Studies of the Uptake of Nitrate in Barley: I. Kinetics of NO(3) Influx

Effects of soil temperature and nitrogen status on kinetics of 15 NO3 - uptake by roots of field-grown Agropyron desertorum (Fisch. ex Link) Schult

Plant NO₈ - acquisition is largely determined by root uptake capacity. Although root uptake capacity has been shown to be sensitive to both root temperature and previous nitrogen (N) supply in hydroponic systems, the uptake capacity response to similar environmental factors under field conditions has not been investigated. Using ¹⁵ NO₃ ^- , root uptake capacities were determined in excised roots of Agropyron desertorum (Fisch. ex Link) Schult grown in the field at two soil temperatures and two N fertilization treatments. Variation in soil and root temperatures was achieved by application of clear plastic film or insulating mulch to the soil immediately around the target plants. Uptake rates were measured at six different assay solution concentrations (from 1 to 1000 μM external ¹⁵ NO₃ ^- concentration range). Two months after the imposition of soil N and temperature treatments, a biphasic transport system (a high-affinity) saturable phase and a low-affinity transport phase) was apparent in low N-treated plants. Nitrate uptake capacity in the low-concentration range (1-500μM) was significantly reduced in N-fertilized plants compared with unfertilized control plants and the effect was more pronounced at high (27 °C) than low (17 °C) soil and assay temperatures. Furthermore, high soil N status inhibited the expression of a low-affinity NO₃ ^- transport system which was clearly apparent at external NO₃ ^- concentration ranges between 500 and 1000 mM in plants grown at low soil N. Prior soil N and temperature history may ultimately determine root ability to exploit NO₃ ^- flushes which can result from changes in soil environmental conditions.

Localization of Nitrate Absorption and Translocation within Morphological Regions of the Corn Root

The absorption of NO(3) (-) was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A x W182E) growing in a complete nutrient solution. Based on changing rates of (15)N accumulation during 15-min time courses, translocation of the concurrently absorbed N through each region of the intact root was calculated and distinguished from direct absorption from the medium. Of the (15)N accumulated in the 5-mm root tip after 15 min, less than 15 and 35% had been absorbed directly from the external solution at 0.1 and 10 mm NO(3) (-) concentration of the external solution, respectively. The characterization of the apical portion of the primary root as a sink for concurrently absorbed N was conconfirmed in a pulse-chase experiment that showed an 81% increase of (15)N in the 5-mm root tip during a 12-min chase (subsequent to a 6-min labeling period). The lateral roots alone accounted for 60% of root influx and 70% of 15-min whole root (15)N accumulation at either 0.1 or 10 mm. NO(3) (-) concentration of the external solution. Because relatively steady rates of (15)N accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying (15)N to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in (15)N during the chase experiment, confirming their role in rapid translocation.

Use of an extracellular, ion-selective, vibrating microelectrode system for the quantification of K(+), H (+), and Ca (2+) fluxes in maize roots and maize suspension cells

An ion-selective vibrating-microelectrode system, which was originally used to measure extracellular Ca(2+) gradients generated by Ca(2+) currents, was used to study K(+), H(+) and Ca(2+) transport in intact maize (Zea mays L.) roots and individual maize suspension cells. Comparisons were made between the vibrating ion-selective microelectrode, and a technique using stationary ion-selective microelectrodes to measure ionic gradients in the unstirred layer at the surface of plant roots. The vibrating-microelectrode system was shown to be a major improvement over stationary ion-selective microelectrodes, in terms of sensitivity and temporal resolution. With the vibrating ion microelectrode, it was easy to monitor K(+) influxes into maize roots in a background K(+) concentration of 10 mM or more, while stationary K(+) electrodes were limited to measurements in a background K(+) concentration of 0.3 mM or less. Also, with this system it was possible to conduct a detailed study of root Ca(2+) transport, which was previously not possible because of the small fluxes involved. For example, we were able to investigate the effect of the excision of maize roots on Ca(2+) influx. When an intact maize root was excised from the seedling at a position 3 cm from the site of measurement of Ca(2+) transport, a rapid fourfold stimulation of Ca(2+) influx was observed followed by dramatic oscillations in Ca(2+) flux, oscillating between Ca(2+) influx and efflux. These results clearly demonstrate that wound or perturbation responses of plant organs involve transient alterations in Ca(2+) transport, which had previously been inferred by demonstrations of touch-induced changes in cytoplasmic calcium. The sensitivity of this system allows for the measurement of ion fluxes in individual plant cells. Using vibrating K(+) and H(+)electrodes, it was possible to measure H(+)efflux and both K(+) influx and efflux in individual maize suspension cells under different conditions. The availability of this technique will greatly improve our ability to study ion transport at the cellular level, in intact plant tissues and organs, and in specialized cells, such as root hairs or guard cells.

Effects of nitrite, chlorate, and chlorite on nitrate uptake and nitrate reductase activity

Effects of NO(2) (-), ClO(3) (-), and ClO(2) (-) on the induction of nitrate transport and nitrate reductase activity (NRA) as well as their effects on NO(3) (-) influx into roots of intact barley (Hordeum vulgare cv Klondike) seedlings were investigated. A 24-h pretreatment with 0.1 mol m(-3) NO(2) (-) fully induced NO(3) (-) transport but failed to induce NRA. Similar pretreatments with ClO(3) (-) and ClO(2) (-) induced neither NO(3) (-) transport nor NRA. Net ClO(3) (-) uptake was induced by NO(3) (-) but not by ClO(3) (-) itself, indicating that NO(3) (-) and ClO(3) (-) transport occur via the NO(3) (-) carrier. At the uptake step, NO(2) (-) and ClO(2) (-) strongly inhibited NO(3) (-) influx; the former exhibited classical competitive kinetics, whereas the latter exhibited complex mixed-type kinetics. ClO(3) (-) proved to be a weak inhibitor of NO(3) (-) influx (K(i) = 16 mol m(-3)) in a noncompetitive manner. The implications of these findings are discussed in the context of the suitability of these NO(3) (-) analogs as screening agents for the isolation of mutants defective in NO(3) (-) transport.

Studies of the Uptake of Nitrate in Barley : IV. Electrophysiology

Transmembrane electrical potential differences (Deltapsi) of epidermal and cortical cells were measured in intact roots of barley (Hordeum vulgare L. cv Klondike). The effects of exogenous NO(3) (-) on Deltapsi (in the concentration range from 100 micromolar to 20 millimolar) were investigated to probe the mechanisms of nitrate uptake by the high-affinity (HATS) and low-affinity (LATS) transport systems for NO(3) (-) uptake. Both transport systems caused depolarization of Deltapsi, demonstrating that the LATS (like the HATS) for NO(3) (-) uptake is probably mediated by an electrogenic cation (H(+)?) cotransport system. Membrane depolarization by the HATS was "inducible" by NO(3) (-), and saturable with respect to exogenous [NO(3) (-)]. By contrast, depolarization by the LATS was constitutive, and first-order in response to external [NO(3) (-)]. H(+) fluxes, measured in 200 micromolar and in 5 millimolar Ca(NO(3))(2) solutions, failed to alkalinize external media as anticipated for a 2 H(+):1 NO(3) (-) symport. However, switching from K(2)SO(4) solutions (which were strongly acidifying) to KNO(3) solutions at the same K(+) concentration caused marked reductions in H(+) efflux. These observations are consistent with NO(3) (-) uptake by the HATS and the LATS via 2 H(+):1 NO(3) (-) symports. These observations establish that the HATS for nitrate uptake by barley roots is essentially similar to those reported for Lemna and Zea mays by earlier workers. There are, nevertheless, distinct differences between barley and corn in their quantitative responses to external NO(3) (-).

Comparative kinetics and reciprocal inhibition of nitrate and nitrite uptake in roots of uninduced and induced barley (Hordeum vulgare L.) seedlings

Nitrate and NO2- transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO3- and NO2- was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO3- and NO2-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO3- and NO2- uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO3- and NO2- uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO3- uptake ranged from 34 to 36 micromolar and of NO2- uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO3- and NO2- inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO3- was a more effective inhibitor than NO2-. Nitrate and NO2- transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited.

Compartmental nitrate concentrations in barley root cells measured with nitrate-selective microelectrodes and by single-cell sap sampling

Nitrate-selective microelectrodes were used to measure intracellular nitrate concentrations (as activities) in epidermal and cortical cells of roots of 5-d-old barley (Hordeum vulgare L.) seedlings grown in nutrient solution containing 10 mol · m(-3) nitrate. Measurements in each cell type grouped into two populations with mean (±SE) values of 5.4 ± 0.5 mol · m(-3) (n=19) and 41.8 ± 2.6 mol · m(-3) (n = 35) in epidermal cells, and 3.2 ± 1.2 mol · m(-3) (n = 4) and 72.8 ± 8.4 mol · m(-3) (n = 13) in cortical cells. These could represent the cytoplasmic and vacuolar nitrate concentrations, respectively, in each cell type. To test this hypothesis, a single-cell sampling procedure was used to withdraw a vacuolar sap sample from individual epidermal and cortical cells. Measurement of the nitrate concentration in these samples by a fluorometric nitrate-reductase assay confirmed a mean vacuolar nitrate concentration of 52.6 ± 5.3 mol · m(-3) (n = 10) in epidermal cells and 101.2 ± 4.8 mol · m(-3) (n = 44) in cortical cells. The nitrate-reductase assay gave only a single population of measurements in each cell type, supporting the hypothesis that the higher of the two populations of electrode measurements in each cell type are vacuolar in origin. Differences in the absolute values obtained by these methods are probably related to the fact that the nitrate electrodes were calibrated against nitrate activity but the enzymic assay against concentration. Furthermore, a 28-h time course for the accumulation of nitrate measured with electrodes in epidermal cells showed the apparent cytoplasmic measurements remained constant at 5.0 ± 0.7 mol · m(-3), while the vacuole accumulated nitrate to 30-50 mol · m(-3). The implications of the data for mechanisms of nitrate transport at the plasma membrane and tonoplast are discussed.

Studies of the Uptake of Nitrate in Barley : II. Energetics

Q(10) values for (13)NO(3) (-) influx were determined in ;uninduced' (NO(3) (-)-starved) and ;induced' (NO(3) (-)-pretreated) roots of barley (Hordeum vulgare L.) plants at various concentrations of external NO(3) (-) ([NO(3) (-)](0)). At 0.02 mole per cubic meter [NO(3) (-)](0), Q(10) values for influx were from 3 to 4 between 5 and 10 degrees C. As [NO(3) (-)](0) increased Q(10) values decreased, reaching values of 1.2 and 2.0, respectively, at 20 moles per cubic meter in uninduced and induced plants. The metabolic dependence of (13)NO(3) (-) influx at low and high [NO(3) (-)](0) (0.1 and 20.0 moles per cubic meter, respectively) in uninduced and induced plants was probed by the use of various inhibitors. These experiments confirmed the findings of the Q(10) studies, demonstrating that at low [NO(3) (-)](0) (13)NO(3) (-) influx was extremely sensitive to metabolic inhibition. By contrast, at high [NO(3) (-)](0), influx was relatively insensitive to the presence of inhibitors.