Nitrate-ammonium synergism in rice. A subcellular flux analysis

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 N-13 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.