Effect of herbivore load on VOC-mediated plant communication in potato

Abstract

VOC emissions increased with herbivore load, but this did not result in concomitant increases in resistance in neighbouring plants, suggesting that communication occurred independently of herbivore load in emitter plants. Herbivore-damaged plants emit volatile organic compounds (VOCs) that can alert neighbours and boost their resistance. While VOC-mediated plant communication has been shown to be herbivore-specific, we know little about its contingency on variation in herbivore load. To address this knowledge gap, we tested herbivore load effects on VOC-mediated communication between potato plants (Solanum tuberosum) using the generalist herbivore Spodoptera exigua. First, we tested whether herbivore load (three levels: undamaged control, low, and high load) affected total VOC emissions and composition. Second, we matched emitter and receiver plants and subjected emitters to the same herbivore load treatments. Finally, we performed a bioassay with S. exigua on receivers to test for induced resistance due to VOC-mediated communication. We found that herbivory significantly increased total VOC emissions relative to control plants, and that such increase was greater under high herbivore load. In contrast, we found no detectable effect of herbivory, regardless of the load, on VOC composition. The communication experiment showed that VOCs released by herbivore-induced emitters boosted resistance in receivers (i.e., lower leaf damage than receivers exposed to VOCs released by control emitters), but the magnitude of such effect was similar for both levels of emitter herbivore load. These findings suggest that changes in VOCs due to variation in herbivore load do not modify the outcomes of plant communication.

Keywords: Herbivore density; Induced resistance; Plant–plant signalling; Solanum tuberosum; Spodoptera exigua; Volatile organic compounds.

Fig. 1

a Total emission of volatile organic compounds (VOCs; measured as normalized peak areas in nanograms per hour) in response to herbivore load treatments in emitter plants (namely: undamaged control, low, and high load of Spodoptera exigua in emitter potato (Solanum tuberosum) plants. Bars are back-transformed least-square means ± SE obtained from a general linear mixed model (n = 20); see Materials and methods. Different letters indicate significant differences (P < 0.05) between least-square means. b Unconstrained ordination (PCoA) showing the effect of the herbivore load treatment on the composition of VOCs for plants from a second experiment aimed to test for effects on emissions. Biplot arrows represent linear associations for the first two most influential VOCs (i.e., those having the strongest associations with the first two ordination axes). Diamonds represent the centroids for each emitter herbivory treatment and associated 95% ellipses

Fig. 2

Percentage of leaf area consumed by Spodoptera exigua on receiver potato (Solanum tuberosum) plants in response to herbivore load treatments in emitter plants (control, low and high load of Spodoptera exigua). Bars are back-transformed least-square means ± SE obtained from a general linear mixed model (n = 20). Different letters indicate significant differences (P < 0.05) between least-square means