Root Tolerance Characteristics Of Phytolacca Americana L. Under ZnONPs/Cd²⁺ Interaction Stress

The risk of soil cadmium(Cd)pollution and nano-contamination is seriously in-tensified in China in recent years,and composite soil pollution of Cd and nano-materi-als is also emerging.As a result,the soil plant growth,nutrient health and physiological structure were adversely affected.Here,research on the combined pollution of nano-material and metal ions becomes a hot topic in eco-toxicity.In this paper,the Phytolacca was served as the research object,and zinc oxide nanoparticles(ZnONPs)/Cd2+ are selected as composite pollutants.Through root scan-ning,inductively coupled plasma mass spectrometry(ICP-MS),enzyme activity assay,biological transmission electron microscopy(TEM),and X-ray energy spectrum,plant physiological indexes,metal accumulation and transport efficiency,root antioxidant ca-pacity,root tip cell structure and element distribution were tested to study plant re-sistance and the responses of organs structure under ZnONPs/Cd2+ interaction stress.The main conclusions of this study are as follows:1.ZnONPs can continuously and slowly affect the growth of stems and leaves of plants under the compiste stress of ZnONPs or Cd2+.The toxic effect of high concen-tration ZnONPs(500 mg/kg)onplants was lower than that of low concentration ZnONPs(200 mg/kg),and it was observed that high concentration of ZnONPs pro-moted stem height and inhibited root length,whereas low concentration of ZnONPs significantly inhibited stem height and root length.With Cd2+ stress alone,low concen-tration of Cd2+(10 mg/kg)promoted plant growth,while high concentration of Cd(100 mg/kg)inhibited plant growth.It is noted that under compound stress(ZnONPs/Cd2+),plant growth was significantly inhibited,and toxic effects of ZnONPslCd2+ on plants were significantly aggravated.2.In all treatments(ZnONPs,Cd2+,ZnONPs/Cd2+),the content of heavy metals in P.americana increased with the pollutant concentration(P<0.01).At the same time,the amount of metal ions in plant increased with pollutant concentration.The heavy metal elements were transported from the underground to the above-ground corresponding to transportation from the roots to the stems and leaves.In this way,plant mitigated the toxicity of heavy metals;3.Under single stress,MDA content of the root was highly promoted at 200 mg/kg ZnONPs and 100 mg/kg Cd2+.POD exhibits the highest activity among three antioxi-dases which indicated that POD played a key role in the root antioxidant system of P americana.The activity of CAT declined along with the time.In addition,in the high combination treatment,the value of MDA content,SOD activity and POD activity ex-tended to the highest and soluable protein content decreased most.4.The root ultrastructure graph showed that both Cd and ZnONPs entered the root cells and detoxified by immobilizing metal ions and nanoparticles through cell wall and vacuoles.With ZnONP stress alone,vacuolization and plasma membrane detachment occurred in the root cell.Low concentrations of ZnONPs were more dispersed in root cells,resulting a higher toxicity.Mitochondria were more seriously damaged by Cd2+in root cells.The toxic effects of ZnONPs were more severe than those of Cd2+.Under stress of combined treatment of ZnONPs/Cd2+,root cells and cell organs suffered to greater damage than ZnONPs or Cd2+ exposured alone.