Phytotoxicity And Long-distance Transport Of CuO Nanoparticles In Maize (Zea Mays L.)

Engineered nanoparticles （ENPs） have been widely applied in our daily life forthe properties of their particular surface effect, volume effect, quantum size effect anddielectric confinement effect. In recent years, they have gradually become the focus ofresearch. While, the industrial scale production and wide variety of applications ofENPs and their possible release into the natural environment might cause potentialrisk to environment. The demand of information referring safety and the biotoxicity ofENPs is increasingly rising. International journals Science and Nature publishedarticles to discuss the biological effect of ENPs in2003. And the environmentalsecurity of ENPs has already cause high attention.Plants are an essential base component of ecosystems and play an important rolein the transport and accumulation of in environment. Up till now, the research ofphytoxicity of ENPs was increasing. CuO ENPs are used extensively in commercialapplications, including gas sensors, photovoltaic cells, heat transfer nanofluids,magnetic phase transitions, catalysts and semiconductors. With the rapid developmentof its application, more and more CuO ENPs will release to the environment.However, knowledge on the phytotoxicity and fate of CuO ENPs in higher plantsremain limited. In this study, the typical plant maize （Zea mays L.） was chosen as testorganism. The aim was to investigate the toxicity of CuO ENPs to plant and themechanism of ENPs translocation in maize, which will provide the basis for theenvironmental safety assessment of ENPs.This research studied the effect of different concentrations of CuO ENPs tomaize seeds. The results showed that CuO ENPs had no obvious inhibition on thegermination of maize seeds. The root elongation of the seeds was inhibited by CuOENPs during the germination period. Under the conditions of soil cultivation, theENPs induced visible chlorosis of maize leaves. The influence of stem lengthdecreased as the extension of exposed time. This may be caused by the complexity of the soil composition. When exposing in solution, CuO ENPs had significant inhibitionon seedling growth. The toxic effect of CuO ENPs is dose and exposing timedepended. No equivalent phytotoxicity of Cu2+or corresponding BPs was evident. Sothe phytotoxicity of CuO ENPs comes from itself. The small size and large surfacearea of NPs might be the reason of the toxic to plant.CuO ENPs can adhere to the surface of maize root under the optical microscope.And then CuO ENPs can be found in the root cells and leaf cells by transmissionelectron microscope （TEM） and energy dispersive spectrometer （EDS）. On the base ofour previous research that CuO ENPs can exist in xylem sap of maize. The resultswere present as follows: CuO ENPs may first adhesion on root surface and then passthrough the root apex where the casparian strip is not yet fully developed and move tothe stele, with subsequent transport to the shoots via xylem. The aggregation withhigh electronic density in the root cells was analyzed by high resolution transmissionelectron microscope （HRTEM）, suggesting that ENPs could be translocated fromshoots back to roots via phloem. During this translocation, some of the CuO ENPswere reduced to Cu₂O and Cu2S. ENPs may be associated with metallothionein andcarbohydrates during normal photosynthetic storage activities. These carbohydratesinclude sugars such as glucose and fructose that can serve as reducing agents.