Abstract

Novel agricultural applications are rapidly emerging to protect crops from weeds or pests in the US and worldwide. Isoxaflutole is a proherbicide considered agriculturally significant because it controls weeds that have developed resistance to other commonly used herbicides. In addition, RNA interference (RNAi) technology is regarded as a next-generation biopesticide because the technology has less toxicity to humans and more specificity to pests than traditional pesticides, which pose risks to the environment and the health of humans and livestock. However, we cannot disregard the possible risks of the emerging proherbicides and biopesticides to humans or non-target organisms. Therefore, the study of the environmental attenuation pathways (e.g., hydrolysis and photolysis) of these emerging agricultural applications is necessary to develop regulations and additional agricultural applications. The first objective examined the transformation of isoxaflutole and its active form, diketonitrile, via abiotic hydrolysis and photolysis at circumneutral pH, which are both key processes impacting the fate of these contaminants. Because isoxaflutole hydrolysis is triggered by hydroxide and buffer ions, the concentration of buffer salts was controlled to ensure that buffer salts hydrolysis did not significantly contribute to isoxaflutole hydrolysis at most pH values. Consequently, isoxaflutole hydrolyzes to diketonitrile with half-lives that are much longer than prior modeling suggested. After correcting for hydrolysis, I found that isoxaflutole photolyzed under simulated sunlight with a steady quantum yield in the buffers and surface waters, corresponding to a predicted near-surface half-life, which was also longer than previously suggested. Diketonitrile, which does not hydrolyze in all matrices, underwent slow photolysis despite significant absorbance within the solar spectrum, resulting in a much lower quantum yield than isoxaflutole quantum yield. To investigate the kinetics of isoxaflutole photolysis, I derived a model showing that diketonitrile was not a primary isoxaflutole photoproduct. Isoxaflutole photolysis generated several photoproducts, which showed different stabilities in the buffers at two pH values. The second objective examined the direct photolysis of dsRNA, the RNAi product, on glass and polytetrafluoroethylene (PTFE) surfaces, which represent the conditions of the leaf surface. The photolysis of dsRNA under simulated sunlight was accelerated by orders of magnitude when dsRNA was dried on surfaces as opposed to solution, when measured using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). I investigated the effect of the absorption of dried nucleic acid on the rapid surface photolysis and concluded that the dried dsRNA did not absorb more light on the surfaces than in solutions, suggesting that this accelerated photodegradation resulted from an increase in the photochemical quantum yield. To classify the photodamage of dsRNA, I analyzed dsRNA degradation using gel electrophoresis, indicating that photodegradation was not attributable to strand breaks. The rapid surface photolysis was more likely associated with photodegradation of nucleobases, resulting in accelerated loss of nucleoside monophosphates (NMPs) from digested dsRNA after its photodegradation on the glass surface compared to in solution. Overall, this study provides a comprehensive photochemical analysis of the emerging agricultural applications under environmental conditions. The photolysis of isoxaflutole and diketonitrile provides the clear kinetics and predicted half-lives of proherbicides under the environmental conditions. The unexpected photolysis behavior of dsRNA on the surfaces shows the importance of dsRNA photolysis in foliar application. This study indicates that the photolysis of the proherbicide and biopesticide is potentially comparable to the other environmental attenuation pathways considered in the prior environmental fate assessment.

Committee Chair

Kimberly Parker

Committee Members

Daniel Giammar; Jeffrey Catalano; Yinjie Tang; Young-Shin Jun

Degree

Doctor of Philosophy (PhD)

Author's Department

Energy, Environmental & Chemical Engineering

Author's School

McKelvey School of Engineering

Document Type

Dissertation

Date of Award

8-18-2025

Language

English (en)

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