Differential patterns of mono- and sesquiterpenes with leaf ontogeny influence pharmaceutical oil yield in Eucalyptus polybractea R.T. Baker

Abstract

Pharmaceutical 1,8-cineole oil is commercially harvested from Eucalyptus polybractea R.T. Baker grown in short rotation coppice cultivation. Harvests occur throughout the year, with no consideration given to the proportion of leaves present at different ontogenetic stages, and the effect leaf ontogeny may have on harvested oil quality and quantity. To determine the optimal leaf stage to harvest pharmaceutical-grade oil, we quantified the relationship between leaf ontogeny and foliar oil quantity and quality, secretory cavity size, and abundance along branches of two E. polybractea clones. The results show that the desirable attributes of oil yield and 1,8-cineole proportion both reach their maximum in fully expanded leaves and that this coincides with maximum secretory cavity size. Across all leaf ages the amount of various monoterpenes were highly correlated with 1,8-cineole, suggesting a single, multi-product monoterpene synthase may be responsible. In the very young leaves, sesquiterpenes such as β-elemene were the most abundant constituents of the essential oil. This study suggests that there may be differential transcriptional regulation of mono- and sesquiterpene synthases with leaf ontogeny such that sesquiterpene biosynthesis is at a maximum only in young leaves, and that monoterpene biosynthesis dominates during leaf and concomitant cavity expansion. Given E. polybractea leaves are produced during major seasonal flushes, there are times throughout the year when the numbers of young leaves with inferior oil quantity and quality are likely to be proportionally decreased. Therefore harvesting times could be manipulated to take advantage of such times and ensure abundant high quality oil is harvested.