A comparison of inducible, ontogenetic, and interspecific sources of variation in the foliar metabolome in tropical trees

Abstract

Plant interactions with other organisms are mediated by chemistry, yet chemistry varies among conspecific and within individual plants. The foliar metabolome-the suite of small-molecule metabolites found in the leaf-changes during leaf ontogeny and is influenced by the signaling molecule jasmonic acid. Species differences in secondary metabolites are thought to play an important ecological role by limiting the host ranges of herbivores and pathogens, and hence facilitating competitive coexistence among plant species in species-rich plant communities such as tropical forests. Yet it remains unclear how inducible and ontogenetic variation compare with interspecific variation, particularly in tropical trees. Here, we take advantage of novel methods to assemble mass spectra of all compounds in leaf extracts into molecular networks that quantify their chemical structural similarity in order to compare inducible and ontogenetic chemical variation to among-species variation in species-rich tropical tree genera. We ask (i) whether young and mature leaves differ chemically, (ii) whether jasmonic acid-inducible chemical variation differs between young and mature leaves, and (iii) whether interspecific exceeds intraspecific chemical variation for four species from four hyperdiverse tropical tree genera. We observed significant effects of the jasmonic acid treatment for three of eight combinations of species and ontogenetic stage evaluated. Three of the four species also exhibited large metabolomic differences with leaf ontogenetic stage. The profound effect of leaf ontogenetic stage on the foliar metabolome suggests a qualitative turnover in secondary chemistry with leaf ontogeny. We also quantified foliar metabolomes for 45 congeners of the four focal species. Chemical similarity was much greater within than between species for all four genera, even when within-species comparisons included leaves that differed in age and jasmonic acid treatment. Despite ontogenetic and inducible variation within species, chemical differences among congeneric species may be sufficient to partition niche space with respect to chemical defense.

Keywords: Coexistence; Community ecology; Intraspecific variation; Mass spectrometry; Metabolomics; Plant-herbivore interactions.

Figure 1. Molecular network indicating the incidence of small molecules in jasmonic acid-treated leaves (magenta), control leaves (cyan), and both (yellow) for four species.

Nodes represent compounds (e.g., caffeine); links between nodes indicate molecular structural similarity between compounds (e.g., caffeine and theobromine). (A) Young and (B) mature leaves of Inga cocleensis, (C) young and (D) mature leaves of Piper cordulatum, (E) young and (F) mature leaves of Protium panamense, and (G) young and (H) mature leaves of Psychotria acuminata.

Figure 2. Chemical similarity within and between jasmonic acid treatment in young and mature leaves.

Chemical similarity between pairs of leaf samples is represented by chemical structural compositional similarity (CSCS) for young (A) and mature (B) leaves and by Bray–Curtis chemical similarity for young (C) and mature (D) leaves. Each point represents the similarity between two leaf samples. Boxplots indicate the median and the first and third quartiles of chemical similarity for pairwise sample combinations.

Figure 3. Chemical similarity within and between leaf ontogenetic stages.

Chemical similarity between pairs of leaf samples is represented by chemical structural compositional similarity (CSCS; A) and Bray–Curtis similarity (B). Boxplots indicate the median and the first and third quartiles of chemical similarity for pairwise sample combinations.

Figure 4. Classical multidimensional scaling (MSDS) of pairwise CSCS similarity among all pairs of leaf samples in four focal species.

Young and mature leaves are indicated by triangles and circles, respectively. Jasmonic acid and control treatment samples are indicated by bold and pale points, respectively. Species is indicated by color.

Figure 5. Chemical similarity within species and between congeneric species.

Chemical similarity between pairs of leaf samples is represented by chemical structural-compositional similarity (CSCS; A) and Bray–Curtis similarity (B). Within-species comparisons include every pairwise combination of samples within each focal species (I. cocleensis, P. cordulatum, P. panamense, and P. acuminata), including within- and between-jasmonic acid treatment and leaf age pairs. Between-species sample pairs include every leaf sample of the four focal species paired with young, untreated leaf samples of each congeneric species, including 14, 10, 2 and 19 species of Inga, Piper, Protium and Psychotria, respectively. Boxplots indicate the median and the first and third quartiles of chemical similarity for pairwise sample combinations.