Variation in the Amount of Yield and in the Extract Composition Between Conventionally Produced and Micropropagated Peppermint and Spearmint

Journal of Essential Oil Research: JEOR, Jan/Feb 2005 by Aflatuni, A, Uusitalo, J, Ek, S, Hohtola, A

Abstract

The quality and quantity of oil obtained from three different peppermint Mentha x piperita L. origins and the spearmint Mentha spicata L. were compared in micropropagated and conventionally propagated plants in northern Finland (64�40'N) in 1997-1998. Each block comprised M. piperita peppermint plants from three different origins; namely, the United States, Bulgaria and Egypt (Black Mitcham), and a spearmint, M. spicata of Egyptian origin. The micropropagated plantlets and conventionally propagated plants were transferred to the experimental field in June.

In the first year, there were no differences in the dry leaf yield between the micropropagated plants and their conventionally produced counterparts, although the former had a higher leaf/stem ratio. In the second year, the dry leaf yield of micropropagated plants, which were left in 1997 in field and developed from the underground stolons, was higher than that of conventionally propagated plants.

The percentage of oil was higher with conventionally propagated plants, but only in the first year. Significant differences were observed between mint origins in the different propagation methods. Moreover, conventionally propagated plants showed a higher menthol percentage. There was no significant difference in the isomenthone content of the plants.

In spearmint, the dry yield was higher in the second year with micropropagated plants, but there were no significant differences in the percentage of carvone between the two propagation methods.

Key Word Index

Mentha piperita, Mentha spicata, Lamiaceae, essential oil composition, menthol, menthone, propagation methods, in vitro propagation.

Introduction

There are many reasons for the vegetative propagation of mint. As a hybrid, peppermint seldom produces seeds that are able to germinate. As a consequence, it is propagated exclusively from its vegetative parts, i.e.,green shoots, underground stolons and rooty turions. In addition, high costs and the plant s high demand for manual labor disallow large-scale propagation with green cuttings, which is appropriate only for the fast propagation of improved clones. Plantations are usually established using underground stolons, although the peppermint also is propagated by means of 8-10 cm high turions, developed from underground stolons (1).

The propagation material, used mostly in the multiplication of peppermint, consists of stolons grown in the soil (green) or on the surface (white). The satisfactory development of stolons during intensive cultivation is hindered by several known and unknown factors. The mass of the root system increases with the age of the plant, whereas the quantity of living stolons decreases. In the cold climate conditions of northern Finland, the best method for the conventional propagation of mints is to cut stolons from the field before frost in autumn and store them in a cool storage in moist peat during winter. Towards the end of April they are transferred to a greenhouse for preliminary growing. The plants are transplanted into a field as early as possible after the frost is over (2).

Several papers (3,4) have reported the influence of m vitro growth conditions on the secondary metabolite biosynthesis of terpenes. Peppermint is micropropagated for a number of reasons, including breeding (5) and the rapid multiplication of elite plants (6). Although a number of micropropagation studies have focused on attaining a mint yield of high quality and quantity, only a few have dealt with differences in the yield and oil quality and quantity between conventionally propagated and micropropagated mints grown under field conditions.

The primary goal of the present study was to assess the impact of two specific propagation methods, referred to as conventional propagation and micropropagation, on the yield and oil quantity and quality of peppermint and spearmint after the first year of cultivation in the field. Another objective was to establish whether the epigenotic effects caused by tissue culture observed in first year growth, disappear in the next generation of growth.

Experimental

Plant material: The field experiments were conducted at the North Ostrobothnia Research Station of the Agricultural Research Centre of Finland, in Ruukki (64�40'N) during 1997-1998.

The experiment was set-up in a randomized complete block design at two locations with four replications. The randomized treatments were the propagation methods and the mint origins. Two propagation methods, conventional and micropropagation, were investigated. Each block comprised M. piperita (peppermint) plants from three different origins; namely, the United States, Bulgaria and Egypt (Black Mitcham), and a spearmint, M. spicata, of Egyptian origin.

Mtcropropagaiion: Having been sterilized in 3.5% sodium hypochlorite for 10 min, nodal cuttings were propagated on MS medium (7) with 0.5 mg/L ldnetin. The plants were grown at a temperature of 22�C (16 h light, 8 h dark) at a light intensity of about 50 W m^sup -2^. The shoots were then transferred into MS multiplication medium supplemented with 0.5 mg/L6-benzylaminopurine (BAP) and 0.25 mg/lindole-3-butyric acid (IBA). Roots were generated on WPM medium (8) containing 0.1 mg/L l^sup -1^ IBA. Finally, following a two-month acclimatization in a greenhouse, the micropropagated plantlets were transferred to the experimental field on June 15.