Allocation of nitrogen to chemical defence and plant functional traits is constrained by soil N

Plants have evolved a vast array of defence mechanisms to avoid or minimize damage caused by herbivores and pathogens. The costs and benefits of defences are thought to vary with the availability of resources, herbivore pressure and plant functional traits. We investigated the resource (nitrogen) and growth cost of deploying cyanogenic glycosides in seedlings of Eucalyptus cladocalyx (Myrtaceae). To do this, we grew the plants under a range of soil N conditions, from levels that were limiting for growth to those that were saturating for growth, and we measured correlations between foliar chemical and performance attributes. Within each N treatment, we found evidence that, for every N invested in cyanogenic glycosides, additional N is added to the leaf. For the lowest N treatment, the additional N was less than one per cyanogenic glycoside, rising to some two Ns for the other treatments. The interaction between cyanogenic glycosides and both condensed tannins and total phenolic compounds was also examined, but we did not detect correlations between these compounds under constant leaf N concentrations. Finally, we did not detect a correlation between net assimilation rate, relative growth rate and cyanogenic glycoside concentrations under any soil N treatment. We conclude that the growth cost of cyanogenic glycosides was likely too low to detect and that it was offset to some degree by additional N that was allocated alongside the cyanogenic glycosides.