Responses of barley and wheat to pathogens, non‐pathogens and wounding as indicated by induced phenylalanine ammonia‐lyase activity

Phenylalanine ammonia‐lyase (PAL) activity was strongly induced in barley and wheat leaves after inoculation with highly or weakly aggressive isolates of Bipolaris sorokiniana. Aggressive isolates, however, generated much stronger early induction of PAL than less aggressive isolates. The non‐pathogens Drechslerei teres (in wheat), Stagonospora nodorum (in barley), Botrytis cinerea, Trichoderma reesei and the root and stem pathogen Fusarium culmorum caused no clear induction of PAL on intact barley or wheat leaves. They were able to induce PAL at the wound margins of barley leaves, although at a lower level than B. sorokiniana. Wounding alone did not induce PAL. Prior inoculation of barley leaves with non‐pathogens provided partial protection against a subsequent challenge with B. sorokiniana seven days later. Induced plants showed unchanged PAL activity levels compared to non‐induced plants after challenge inoculation. The results suggest that PAL plays a role in the active defences of barley and wheat in response to pathogen attack, but apparently not in response to non‐pathogens, wounding, or in plants expressing induced resistance.