464 Handbook of herbs and spices
between 0.40–0.45%. Out of these, CIM Indus provides menthofuran rich oil, while
other varieties yield sweet smelling oil usually rich in menthol with a balanced
quantity of menthone, menthyl acetate and menthofuran.
The chromosomes in Mentha are of relatively small size, responsible for its wide
adaptability and world-wide distribution. Shehudka and Korneva (1980) used the seed
produced through free pollination of M. piperita, characterized by different degrees
of seed productivity and made selection of seedlings and clones with high essential
oil content (up to 4%) and menthol (up to 65%). Induction of variability through
physical and chemical mutagens has also been attempted by several workers. Only a
few mutants have reached the stage of advanced field testing. A broad-leafed natural
mutant in peppermint crop was identified in a population of Mitcham mint by Singh
et al. (1982), who found it to grow better under sub-tropical conditions in India.
Interspecific somatic hybridization by protoplast fusion was carried out between
M. piperita L. cv. Black mint and ginger mint (M. gentilis L. cv. Variegata). These
protoplasts divided to form calli under the conditions developed for peppermint
protoplast culture. Callus-derived shoots were grown to whole plants, and some with
intermediate character between the parental plants were tested for their volatile
constituents by gas chromatography. Among the four plants investigated, one had
three major volatile constituents, menthone, menthol and linalool (the major component
of ginger mint oil). Chromosome counts and random amplified polymorphic DNA
analysis indicated that it was an inter-specific somatic hybrid between two species
(Satoa et al., 1996).
Jullien et al. (1998) developed an efficient protocol for plant regeneration from
protoplasts of peppermint by stepwise optimization of first cell division, microcalli
formation and shoot differentiation. Transgenic peppermint plants were obtained by
using Agrobacterium tumefaciens-mediated gene transfer. Transgenic plants were
successfully acclimatized in the greenhouse (Diemera et al., 1998). Genetic
transformation of Mentha arvensis and M. piperita with the neomycine
phosphotransferase marker gene and a 4S-limonene synthase cDNA from M. spicata
led to the regeneration of 47 transformed plants. Quantitative and qualitative
modifications in monoterpene levels were observed in transgenic plants. Four M.
piperita transgenic plants contained higher levels of total monoterpenes, whereas
monoterpene levels were lower in two M. arvensis transgenic lines. In both M.
piperita and M. arvensis, transgenic lines, altered levels of compounds formed directly
from geranyl diphosphate such as cineole or ocimene and monoterpene end-products
such as pulegone or piperitone were observed (Diemera et al., 2001). These data
demonstrate the feasibility of modifying essential oil content by the introduction of
a cDNA encoding an enzyme involved in monoterpene metabolism.
In temperate conditions, a plantation lasts about four years, the best output being
the second year. The fourth-year crop is rarely good. A crop that yields a high
percentage of essential oil exhausts the ground as a rule, and after cropping with
peppermint for four years, the land must be put to some other purpose for at least
seven years. The monocultures of peppermint pose the potential hazards of continuous
cropping including eventual suppression of soil fertility, productivity, soil structure,
and microbial activity. Intercropping peppermint with soybean in Italy resulted in
yield and quality increases in the essential oil, compared to sole peppermint cultivation.
The yield was higher by about 50% on an equal land area basis and higher percentages
of menthol and lower percentages of menthofuran and menthyl acetate improved the
quality of the oil (Maffei and Mucciarelli, 2003). Peppermint is grown as an annual