Novel aspects of cyanogenesis in Eucalyptus camphora subsp. humeana

Cyanogenesis is the release of cyanide from certain organisms upon tissue disruption. Tissue disruption, such as that caused by folivory, brings cyanogenic glycosides into contact with catabolic enzymes and toxic HCN is subsequently released. The process provides a measure of defence against generalist herbivores. Within the genus Eucalyptus, several species have been identified as cyanogenic and all of these store cyanide exclusively in the form of the cyanogenic glycoside prunasin. Here we report for the first time cyanogenesis in Eucalyptus camphora subsp. humeana L.A.S. Johnson & K.D. Hill. We found that foliage contains at least five different cyanogenic glycosides, three of which were purified and identified (prunasin, sambunigrin and amygdalin). Two natural populations of E. camphora trees were screened for cyanogenesis, and quantitative polymorphism was measured at both sites. Trees varied in their capacity for cyanogenesis from 0.014 to 0.543 mg CN g^-1 DW in one population and from 0.011 to 0.371 mg CN g^-1 DW in the other. A progeny trial, testing both cyanogenesis and carbon-based defence (namely total phenolics and condensed tannins), was performed with seed sourced from two cyanogenic, open-pollinated maternal trees. Interestingly, the seedlings exhibited markedly lower levels of cyanogenesis and condensed tannins than the adult population, with some individuals completely lacking one or both of the chemical defences. Total phenolic concentrations, however, were significantly higher in the seedlings than in the parental population from which the seed was sourced. Eucalyptus camphora is relatively unique among cyanogenic trees having multiple foliar cyanogenic glycosides and an apparently marked ontogenetic regulation of cyanogenic capacity.