Polymorphism in cyanogenic glycoside content and cyanogenic beta-glucosidase activity in natural populations of Eucalyptus cladocalyx

Cyanogenesis is the process by which plants release hydrogen cyanide (HCN) from endogenous cyanide-containing compounds and is thought to play a role in plant defence against generalist herbivores. Cyanogenesis is poorly understood in natural populations, and has been little studied in tree species. In this paper we present the first systematic survey of cyanogenesis in the economically and ecologically important genus Eucalyptus. We document variability in both the concentration of the cyanogenic glycoside, prunasin, and the accompanying degradative beta-glucosidase in a woody plant for the first time. Leaves of 96 E. cladocalyx F. Muell. trees growing in natural populations on Kangaroo Island, South Australia were analysed. All trees were cyanogenic, containing both cyanogenic glycosides and active beta-glucosidase. Cyanogenic glycoside concentration varied by over two orders of magnitude. The beta-glucosidase activity varied widely as well, but plants high in cyanogenic glycosides did not necessarily have higher enzyme activity. A significant proportion of the variation in cyanogenic glycoside concentration can be accounted for by the variation in leaf nitrogen. Most of the variation, however, appears to be the result of genetic polymorphism, which is inherited independently of the level of activity of the degradative beta-glucosidase.