Date of Award

2018-01-01

Degree Name

Doctor of Philosophy

Department

Environmental Science and Engineering

Advisor(s)

Jorge L. Gardea-Torresdey

Abstract

Engineered nanomaterials (ENMs) have proven to be one of the emerging chemicals of concern in the contemporary times. Soil acts as one of the major sinks of these ENMs. Reports have shown that ENMs have varied effects on soil biota. Particularly, their effects on plants are inconsistent. Amongst these ENMs, ZnO nanoparticles (nZnO) are the fourth largest raw materials in the nanotechnology industry. Globally, it is estimated that around 34,000 tons of n ZnO are utilized per year. The nZnO exposure on terrestrial plants yielded both beneficial as well as detrimental effects. Recently, there is an emerging evidence about the scope of nZnO as a nanofertilizer. The beneficial effect is attributed to sustained Zn2+ release property of the nZnO compounds. On the other hand, at elevated concentrations it has proven to exert oxidative stress upon plants. Thus affecting their ambient growth and development. The factors that contribute to these conflictory findings are yet to be known. It is also necessary to determine if application of antioxidants such as L-ascorbic acid would potentially alleviate oxidative stress exerted by nZnO exposure. Most studies reported in literature are been carried out in soil-grown plants. Considering the changes in recent agricultural practices, we lack knowledge about the ZnO exposure towards plants grown in soil-less media such as hydroponics and nutrient media studies. Thus, there are more questions than answers concerning the role of concentrations, media, and plant response in overall assessment of nZnO exposure.

Cilantro (Coriander sativum) and radish (Raphanus Sativus) are herb plants widely used since immemorial times in various cuisines across the globe. These plants are edible both cooked as well as in raw form (roots, leaves, and seeds). Besides nutritional components, the herbs possess anti-oxidant and metal chelating properties that help maintain a good health. In order to enhance our knowledge about the impact of nZnO exposure on these plants, the research project was carried out in three parts.

In the first part, cilantro plants were cultivated for 35 days in soil amended with ZnO nanoparticles (nZnO), bulk ZnO (bZnO) and ionic ZnCl2 (Zn2+) at 0-400 mg/kg. This study was aimed to assess the metallomics, 1NMR metabolic profiling and biochemical alterations upon the aforementioned exposure.

Part two was aimed to study the impact of soil amended nZnO and Zn (NO3)2 (Zn2+) and foliar L-Ascorbic acid (Asc) exposure on cilantro at concentrations 500 mg/kg and 200 mg/L respectively. At the seed development stage (35 days), the plants were harvested. The biomass, pigment contents, stress enzymes, and metallomics were evaluated. In the third part, the radish seeds were exposed to nZnO and ZnCl2 (Zn2+) suspensions/solutions at concentrations 0-400 mg/kg and the sprouts allowed to growth for 8 days. At harvest, germination, biomass, metallomics and FTIR-based biomolecule conformational changes in plant tissues were evaluated.

Results from the first part, indicates n400 and b400 treatments increased chlorophyll content in cilantro at least by 50%, compared with control (p > 0.05). Additionally, nZnO at 400 mg/kg decreased the lipid peroxidation by 70%, compared with control. The highest Zn uptake in roots was observed with b400, while and shoots and Zn2+ 100 treatments, respectively. Finally, the 1NMR data showed alterations in carbinolic regions (pertaining to lipids) of the plant metabolites. In part two, all the treatments nZnO, Asc + nZnO, Zn2+ and Asc+ Zn2+ did not affect the chlorophyll or lipid peroxidation compared with control. Asc+nZnO decreased carotenoid content by 47% in comparison to control (p < 0.1). Furthermore, the same treatment increased the dry biomass and catalase content by 300% compared with control. The highest Zn uptake was observed for the Zn2+treatment, while the lowest Zn uptake was obtained with Asc+ nZnO treatment. In part three, all treatments biomass accumulation of radish seedlings by 70 and 58% were observed for Zn2+ 200 and Zn2+ 400 treatment (p > 0.05). The nZnO and Zn2+ at 400 mg/L reduced seed germination by 50%, compared with control. Highest Zn uptake in radish seedlings was observed at Zn2+100 and Zn2+ 400 treatments. Finally, FTIR spectra of all plant tissues have revealed functional group based conformational changes pertaining to lipids, carbohydrates and proteins. The Zn2+ at concentrations of 200 and 400 mg/kg had clearly caused band shifts in the spectra.

Above results suggest that ENMs such as nZnO at concentrations (< 400 mg/kg) may be beneficial to soil grown cilantro as it can improve pigment content as well induce enhanced stress response.. Furthermore, the application of ascorbic acid in combination with nZnO can enhance the reduce the metal uptake and enhance the plant biomass to a good extent. However, aforementioned beneficial results may be plant and media specific. The beneficial results may not be replicated in soil-less medium such as hydroponics and media cultures. In the case of radish seedlings study, the nano and ionic treatments at 400 mg/L have led to deleterious effects on seed germination, biomass and caused band shifts in the regions pertaining to lipids, proteins and carbohydrates of FTIR spectra.

Language

en

Provenance

Received from ProQuest

File Size

114 pages

File Format

application/pdf

Rights Holder

Venkata Laxma Reddy Pullagurala

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