School of BioSciences - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemNon-volatile secondary metabolites in foliar oil glands of Eucalyptus species( 2020)Plants synthesise a vast range of secondary metabolites that are stored in specialised cells or organs. The presence of sub-dermal glands rich in volatile terpene essential oils is characteristic of the trees of the genus Eucalyptus (Myrtaceae). Recent studies showed that non-volatile compounds (NVCs), particularly monoterpene acid glucose esters (MAGEs), co-occur with volatile components in Eucalyptus foliar oil glands. The principal aim of this thesis is to characterise MAGEs and other non-volatiles localised to Eucalyptus oil glands and to explore their relationships to the co-housed oil components. Glandular extracts from a range of Eucalyptus species belonging to the two major subgenera, Symphyomyrtus and Eucalyptus, were investigated in Chapter 2. Non-volatiles were extracted from enzymatically isolated glands and analysed using high-performance liquid chromatography (HPLC) and mass spectrometry (LC-MS). MAGEs were identified based largely on diagnostic mass spectral fragmentation patterns. MAGEs were the dominant NVC in Symphyomyrtus species. In contrast, the sampled subgenus Eucalyptus species lacked MAGEs, but were rich in phenolics. Volatile oil components were also analysed using gas chromatography with flame ionisation detection and mass spectrometry (GC-FID and GC-MS). Monoterpenes and sesquiterpenes were identified and quantified for each species. In addition, cell suspensions of E. polybractea were successfully established from leaf-derived callus as a potential tool to investigate the biosynthesis of MAGEs. Glandular extracts from subgenus Eucalyptus species rich in non-volatiles containing phenolic moieties were further analysed in Chapter 3. A suite of unsubstituted B-ring flavanones was identified as the dominant glandular NVCs. In addition, flavones, flavanone-O-glucosides, flavanone-b-triketone conjugates, triketone heterodimers and chromone-C-glucosides were also identified. Flavanones were quantified and species-specific variations were observed. This chapter also showed that flavanones are exclusively localised to the glands rather present throughout leaf tissues. A positive correlation was observed between some monoterpenes and sesquiterpenes with total flavanones and particularly with pinostrobin. Interestingly, b-triketones were also found in the volatile extracts of glands from E. suberea and E. brevistylis. The presence of glandular b-triketones was further explored in Chapter 4. A major discovery of this thesis was the occurrence of two gland types in E. brevistylis that differ in their colour and importantly, in their metabolic contents. ‘Sesquiterpene glands’, which are translucent-white in appearance, contained sesquiterpene alcohols. ‘Triketone glands’, which are golden-brown in appearance, contained mostly the b-triketone conglomerone and sesquiterpene hydrocarbon caryophyllenes in lower abundance. None of the glands contained the NVCs identified from the other species in this thesis. The results were consistent in trees from a natural population of E. brevistylis and in glasshouse-grown seedlings and saplings. In addition, ‘Triketone glands’ seem to develop earlier than ‘sesquiterpene glands’ in leaf ontogeny. This is the first identification of such metabolic differentiation of embedded glands from any tree species. The work presented in this thesis revealed many novel aspects of oil glands, particularly in relation to their non-volatile and volatile constituents and how they are related to one other. Overall, the findings of this thesis contribute significantly to the knowledge on Eucalyptus foliar oil gland chemistry and biology.