Is there a trade-off between glucosinolate-based organic and inorganic defences in a metal hyperaccumulator in the field?

Several plant species are able to not only tolerate but also hyperaccumulate heavy metals in their aboveground tissues. Thus, in addition to secondary metabolites acting as organic defences, metal hyperaccumulators possess an elemental defence that can act as protection against antagonists. Whereas several laboratory studies have determined potential relationships or trade-offs between organic and inorganic defences, little is known about whether these traits are interconnected in the field and which factors determine the compositions of organic defences and elements of leaf tissues most. To target these questions, we collected young leaves of Arabidopsis halleri, a Brassicaceae capable of hyperaccumulating Cd and Zn, as well as soil samples in the field from 16 populations. We detected wide variation in the composition of glucosinolates—the characteristic secondary metabolites of this plant family—among plants, with two distinct chemotypes occurring. Distance-based redundancy analyses revealed that variation in glucosinolate composition was determined mainly by population affiliation and to a lesser degree by geographic distance. Likewise, elemental composition of the leaves was mainly influenced by the location at which samples were collected. Therefore, the particular abiotic and biotic conditions and potential genetic relatedness at a particular locality affect the plant tissue chemistry. A slight indication of a trade-off between glucosinolate-based organic and inorganic defences was found, but only in the less abundant chemotype. A large variation in defence composition and potential joint effects of different defences may be highly adaptive ways of protecting against a wide arsenal of biotic antagonists.