Plant secondary metabolite diversity reflects both phylogeny and ecological adaptation

A phylogenetic framework explaining plant secondary metabolite diversity is lacking, but metabolite classes could represent adaptations to habitat resource availability. We test the hypothesis that primary adaptive strategies (competitors, C; stress-tolerators, S; ruderals, R) are associated, respectively, with nitrogenous metabolites synthesized in persistent organs (alkaloids), nitrogen-lacking aromatic terpenes and phenolics, and nitrogenous compounds prevalent in reproductive tissues (cyanogenic glucosides and glucosinolates). A matrix was compiled of 1019 species for which secondary metabolite pathways and CSR strategies are known. Accounting for phylogenetic relatedness and native biomes, we found that most phytochemical pathways did not correlate with strategy axes, but certain key associations were evident. C-selection was positively associated with amino acid-derived phenylpropanoids (low phylogenetic relatedness; λ<0.5) and pyrrolizidine alkaloids and galloyl derivatives (high λ), and negatively with N-lacking linear monoterpenes (low λ). Nitrogenous cyanogenic glucosides positively correlated with R-selection (low λ). Terpenoids were widely distributed, but correlated positively with S- and negatively with R-selection (low λ). Twenty-six correlations between phytochemicals and biomes (low λ) were evident. Most secondary metabolite synthesis pathways are widespread, reflecting common roles and obligate defence, and strong phylogenetic effects are often evident. However, the character of phytochemical/adaptive strategy associations agrees with ecological theory and thus reflects adaptation.