The simultaneous inducibility of phytochemicals related to plant direct and indirect defences against herbivores is stronger at low elevation

Abstract

1. Ecological theory indicates that warmer and more stable climates should result in stronger biotic interactions. Therefore, plant species growing at lower elevations and experiencing greater herbivore pressure should invest in higher levels of defences than those at higher elevations. Nonetheless, there are a number of studies that have found no effect of elevational gradients on plant defensive traits. Several factors might explain the lack of consistency for the altitude-defence relationships, including (i) the reduction of all defensive traits into one measure of resistance; (ii) not considering plant defence as the simultaneous expression of several defensive traits; and (iii) not considering the relative influence of biotic (e.g. herbivory) and abiotic (e.g. climate and soil conditions) factors associated with the ecological gradient.2. Here, we present a comprehensive test of the effects of elevation and its associated biotic and abiotic factors on the individual and simultaneous expression of constitutive direct and indirect defences and their inducibility (i.e. expression of defences after herbivore attack). Specifically, we estimated climatic and soil variables and measured herbivore damage and constitutive and jasmonic acid-induced glucosinolate levels in the leaves as a proxy for direct defences, and volatile emission as a proxy for indirect defences in 16 Cardamine species naturally growing along the steep elevational gradient of the Alps. 3. Within a phylogenetic comparative framework, we found that species growing at lower elevations invested more in the simultaneous inducibility of both direct and indirect defences, whereas species growing at higher elevations invested more in constitutive direct defences. Although we found strong elevational gradients in herbivory and climatic and soil variables, these biotic and abiotic factors only partially explained elevational patterns in plant defences. 4. Synthesis. These results highlight that the complex regulation of multiple defence traits strongly vary across elevational gradients and build towards a better understanding of the multiple mechanisms underlying trait evolution and species interactions along ecological gradients.

Elevational gradients in plant defences and insect herbivory: recent advances in the field and prospects for future research

“…1B-C). With respect to plants, this includes variation in chemical and physical traits associated with resistance or food quality to insect herbivores (Pellissier et al 2016). With respect to herbivores, this includes traits such as foraging patterns, feeding behaviour, and diet breadth (Pellissier et al 2012, Rasmann et al 2014a).…”

“…Consequently, plant species adapted to environments with limiting abiotic conditions are predicted to increase allocation to expensive constitutive defences (relative to induced defences), as the energy demand of replacing tissues consumed by insect herbivores is higher under such stressful conditions (Moreira et al 2014, Pellissier et al 2016 (Fig. 1B).…”

“…Plant defence strategies are also expected to respond to changes in plant-associated multi-trophic communities along elevational gradients. For example, changes in the ratios of insect herbivores to predators may underlie shifts in plant allocation to traits associated to direct versus indirect defence , Pellissier et al 2016, Rodríguez-Castañeda et al 2016. Predator diversity and abundance are generally higher at low elevations (Hodkinson 2005, Rasmann et al 2014a), such that low-elevation plants are expected to invest more in traits that attract predators (i.e.…”

“…These so-called 'defence syndromes' can vary both among and within plant species depending on the ecological context (Callis-Duehl et al 2017). To our knowledge, only two studies to date have looked at the influence of elevation on plant defensive syndromes (Dostálek et al 2016, Pellissier et al 2016. In one of these, Pellissier et al (2016) found a positive correlation between inducibility of direct and inducibility of indirect defences among herbaceous species, and that the simultaneous expression of these two strategies was stronger at lower elevations.…”

Plant Antiherbivore Defense in Diverse Environments

“…Then, synergistic interactions among various defensive traits offer an effective resistance, which is reflected with an increase in the simultaneous expression for direct and indirect defenses [73]. When plants exceed the capacity to store, constitutive secondary metabolites could avoid autotoxicity [74].…”