EFFECTS OF OSMOTIC-STRESS ON THE ESSENTIAL OIL CONTENT AND COMPOSITION OF PEPPERMINT

The essential oil content and composition of hydroponically-grown peppermint (Mentha x piperita L.) plants (29- and 36-day-old cuttings) were each affected by growth solution potential (ψs) and duration of exposure to mild and moderate osmotic stress. After one week of treatment, the essential oil content of leaves increased from 44.0 to 61.1 ul μl g-1 leaf dry wt as ψs decreased from -0.05 to -0.6 MPa, and no differences in shoot biomass, leaf area, or total oil production by leaves were evident in plants grown at these treatment levels. The same levels of ψs applied for two weeks resulted in larger differences in plant growth and leaf oil content. When plants were subjected to increasing levels of osmotic stress, leaf essential oil content increased linearly from 44.0 to 70.8 μl g-1 dry wt, but shoot biomass and leaf area each decreased exponentially. Total essential oil yield of plants subjected to two weeks of treatment was significantly reduced as stress increased. The major constituents of essential oil, menthone and menthol, together accounted for ca 80% of the total monoterpenes, and this proportion was unaffected by treatment. Few consistent trends in oil composition among stress treatments were evident, although the relative proportion of sesquiterpenes increased with osmotic stress. © 1990.