Q. Can you explain Secondary Metabolites?
Secondary metabolites or secondary plant products are those macromolecules that lack nitrogen and arc of restricted occurrence and therefore of greater taxonomic importance the an primary metabolites. This group includes different kinds of compounds such as phenolics, alkaloids, terpenoids, etc. They are usually not involved in vital functions and are largely storage products or pigments.
Amongst the secondary metabolites, flavonoids, which are the commonest phenolic compounds of leaves, have been very useful for chemotaxonomic purposes. Both monocots and dicots have been extensively surveyed for these compounds which show structural variability and chemical stability besides widespread distribution. They can be rapidly and easily identified and provide important chemical characters for
taxonomic purposes. For example, 80 species of plants from the family Ulmaceae were investigated for their flavonoid chemistry by Ciannasi (1978). A majority of the species contain flavonols, but a few species have glyco-flavonol and these two types of flavonoid compounds are never present together in any species. Interestingly, enough, in most classical systems of classification, the family Ulmaceae is divided, into two subfamilies called Ulmoideae and Celtoideae which are aiso distinguishable
by the flavonoid chemistry. Therefore, morphological criteria combined with flavonoid dichotomy can be used to divide the family Ulmaceae (sensu lato) into two distinct families: family Ulmaceae (sensu stricto) characterised by the presence of flavonols, and family Celtaceae. characterised by the presence of glucoflavonols. Several other studies have used flavonoid chemistry for taxonomic purposes in families such as Arilidaceae, Cornaceae, Labiatae (Lamiaceae), Leguminosae (Fabaceae), Orchidaceae, Rutaceae, Lemnaceae and others.
A second group of secondary metabolites commonly examined by chemotaxonomists are the terpenes. Chemically speaking, these compounds can be classified on the basis of their molecular structure into monoterpenes, diterpenes, triterpenes, , sesquiterpenes, etc. and each group can be used for taxonomic purposes. For examp1e;;'the genus Salvia, 19 species could be distinctly identified and classified on the basis of their monoterpenes. The terpene composition was as 'useful as the morphological' characteristics in the analysis of introgression and hybridisation within the genus. Similarly, triterpenes and sesquiterpenes have been particularly important and useful in the classification of the families Cucurbitaceae and Coinpositae (Asteraceae) respectively.