Shifts in N and δ 15 N in wheat and barley exposed to cerium oxide nanoparticles.

The effects of cerium oxide nanoparticles (CeO2 -NPs) on 15 N/14 N ratio (δ15 N) in wheat and barley were investigated. Seedlings were exposed to 0 and 500 mg CeO2 -NPs/L (Ce-0 and Ce-500, respectively) in hydroponic suspension supplied with NH4 NO3 , NH4 + , or NO3 - . N uptake and δ15 N discrimination (i.e. differences in δ15 N of plant and δ15 N of N source) were measured. Results showed that N content and 15 N abundance decreased in wheat but increased in barley. Ce-500 only induced whole-plant δ15 N discrimination (-1.48‰, P ≤ 0.10) with a simultaneous decrease (P ≤ 0.05) in whole-plant δ15 N (-3.24‰) compared to Ce-0 (-2.74‰) in wheat in NH4 + . Ce-500 decreased (P ≤ 0.01) root δ15 N of wheat in NH4 NO3 and NH4 + (3.23 and -2.25‰, respectively) compared to Ce-0 (4.96 and -1.27‰, respectively), but increased (P ≤ 0.05) root δ15 N of wheat in NO3 - (3.27‰) compared to Ce-0 (2.60‰). Synchrotron micro-XRF revealed the presence of CeO2 -NPs in shoots of wheat and barley regardless of N source. Although the longer-term consequences of CeO2 -NP exposure on N uptake and metabolism are unknown, the results clearly show the potential for ENMs to interfere with plant metabolism of critical plant nutrients such as N even when toxicity is not observed.