The inducibility and kinetics of the NO3-, NO2-, and NH4+ transporters in roots of wheat seedlings (Triticum aestivum cv Yercora Rojo) were characterized using precise methods approaching constant analysis of the substrate solutions. A microcomputer-controlled automated high performance liquid chromatography system was used to determine the depletion of each N species (initially at 1 millimolar) from complete nutrient solutions. Uptake rate analyses were performed using computerized curve-fitting techniques. More precise estimates were obtained for the time required for and the extent of the induction of each transporter. Up to 10 and 6 hours, respectively, were required to achieve apparent full induction of the N03-and NO2-transporters. Evidence for substrate inducibility of the NH4+ transporters requiring 5 hours is presented. The transport of N03-was mediated by a dual system (or dual phasic), whereas only single systems were found for transport of NO2-and NH4+.The K. values for N03-, NO2-, and NH4I were, respectively, 0.027, 0.054, and 0.05 millimolar. The K,,, for mechanism II of NO3-transport could not be defined in this study as it exhibited only apparent first order kinetics up to I millimolar.Apparent substrate inducibility of a root NO3-transporter in many higher plants has been known for over a decade (16). The reported induction periods required to achieve a steady state uptake rate differ: 1 to 2 h, corn roots (22); 3 to 4 h, barley seedlings (23); 4 h, wheat seedlings (16); 6 to 7 h, dwarf bean seedlings (5). Much less work has been done on the induction of a NO; transporter; its apparent induction by NO;2, requiring about 4 h, has been reported (17). Ammonium uptake by plant roots during early exposure to NH4' has been characterized as an initial brief phase of rapid uptake confounded by free space and ion exchange, followed by a slower linear rate (20). Information on substrate inducibility of the NH4+ transporter has not been previously reported.The uptake of inorganic N species during the induction of their transporters is a dynamic process with an increasing catalyst and a decreasing ion concentration in the ambient solution. The resultant uptake during that time requires further definition. It is important to characterize the induction phase of a transporter since it should be fully induced before evaluating kinetic parameters. ' Supported in part by a grant from the United States National Aeronautics and Space Agency (NASA NCC 2-99).Conclusions about the induction periods have often been based on accumulative uptake as a function of time. This method does not offer the degree of resolution required to precisely define the extent of induction or the time required for full induction. Small changes in uptake rates are masked in such plots. A rate analysis as a function of time allows a higher resolution of the induction of a transporter. This requires a sensitive and precise analytical method to establish rates over small changes in concentration of the ambient solution. A microcompute...