The Uptake of NO3−, NO2−, and NH4+ by Intact Wheat (Triticum aestivum) Seedlings

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 the extent of the induction of each transporter. Up to 10 and 6 hours, respectively, were required to achieve apparent full induction of the NO3- and NO2- transporters. Evidence for substrate inducibility of the NH4+ transporters requiring 5 hours is presented. The transport of NO3- was mediated by a dual system (or dual phasic), whereas only single systems were found for transport of NO2- and NH4+. The Km values for NO3-, NO2-, and NH4+ were, respectively, 0.027, 0.054, and 0.05 millimolar. The Km for mechanism II of NO3- transport could not be defined in this study as it exhibited only apparent first order kinetics up to 1 millimolar.