Respiratory burst oxidases and apoplastic peroxidases facilitate ammonium syndrome development in Arabidopsis

•Distinct nitrogen sources facilitated the identification of apoplastic ROS accumulation as a major symptom under ammonium toxicity.•An apoplastic ROS burst under NH4+ toxicity conditions is the product of respiratory burst oxidase and extracellular peroxidase activities.•Apoplastic H2O2 is utilized by cell wall peroxidases engaged in lignification, which may limit expansion growth of cells in response to NH4+.•Ammonium perception is actively processed in the extracellular space and omits activation of typical ROS-signalling routes.