Monoterpene content in Origanum syriacum as affected by environmental conditions and flowering

A high content of essential oil in the foliage of two chemotypes of field-grown Origanum syriacum L. was found between August and October. The content of phenolic monoterpenes in the essential oil was low from November to January, while the content of p-cymene was high. The effect of environmental conditions and flowering on the two chemotypes was similar. Controlled experiments revealed that under long photoperiods the content of essential oils in the foliage and of phenolic monoterpenes in the oil increased, while that of p-cymene decreased. A similar effect was observed when an 8 h natural light day was extended by 8 h low-intensity incandescent light. When the 16 h day was composed of 12 h natural light, the effect on oil content and composition was intensified. Under short-day photoperiods, increasing temperature or light intensity enhanced the relative content of p-cymene, while that of phenolic monoterpenes and the γ-terpinene generally decreased. Flowering decreased the essential oil content in the leaves and the relative content of the phenolic monoterpenes in the essential oil. Since environmental conditions influence flowering, they had both direct and indirect effects on the essential oil content and composition.