Systemic induced resistance: a risk‐spreading strategy in clonal plant networks?

Abstract

Summary• Clonal plant networks consist of interconnected individuals (ramets) of different sizes and ages. They represent heterogeneous ramet assemblages with marked differences in quality and attractiveness for herbivores. Here, feeding preferences of a generalist herbivore (Spodoptera exigua) for differently-aged ramets of Trifolium repens were studied, and changes in herbivore preference in response to systemic defense induction were investigated.• Dual-choice tests were used to assess the preference of herbivores for young versus mature ramets of induced and uninduced plants, respectively. Additionally, leaf traits related to nutrition, biomechanics and chemical defense were measured to explain variation in tissue quality and herbivore preference.• Young ramets were heavily damaged in control plants. After systemic defense induction, damage on young ramets was greatly reduced, while damage on mature ramets increased slightly. Defense induction increased leaf strength and thickness, decreased leaf soluble carbohydrates and substantially changed phenolic composition of undamaged ramets connected to attacked individuals.• Systemic induced resistance led to a more dispersed feeding pattern among ramets of different ages. It is proposed that inducible defense acts as a risk-spreading strategy in clonal plants by equalizing herbivore preference within the clone, thereby avoiding extended selective feeding on valuable plant tissues.

Defense signaling among interconnected ramets of a rhizomatous clonal plant, induced by jasmonic-acid application

“…5). A similar effect was reported for T. repens, in which phenolic profiles and foliage quality measured as foliage carbohydrate and nitrogen contents were different between young and old ramets (Gómez et al, 2008).…”

“…Therefore, young ramets of clonal plants are the most valuable tissues for growth and fitness and should hence be especially well protected (Mckey, 1974). C. alrofusca can share defense signals through belowground rhizome connections and its young ramets are more responsive than middle-age or old ramets, which selectively enhances the protection of more vulnerable and valuable plant tissues and confers a significant benefit to clonal plants by employing a modular risk-spreading strategy, equalizing ontogenetic ramet differences in chemical defense compounds and nutritional properties of tissue (Gómez et al, 2008).…”

“…Evidence on the integration of the non-resource substances among interconnected ramets has recently been gathered from studies on a stoloniferous clonal plant Trifolium repens (Gómez and Stuefer, 2006;Gómez et al, 2007Gómez et al, , 2008. These studies indicated that defense signaling could allow undamaged members of a clonal fragment to prepare themselves for impending herbivore attacks by up-regulating the expression of chemical and other defense traits.…”

“…1). The leaves of the untreated young, middle-aged and old ramets of the same clonal fragment were put into a petri dish and a fourth instar caterpillar was placed in the middle of the leaves, which is similar to the method described by Gómez et al (2008). The caterpillar was allowed to feed for a fixed amount of time (24 h).…”

“…However, animal grazing may alter the quality of hosteplant foliage, by reducing foliar nitrogen and carbohydrate concentrations. The nutritional properties of tissue may affect the feeding preferences and/or performance of herbivores so that they should be regarded in the context of inducible defense strategies (Jones and Roberts, 1991;Jensen, 1993;Redak and Capinera, 1994;Brathen et al, 2004;Gómez et al, 2008).…”