Genetic determinants of oil yield in Eucalyptus polybractea R.T. Baker

Abstract

Eucalyptus polybractea R.T. Baker is the key species used in Australia to produce commercial quantities of pharmaceutical-grade eucalyptus oil. Much of the variability in oil yield observed in coppiced stands of the species is considered genetically determined. Nonetheless it is not known how genetics acts on leaf and secretory cavity morphology to influence oil yield. To examine this, we first chose similar-sized mature leaves of different genotypes and determined that secretory cavity size accounted for 84 % of the variation in oil yield between genotypes. We then examined natural variation in mature leaf size found along branches and observed oil yield to increase in a linear, but non-proportional fashion with increasing leaf size, suggesting leaves of different final sizes on a given tree can have different volumetric oil proportions. From these analyses, we calculated that a minimum of 2.2 μl of photosynthetic tissue is required to service each additional μl of secretory cavities as leaves expand. This suggests there is likely to be a maximum oil concentration obtainable in E. polybractea leaves for a given leaf size increase. Such information helps to understand the factors that underlie oil yields and provides a potential framework for selection strategies aimed at maximising harvestable oil yields.