Genetic and Environmental-Based Variability in Secondary Metabolite Leaf Content of Adenostyles alliariae and A. alpina (Asteraceae). A Test of the Resource Availability Hypothesis

To test the resource availability hypothesis we compared the leaf content in carbon-and nitrogen-based allelochemicals between heavily and lightly shaded plants of <i>Adenostyles alliariae</i> and <i>A. alpina</i> (Asteraceae). Both species contain pyrrolizidine alkaloids (PAs) but only <i>A. alpina</i> also contains sesquiterpenes in its leaves. In <i>A. alliariae</i> we found no difference in leaf alkaloid content between the two treatments. In A. alpina alkaloid content tended to be higher in the heavily shaded treatment. One sesquiterpene, cacalol-trimer, was present in higher concentrations in the heavily shaded leaves, whereas concentrations of the other sesquiterpene, cacalol, were reduced. Under light-(carbon)limiting conditions the resource availability hypothesis predicts an increase in nitrogen-based defenses and a decrease in carbon-based defenses; these predictions were met in <i>A. alpina</i> with the exception of the carbon-based cacalol-trimer. Leaf nitrogen content was strongly increased in heavily shaded plants of both species. We found significant genetic variation in PA content in one out of four populations of <i>A. alliariae</i>. For PA content in <i>A. alpina</i> we found significant genetic variation in two out of four populations and for cacalol content in three populations. We therefore conclude that selection on allelochemical content is possible in some populations, whereas in other populations evolutionary processes must have fixed the level of allelochemical content in the two species.