Influence of water stress on cyanogenic capacity in Eucalyptus cladocalyx

Cyanogenesis in many plant species is an effective herbivore deterrent, which appears to be influenced by a range of environmental variables. There is evidence that one such variable, soil water availability, increases cyanogenic capacity (i.e. leaf cyanogenic glycoside concentration), but it is not clear whether this is a relatively direct or indirect effect. To shed light on this issue, we compared the cyanogenic capacity of individuals from two populations of Eucalyptus cladocalyx F.Muell. from areas of South Australia that differ markedly in rainfall. Stable carbon isotope analysis confirmed that trees at the drier site were more water-stressed. We found a large range in leaf cyanogenic capacities, from 0 to 1.01 mg cyanide g(-1) dry weight. Importantly, this is the first record of acyanogenic E. cladocalyx. Mean cyanogenic capacity was 30% higher in trees from the drier site, and they suffered less damage from herbivores. However, these trees also contained higher concentrations of leaf nitrogen (N). Correlative analysis of data for individual plants from both sites showed that leaf N was able to account for a significant amount of the variation in cyanogenic glycoside concentration (28%). Water availability on its own, however, was not able to account significantly for any such variation. We conclude that most of the variation in cyanogenic capacity is due to genetic differences between individuals, while the remaining variation is due to differences in leaf N.