Functional Ecology 2003 17, 549–554 © 2003 British Ecological Society 549 Blackwell Publishing Ltd. Exogenous jasmonic acid mimics herbivore-induced systemic increase in cell wall bound peroxidase activity and reduction in leaf expansion JASON P. MOORE*, NIGEL D. PAUL, JOHN B. WHITTAKER and JANE E. TAYLOR Department of Biological Science, Institute of Environmental and Natural Sciences, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, UK 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 obtusi- folius, 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 epi- dermal cell numbers remained unchanged. 5. Cell wall bound peroxidase activity was measured in leaf 8 over a 6-day period fol- lowing 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 ecolo- gical cost in the form of reductions in subsequent leaf expansion. Key-words: Jasmonic acid, ecological costs, Rumex obtusifolius, systemic induced resistance Functional Ecology (2003) 17, 549–554 Introduction Phenotypically plastic traits such as induced plant defences to insect herbivores are costly in terms of the resources required to activate them (Agrawal, Strauss & Stout 1999) and are mediated, at least in part, by jas- monic acid (JA) (McConn et al . 1997). The idea that these induced defences to insect herbivores not only benefit plants at the time of attack by negatively affecting the performance of the attacking herbivore, but incur costs when limited resources are reallocated away from growth and/or reproduction to defence is a common assumption in plant defence theory (Coley, Bryant & Chapin 1985; Feeny 1976). In turn, this assumption is based on the idea that plants have a limited stock of resources that can be utilised either in growth and / or reproduction, or for defence (Coley et al . 1985). In a previous study (Moore et al . 2003) we re- ported that limited grazing of Rumex obtusifolius *Author to whom correspondence should be addressed. E-mail: j.p.moore@lancaster.ac.uk