Date of Award
Doctor of Philosophy
Rapid production and usage of engineered nanomaterials (ENPs) in the recent years has highlighted the need to address their potential impact to biological systems. The interaction of ENPs with plant systems is not well known or understood. As plants are the base of all ecosystems, there is a possible threat of ENPs entering food webs and the human food supply. In addition, the mechanism of nanoparticle uptake and toxicity in plant systems is not fully understood so there is considerable opportunity for investigation in this area. To provide an assessment of the potential risk of ENPs to plants, the proposed research has examined the interaction of CuO and ZnO ENPs with carrot, parsnip and wheat. This research was completed in three phases in which different aspects of nanoparticle uptake and toxicity were addressed. Phase I analyzed the uptake and accumulation of CuO and ZnO ENPs in carrot and parsnip and gave information regarding physiological impact. Phase II was aimed at the evaluation of CuO and ZnO ENPs as potential micronutrient fertilizers using nutritionally altered carrots. Phase III was focused on comparing the uptake characteristics of CuO or ZnO ENPs to Cu2+ or Zn2+ ions using specific membrane transport inhibitors in wheat. To accomplish goals of phase I, a preliminary experiment was performed to evaluate the behavior of CuO and ZnO ENPs in aqueous suspensions and get information on the extent of dissolution at increasing concentrations of ENPs. For the main experiment, 192 d old hydroponically grown carrot or parsnip plants were treated for 10 d with increasing concentrations of one of the two ENPs with increasing concentrations of the corresponding metal (i.e. 0, 10, 100, 500, or 1000 mg L-1) in deionized water. An ionic treatment was also included, based on preliminary experiment, to distinguish between ion related and nanoparticle-specific toxicity. Plants of both species accumulated the corresponding metals from ENPs in their storage organs. The metal concentration changed in a dose dependent manner and showed saturation at higher concentration treatments. The outer layer of storage organs or peels for both the plant species accumulated significantly higher concentrations of metal as compared to the flesh. There was a significant difference in the total amount of water transpired during the treatment period, suggesting an imbalance in water relations. Overall, this study has provided information describing the impact of ENPs on these two plants and the accumulation of Cu or Zn from CuO or ZnO ENPs. In Phase II, carrot plants were hydroponically grown for 158 d under normal conditions and subsequently subjected to nutrient regime change in a 30 d pre-treatment. Based on the initial level of Cu or Zn present in nutrient solution, plants were grown in similar nutrient solution except for increasing concentrations of CuSO4.5H2O (0, 0.5 and 2.5 μM) or ZnSO4.7H2O (0, 1 and 5 μM) to generate Deprived, Replete and Surplus sets of plants in pre-treatments. The plants from each of the sets were divided into three subsets to get control, ionic or ENP treated plants and treatments were enforced. Control plants received no Cu or Zn whereas ionic and ENP treated plants received 50 mg Cu L-1 or 100 mg Zn L-1 in sulfate salt and nanoparticle form during the 7d treatment period. The visual results show that ZnO ENPs had similar performance as compared to the ion group whereas in CuO ENPs showed a slightly greater phytotoxic effect(s) as compared to ion treatments. The plant’s Zn nutritional status had a significant effect on uptake of Zn in all analyzed tissues, whereas the effect on the plant’s Cu nutritional status had a mixed response to Cu source and trends showed significant variability. The presence of plants significantly promoted dissolution of ENPs as compared to controls, but the change in nutrient regime had no significant effect on the dissolution behavior of ENPs. Overall, our results show that use ZnO ENPs can be a promising way of providing Zn to plants, but the same may not be true in case of CuO ENPs for the supply of Cu. In Phase III, 25 d hydroponically grown wheat seedlings were exposed to specific membrane transport inhibitors (carbonylcyanide-m-chlorphenylhydrazole, vanadate, lanthanum) in the presence of either ions (Cu2+ or Zn2+) or ENPs (CuO or ZnO) in a 3 d treatment period. Separate treatments consisting of either membrane transport inhibitors or ions or ENPs were also included to serve as a reference along with controls which received deionized water only. In the Cu experiment, elemental analysis of whole plant showed significant difference only in case of CCCP and vanadate with a decrease in Cu concentration in CuO treatments which suggests the transport of CuO ENPs is reduced by the presence of a protonophore or P-type ATPase. The result of Zn experiment found significant differences in Zn accumulation only in case of CCCP in Zn2+ treatments with a decrease in Zn uptake which implies only Zn2+ ions were affected by the presence of a protonophore (CCCP). The results of these experiments provided a mechanistic insight to the uptake of ENPs and ions in the wheat plant. To sum up, the results gathered in this dissertation has provided a comprehensive understanding of the metal uptake characteristics from the corresponding ENPs, given information on the physiological impact of ENPs to selected plants and analyzed a potential use of ENPs to address a global nutritional problem. The results of this study have potential implications for agriculture, food safety, environment, and human health.
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