Florida State University Tallahassee, FL, United States
Abstract: Plants have many adaptations for life as sessile organisms, including the ability to defend themselves against mobile herbivores and pathogens. Upon damage, plants can exhibit inducible defenses (defense plasticity), whereby they increase responses against an attacker via the modification of plant metabolites. Additionally, plants can recognize herbivore-induced plant volatiles (HIPVs) released from damaged neighbors as a cue to prime defenses to respond stronger and/or more quickly to subsequent damage. Many ecological and evolutionary studies of induced resistance have modeled responses within individual non-clonal annual plants, or have assumed individual plant defense responses occur independently in the field. However, clonal perennials are widespread in nature and tend to be physiologically integrated at some level belowground so responses of ramets might not be independent.
Clonal ramets can vary in their distribution of defenses to ramets through space and time. Although below-ground signaling is known to occur, we do not know the distances at which induced defense signals travel through root connections among physiologically integrated ramets. In addition to exchanging induced defense signals through root connections, HIPVs released from a damaged ramet could influence how nearby ramets respond to subsequent herbivory by causing priming. We examined the distance at which induced defense signaling occurs among interconnected ramets of the clonal species Solanum carolinense in response to damage by the specialist beetle, Leptinotarsa juncta. We planted connected networks of ramets in small pools, damaged one ramet, and measured resistance in ramets at varying distances from the damaged ramet with beetle feeding trials. We also examined the relative contribution of direct root connections and volatile-mediated priming mechanisms to induced defense signaling by placing unconnected, potted S. carolinense near damaged plants.
Our results suggest induced resistance responses grew weaker as ramet distances from the initially damaged ramet increased. Clonal induced resistance and unconnected but HIPV-primed induced resistance did not significantly differ from one another, suggesting volatile-mediated priming mechanisms may play as large a role in systemic induction as root connections, because induction patterns were qualitatively similar regardless of whether plants were connected below-ground. This work contributes to our understanding of how induced defenses are distributed across a clonal plant and provides qualitative evidence that ramets of clonal plants in the field do not respond independently to damage.
