Composition and seasonal variation of essential oil in Alpinia zerumbet from Okinawa Island.

The composition of steam-distilled essential oils from the leaves of Alpinia zerumbet (Pers.) B. L. Burtt. et R. M. Sm. cultivated in Okinawa Island was investigated by GC-MS analysis. The seasonal variation of the main volatile constituents was also investigated. Analysis of the essential oil identified 17 compounds. It showed the predominant presence of monoterpenic constituents, representing 95% of the essential oil. p-Cymene was the most abundant compound, followed by 1,8-cineole, terpinen-4-ol, alpha-pinene, beta-pinene and limonene. The amount of sesqiterpenic content of the essential oil was small, mostly represented by beta-caryophyllene and alpha-caryophyllene. One of the phenylpropanoid derivatives, methyl cinnamate, was also detected. The essential oils from the leaves collected at ten different collection periods for over 5 years showed significant differences in their compositions. p-Cymene, terpinen-4-ol and 1,8-cineole were the most predominant constituents over the periods with a few exceptions. The content rates of p-cymene were abundant in summer, still high in late autumn and early winter, and decreased in mid-winter and early spring. In contrast, the contents of terpinen-4-ol and 1,8-cineole were high in winter, but decreased around summer. On enantiomeric distribution, terpinen-4-ol and alpha-pinene were suggested to relate to seasonal transformation. (+)-Terpinen-4-ol had a tendency to decrease in high temperatures and huge precipitations. It was also suggested that (-)-alpha-pinene was more sensitive to climate change than (+)-alpha-pinene.