Transport And Toxicological Effects Of Lead Oxide (PbO) Nanoparticles In Maize

Nanotechnology is widely used in industry,medicine,energy,agriculture and other fields.Nano-metal oxide particles have unique catalytic,electrical,optical and magnetic properties,which have great development potential and application prospect.For example,nano lead oxide（PbO,PbO NPs）is widely used in catalytic battery,glass,ceramics,paint and so on.Nanoparticles（NPs）can enter the atmosphere,aquatic and terrestrial environments through various pathways.The potential harm and ecological toxicity of NPs in the environment have been paid more and more attention.The effects of NPs on plants deserve special attention:on the one hand,plants interact closely with environmental elements such as soil,water and atmosphere;On the other hand,plants are an important part of the ecosystem and also the starting point of the biological accumulation of toxic substances.NPs is enriched in the food chain step by step to the high nutritional level,which may have toxic effects on the higher organisms in the food chain,and thus bring risks to human health.It is of great significance to evaluate the environmental safety risks of nanoparticles to study the toxic effects,absorption,accumulation and transport of nanoparticles on plants.In this study,maize was used as the test plant for the first time,and lead oxide was used as the research object to investigate the toxic effects of different concentrations of PbO NPs（5,10,20,30,40,50 mg·L-1）and Pb ions released by nano-size lead oxide（1 mg·L-1）on maize and the mechanism of accumulation,migration and transport in maize.XRD,SEM,TEM and EDS were used to characterize the morphology of PbO NPs and the differences in absorption and accumulation characteristics of maize.Plant biomass,root morphology,Malonaldehyde（MDA）,Hydrogen peroxide（H2O2）,chlorophyll,Superoxide dismutase（SOD）,Catalase（CAT）,and Peroxidase（POD）were measured to investigate the toxicity mechanism of PbO NPs to maize.The accumulation level and migration mechanism of PbO NPs in maize were investigated by ICP-AES and TEM.The specific research conclusions are as follows:（1）PbO NPs with particle size of～20 nm was prepared by biosynthesis method.The precipitation kinetics of PbO NPs colloid solution showed that the precipitation occurred after 4h.The ICP-AES test showed that the maximum concentration of Pb ions released from the colloid solution within 4h was 0.437 ± 0.03 mg·kg-1.（2）Corn exposure experiments with different concentrations of PbO NPs（5,10,20,30,40,50 mg·kg-1）showed that PbO NPs completely inhibited the growth status,biomass,root length,chlorophyll,H2O2,MDA and related enzyme activities of corn at the experimental concentration.Compared with the control group,the biomass,root length,chlorophyll,activities of SOD and CAT in all exposed groups showed a general decreasing trend,while the activities of H2O2,MDA and POD enzymes showed an increasing trend.In 50 mg·L-1 processing,the degree of lipid peroxidation of corn leaf,form of H2O2 and MDA content increased dramatically,and corn leaf chlorophyll content in the sharply reduced,plants were under stress,affected the corn of photosynthesis,leaf appeared yellow,green,and curly and tip the phenomenon such as dry,its root color darker,diameter increased,the phenomenon such as the decrease in the number of capillary roots.The results showed that maize seedlings were severely stressed by reactive oxygen species（ROS）and had oxidative stress response under PbO NPs.The toxicity of most indexes in the Pb ion group was higher than that in the CK group,but lower than that in the NPs treatment group,which indicated that the growth of maize under Pb ion treatment was inhibited to a certain extent,but the toxicity effect was lower than that of PbO NPs.TEM results showed that the cell membrane and organelles of maize roots and leaves were damaged to varying degrees.The higher the concentration of NPs,the more serious the damage was.The main cause of plant toxicity was PbO NPs itself.（3）Inductively coupled plasma atom emission spectrometer（ICP-AES）,TEM and EDS were used to confirm that the absorption and accumulation amount of PbO NPs in maize was much higher than that in Pb ion treatment group,and PbO NPs could be absorbed and accumulated by maize roots and migrated to stems and leaves.TEM-EDS results showed that PbO NPs deposits were present in both rhizome and leaf of maize under PbO NPs treatment,and the main layers were accumulated in the intercellular space and near the cell wall.In addition,PbO NPs in maize mainly migrated through symplasts,such as plasmodesmata,endocytosis and cell membrane damage caused by nanoparticles.In addition,the toxic effects of PbO NPs on plants caused low transport efficiency of nutrients needed by plants（K,Ca,Cu,Fe,Mg,Mn,Zn）,which indirectly led to the damage of maize seedlings.On the other hand,the nanoparticles themselves into the plant will lead to the surge of reactive oxygen species,thus causing damage to the plant.Both of them may occur simultaneously in maize seedlings,but PbO NPs is the main cause of maize stress.