Finlay, Christine Jane
Pectic methyl and non-methyl esters and the environmental implications of methanol emissions from plants.
PhD thesis, University of Glasgow.
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Pectin methyl esterase (PME) enzymes are produced by bacteria, fungi and higher plants, and hydrolyse methyl ester groups present on the backbone of pectic polysaccharides found in the primary cell wall of plants.
Pectic polysaccharides are required for intercellular adhesion in dicotyledonous plants. To fulfil this function, they need to be cross-linked, covalently or non-covalently. The texture of plant-based foods is affected by the location and extent of these linkages, ultimately determined by chemical and subsequent structural modifcations undergone by the methyl ester groups on the pectic polysaccharide backbone during growth, ripening, storage, cooking and processing. These processes result in spatial variations in cell separation and adhesion betwen the walls of adjacent cells. These modifications are due to the action of PME enzymes. It has been suggested that covalent intercellular linkages are formed by glycosidic bonds between xyloglucan and acidic pectins. However, the presence of non-methyl esters has been confirmed and these linkages are candidates for the role of intercellular adhesion. The correlation between the quantity of these non-methyl ester groups and the deterioration of potato tuber texture during the period of storage following harvest was explored in two potato cultivars using titrimetric techniques and atomic absorption spectrometry.
Pectic methyl ester groups in plant material are a major reservoir of methanol in the biosphere. PME enzymes are responsible for the cleavage of the ester bond between theses methyl groups and the pectic polysaccharide backbone, resulting in the release of methanol and acidic pectin. Methanol from both anthropogenic and biogenic sources is an important precursor of the gaseous pollutant, tropospheric ozone. Thus the accurate quantification of methanol emitted to the troposhere from both growing and decaying plant material is essential as the global balance of gases in the Earth's atmosphere is continuously skewed as a result of anthropogenic activities. Particular principles and techniques used in the quantification of methyl ester groups in potato tubers were developed and utilised in order to quantify the amount of methanol contained in the leaves from tree species native to Great Britain, in the form of pectic methyl esters, that could potentially be emitted to the troposphere as a consequence of the senescence of these leaves.
The potential impact on the quantity of methanol in the troposphere of the growth of certain crop species grown commercially in the UK was investigated. A modified bell jar headspace capture and purge emthod and thermal desorption technique, in conjunction with gas chromatographic (GC) quantification, were developed and used to quantify methanol emitted from the developing tissues of plant species with significant agronomical importance, both in the UK and globally.
Results obtained from the determination of pectic methyl esters contained in mature leaves of tree species, native to Great Britain and growing sites in Scotland, were extrapolated using available leaf litterfall and leaf area index (LAI) data to account for total area of trees of the same species growing across Great Britain and the globe. The results obtained following the quantification of methanol emitted from growing plant species of agricultural significance were extrapolated to ultimately estimate the quantity of methanol being emitted from these plant species on a global scale annually.
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