A comparative study of the seedling leaf oils of the spotted gums: species of the Corymbia (Myrtaceae), section Politaria

Abstract

This study examines variation in seedling leaf oil composition and yield within and between populations of the spotted gum group of eucalypts, with the aim of better defining relationships between species in this taxonomic grouping of economically important forest trees. Eleven provenances from across the geographical distribution, consisting of three provenances of C. maculata (Hook.) K. D. Hill & L. A. S. Johnson (syn. Eucalyptus maculata), four provenances of C. variegata (F. Muell.) K. D. Hill & L. A. S. Johnson, one provenance of C. henryi (S. T. Blake) K. D. Hill & L. A. S. Johnson (syn. E. henryi) and three provenances of C. citriodora (Hook.) K. D. Hill & L. A. S. Johnson (syn. E. citriodora) were analysed. Chemometric analysis (PCA and LDA) of oil compositional data gave effective discrimination between all four species. The major findings were the clear separation of C. maculata populations from other members of the group based on chemical composition of their seedling leaf oils. Two chemotypes were identified in C. maculata, one with the cadinol–muurolol complex prominent and a second, less frequent type in which the eudesmols and elemol were prominent. Corymbia citriodora was also unambiguously separated from other group members through the presence of citronellal and citronellol in all amples tested, although abundance of these compounds in individual oils varied between regions of provenance (north cf. south). The oils from some trees in the southern most provenance comprised of low levels of these compounds, with an increase in α -pinene suggesting introgression with C. variegata. The seedling oils of C. variegata were mostly dominated by α -pinene and several sesquiterpene alcohols, while the oils of the single population of C. henryi studied were generally low in α-pinene and high in sesquiterpenes. Oil composition of C. henryi and the southernmost population of C. variegata showed affinities, suggesting intergradation of oil characteristics between these nearby stands. More intensive sampling of C. henryi and C. citriodora and populations in the zones of suspected intergradation between C. henryi–C. variegata–C. citriodora would be helpful in clarifying patterns of variation within and between these taxa.