Spearmint 503
31.2 Chemical composition, biosynthesis and genetics of
essential oil
31.2.1 Chemical composition
Natural population of M. spicata
As the plant is liable to give hybrids through spontaneous out-crossing, the essential
oil constituents (terpenes) in the natural populations frequently fluctuate with the
result that a total of nine types of M. spicata oil have been reported to date (Hocking,
1949; Bhattacharya and Chakravorty, 1955; Dhingra et al., 1957; Shimadzu and
Nagamori, 1961; Baslas and Baslas, 1968; Misra et al., 1989; Garg et al., 2000).
These nine types are: (i) carvone and limonene type, (ii) piperitone-oxide type, (iii)
piperitenone-oxide type, (iv) menthone and piperitone type, (v) glyoxal and 1, 8-
cineole type, (vi) linalool, 1,8-cineole and carvone type, (vii) piperitenone oxide
and 1,8-cineole type, (viii) piperitenone and carvone type and (ix) piperitenone and
limonene type.
Cultivated varieties
The cultivated varieties and genetic stocks, the essential oils of which are traded in
the world, always fall in the carvone and limonene rich category of M. spicata
(Tucker, 1992). The main constituent on the basis of their relative concentrations in
the essential oil of the normal varieties/genetic stocks, are: carvone, limonene, linalool,
a terpenic glyoxal C 10 H 14 O 21 , peperitenone oxide, peperitone oxide, menthone, 1,8-
cineole and carvacrol (Garg et al., 2000) (discussed in detail in ‘Quality issues’).
31.2.2 Biosynthesis and molecular genetics
The constituents of the essential oil belong to terpenoids. In general, monoterpenes
(C 10 ) belong to the large class of isoprenoids and are synthesized from five carbon
units of isopentenyl pyrophosphate (IPP) which is produced in plastids by the
methylerythritol phosphate pathway (Flesh and Rohmer, 1988; Brun et al., 1991;
Litchtenthaler et al., 1997). In contrast to the long-standing misconception (prevalent
for about 40 years) that the isoprenoids in living organisms are synthesized only
through the single pathway, i.e., acetate/mevalonate pathway of cytoplasm, Litchtenthaler
et al. (1997) on the basis of extensive inhibitor and precursor studies have discovered
that apart from the cytosolic acetate/mevalonate pathway, there exists an alternative
novel plastidic pathway (GAP/pyruvate pathway) for the synthesis of terpenoids in
higher plants including the medicinal ones, Taxus chinensis and Ginkgo biloba (see
also Schwender et al., 1996).
In mints, including spearmint, monoterpenes are synthesized and accumulated in
the secretory cells of glandular trichomes located mainly in leaves (Gershenzon et al.,
2000). The biosynthetic pathways leading to different monoterpenes have been well
characterized in mints (Fig. 31.1). They are localized in two sub-cellular compartments:
limonene is synthesized in the leucoplasts and subsequent biosynthetic transformations
occur in the cytoplasm. Diemer et. al. (2001) reported that the enzymatic steps are
divided into three stages (Gershenzon and Croteau, 1993). The first stage is the
condensation of IPP with dimenthylallyl diphosphate yielding geranyl diphosphate
(GDP), the universal monoterpene precursor with the interaction of a prenyltransferase
(GPP synthase). In the second stage, this acyclic intermediate is transformed by various
monoterpenes synthases, such as sabinene synthase, cineole-1, 8 synthase, linalool