Nanotoxicology, December 2011; 5(4): 743–753 Phytotoxicity and biotransformation of La 2 O 3 nanoparticles in a terrestrial plant cucumber (Cucumis sativus) YUHUI MA 1 , XIAO HE 1 , PENG ZHANG 1 , ZHIYONG ZHANG 1 , ZHI GUO 2 , RENZHONG TAI 2 , ZIJIAN XU 2 , LIJUAN ZHANG 2 , YAYUN DING 1 , YULIANG ZHAO 1 ,& ZHIFANG CHAI 1 1 Key Laboratory for Biological Effects of Nanomaterials and Nanosafety, Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, and 2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, P. R. China (Received 9 November 2010; accepted 19 November 2010) Abstract With the increasing applications of metal-based nanoparticles in various commercial products, it is necessary to address their environmental fate and potential toxicity. In this work, we assessed the phytotoxicity of lanthanum oxide (La 2 O 3 ) NPs to cucumber plants and determined its distribution and biotransformation in roots by TEM and EDS, as well as STXM and NEXAFS. LaCl 3 was also studied as a reference toxicant. La 2 O 3 NPs and LaCl 3 were both transformed to needle-like LaPO 4 nanoclusters in the intercellular regions of the cucumber roots. In vitro experiments demonstrated that the dissolution of La 2 O 3 NPs was significantly enhanced by acetic acid. Accordingly, we proposed that the dissolution of NPs at the root surface induced by the organic acids extruded from root cells played an important role in the phytotoxicity of La 2 O 3 NPs. The reactions of active NPs at the nano-bio interface should be taken into account when studying the toxicity of dissolvable metal-based nanoparticles. Keywords: La 2 O 3 nanoparticles, phytotoxicity, dissolution, biotransformation Introduction Recently, engineered nanoparticles (NPs) have been widely used in various fields, such as engineering, electronics, textile, chemical industry, environment protection, medicine and military equipment. Pro- blems of NPs related to environmental significance and biological effects have attracted much attention from scientists. When discharged into the environ- ment, NPs may alter mobility through physical, biochemical, and biological transformations, thereby posing a threat to ecological species (Oberdörster et al. 2005; Nel 2006; Maynard et al. 2006; Wiesner et al. 2006). Plants are an important component of the ecologi- cal system and serve as a potential pathway for the transporting of NPs. Through the food chain, NPs might be accumulated in higher-level organisms (Zhu et al. 2008). Assessing the impact of NPs on terrestrial plants will provide insight into the risk of these materials to ecological species, as well as to human beings. At the initial stage, seed germination and root elongation were examined as standard tests to evaluate the phytotoxicity of NPs on higher plants. Several NPs, such as nano-Cu, nano-ZnO and carbon nanomaterials were found to be toxic to some plant species (Yang and Watts 2005; Lin and Xing 2007; Cañas et al. 2008; Lee et al. 2008; Barrena et al. 2009; Ma et al. 2010), but the mechanism of toxicity of NPs is not well understood. Given the well known toxicity of the ionic forms of some metals (An 2004), the solubility of metal-based nanoparticles may require particular attention. One possible cause of nanotoxi- city is the release of toxic ions when the thermody- namic properties favor particle dissolution in the biological environment. Franklin et al. (2007) com- pared the toxicity of ZnO NPs with that of Zn 2+ to a freshwater microalga and found that toxicity of ZnO NPs might be related to the solubility of the NPs. This was confirmed by the work of Xia et al. (2008), which Correspondence: Dr Zhiyong Zhang, Institute of High Energy Physics, Academia Sinica, 19B Yu Quan Lu, Shijingshan District, Beijing, 100049, P. R. China. Tel: +86 10 88233215. Fax: +86 10 88235294. E-mail: zhangzhy@ihep.ac.cn ISSN 1743-5390 print/ISSN 1743-5404 online Ó 2011 Informa UK, Ltd. DOI: 10.3109/17435390.2010.545487