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Posted: Mar 27, 2007

Nanoparticles could have a negative effect on plant growth

(Nanowerk Spotlight) Nanomaterials, with at least one dimension of 100 nanometers or less, are increasingly being used for commercial purposes such as fillers, opacifiers, catalysts, semiconductors, cosmetics, microelectronics, and drug carriers. Nanoparticles with a size of between 1 and 100 nanometers fall in the transitional zone between individual atoms (or molecules) and the bulk material. Because the physicochemical properties of material on this scale can greatly differ from the corresponding bulk material, these nanomaterials can have the potential to generate unknown biological effects in living cells. As the discussion on potentially undesired side effects of engineered nanoparticles heats up there is an increasing amount of nanotoxicology research that gets undertaken and published. However, very few studies have been conducted to assess the toxicity of nanomaterials to ecological terrestrial species, particularly plants. In order to develop a comprehensive toxicity profile for manufactured nanoparticles, their phytotoxicity – the ability to cause injury to plants – has to be investigated. A new study examined the effects of five types of nanoparticles on seed germination and root growth of six higher plant species and observed that several types of the particles had significant inhibition on seed germination and root growth of the six plants. If confirmed, these results are significant in terms of use and disposal of engineered nanoparticles.
Only a handful of studies are available on the effects of nanoparticles on higher plants. There are reports of negative effects, such as a 2005 study ("Particle surface characteristics may play an important role in phytotoxicity of alumina nanoparticles") that showed that aluminum oxide nanoparticles, commonly found in everything from cosmetics to environmental catalysts that reduce pollution, can stunt root growth in five plant species (corn, cucumber, soybean, carrot and cabbage), although preliminary findings suggest extremely high concentrations of such particles are necessary for such damage. This study did not clearly address the possible aluminum ion toxicity as a result of particle dissolution.
There are also reports of positive effects where for instance one report (" Effect of Nano-TiO2 on Strength of Naturally Aged Seeds and Growth of Spinach") showed that titanium dioxide nanoparticles promote photosynthesis and nitrogen metabolism, and then greatly improved growth of spinach at a proper concentration.
Nanoparticles can be made from an enormous variety of materials and there are many unresolved issues and challenges concerning the biological effects of these nanoparticles. A new study aimed to provide new information about phytotoxicology of nanoparticles (multi-walled carbon nanotube, aluminum, alumina, zinc, and zinc oxide) on seed germination and root growth of six higher plant species (radish, rape, ryegrass, lettuce, corn, and cucumber). Titled "Phytotoxicity of nanoparticles: Inhibition of seed germination and root growth", this paper is available online as a corrected proof on Environmental Pollution's website.
"This research revealed that manufactured nanoparticles can have negative effect on plant growth" Dr. Baoshan Xing explains to Nanowerk. "Our data indicates that several types of manufactured nanoparticles inhibited seed germination and root growth, particularly zinc and zinc oxide nanoparticles."
Root growth inhibition of rape seeds by  Zn and ZnO nanoparticles
Root growth inhibition of rape seeds by zinc and zinc oxide nanoparticles. (Image: Dr. Baoshan Xing/University of Massachusetts)
Xing is a professor in the Department of Plant, Soil & Insect Sciences at the University of Massachusetts. In a previous Nanowerk Spotlight (" The flip side of using carbon nanotubes for environmental pollutants removal") we covered his study on the interactions of organic contaminants with carbon nanomaterials in water.
"Our study aimed to study the phytotoxicity of nanoparticles in higher plants" says Xing. "This work will help to better understand phytotoxicity of various nanomaterials, pointing to the need to investigate the mechanism of phytotoxicity and the potential impact of nanomaterials on the environment and public health. A long-term goal is to ensure sustainable development and use of nanotechnology."
In testing the five types of nanoparticles, Xing and his co-author Dr. Daohui Lin from the Department of Environmental Science at Zhejiang University in Hangzhou, PR China, observed that the phytotoxicity of zinc (Zn) and zinc oxide (ZnO) nanoparticles was evident – their suspensions significantly inhibited root growth of corn and practically terminated root development of the other five plant species. The carbon nanotube suspension did not show any significant difference from the control (water). Alumina nanoparticles had no phytotoxicity except for corn whose root elongation was reduced by 35%. Aluminum nanoparticles had no obvious effect on cucumber, but, promoted the root growth of radish and rape, and significantly retarded root elongation of ryegrass and lettuce.
Researchers still don't understand the specific mechanism of nanotoxicity but it is a fair assumption that it would be closely related to the chemical composition, chemical structure, particle size and surface area of the nanoparticles.
Xing points out that the mechanism of the nanotoxic effects of zinc and zinc oxide nanoparticles in their experiments also remains unknown. "It may be attributed to two different modes of action" he says: 1) a chemical toxicity based on the chemical composition, e.g., release of (toxic) ions; and 2) stress or stimuli caused by the surface, size and/or shape of the particles. It was reported that solubility of oxide nanoparticles greatly affected the cell culture response. Thus, we may wonder if the phytotoxicity of nano-Zn and nano-ZnO were directly from its dissolution in culture media (solution)."
The researchers conducted two further experiments to exclude zinc ion in culture media from the phytotoxicity of nano-Zn and nano-ZnO and concluded that phytotoxicity of nano-Zn and nano-ZnO was not directly from ion dissolution in culture media, though it might be related to the behavior of nanoparticles on root surfaces.
Xing's findings will be helpful to develop a comprehensive toxicity profile for nanoparticles and are significant in terms of future use and disposal of engineered nanoparticles.
"Future studies should be directed into phytotoxic mechanisms" says Xing, "for example, can nanoparticles be directly taken up and translocated by a plant? What happens once nanoparticles are adsorbed on the root surface? One of the main challenges will be how to detect and determine nanoparticles in plants – their concentrations, forms, locations, and chemical behavior if they can be uptaken."
By . Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology (RSC Nanoscience & Nanotechnology) and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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