Nitrate Uptake of Rice as Affected by Growth Stages and Ammonium

Solution culture experiments were carried out to study the NO3- uptake kinetics by rice roots and the nitrate reductase activity (NRA) in leaves of four typical rice genotypes (conventional indica, conventional japonica, hybrid indica, and hybrid japonica) at different growth stages. The effects of NH4+ on the NO3- uptake kinetics was also studied. The results indicated that the four genotypes responded differently to NO3- and NH4+. Comparing indica rice with japonica rice, hybrid rice with conventional rice, formers absorbed NO3-more rapidly than laters. The superiority of indica rice and hybid rice for NO3- uptake was mainly due to the higher Vmax at the early growth stage (20 days) and the higher affinity of nitrate transporters for NO3- at the middle growth stage (50 days). At both growth stages NH4+ significantly inhibited NO3- uptake for japonica rice, but showed little effect for indica rice. Conversely, NH4+ stimulated the uptake of NO3- for hybrid rice. NH4+ could repress NRA in leaves of all four genotypes with a sensitive order as following: hybrid japonica > hybrid indica > conventional indica > conventional japonica.