmium treatments, the net photosynthesis rates of Perillafrutescens decreased obviously. Higher resistance performed by adjusting the oxygen protectiveenzyme activity and GSH content. Meanwhile, low concentration cadmium treatment can induce thesecretion of the hormonehormone with high concentration cadmium treatment inhibit the producing ofhormonehormone in Perilla frutescens. In cooperation with its response in protein level, such asmanganese superoxide dismutase upregulated under cadmium stress, which enhanced the ability ofclearing the free radical; Ascorbic acid peroxidase upregulated under cadmium stress, which couldincreased the GSH content, reduced the toxicity of cadmium; Isoflavone reductase was upregulatedunder cadmium stress, which enhanced the disease-resistant ability of Perilla frutescens in root. Aswell as the results of applied Perilla frutescens to remediatethe cadmium polluted soil, proving thatPerilla frutescens had higher phytoremediation effects on cadmium polluted soil, and a bestphytoremediation pattern was summarized for the plant cultivation was found, which mainly includingdirect sowing, the plant spacing25cm×20cm, and recommomed fertilizer application is organicfertilizer combinded with45%compound fertilizer. |
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