Nitrate-Ammonium Synergism in Rice. A Subcellular Flux Analysis

Many reports have shown that plant growth and yield is superior on mixtures of NO(3)(−) and NH(4)(+) 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 (13)N to measure cellular turnover kinetics, patterns of flux partitioning, and cytosolic pool sizes of both NO(3)(−) and NH(4)(+) in seedling roots of rice (Oryza sativa L. cv IR72), supplied simultaneously with the two N sources. We show that plasma membrane fluxes for NH(4)(+), cytosolic NH(4)(+) accumulation, and NH(4)(+) metabolism are enhanced by the presence of NO(3)(−), whereas NO(3)(−) fluxes, accumulation, and metabolism are strongly repressed by NH(4)(+). However, net N acquisition and N translocation to the shoot with dual N-source provision are substantially larger than when NO(3)(−) or NH(4)(+) is provided alone at identical N concentrations.