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

Abstract

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...

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

“…This model conformed best with the measured data with regard to the obtained standard errors (data not shown) and the fitting of curves to the means of uptake rates (Fig. 3) Aslam et al 1992), but lower than those found in maize (24 mmol m -3 , Hole et al 1990), in wheat (27 mmol m -3 , Goyal & Huffaker 1986b) and in Arabidopsis (40 mmol m -3 , Doddema & Telkamp 1979) or in excised barley roots in earlier reports (110 mmol m -3 , Rao & Rains 1976a). Similar agreements were found for the 13 N-NO 3 -influx (Lee & Drew 1986: 14 mmol m -3 ;Siddiqi et al 1990: between 30 and 79 mmol m -3 depending on the N status of the plants).…”

“…The HATS is described as a saturable system, whereas the nature of the LATS is still not clear. Doddema & Telkamp (1979) and Goyal & Huffaker (1986b) found a second saturable system, but Siddiqi et al (1990) and Aslam, Travis & Huffaker (1992) observed a linear non-saturable component under higher concentrations of nitrate. Aslam et al (1992) even found two saturable, possibly constitutive systems in the low concentration range and a linear system at more than 500 mmol m -3 nitrate.…”

Comparative Induction of Nitrate and Nitrite Uptake and Reduction Systems by Ambient Nitrate and Nitrite in Intact Roots of Barley (Hordeum vulgare L.) Seedlings

“…The induction of the Nos-and NOz-uptake systems in the presence of their respective substrates is well established (Jackson et al, 1973(Jackson et al, , 1974a(Jackson et al, , 1974bGoyal and Huffaker, 1986b;Aguera et al, 1990;Siddiqi et al, 1990). In addition, both uptake systems clearly appear to be induced by Nos- (Aslam et al, 1992a).…”

Preferential expression of an ammonium transporter and of two putative nitrate transporters in root hairs of tomato.

“…The available data indicate that entry of NH' and NO-into roots apparently is mediated by more than one transporter specific for each ion (11,36). To dissect nitrogen transport systems at the molecular level, genes encoding putative transporters for mineral nitrogen were cloned.…”