3.2 Plant diversity or soil legacy effects on herbivore-induced
responses
Both in the PPI (Fig
3a-c ) and the PSI (Fig 3d-f ) experiment, we discovered
significant differences in the foliar metabolome composition across all
plant diversity levels and soil legacies between control and
herbivore-induced plants in all plant species. When we tested for the
regulation of metabolites between control and induced plants, we found
that the total number of up-regulated metabolites was higher than the
total number of down-regulated metabolites across all species
(Appendix Fig A2 ). Furthermore, we observed that the
absolute number of regulated metabolites was highest when plants had
grown in different soil legacies in the PSI experiment. This effect was
strongest for L. vulgare , while R. acris showed the
overall strongest response in numbers of regulated metabolites in both
the PPI and PSI experiment (Appendix Fig A2 ).
In contrast, we found no
significant effect of plant diversity in the PPI experiment and of soil
legacy in the PSI experiment on the induced metabolome in either species
(Table 1 ). However, when we compared foliar metabolomes of
herbivore-induced plants grown in monocultures with conspecifics growing
in mixtures, we discovered a total of 141 significantly up- or
down-regulated metabolites (Fig. 4 ). Both heterospecific
plant-plant and plant-soil interactions affected the induction of
metabolites compared to conspecific plant-plant or plant-soil
interactions. Overall, heterospecific plant-plant interactions regulated
more induced metabolites than plant-soil interactions in leaves ofL. vulgare (26 vs. 14) and R. acris (40 vs.24). In comparison, heterospecific plant-soil interactions had a
stronger effect on the regulation of herbivore-induced metabolites in
leaves of G. pratense than heterospecific plant-plant
interactions (21 vs. 16; Fig. 4 ). In R. acris , we
discovered that heterospecific plant-plant and plant-soil interactions
had contrasting effects on the regulation of induced metabolites.
Heterospecific plant-plant interactions strongly down-regulated the
induction of metabolites, while plant-soil interactions strongly
up-regulated the induction of metabolites (Fig. 4 ). In
contrast, these modulating effects of heterospecific plant-plant or
plant-soil interactions on the induction of metabolites were mostly
similar or less pronounced in herbivore-induced plants of G.
pratense or L. vulgare (Fig. 4 ). Across all species and
both experiments, we found no de-novo regulated metabolites in
herbivore-induced plants (Appendix Fig. A3 ); all up- and
down-regulated metabolites were present in control plants as well.
Similar to the analysis of regulated metabolites in leaves and roots,
the tentatively assigned metabolites in herbivore-induced plants mostly
belonged to the family of phenolics, in particular flavonoids, their
precursors, and derivatives. Besides, we tentatively assigned two
metabolites in L. vulgare as an iridoid and an alkaloid glycoside
(Table 2 ).