Date of Award

2020-01-01

Degree Name

Doctor of Philosophy

Department

Chemistry

Advisor(s)

Jorge L. Gardea-Torresdey

Abstract

With the recent increase of nanomaterial production, nano copper oxide (nCuO) and surface-modified titanium dioxide nanoparticles (nTiO2) are among the most widely applied nanoparticles in industry and daily lives. Their use has resulted in accumulation in soils as a consequence of their direct or indirect release. Hence, these NPs may raise a potential risk to crops cultivated in soils. Moreover, the physiological effects of nCuO on green onion (Allium fistulosum) and surface-coated nTiO2 on full-grown carrot (Daucus carota L.) are still unknown. Green onion is characterized by its high content of the antioxidant allicin, and carrot is a worldwide economic and nutritionally important taproot crop. After environmental release, NPs may remain in the soil for a long time and undergo surface chemical/physical changes. Currently, no studies have addressed the soil weathering effects on the chemistry of surface-coated TiO2 and their implications on belowground carrots. Thus, this research was aimed at evaluating the physiological and biochemical responses of green onion and carrot plants after being exposed to nCuO and surface-coated nTiO2, respectively.

This research was performed into three phases. In phase I, green onions were harvested after growing for 80 days in soil treated with nano (nCuO), bulk (bCuO), and CuSO4 at 75–600 mg/kg]. Two-photon microscopy images confirmed the Cu uptake in nCuO and bCuO-treated roots. Plants cultivated with soils at 150 mg/kg of the Cu-based compounds exhibited a higher root Cu content when treated with nCuO, in comparison to bCuO, CuSO4, and control (p ≤ 0.05). The nCuO increased root Ca and Fe, bulb Ca, and Mg, while bCuO reduced root Ca and Mg, compared with controls (p ≤ 0.05). At all evaluated concentrations, nCuO and CuSO4 augmented leaf allicin compared with control. The activity of antioxidant enzymes was differentially affected by the treatments. In phase II, carrots grew in soils amended with nTiO2 at 0, 100, 200, and 400 mg/kg for 115 days until plant maturity. Three types of nTiO2 in the form of pristine, hydrophilic, and hydrophobic surface-coated were applied. The fresh biomass of the taproot was significantly decreased by all nTiO2 forms at 400 mg/kg, compared to control. Remarkably, an abnormal increase of taproot splitting was found in plants treated with all nTiO2 forms. It was more significant in plants exposed to pristine nTiO2 treatments. In carrots treated with the surface-coated nTiO2, the accumulation of Ca, Mg, Fe, and Zn increased in leaves; but Mg, Mn, and Zn decreased in taproots. In phase III, surface-coated nTiO2 were aged in the soil for four months at 0, 100, 200, and 400 mg/kg, and their effect on carrot development was investigated. The aged nTiO2 with surface coatings improved taproot and leaf fresh biomass, plant height, total taproot length, and taproot top perimeter, compared to control. Hydrophilic and hydrophobic-coated nTiO2 showed more stimulation effects on plant development. Moreover, changes in surface charges of aged pristine, hydrophilic, and hydrophobic surface-coated nTiO2 were detected. Additionally, the accumulation of nutrient elements such as Ca, Zn, and K in roots, Fe in leaves, and Mg in taproots was enhanced by aged nTiO2.

Overall, our results demonstrated that nCuO improved the nutrient and allicin contents in green onion, which suggests they might be used as a nanofertilizer. Meanwhile, the overall growth of carrots was inhibited by the unaged nTiO2, while benefited by the aged nTiO2. The future regulation of nTiO2 release into soils should consider its surface coating properties since plant responses depended on the nTiO2 outer structure. These studies provide valuable insights into the interaction of nCuO with green onion, and aged and un-aged surface-coated nTiO2 with carrots.

Language

en

Provenance

Received from ProQuest

File Size

135 pages

File Format

application/pdf

Rights Holder

Yi Wang

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