Date of Award

2015-01-01

Degree Name

Doctor of Philosophy

Department

Chemistry

Advisor(s)

Jorge Gardea-Torresdey

Abstract

Previous studies investigating the effects of cerium oxide nanoparticles (nanoceria, nCeO2) on plants have primarily focused on the physiological and biochemical changes at early growth stages. Comprehensive information on the effects of nCeO2 through the entire life cycle of plants and the nutritional quality of the edible tissues is limited. No studies have been reported on the interactions between nCeO2 and common beans (Phaseolus vulgaris). Common beans are leguminous crops, which are societally important due to their nutritional benefits. The beans are rich in proteins and essential nutrients like folate, iron, zinc, molybdenum and magnesium, and are consumed worldwide. This research was performed to comprehend the impact of nCeO2 on plant health, defense mechanisms, yield and nutritional quality of P. vulgaris var. red hawk kidney beans, and further transfer of nCeO2 to a primary consumer in a terrestrial food chain. The broad scope of this research was divided into four major phases. Phase I was focused towards the mechanism of uptake and toxicity in a hydroponic system. Phase II evaluated soil organic matter as a factor towards the impact of nCeO2 on plant physiology, metabolism, productivity, and bean nutritional quality. Phase III involved exploring the molecular mechanisms responsible for modulation of bean nutritional quality by nCeO2. In Phase IV, the possible trophic transfer of nCeO2 from the plant to a primary consumer was examined. To accomplish the goals of Phase I, plants were exposed to nCeO2 suspensions (0, 62.5, 125, 250 and 500 mg/L) in hydroponics and analyzed for Ce uptake and translocation after 1, 7, and 15 days of exposure, using ICP-OES. Cerium localization and speciation in roots were studied by using scanning electron microscopy, synchrotron µ-XRF mapping/µ-XANES. Primary indicators of stress like oxidative stress, antioxidant enzyme activities, soluble protein and chlorophyll content were evaluated using biochemical assays. Synchrotron µ-XRF/µ-XANES provided evidence that Ce enters through the root epidermis maintaining its oxidation state, Ce(IV), as in nCeO2, and reaches the vascular system through the region of emergence of lateral roots, due to the gaps in Casparian strip. Prolonged exposure to 500 mg/L nCeO2 negatively affected the radical scavenging enzymes in the roots, whereas, guaiacol peroxidase played a major role in the leaves to combat stress. In Phase II, the plants were grown in soils varying in their organic matter content (low organic matter soil: 4%, LOMS and organic matter enriched soil: 10%, OMES), amended with 0 to 500 mg/kg nCeO2 through their complete life cycle. Plant tissues were analyzed for Ce accumulation, leaf area, photosynthetic pigments and metabolic activities like net photosynThesis, transpiration, and stomatal conductance. The matured beans harvested upon treatments were assessed for the mineral nutrients and macromolecular composition using ICP-MS, FTIR and colorimetric assays. Organic matter content in soil influenced the response of nCeO2 on metabolic activities and bean quality. Although Ce accumulation in tissues was dose-dependent, Ce translocation to the leaves was significantly higher in plants grown in OMES by 71%, than in LOMS at 500 mg/kg nCeO2. PhotosynThesis and transpiration increased significantly compared to control upon nCeO2 exposure at 62.5, 125 and 250 mg/kg nCeO2 in OMES. Plant productivity in the presence of less organic matter was enhanced when amended with 250 and 500 mg nCeO2/kg, but a bell-shaped curve with increasing nCeO2 concentration was noted in the plants grown in OMES. Nanoceria did not affect the macronutrient content in the beans, however Mo was reduced by 38-61% with respect to control, upon nCeO2 exposure in LOMS. On the other hand, in the seeds from OMES, Na content was reduced by 18-31% with respect to control. FTIR studies showed alteration in the carbohydrates, lipids, and amides in the beans harvested from plants exposed OMES amended with nCeO2. However in the beans from LOMS, only the amides were affected at 62.5 mg/kg nCeO2. In Phase III, proteomic analyses performed using LC MS/MS tandem spectrometry on the beans harvested upon nCeO2 exposure revealed that at 125 and 250 mg/kg, nCeO2 induced two proteins, defensin and purple acid phosphatase, responsible for stress response and metabolism, respectively. However, the number of downregulated proteins increased, with increasing nCeO2 exposure concentration. At 500 mg/kg nCeO2, eighteen proteins associated with protein storage, carbohydrate metabolism, and ATP/GTP binding activities were downregulated. Phase IV of this Dissertation was accomplished by infesting plants exposed to 1000 and 2000 mg/kg nCeO2 with Mexican bean beetles (Epilachna varivestis). The beetles were allowed to grow through their entire life cycle, feeding on nCeO2 exposed plants. Then they were analyzed for Ce accumulation at the various stages of development. Beetles were shown to accumulate Ce in tissues, depending on the exposure concentration, and their food assimilation habits at different developmental stages. Beetles at the pupal stage feeding on 2000 mg/kg nCeO2 accumulated the highest concentration (1300 µg Ce/kg d wt), which decreased to 400 µg Ce/kg d wt in adult tissues. This Dissertation thus provides a holistic understanding of the interaction of nCeO2 with kidney beans at physiological, biochemical and molecular level, and their possible transfer along a terrestrial food chain.

Language

en

Provenance

Received from ProQuest

File Size

143 pages

File Format

application/pdf

Rights Holder

Sanghamitra Majumdar

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