White micromeria [ Micromeria fruticosa (L.) Druce, Lamiaceae] is a dwarf evergreen shrub endemic to Israel and the eastern Mediterranean. The essential oil of M. fruticosa largely comprises the monoterpenes (+)-pulegone and isomenthol. Seasonal variations in the levels and composition of the monoterpene components of the essential oil of M. fruticosa were noted. During the summer months, when growth rates are maximal, (+)-pulegone constituted up to 80% of the essential oil, while in early winter, a period of growth-rest in Mediterranean climates, (+)-pulegone levels dropped dramatically to a few percent, while isomenthol constituted up to 80% of the essential oil. Experiments in which plants were grown under controlled temperature and photoperiodic regimes indicated that the variation was not directly associated with environmental conditions, but the composition of the monoterpenes obtained from mature flowering branches was strikingly different to that obtained from young vegetative branches. Additionally, there were marked differences in the extracts obtained from individual leaf pairs from the same plant. In young upper leaves, the main component was (+)-pulegone, constituting up to 70% of the total essential oil extracted. During maturation, levels of this component dropped steadily, becoming negligible in older leaves. Reciprocally, levels of isomenthol increased steadily with leaf position, from 0% in young leaves to more than 60% in older leaves. Less pronounced but significant decreases in the levels of limonene, isopulegone, piperitenone oxide, germacrene D and bicyclogermacrene, accompanied by increases in neoiso-isopulegol, isopulegol, neoisomenthol and pulegol were noted. This study indicates that the strong seasonal variation observed in the chemical composition of M. fruticosa is primarily due to leaf maturation. Copyright 2001 Annals of Botany Company

Received: 2 February 2001 ; Returned for revision: 6 April 2001 . Accepted: 27 April 2001


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