Nicholson, Adrian

2019-02-15T14:15:02Z

2019-02-15T14:15:02Z

1959

The plant gums have been defined (1) as "those substances of plant origin which are obtained as exudations from the fruit, trunks or branches of trees spontaneously or after mechanical injury of the plant by incision of the bark or by removal of a branch, or after invasion by bacteria or fungi ". The exudates harden on drying, forrdng nodules of polysaccharide material, with various impurities such as lignin, protein, resinous material and bits of bark.

The gums show many similarities in properties and in molecular structure to the plant mucilages, and are normally reviewed together (1 -6). They are usually distinguished, in that the mucilages exist inside the plant in the form of membrane thickening material, or intracellular substances, and are thought to act either as food stores, or as a means of retaining water. They have also been divided because of their different solubilities in water, the gums generally dissolving to give clear solutions and the mucilages forming colloids.

The plant gums are found on trees and shrubs of many species, but are most commonly produced by fruit trees, and by the small thorny trees which are widespread in hot, dry countries. Gum arabic is the best known and most important commercially of the gums. It has been in use for many centuries, the Egyptians employing it as a paint thickener, as far back as 2000 B.C. It is now harvested in considerable amounts in the Near East, often from trees specially grown in gardens for the production of the gum.

Under the name of gum arabic go secretions from trees of the various species of the genus Acacia. This leads to confusion in structural work, and unless a gum has a known botanical origin, precise conclusions cannot be drawn about the molecular structure of the species. The commercially highest quality gtmì comes from Acacia senegal and the classical work of Smith (24,27,30,31,33,34) on the molecular composition of arabic acid, was carried out on gum of high quality known as Gum Kordofan.

The cause of formation of the gums is still in doubt. Although they seem to be exuded exclusively to seal off the attacked or invaded area, it is unknown whether they are produced by invading micro- organisms, or by the plant to protect itself against such attack, or if they are merely natural secretions of the tree. It has been found that some fungi will induce the formation of water soluble gums on Acacia trees (7), but, on the other hand, in the case of other gums, there is evidence against the secretions being bacterial products. For example, gum tragacanth is produced immediately the bark of the tree is cut. In addition the exudates from different trees of the same species, are remarkably similar in their properties, whereas, one might expect different bacteria to form different gums on trees of the same species. It is also interesting to note that gums are produced mainly by trees which are old, in poor health or short of water. Healthy trees with a plentiful supply of wzter, produce little gum.

http://hdl.handle.net/1842/33390

The University of Edinburgh

Annexe Thesis Digitisation Project 2019 Block 22

Molecular structure of galactose containing polysaccharides of the plant gum and hemicellulose groups

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy