Volatiles of myrmecophytic Piper plants signal stem tissue damage to inhabiting Pheidole ant‐partners

Abstract

Summary The major aim of this paper was to explore the mechanistic basis of induced defensive ant response in myrmecophytic Piper species and their ant‐partner Pheidole bicornis by combining experimental trials and chemical analysis. In field experiments, the response of Pheidole ants to simulated stem‐damage of their host plant was documented. Their reaction to other myrmecophytic and non‐myrmecophytic Piper species was explored by fixing respective stem sections onto inhabited Piper fimbriulatum individuals. After artificial damage to stems of P. fimbriulatum and stem sections from other myrmecophytic Piper, the number of ants increased to a maximum of 63 individuals on the damaged internode within the first 10 min. By contrast, no or only a poor reaction was elicited by stem sections from the selected non‐myrmecophytic Piper species. Solid‐phase microextraction/gas chromatography–mass spectrometry identified 68 volatile organic compounds (VOCs) which were emitted from wounded stem bark of three myrmecophytic (P. fimbriulatum, P. obliquum, P. sagittifolium) and two non‐myrmecophytic (P. hispidum, P. riparense) Piper species, comprising monoterpenes, sesquiterpenes, simple aromatic compounds, and green leaf volatiles (GLVs, oxylipins). Despite considerable variation in the VOC profiles, an analysis of similarities (anosim) classified the accumulation of the sesquiterpenes β‐caryophyllene, germacrene D and emission of the oxylipin reactive electrophilic species (RES) compounds hexanal, cis‐3‐hexene‐1‐ol, 2‐hexenal, 2‐undecanone and 1‐hexanol as characteristic of the myrmecophytic species. Synthesis. Field experiments showed that mechanical disturbance of myrmecophytic Piper stems is not sufficient to elicit recruitment and alertness of the ant‐partner Ph. bicornis, but that specific volatiles released from the host plant are responsible. Analyses of the VOCs identified after simulated stem damage of three myrmecophytic and two non‐myrmecophytic Piper species revealed differences in their VOC emission profiles: Only in the myrmecophytic species were induced stress compounds found, such as various oxylipin RES or the sesquiterpene β‐caryophyllene, which are typically emitted after plant damage for interplant signalling. Some of these compounds (e.g. hexanal, 1‐hexanol) are known to induce alerting and alarm behaviours in several ant species. Our study provides evidence that the VOCs of the Piper host plant may signal stem damage and induce recruitment and alert behaviour of associated Pheidole ants.