Reduced growth and seed set following chemical induction of pathogen defence: does systemic acquired resistance (SAR) incur allocation costs?

1 Although most theories on plant defence assume that costs will result fro m the production and maintenance of defensive traits, studies on the costs of induced defence against pathogens are comparatively rare. 2 We focus on fitness costs resulting from the chemical induction of system ic acquired resistance (SAR), a rather unspecific form of defence, which ca n be induced by and is effective against a broad spectrum of bacteria, fung i and viruses. 3 We used a model system in which we treated wheat plants that were protect ed against fungi by 'traditional' fungicides with BION(R) (a benzothiadiazo le which induces pathogen resistance). Treated plants were therefore compel led to invest in defence without gaining any profit from the induction. 4 Treated plants achieved lower biomass than untreated controls, and develo ped fewer shoots and ears and therefore produced fewer seeds. The effects w ere most pronounced in plants that suffered from a shortage of nitrogen, an d were observed only when pathogen resistance was induced during lateral sh oot production. Later treatment revealed no significant effects. 5 We discuss whether the differences between treated and control plants can be interpreted as a consequence of allocation costs. Such costs could resu lt from metabolic competition between processes involved in plant growth an d the synthesis of defence-related compounds.