Summary 1. Jasmonic acid (JA), a ubiquitous regulator of the wound response in plants, is part of a long distance defence signalling pathway and when applied exogenously induces several defence related responses in many species including the activation of proteinase inhibitor (PIN) proteins and defence-related metabolites. 2. There is evidence for an induced systemic resistance mechanism linking increased cell wall peroxidase activity with reductions in leaf expansion rates in Rumex obtusifolius, following limited grazing by the chrysomelid beetle Gastrophysa viridula. We hypothesised that herbivory induces strengthening of cell walls in distal, non-damaged tissue through increases in peroxidase activity and that this mechanism was mediated systemically by JA. 3. JA was applied to the fully expanded fourth leaf of R. obtusifolius and expansion of leaf 8 was measured over 19 days. Treatment with exogenous JA induced a temporary reduction in the expansion rate of leaf 8 beginning 6 days after treatment. This reduction continued until day 7 and reduced final leaf areas in treated plants by approximately 20%. 4. Final epidermal cell areas in leaf 8 were reduced by approximately 25%, while epidermal cell numbers remained unchanged. 5. Cell wall bound peroxidase activity was measured in leaf 8 over a 6-day period following application of JA to leaf 4. Activity increased approximately 9-fold 2 days after treatment, before returning to control activities on day 3. 6. Treatment of leaf 4 with JA reduced the gregariousness of G. viridula larvae on the fully expanded leaf 8. 7. We postulate that JA acts as a signalling molecule in a long-distance pathway responsible for inducing resistance to future attack through increases in cell wall bound peroxidase activity leading to cell wall toughening, while incurring a potential ecological cost in the form of reductions in subsequent leaf expansion.