Appropriate Supply of Ammonia and Ammonium Nitrate Reduce Cadmium Content in Rice Seedlings by Inhibiting Cadmium Uptake and Transport

Reasonable nitrogen (N) application is a promising strategy for reducing crop cadmium (Cd) toxicity. However, which state of N and how much N affects Cd tolerance and accumulation in rice remains unclear. This study explored the influence of different N-fertilizer forms (NH4NO3, NH4Cl, and KNO3) and dosages on Cd tolerance and absorption in Cd-stressed N-sensitive and insensitive indica accessions. The results indicated that the Cd tolerance of N-sensitive indica accessions is more robust than N-insensitive indica accessions. Furthermore, the shoot Cd contents and Cd translocation rates of both N-sensitive and insensitive indica accessions were decreased with an appropriate supply of NH4NO3 and NH4+, whereas comparable or slightly improved with increased NO3-. Unfortunately, we did not find significant and regular differences in Cd accumulation or translocation between N-sensitive and N-insensitive rice accessions. Consistent with the reduction of shoot Cd content, adding NH4NO3 and NH4+ also inhibited the instantaneous root Cd2+ uptake. The expression changes of Cd transport-related genes under different N forms and dosages suggested that the decreased shoot Cd content, caused by increased NH4NO3 and NH4+, is likely achieved by reducing the transcription of OsNRAMP1 and OsIRT1. In summary, our findings reveal that the appropriate supply of NH4NO3 and NH4+ could reduce Cd uptake and transport in rice seedlings and suggest that rational N management could reduce Cd risk in rice production.