Short‐term ammonium supply induces cellular defence to prevent oxidative stress inArabidopsisleaves

Plants can assimilate nitrogen from soil pools of both ammonium and nitrate, and the relative levels of these two nitrogen sources are highly variable in soil. Long‐term ammonium nutrition is known to cause damage toArabidopsisthat has been linked to mitochondrial oxidative stress. Using hydroponic cultures, we analysed the consequences of rapid shifts between nitrate and ammonium nutrition. This did not induce growth retardation, showing thatArabidopsiscan compensate for the changes in redox metabolism associated with the variations in nitrogen redox status. During the first 3 h of ammonium treatment, we observed distinct transient shifts in reactive oxygen species (ROS), low‐mass antioxidants,ROS‐scavenging enzymes, and mitochondrial alternative electron transport pathways, indicating rapid but temporally separated changes in chloroplastic, mitochondrial and cytosolicROSmetabolism. The fast induction of antioxidant defences significantly lowered intracellularH2O2levels, and thus protectedArabidopsisleaves from oxidative stress. On the other hand elevated extracellularROSproduction in response to ammonium supply may be involved in signalling. The response pattern displays an intricate plasticity ofArabidopsisredox metabolism to minimise stress in responses to nutrient changes.