RESUMEN
Plants employ diverse anti-herbivore defences that can covary to form syndromes consisting of multiple traits. Such syndromes are hypothesized to impact herbivores more than individual defences. We studied 16 species of lowland willows occurring in central Europe and explored if their chemical and physical traits form detectable syndromes. We tested for phylogenetic trends in the syndromes and explored whether three herbivore guilds (i.e., generalist leaf-chewers, specialist leaf-chewers, and gallers) are affected more by the detected syndromes or individual traits. The recovered syndromes showed low phylogenetic signal and were mainly defined by investment in concentration, richness, or uniqueness of structurally related phenolic metabolites. Resource acquisition traits or inducible volatile organic compounds exhibited a limited correlation with the syndromes. Individual traits composing the syndromes showed various correlations to the assemblages of herbivores from the three studied guilds. In turn, we found some support for the hypothesis that defence syndromes are composed of traits that provide defence against various herbivores. However, individual traits rather than trait syndromes explained more variation for all studied herbivore assemblages. The detected negative correlations between various phenolics suggest that investment trade-offs may occur primarily among plant metabolites with shared metabolic pathways that may compete for their precursors. Moreover, several traits characterizing the recovered syndromes play additional roles in willows other than defence from herbivory. Taken together, our findings suggest that the detected syndromes did not solely evolve as an anti-herbivore defence.
RESUMEN
Chemical variation is a critical aspect affecting performance among co-occurring plants. High chemical variation in metabolites with direct effects on insect herbivores supports chemical niche partitioning, and it can reduce the number of herbivores shared by co-occurring plant species. In contrast, low intraspecific variation in metabolites with indirect effects, such as induced volatile organic compounds (VOCs), may improve the attraction of specialist predators or parasitoids as they show high specificity to insect herbivores. We explored whether induced chemical variation following herbivory by various insect herbivores differs between VOCs vs. secondary non-volatile metabolites (non-VOCs) and salicinoids with direct effects on herbivores in six closely related willow species. Willow species identity explained most variation in VOCs (18.4%), secondary non-VOCs (41.1%) and salicinoids (60.7%). The variation explained by the independent effect of the herbivore treatment was higher in VOCs (2.8%) compared to secondary non-VOCs (0.5%) and salicinoids (0.5%). At the level of individual VOCs, willow species formed groups, as some responded similarly to the same herbivores. Most non-VOCs and salicinoids were upregulated by sap-suckers compared to other herbivore treatments and control across the willow species. In contrast, induced responses in non-VOCs and salicinoids to other herbivores largely differed between the willows. Our results suggest that induced responses broadly differ between various types of chemical defences, with VOCs and non-VOCs showing different levels of specificity and similarity across plant species. This may further contribute to flexible plant responses to herbivory and affect how closely related plants share or partition their chemical niches.