Secondary metabolites play an important role in plant adaptation because
they can mitigate biotic and abiotic environmental stresses. However,
their production and allocation incur different costs and benefits and are
therefore subject to trade-offs, which are less studied. To understand
large-scale geographic patterns of secondary metabolites, and their
environmental drivers and trade-offs, we studied 39 natural populations of
the perennial herb Japanese knotweed (Reynoutria japonica) along a large
latitudinal gradient in China. We measured the concentrations of six
polyphenols in leaves and rhizomes of R. japonica and associated the
variation in these metabolites with biotic and abiotic environmental
factors as well as with functional plant traits and putative costs of
secondary metabolites. We found that climate was an important driver of
variation in secondary metabolites, both above- and belowground.
Remarkably, the patterns of association differed between leaves and
rhizomes, as well as between putative low-cost vs. high-cost compounds.
While annual mean temperature was a stronger predictor of aboveground
metabolites, annual precipitation was more frequently associated with
variation in belowground metabolites. Moreover, annual temperature was
positively associated with high-cost metabolites, but negatively with
low-cost metabolites. Aboveground secondary metabolites were generally
more strongly associated with functional traits (e.g., specific leaf area)
than belowground metabolites, and in all cases, the directions of
correlation were opposite for low-cost versus high-cost metabolites
aboveground. The patterns of association also varied with latitude such
that leaf concentrations of low-cost metabolites (quercetin) increased but
those of high-cost metabolites (resveratrol, piceid and emodin) decreased
at higher latitudes. In rhizomes, in contrast, the concentrations of
high-cost metabolites (piceid and emodin) increased with latitude.
Synthesis. Our findings indicate that allocation strategies differ between
above- and belowground tissues of R. japonica. As latitude increases, R.
japonica invests relatively more into belowground metabolites. We propose
that reduced high-cost metabolites in the leaves at higher latitudes may
help to conserve nutrients after defoliation, while maintaining high-cost
metabolites in rhizomes may be important for persistent allelopathic
effects and resource conservation belowground. T...