Uptake, transport and toxicity of engineered nanomaterials (ENMs) into plant cells are complex processes that are currently still not well understood. Parts of this problem are the multifaceted plant anatomy, and analytical challenges to visualize and quantify ENMs in plants. We critically reviewed the currently known ENM uptake, translocation, and accumulation processes in plants. A vast number of studies showed uptake, clogging, or translocation in the apoplast of plants, most notably of nanoparticles with diameters much larger than the commonly assumed size exclusion limit of the cell walls of ∼5-20 nm. Plants that tended to translocate less ENMs were those with low transpiration, drought-tolerance, tough cell wall architecture, and tall growth. In the absence of toxicity, accumulation was often linearly proportional to exposure concentration. Further important factors strongly affecting ENM internalization are the cell wall composition, mucilage, symbiotic microorganisms (mycorrhiza), the absence of a cuticle (submerged plants) and stomata aperture. Mostly unexplored are the roles of root hairs, leaf repellency, pit membrane porosity, xylem segmentation, wounding, lateral roots, nodes, the Casparian band, hydathodes, lenticels and trichomes. The next steps towards a realistic risk assessment of nanoparticles in plants are to measure ENM uptake rates, the size exclusion limit of the apoplast and to unravel plant physiological features favoring uptake.

ii ACKNOWLEDGEMENTS I would like to thank my advisor Jerry Schnoor for allowing me this opportunity to come here as well as providing the grants and supports to fund me. I would like to thank Guangshu Zhai and Nastaran Moradi for their help and direction during my research, and Kathy Walters who graciously walked me through the TEM process and helped me understand the images. I also thank Dr. David Peate for patiently assisting me during my time prepping, measuring, and analyzing my samples with the ICP--MS. Finally, I would like to thank my family and friends for supporting me during my time here at The University of Iowa. I would especially like to thank my mom, Marriah Kern, Melanie Kern--Kuhn, and Melissa Karo.iii PUBLIC ABSTRACT By the year 2020, the nanotechnology market is expected to be three trillion dollars. With a quasi--exponential increase in consumer products, which contain nanomaterials, there is likely to be an equal increase in nanoparticles entering the environment. As a result, it is imperative to fully understand the relationship between nanomaterials and the food chain, including plants.In this study, the relationship between gold nanomaterials and the edible food crop, Helianthus annuus was investigated. First, an attempt to inhibit the uptake of nanoparticles into the roots of H. annuus was investigated by decreasing temperature. Second, the interactions between citrate--stabilized 20 nm diameter Au nanoparticles and sunflower seedlings were explored by exposing sunflower to a range of concentrations (3.0--40.0 mg/L). Nanoparticle sorption to roots was estimated using a linear isotherm with a distribution coefficient, Kd. Finally, sunflowers were exposed to 20 nm Au nanoparticles and 25x69 nm CTAB--stabilized Au nanorods. Results showed there was no change in biomass growth and transpiration between sunflowers that were exposed to nanoparticles and the unexposed controls. Thus Au gold nanoparticles (20 nm) were shown to have no phytostimulatory or phytotoxic effect on sunflower seedlings during eight to ten day exposure experiments. However, 25x69 nm gold nanorods were phytotoxic to sunflowers at 6.0 mg/L, indicating a potential charge or chemical effect of the surface coating of the nanorods compared to the spherical gold nanoparticles iv SCIENTIFIC ABSTRACTBy the year 2020, the nanotechnology market is expected to be three trillion dollars. Inevitably, these man--made materials will accumulate in the environment and it is imperative to fully understand the relationship between nanomaterials and the food chain, including plants.In this study, the relationship between gold nanomaterials and the edible food crop, Helianthus annuus were investigated. First, an attempt to inhibit endocytosis and uptake into H. annuus root cells was investigated by decreasing temperature. Second, the interactions between citrate--stabilized 20 nm diameter Au nanoparticles and sunflower seedlings were explored by exposing sunflower seedlings to a range of concentrations (3.0--40.0 mg/L). Nanoparticle sorption to...