The impact of metal nanoparticles (NPs) on biological systems, especially plants, is still not well understood. The aim of this research was to determine the effects of zinc oxide (ZnO) NPs in velvet mesquite (Prosopis juliflora-velutina). Mesquite seedlings were grown for 15 days in hydroponics with ZnO NPs (10 nm) at concentrations varying from 500 to 4000 mg L-1. Zinc concentrations in roots, stems and leaves were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Plant stress was examined by the specific activity of catalase (CAT) and ascorbate peroxidase (APOX); while the biotransformation of ZnO NPs and Zn distribution in tissues was determined by X-ray absorption spectroscopy (XAS) and micro X-ray fluorescence (mu XRF), respectively. ICP-OES results showed that Zn concentrations in tissues (2102 +/- 87, 1135 +/- 56, and 628 +/- 130 mg kg(-1) d wt in roots, stems, and leaves, respectively) were found at 2000 mg ZnO NPs L-1. Stress tests showed that ZnO NPs increased CAT in roots, stems, and leaves, while APDX increased only in stems and leaves. XANES spectra demonstrated that ZnO NPs were not present in mesquite tissues, while Zn was found as Zn(II), resembling the spectra of Zn(NO3)(2). The mu XRF analysis confirmed the presence of Zn in the vascular system of roots and leaves in ZnO NP treated plants. (C) 2011 Published by Elsevier B.V.