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 i n carbon-and nitrogen-based allelochemicals between heavily and lightly sha ded plants of Adenostyles alliariae and A. alpina (Asteraceae). Both specie s contain pyrrolizidine alkaloids (PAs) but only A. alpina also contains se squiterpenes in its leaves. In A. alliariae we found no difference in leaf alkaloid content between the two treatments. In A. alpina alkaloid content tended to be higher in the h eavily 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)limi ting conditions the resource availability hypothesis predicts an increase i n nitrogen-based defenses and a decrease in carbon-based defenses; these pr edictions were met in A. alpina with the exception of the carbon-based caca lol-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 pop ulations of A. alliariae. For PA content in A. alpina 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 evolu tionary processes must have fixed the level of allelochemical content in th e two species.