Resin is sometimes referred to in a general manner, such as sap or exudate, both of which include numerous substances from plants. Throughout written history there has been a tendency to characterize resin vaguely as any sticky plant exudate. In some dictionaries, this definition has been extended to include substances that are mainly insoluble in water and that ultimately harden when exposed to air. Nevertheless, the vagueness of even this amended definition has led to continued confusion with other plant exudates, including gums, mucilages, oils, waxes, and latex. Some terms such as gum have often been used synonymously with resin; in fact, one prominent forest products researcher has referred to the use of these terms as "haphazard" (Hillis 1987). A better definition of resin, however, has awaited more knowledge about their chemistry, secretory structures, and functions in the plant.
Interest in the chemistry of resins and the secretory structures in which they are synthesized and stored began in the later 19 th century in Germany. A pioneering book, Die Harze und die Harzebehalter, resins and resin-containing structures, was published by Tschirch and his students in 1906. Recognition that detailed chemical knowledge of plant exudates would be valuable, perhaps essential, for their commercial utilization led to the voluminous publications in the 1930s by Tschirch and Stock (1933-36) and others (e.g., Barry 1932). Nonetheless, only with the advent of various kinds of chromatography and spectroscopy in the 1940s and 1950s was real progress made in identifying the chemical constituents of resins and quantifying their composition. All the exudates that have been confused with resin in the past can now be distinguished from resin in their pure form by chemical composition and by the bio-synthetic pathways through which they are formed. Information about resin secretory structures has become available through advances in plant anatomy, including electron microscopy (Chapter 3), and from ecological studies regarding the survival roles played by resins (Chapter 5). Together, these data provide criteria for a definition to minimize the confusion surrounding the term resin.
Thus in Plant Resins, plant resin is defined operationally as primarily a lipid-soluble mixture of volatile and nonvolatile terpenoid and/or phenolic secondary compounds that are (1) usually secreted in specialized structures located either internally or on the surface of the plant and (2) of potential significance in ecological interactions. Note that resins consist primarily of secondary metabolites or compounds, those that apparently play no role in the primary or fundamental physiology of the plant. In addition to being preformed and stored in secretory structures, resins sometimes may be induced at the site of an injury without forming in a specialized secretory structure. Moreover, resin occurs predominantly in woody seed plants. Amber is fossilized resin (Chapter 4).
Although terpenoid resins constitute the majority of copious internally produced resins that have been used commercially, some important resins are phenolic. Phenolic resin components occurring on the surfaces of plant organs have been used, particularly in medicines, and may be useful as a bio-
mass source of fuel; however, their overall significance is probably greater as protection for vulnerable plant surfaces. Resin components are derived from photosynthetically produced carbohydrates that are broken down to produce simpler compounds (pyruvate products); terpenoid and phenolic compounds are then synthesized via different metabolic pathways (Figure 1-1).
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