Plant secondary metabolites

The distribution of secondary metabolites in plants is far more restricted than that of primary metabolites; a compound is often only found in a few species, or even within a few varieties within a species. Though their function in plant metabolism is unclear, nevertheless they may have an ecological role, e.g. as sexual attractants for pollinating insects or in defence mechanisms against predators (Grisebach, 1988). Secondary metabolites often accumulate in the plant in small quantities sometimes in specialized cells. Hence their extraction is often difficult. Among them are many compounds which are commercially important as medicinal substances, fragrances, food additives (pigments, flavouring and aromatic compounds) and pesticides (Heble & Chadha, 19856; Kurz, 1989).

In spite of the progress made in organic synthesis or semi-synthesis of a wide range of compounds similar to those produced by the plants, extraction of secondary metabolites from plants is still of considerable commercial importance. A large number of these metabolites are difficult or virtually impossible to synthesize at economic values. In several cases, the natural product is more easily accepted by consumers than an artificially produced one. The term ‘secondary metabolite’ still applies to a large number of aromas or fragrances which are mixtures of hundreds of different compounds. This is the case with many essential oils, flavours and fragrances. Many of the components, moreover, have complex molecular structures (e.g. some alkaloids and glycosides).

There is great interest in developing alternatives to the intact plant for the production of plant secondary metabolites.