Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton

Abstract

The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number of studies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap, here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum).• We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant.• Single events of 25% and 50% leaf damage did not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar production was stronger than systemic induction, whereas the reverse pattern was observed for phenolics.• Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.

Effects of soil salinity on the expression of direct and indirect defences in wild cotton Gossypium hirsutum

“…phenolic compounds) and indirect (e.g. EFN, VOCs) defensive traits in wild cotton, which is consistent with previous work of ours with wild (Abdala‐Roberts, Pérez‐Niño, Moreira, et al, 2019; Abdala‐Roberts, Quijano‐Medina, Reyes‐Hernández, et al, 2019) as well as studies addressing leaf herbivory with cultivated cotton (e.g. EFN: Wäckers & Wunderlin, 1999; VOCs: Paré & Tumlinson, 1997; Röse & Tumlison, 2005).…”

“…leaf buds) at the moment of damage application (totally or almost totally expanded at the time of collection) and was aimed at quantifying induction in new leaf tissues grown after induction. We followed this approach as previous work has shown that phenolic compounds are not only induced a few days after leaf damage in pre‐existing tissue, but also exhibit induced levels in new tissues grown several weeks after damage (Abdala‐Roberts, Pérez‐Niño, Moreira, et al, 2019; Abdala‐Roberts, Quijano‐Medina, Reyes‐Hernández, et al, 2019). We used a different subset of plants for each time point because mechanical damage from short‐term leaf collection would induce defences and therefore affect long‐term measurements.…”

“…(Heil et al., 2012), various studies have demonstrated that combining mechanical leaf removal and caterpillar secretions provides an effective proxy of natural damage in several cultivated species (e.g. maize: Turlings et al., 1993; McAuslane et al., 1997; tobacco: Halitschke, Schittko, Pohnert, Boland, & Baldwin, 2001), including cultivated (Röse & Tumlinson, 2005) and wild cotton (Abdala‐Roberts, Pérez‐Niño, Moreira, et al, 2019; Abdala‐Roberts, Quijano‐Medina, Reyes‐Hernández, et al, 2019; Chappuis & Egger, 2016).…”

Host-mediated, cross-generational intraspecific competition in a herbivore species

“…Previous herbivory generally reduces the performance of later arriving herbivores on the same plant (Moreira et al, 2018), although the opposite effect can also be observed (Sarmento et al, 2011;Godinho et al, 2016). Reduced performance of herbivores on previously damaged plants could occur through both exploitative competition -whereby the first attacking herbivore depletes the resource available to forthcoming herbivores (Kaplan & Denno, 2007) -or through changes in plant traits (Hilker & Fatouros, 2015;Abdala-Roberts et al, 2019;Marchand & McNeil, 2004;Blenn et al, 2012;Fatouros et al, 2012). By triggering an hormonal response and inducing the production of anti-herbivore defenses as well as resource reallocation in plant tissues, herbivory may induce changes in plant quality that generally reduce the performance of late coming herbivores (Agrawal, 1999;Abdala-Roberts et al, 2019;Wratten et al, 1988).…”

“…Reduced performance of herbivores on previously damaged plants could occur through both exploitative competition -whereby the first attacking herbivore depletes the resource available to forthcoming herbivores (Kaplan & Denno, 2007) -or through changes in plant traits (Hilker & Fatouros, 2015;Abdala-Roberts et al, 2019;Marchand & McNeil, 2004;Blenn et al, 2012;Fatouros et al, 2012). By triggering an hormonal response and inducing the production of anti-herbivore defenses as well as resource reallocation in plant tissues, herbivory may induce changes in plant quality that generally reduce the performance of late coming herbivores (Agrawal, 1999;Abdala-Roberts et al, 2019;Wratten et al, 1988). Such an affect has long been documented in interspecific interactions (Kaplan & Denno, 2007;Moreira et al, 2018), but also in intraspecific interactions.…”