INFLUENCE OF TIO2 ENGINEERED NANOPARTICLES ON PHOTOSYNTHETIC EFFICIENCY AND CONTAMINANT UPTAKE

The production of engineered nanoparticles (ENPs) is growing at an incredibly fast rate and will soon become a trillion dollar industry. At this rate of production, there is a great potential for engineered nanomaterials to be released into the environment, both intentionally and unintentionally. TiO2 ENPs are one of the most widely produced nanoparticles with a broad range of applications in paints, inks, sunscreens, cosmetics, astronautics, and air/water purification. TiO2 ENPs have been proposed for their use in agricultural settings as a UV protectant, a defense against harmful bacteria and fungi, or a catalyst for the degradation of pesticides and herbicides. Furthermore, it has been shown to increase several aspects of photosynthesis in spinach including Rubisco and Rubisco activase activity, chlorophyll synthesis, and oxygen evolution. Foliar application of TiO2 ENPs on spinach resulted in a significant increase in plant fresh weight, dry weight, chlorophyll content, net photosynthetic rate, and carboxylase activity of Rubisco. These findings have prompted investigations for the use of TiO2 ENPs as a foliar spray to promote plant growth and yield. The first main objective of this research was to determine if TiO2 ENPs has the capabilities to increase photosynthetic production in Zea mays at concentrations similar to that of the experiments performed with spinach. Secondly, it was examined if the size of the TiO2 was a factor in the increased photosynthetic response by comparing TiO2 ENPs with bulk TiO2. Finally, the determination of whether the boost in photosynthesis resulted in an increased seed quality/quantity. Another aspect of this research was to determine how the interaction of TiO2 ENPs with inorganic contaminants may affect the uptake and accumulation of the contaminants in plants. Cadmium and arsenic are two of the top ten most hazardous substances on the priority list of the Agency for Toxic Substances and Disease Registry. Sources for Cd and As contamination include atmospheric deposition resulting from mining, smelting, and fuel combustion, phosphate fertilizers, and sewage sludge. Both of these contaminants can be taken up by plant roots and translocated to the leaves and fruits, thus entering the food chain. The release of TiO2 ENPs into domestic and industrial wastewaters is expected to represent the largest release of these nanoparticles. There has been data showing that up to 99% of TiO2 ENPs that enter wastewater treatment plants are retained in the sludge. In addition, TiO2 ENPs are being used at some water treatment plants because of their strong adsorption strength for hazardous materials, such as cadmium, arsenic, and copper and also the photocatalytic breakdown of harmful organic compounds. Since sewage sludge from wastewater treatment plants is applied to agricultural lands as a soil conditioner and fertilizer, this has resulted in the introduction of an estimated 120 g kg-3 per year of TiO2 ENPs. With sewer sludge being the common factor for contamination of agricultural fields, there is a high potential for the simultaneous introduction of TiO2 ENPs and heavy metal contaminants. To date, there has been very little research done for ENP and contaminant interactions. Of the research that has been performed on the subject, the majority of it was conducted using aquatic systems involving fish and daphnids. This research has shown that the interaction of TiO2 ENPs and metal contaminants generally increases the concentration of the contaminant in the organism, however it is still unclear whether the contaminant is biologically available or if it is adsorbed to the surface of the TiO2 ENPs. This information gives rise to two alternative hypotheses on how TiO2 ENPs may affect the fate of heavy metal contaminants in a single substrate growth media. The first is that the TiO2 ENPs may sequester the heavy metals in the soil thus decreasing the amount of the heavy metals that can be taken up by the plant. The alternative is that the TiO2 ENPs could act as a carrier of the metals i.e. if the plant is able to take up the intact TiO2 ENP with heavy metals adsorbed to the surface, it could potentially increase the amount of the metals that enter the plants. The main objective of this study was to determine which of these scenarios is true for broccoli plants that were grown in cadmium and arsenate contaminated growth media