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 from 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 systemic acquired resistance (SAR), a rather unspecific form of defence, which can be induced by and is effective against a broad spectrum of bacteria, fungi and viruses. 3 We used a model system in which we treated wheat plants that were protected against fungi by 'traditional' fungicides with BION(R) (a benzothiadiazole which induces pathogen resistance). Treated plants were therefore compelled to invest in defence without gaining any profit from the induction. 4 Treated plants achieved lower biomass than untreated controls, and developed fewer shoots and ears and therefore produced fewer seeds. The effects were most pronounced in plants that suffered from a shortage of nitrogen, and were observed only when pathogen resistance was induced during lateral shoot 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 result from metabolic competition between processes involved in plant growth and the synthesis of defence-related compounds.