Peppermint (Mentha piperita L.) was grown under growth room conditions with two photoperiodic treatments, short day and long day. Each treatment received a total of 13 hours light per 24 hour cycle, either continuously (13H) or as an interrupted night treatment (131) with 1 hour of light in the middle of the dark period. In addition to the previously reported changes in dry matter yield of herb, oil yield, growth habit and flowering, the photoperiodic treatments strongly influenced the proportions of several individual monoterpenes in peppermint. The long day treatment resulted in reduced levels of menthofuran, pulegone, menthyl acetate and limonene as well as increased levels of menthone, menthol, neo menthol acetate (+ unknown), trans-sabenine hydrate, cineole and β pinene + sabenine.
Oil yield of peppermint (Mentha piperita L.) per unit area obtained from plant density treatments 30 and 40 plants/m2, reached a maximum early in the growing season, whereas oil yield from the lower density treatment, 10 plants/m2, continued to increase even at a menthol content of 50%. The latter density treatment yielded less oil per unit area. At the 2 highest densities, herb harvested at a stage when oil contained 45% free menthol resulted in maximum oil yield and optimum oil quality. Delaying harvest once the above stage had been reached resulted in increased levels of menthol but at the expense of increased levels of menthofuran and decreased oil yields. As the growing season progressed, menthol and menthyl acetate contents of oil increased while menthone decreased. This effect was accelerated at the high plant densities.