Nitrate-Ammonium Synergism in Rice. A Subcellular Flux Analysis1

Many reports have shown that plant growth and yield is superior on mixtures of NO3- and NH4+ compared with provision of either N source alone. Despite its clear practical importance, the nature of this N-source synergism at the cellular level is poorly understood. In the present study we have used the technique of compartmental analysis by efflux and the radiotracer 13N to measure cellular turnover kinetics, patterns of flux partitioning, and cytosolic pool sizes of both NO3- and NH4+ in seedling roots of rice (Oryza sativa L. cv IR72), supplied simultaneously with the two N sources. We show that plasma membrane fluxes for NH4+, cytosolic NH4+ accumulation, and NH4+ metabolism are enhanced by the presence of NO3-, whereas NO3- fluxes, accumulation, and metabolism are strongly repressed by NH4+. However, net N acquisition and N translocation to the shoot with dual N-source provision are substantially larger than when NO3- or NH4+ is provided alone at identical N concentrations.