On the impact of drought-induced abiotic stress on the composition  of Douglas-fir lignin for valorization
Creators: Barker-Rothschild, Daniel
Publisher: name: University of Alberta Library
Year: 2022

<p>Rising interest in the transition to renewable energy, materials, and chemicals ushers biorefining strategies that effectively utilize all components of lignocellulosic biomass feedstocks. <br>
Concurrently, more frequent and severe drought brought on by accelerating climate change pose  questions regarding the reliability of biomass feedstocks. Lignin is a phenolic branched copolymer deposited mainly in the secondary plant cell wall of some plant cells providing strength, rigidity,  and pathogen resistance to most terrestrial plants. Lignin also accounts for ~15-30% of  lignocellulosic biomass composition and up to 40% of its energy content. Lignin has been correlated with drought stress response mechanisms of plants due to the various properties it  confers to the plant cell wall. Lignin has also become a center-point of contemporary biorefining <br>
schemes due to its impact on biomass processing chemistry and inherent potential for various high-value products as the most abundant renewable aromatic carbon source. </p>

<p>One of the most ecologically and economically important conifers in North America known for its large size, high lignin content, and drought resistance; Douglas-fir, is a promising feedstock for contemporary biorefineries. This thesis has investigated the impact of drought  induced stress on the composition of Douglas-fir lignin to assess to what degree lignin may play a role in its drought stress tolerance mechanisms. In particular, this work has looked at lignin content and structural composition which may be useful indicators for assessing biomass feedstock quality.</p>

<p>Wood from the outer rings of stems of Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco] seedlings grown in either severe drought or under normal conditions for one year have been harvested for analysis. Lignin has been extracted from the wood samples using a mild organic solvent extraction approach for more detailed analysis of its structural features. Wood and lignin samples were analysed with wet chemical approaches, attenuated total reflectance Fourier transform infrared spectroscopy, pyrolysis-gas chromatography – mass spectrometry, quantitative phosphorus-31 nuclear magnetic resonance spectroscopy and quantitative solid-state multiple-cross polarization carbon-13 nuclear magnetic resonance spectroscopy. While wet chemical analysis did not discern differences in lignin content, quantitative solid-state carbon-13 nuclear magnetic resonance spectroscopy of wood revealed a 5% higher lignin content in the drought stressed wood compared to the control. This result provides some evidence to suggest lignin biosynthesis may be associated with the drought stress response mechanism in Douglas-fir. Furthermore, quantitative phosphorus-31 and solid-state carbon-13 nuclear magnetic resonance spectroscopy of the isolated lignin indicated slight variations in concentration of distinct hydroxyl group functionalities and other structural features between the samples. Altogether, while this thesis has provided encouraging evidence for associating lignin content and composition with the drought stress response of Douglas-fir, further investigation will be necessary to better understand this mechanism. The findings of this thesis have aided in identifying promising pathways for more comprehensive and sophisticated investigations. The limitations of the current work and new directions for future studies have been discussed in detail.</p>

<p>The first five chapters of this document provide a detailed background of the key areas related to the investigation carried out for this the thesis while chapter 6 describes the experimental work. Chapter 1 provides an introduction and background to the subject along with characterizing the status of the field, defining challenges to be addressed, and presenting a hypothesis for this thesis. Chapter 2 reviews the literature regarding the association between plant drought stress tolerance and lignin, discussing the approaches, current understandings, challenges and limitations. Chapter 3 provides an overview of lignocellulosic biomass fractionation strategies, covering both the industrial approaches as well as laboratory methods. Chapter 4 reviews lignin characterization and the analytical tools for elucidating structural features. In chapter 5, a review of chemometric approaches applied for lignocellulosic biomass and lignin characterization is presented including a detailed background on chemometrics and an organized assessment of approaches applied to different analytical challenges in the field. Chapter 6 describes and discusses the experimental work performed for this investigation, including an analysis of the results and the conclusions regarding the effects of drought stress on the composition of Douglas-fir wood observed in this work. Finally, chapter 7 summarizes the work done for this thesis, presents key findings, limitations, and prospects for future work.</p>