University of Tasmania
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Phytochemistry of Eucalyptus spp. and its role in insect-host-tree selection

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posted on 2023-05-26, 08:46 authored by Li, HX
Leaf oils and waxes of 29 native Tasmanian Eucalyptus species were studied from primarily a taxonomic aspect and their effects on insect-plant selection were evaluated. E. nitens, an important introduced pulpwood species that is not indigenous to Tasmania was also investigated in detail. The chemical compositions of leaf oils and waxes of both juvenile and adult leaf samples of each species were examined by GC-MS. Results indicated that the distribution of both oil and wax components within Tasmanian eucalypts and their relative proportions between subgenera and species have taxonomic importance. A chemotaxonomic classification based on these data was constructed and this classification more functionally describes insect-host-tree relationships as opposed to the classification based on morphological differences. The results of this study indicated that differences in leaf oils of eucalypt species are correlated to genetic divergence in the biosynthesis of terpenoid compounds and that biochemically related terpenoid compounds provide sensitive taxonomic characteristics for the identification of eucalypt species. A study, using material from species/provenance trials, indicated that seasonal variation in leaf oil chemicals was influenced by time, ontogenetic and physiological aging effects. The variation pattern of leaf oils between different species was found to be under strong genetic control and environmental factors tended to affect leaf oils quantitatively but not qualitatively. For the first time the chemical composition of eucalypt leaf waxes of trees from a large number of species localities has been determined by the direct analysis of crude wax. In a representative number of species localities, wax morphology was examined by scanning electron microscopy (SEM). Two general wax categories, amorphous and structured were found, the latter including three types. The results indicated that wax structure was correlated with chemical composition and that the distribution and variation ,- in both wax chemicals and wax types has relevance at all levels of the taxonomic hierarchy. The chemical composition of leaf oils and waxes among most provenances and localities of E. nitens were examined. The differences in leaf oil characteristics of two forms of E. nitens has supported the separation of the two morphological forms into separate species, E. nitens and E. denticulata. Results indicated that the chemical composition of leaf oils and waxes of E. nitens was similar to most Symphyomyrtus species but that oil yields from all E. nitens populations were significantly lower. Commercially important eucalypt species are attacked by two paropsine defoliating beetles: Chrysophtharta bimaculata and C. agricola (Coleoptera: Chrysomelidae, Paropsini). The feeding and oviposition preferences of these beetles and the feeding response of their larvae to eucalypt leaves were examined in laboratory bioassay in which the effects of leaf oils and waxes were evaluated. The results indicated that leaf acceptance or rejection by C. bimaculata and C. agricola was correlated to differences in leaf oils and waxes which were chemotaxonomically correlated among Tasmanian eucalypt species. The toxic effects of 1,8-cineole and related chemicals on C. bimuculata beetles were confirmed by leaf extract and chemical bioassay. It was found that the leaves of E. nitens were successfully attacked by C. bimuculata through the lack of chemical defence, notably low levels of 1,8-cineole. The wax bloom on juvenile foliage acts also as a physical defence against adult C. bimaculata beetles. Results also indicated that leaf preference of C. agricola was related more to variation in the triterpenoid class of wax chemicals. The inheritance of leaf oils and waxes in interspecific F1 hybrids of Eucalyptus was analysed. Results indicated that both oil and wax chemicals were strongly inherited in F1 hybrids and were valuable for identification of tree hybrids. Bioassay results indicated that feeding resistance was related to the inheritance of leaf oils in interspecific F1 hybrids. Hybrids between high and low cineole species had intermediate cineole levels and were therefore more resistant to C. bimaculata attack than the low cineole parent.


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