Induction and characterisation of tetraploidy in Eucalyptus polybractea R.T. Baker

Eucalyptus polybractea (blue mallee) is the main commercial source of pharmaceutical grade eucalyptus oil in Australia. In foliage, the oil is stored in large sub-dermal foliar glands, which have a diameter that is sometimes comparable to leaf thickness. Given that ploidy increases are often associated with an increase in leaf thickness, it was reasoned that polyploid blue mallee may have larger glands and thus be superior for essential oil production. Our aim was to induce tetraploidy in blue mallee and determine the effect on growth rate, vegetative and reproductive morphology and oil quality and yield under both controlled and field conditions. In vitro cultures of five high-oil-yielding clones were used for the study. Nodal explants were treated with 0.05 to 0.5% colchicine for two to twelve days, resulting in six tetraploid lines from three clones. The most effective concentration of colchicine was 0.5%, but the most effective exposure time and method of nodal explant preparation varied between clones. Tetraploids of one line, and their respective diploid controls, were rooted, acclimatised and transferred to a glasshouse and a field plot for further study. Tetraploids had a significantly slower growth rate in both the glasshouse and field, which may in part be due to a lower root to shoot ratio. Tetraploid leaves from both the glasshouse and field-grown plants were significantly thicker and had larger stomata at a lower density than diploids. Similarly, tetraploid leaves had significantly larger oil glands at a lower density, but the total oil content of tetraploids was significantly lower than that of diploids. Field-grown tetraploids had larger flowers than diploids, but the size differences are unlikely to preclude pollination and fertilization. Successful breeding of diploids and tetraploids will pave the way for the creation of blue mallee triploids and thereby the production of sterile seeds, an attractive possibility for the eucalyptus oil industry.