A study of some factors affecting the yield and composition of peppermint oil (mentha piperita L.)

This study attempted to define some of the factors or groups of factors which together determine the yield and composition of peppermint oil. By investigating the effect of these factors on plants under glasshouse-growth room conditions, an attempt was made to understand the factors influencing oil yield and composition under field conditions. The manipulation of the field situation to increase oil yield without adversely affecting oil composition was investigated. The oil content of peppermint leaves increased from basal to midstem leaves and decreased from midstem to apical leaves. Oil accumulation corresponded to the period of leaf expansion, during which glandular trichomes were observed to fill with oil. Midstem leaves accumulated maximum amounts of oil at the time inflorescences were observed on plants growing under long day-low night temperature (LD x LNT) conditions. Basal and apical leaves reached their maximum oil content prior to and following the appearance of inflorescences, respectively. Oil accumulation was favoured by LD x LNT conditions relative to SD x HNT (short day-high night temperature) conditions. The decreased oil accumulation under SD x HNT conditions did not appear to be associated with a deficiency of photosynthate, since oil maturation occurred to the same extent under both LD x LNT and SD x HNT conditions. The results presented support previous reports of a true photoperiodic effect on dry matter, oil yield, growth habit and flowering. Furthermore, it appeared that there exists a true photoperiodic effect on the monoterpene composition of peppermint oil. Day length, night temperature, day temperature and light intensity were also important interacting factors determining oil yield and composition, under glasshouse-growth room conditions. The photosynthate model proposed by Burbott and Loomis (1967) explained the effect of environmental factors with respect to pulegone, menthone and menthofuran. Factors favouring the maintenance of high levels of photosynthate resulted in high concentrations of menthone and low concentrations of pulegone and menthofuran. The photosynthate model did not explain the effect of environmental factors on several other monoterpenes of peppermint oil. An investigation of the net CO 2 exchange characteristics of peppermint indicated that light saturation occurred between 400 and 500 pEM- 2s -1 in attached fully expanded leaves of peppermint. Maximum rates of 'apparent' photosynthesis occurred at 20°C. The important determinants of 'apparent' photosynthesis were an increase in 'true' photosynthesis when temperature was increased to 25° C, a steady increase in dark respiration with increased temperature, and a rapid increase in photorespiration between 15 ° C and 30°C. Such net CO 2 exchange characteristics of peppermint support the photosynthate model proposed to explain environmental effects on oil composition. With respect to the field situation in Tasmania, provided that areas with reasonably high plant densities were considered, oil yield per unit area reached a maximum early in the growing season. Oil yield per unit area remained at the maximum level for a considerable period (5 to 6 weeks) with the only significant change being a final decrease in yield towards the end of the growing season. During the period of maximum otl yield the percentage menthol increased from approximately 40% to 45%. Delaying harvest once the percentage menthol reached the required 45%, resulted in further increases in the percentage menthol, but at the expense of increased percentage menthofuran and decreased oil yields. In addition to the above study of harvest date, the relationship between nitrogen application and irrigation rate and timing, on the yield and composition of peppermint oil and the possibility of obtaining two harvests of peppermint in one season, were investigated. High yields of oil were associated with high applications of nitrogen and high levels of irrigation, particularly throughout the last half of the growing season. The composition of oil extracted from herb at the commercial harvest date (approximately 45% menthol) was not significantly affected by either nitrogen or irrigation treatments. The oil yield from regrowth within the same growing season was significantly affected by irrigation and nitrogen treatments applied prior to the first harvest. When 300kg N/ha and 50mm of irrigation weekly (during the last half of the growing season) were applied, the oil yields from regrowth approached the yield normally obtained at the commercial harvest date. Oil from regrowth contained high concentrations of menthol, menthyl acetate, menthofuran and limonene, and low concentrations of menthone and cineole, relative to peppermint oil typical of Tasmanian production areas. In a subsequent trial involving the manipulation of harvest date, nitrogen and irrigation, the first harvest was timed to coincide with maximum oil yield per unit area (40% menthol) and the second harvest occurred when the concentration of menthol exceeded 50%. In this way the total yield of oil per unit area was increased significantly without adversely affecting oil quality. By comparing the composition and yield potential of peppermint oil under Tasmanian conditions with that reported for other world production areas, it is concluded that Tasmania is well suited to the production of high yields of high quality peppermint oil.