In defence of flowers: The ecology and biochemistry of a chemical defence, cyanogenesis, in Proteaceae
AffiliationSchool of Ecosystem and Forest Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-09-23. This item is currently available to University of Melbourne staff and students only, login required.
© 2019 Edita Ritmejeryte
As sessile organisms, plants produce a remarkable diversity of chemicals to defend their tissues against herbivory. The large body of research into plant defences has primarily focused on plant-herbivore interactions and foliar defences, whereas floral chemical defences have received little attention. This is despite the fact that flowers of many species are known to contain defence metabolites, the sizeable investment of resources in floral structures, the significance of flowers to fitness, and the importance of floral chemistry for both florivores and pollinators. This thesis investigates patterns in floral chemical defence in the Proteaceae, a dominant plant family in Australia characterised by diverse floral traits and a high frequency of floral cyanogenesis - a constitutive nitrogen-based chemical defence involving the release of toxic hydrogen cyanide from endogenous cyanogenic glycosides (CNglycs) upon tissue damage. There were two main aspects to this research, (1) investigations into patterns in the distribution of CNglycs within flowers of different species, and (2) investigations into patterns in whole-plant chemical defence using Telopea speciosissima. In Chapters 2 and 3 I quantified CNglycs in dissected floral tissues and used a new MALDI-MS imaging method to investigate and compare fine-scale patterns in the distribution of CNglycs within flowers of eight congeneric Lomatia species (Chapter 2), and 11 species from different genera from across the family phylogeny (Chapter 3). Together these studies identified: (1) variation in floral CNglyc concentrations over three orders of magnitude across species, (2) unprecedented concentrations of CNglycs in some floral tissues (e.g. Lomatia stigmatic cells and pollen were 11% CNglyc by mass), (3) highly tissue-specific distributions of CNglycs within flowers, (4) substantial interspecific differences in patterns of within-flower CNglyc distribution, and (5) that differences in floral CNglycs across species were not correlated with other traits (e.g. colour, disturbance response) or phylogenetic position. Findings demonstrate the power of MALDI-MSI to reveal dynamic and fine-scale spatial patterns in the distribution of different CNglycs. Data are discussed in relation to defence theories and potential multiple functions of CNglycs in plants. In Chapter 4 I used T. speciosissima, which has CNglycs in all tissues, to compare floral and foliar CNglycs in seven populations from three distinct genetic groups, which also differ in environmental conditions. There was no consistent relationship between floral and foliar cyanogenic capacity within populations; however, mean floral and foliar CNglyc concentrations differed between genetic groups. Correlations between CNglycs and δ13C, N and physical leaf traits suggest these differences reflect both genetic and environmental effects. In Chapter 5, a glasshouse study investigating the effects of N and P supply on above- and below-ground CNglycs in T. speciosissima seedlings identified that root and shoot chemistry responded differently to N and P treatments. The higher CNglyc concentrations in roots may reflect greater importance of root chemical defence in a non-mycorrhizal species that relies on basal resprouting following fire. Together this research provides evidence that foliar chemistry cannot be assumed to reflect whole plant chemistry (floral, root) and that both whole plant and fine-scale tissue-specific studies are required to understand the regulation and roles of CNglycs in plants.
Keywordsconstitutive defence; cyanogenesis; cyanogenic polymorphism; cyanide; Proteaceae; nitrogen allocation; optimal defence theory; apparency Theory; resource allocation; florivory; flower defence; spatial metabolomics; dhurrin; proteacin; MALDI
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