Quantitative and qualitative shifts in defensive metabolites define chemical defense investment during leaf development inInga, a genus of tropical trees

Abstract

Selective pressures imposed by herbivores are often positively correlated with investments that plants make in defense. Research based on the framework of an evolutionary arms race has improved our understanding of why the amount and types of defenses differ between plant species. However, plant species are exposed to different selective pressures during the life of a leaf, such that expanding leaves suffer more damage from herbivores and pathogens than mature leaves. We hypothesize that this differential selective pressure may result in contrasting quantitative and qualitative defense investment in plants exposed to natural selective pressures in the field. To characterize shifts in chemical defenses, we chose six species of Inga, a speciose Neotropical tree genus. Focal species represent diverse chemical, morphological, and developmental defense traits and were collected from a single site in the Amazonian rainforest. Chemical defenses were measured gravimetrically and by characterizing the metabolome of expanding and mature leaves. Quantitative investment in phenolics plus saponins, the major classes of chemical defenses identified in Inga, was greater for expanding than mature leaves (46% and 24% of dry weight, respectively). This supports the theory that, because expanding leaves are under greater selective pressure from herbivores, they rely more upon chemical defense as an antiherbivore strategy than do mature leaves. Qualitatively, mature and expanding leaves were distinct and mature leaves contained more total and unique metabolites. Intraspecific variation was greater for mature leaves than expanding leaves, suggesting that leaf development is canalized. This study provides a snapshot of chemical defense investment in a speciose genus of tropical trees during the short, few‐week period of leaf development. Exploring the metabolome through quantitative and qualitative profiling enables a more comprehensive examination of foliar chemical defense investment.

Chemocoding as an identification tool where morphological‐ andDNA‐based methods fall short:Ingaas a case study

“…Most often, rainforest plants do not have expanding leaves, restricting chemocoding to a minority of saplings at any given point in time and reducing the utility of chemocoding. We found that mature leaves have most, but not all, of the chemical signals found in expanding leaves (Lokvam et al ., ; Wiggins et al ., ), so the use of mature leaves should be feasible. Additionally, we used UPLC with a 150‐mm column, followed by detection with a high‐resolution time‐of‐flight mass spectrometer, an expensive analysis.…”

“…For each species, samples of expanding leaves were collected from five saplings, 0.5-4 m in height, in the shaded understory. We focused on expanding leaves as part of a study of plant-herbivore interactions and also because secondary metabolites are at greater concentration during the expansion stage than in leaves that have matured and toughened (Wiggins et al, 2016). For each sapling, we collected leaves that were between 20% and 80% of the average maximum size.…”

Physical, but not chemical, antiherbivore defense expression is related to the clustered spatial distribution of tropical trees in an Amazonian forest

“…The methodology described below is specific for extracting secondary soluble metabolites (no primary metabolites were extracted nor analyzed). Those compounds covalently bound to the cell wall were also excluded (Endara et al, ; Wiggins, Forrister, Endara, Coley, & Kursar, ). Preliminary chromatographic analyses were performed to detect the optimal number of samples required for chemical analyses (Supporting Information Appendix ).…”

Macroevolutionary patterns in overexpression of tyrosine: An anti‐herbivore defence in a speciose tropical tree genus, Inga (Fabaceae)

“…As a consequence, there is strong selection to invest in chemical defences, and in Inga, secondary metabolites comprise over 50% of the dry weight of expanding leaves. Once the leaf is full size and can toughen, investment drops by more than half (Wiggins, Forrister, Endara, Coley, & Kursar, 2016).…”