Characteristic Components

Secondary compounds such as those constituting resin differ from primary metabolites in having a restricted distribution in the plant kingdom. Usually, they occur only in particular groups of related plants. Terpenoid resin occurs in most conifer families but is widely scattered among the major evolutionary lineages of angiosperms (Chapter 2). Specific terpenoid skeletal types, however, often characterize taxa such as particular families and genera; thus it has been assumed that the evolutionary history of various taxa can be significant to the understanding of the taxonomic distribution of some of these chemicals (Gershenzon and Mabry 1983).

I introduce a few skeletal structures in this chapter to exemplify components of resins in important conifer and angiosperm plant families, discussed as of value either to the plants themselves or to humans in later chapters. Conifers only produce internally secreted terpenoid resin whereas angio-sperms produce both terpenoid and phenolic resins, which may be secreted internally or on the surface of the plant. This is discussed in detail in Chapter 3.

In addition to the skeletal structure of the compounds, the complexity of the mixture of compounds constituting a resin is important for ecological interactions and human use. In general, among the 20-50 or more compounds that constitute a resin, only a few occur in high concentration. The relative proportions of the compounds in the mixture are called its composition, which may differ in constitutive and induced resins. Because this mixture involves volatile and nonvolatile fractions, the composition of either fraction (or just part of it) or both fractions may be analyzed and compared.

The volatile fraction, which has been most intensively studied, usually consists of mono- and/or sesquiterpene hydrocarbons with some oxygenated forms and, occasionally, diterpene hydrocarbons. The nonvolatile fraction of resin is primarily composed of di- or triterpene acids with some alcohols, aldehydes, and esters in addition to amorphous, neutral substances. The relative proportion of volatile to nonvolatile compounds, which can vary even between species of the same genus, determines a resin's fluidity, viscosity, and polymerization rate. These in turn influence its ecological properties (Chapter 5) as well as the methods used by humans to collect it (Chapters 7-10).

Conifer Resins

Conifer resins, such as those of the pine family (Pinaceae), are characterized by a large volatile fraction (20-50%) with monoterpenes predominating over sesquiterpenes. Both classes most commonly occur as hydrocarbons with a few oxidized forms, often as trace components. Under natural conditions, monoterpenes volatilize with varying degrees of rapidity, providing, for example, the fragrant aromas in conifer forests during warm weather and those from indoor Christmas trees. In fact, monoterpene hydrocarbons from these resins may reach significant proportions in our atmosphere and become troublesome as pollutants. In the soil, monoterpenes from resin may play a role in the nitrogen cycle in conifer forests by inhibiting nitrification. On the other hand, some may supply an energy source for forest soil microbes (Shukla et al. 1968), and others washed from conifer forest soils into estuaries may provide energy for marine microbes (Button 1984). These volatile components of ter-

penoid resin (both mono- and sesquiterpenes) play a major defensive role against insects and pathogens in amazingly intricate ways (Chapter 5). In commercial use in the naval stores industry, the volatile mono- and sesquiterpenes of pine resin produce turpentine, a product used worldwide in solvents and as a feedstock for the flavor and fragrance industries (Chapter 7). Sesquiterpenes (e.g., cedrene) are used as cedarwood oil, again particularly in the aroma industry. Structures of some of the most common volatile mono- and sesqui-terpenes in various conifer resins are shown in Figure 1-3. Note that the abundant monoterpenes are often the ones produced by multiproduct synthases.

Abietane Type

Abietane Type

COOH Abietic acid

Pimarane Type

Pimarane Type

COOH Pimaric acid

COOH Pimaric acid

Labdane Type

'COOH Communic acid

'COOH Communic acid

COOH Ozoic acid

COOH Ozoic acid

Clerodane Type

Clerodane Type

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