Effect of exogenous and endogenous nitrate concentration on nitrate utilization by dwarf bean

An Investigation of a Root Zone Heating System and Its Effects on the Morphology of Winter-Grown Green Peppers

The winter season in Nanjing is from December to February, with extremely low temperature and high humidity due to seasonal snowfall. During these extreme cold climatic conditions, plants have to survive severe heat stress conditions, even if they are being kept in greenhouses. The objective of this study was to investigate a heating system that can provide heat directly to the root zone instead of heating the entire greenhouse, which is a viable option to reduce energy consumption. Root zone heating could be an effective alternative for the sustainable development of plants during the winter. A novel type of root zone heating system was applied to evaluate the energy consumption during different greenhouse ambient temperature conditions, the effects of root zone heating systems on pepper plant morphology, and heat transfer rates to plant canopy in the greenhouse. The temperature treatments in root zone heating system were T-15, T-20, T-25, T-30, and a control treatment (TC) at 15 °C, 20 °C, 25 °C, and 30 °C, respectively, while TC received no heat. A simulation study was carried out to validate the root zone temperature. The results of the current investigation revealed that energy consumption has an inverse relationship to the ambient temperature of the greenhouse, while temperature gradients to the plant canopy observed from the lower to the upper part of the plant and the upper canopy experienced less temperature fluctuation as compared to the lower part of the plant. The results also showed that treatment T-20 had the maximum in terms of the leaf dry weight, stem diameter, and the number of leaves, while T-25 showed the maximum root dry weight and stem dry weight; T-30 and T-15 had minimum dry weights of plant segments among all treatments. Control treatment (TC) showed a minimum dry mass of plant. The root zone heating with optimal root zone temperature was found to be a viable and adaptable option as this leads to improved energy consumption patterns for the sustainable growth and development of plants in greenhouses during extremely low temperatures.

Quantifying N-loss by root abscission: consequences for wheat N budgets and δ15N values

Lower plant δ¹⁵N values relative to source δ¹⁵N are commonly attributed to ¹⁵N efflux. We determined the extent to which root abscission contributes to plant N-loss and consequences for plant δ¹⁵N. Wheat (Triticum aestivum L. cv. SST015) was grown in hydroponics with direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand, representing three levels of stability for root growth. The δ¹⁵N of nutrient solutions and root fragments were periodically determined, as well as root and shoot δ¹⁵N. Plants in solution had significantly more negative δ¹⁵N (-8.9 and -9.2‰) than plants in sand (-6.9‰), suggesting greater ¹⁵N-loss; root fragments were major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathways in solution. These plants had more ephemeral roots and two-fold more root tips than the sand treatment. We estimated that root fragment loss decreased plant δ¹⁵N by at least -3.7, -2.6 and -1.0‰ in the direct, pipe and sand treatments, respectively. Positive nutrient solution δ¹⁵N in all treatments relative to the source δ¹⁵N suggests that plant N, probably derived from efflux, was present in solution. Despite this, root abscission and root turnover are also important N-loss pathways in plants, while plant δ¹⁵N values are probably influenced by a combination of root abscission and N efflux.

Plasma membrane anion channels in higher plants and their putative functions in roots

Recent years have seen considerable progress in identifying anion channel activities in higher plant cells. This review outlines the functional properties of plasma membrane anion channels in plant cells and discusses their likely roles in root function. Plant anion channels can be grouped according to their voltage dependence and kinetics: (1) depolarization-activated anion channels which mediate either anion efflux (R and S types) or anion influx (outwardly rectifying type); (2) hyperpolarization-activated anion channels which mediate anion efflux, and (3) anion channels activated by light or membrane stretch. These types of anion channel are apparent in root cells where they may function in anion homeostasis, membrane stabilization, osmoregulation, boron tolerance and regulation of passive salt loading into the xylem vessels. In addition, roots possess anion channels exhibiting unique properties which are consistent with them having specialized functions in root physiology. Most notable are the organic anion selective channels, which are regulated by extracellular Al3+ or the phosphate status of the plant. Finally, although the molecular identities of plant anion channels remain elusive, the diverse electrophysiological properties of plant anion channels suggest that large and diverse multigene families probably encode these channels.

Evidence for Substrate Induction of a Nitrate Efflux System in Barley Roots

Induction of an NO3- efflux system in intact barley (Hordeum vulgare L.) roots was demonstrated. Since the measurement of NO3- efflux is dependent on its accumulation, experiments were devised to facilitate accumulation under noninducing conditions. This was accomplished by incubating seedlings in 10 mM NO3- in the presence of RNA and protein synthesis inhibitors. Under these conditions NO3- uptake is mediated by constitutive high- and low-affinity transport systems. Control roots were incubated with 1.0 mM NO3-. This resulted in the accumulation of similar levels of NO3- in both treated and control roots; however, cytoplasmic NO3- efflux from inhibitor-treated roots was much lower than from control roots. Following a brief lag period, efflux rates increased rapidly in the presence of NO3- for 8 to 12 h. The NO3- efflux system was also induced by ambient NO2-. After induction the efflux system was relatively stable in the presence of RNA and protein synthesis inhibitors as long as NO3- or NO2- was present. These results suggest that NO3- efflux may be an inducible system requiring both RNA and protein synthesis, as does induction of the uptake system. The efflux system, however, has a much slower turnover rate than the uptake system.

Kinetics of NO3- Influx in Spruce

Influxes of 13NO3- across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Three kinetically distinct uptake systems for NO3- were identified. In seedlings not previously exposed to external NO3-, a single Michaelis-Menten-type constitutive high-affinity transport system (CHATS) operated in a 2.5 to 500 [mu]M range of external NO3- [NO3-]o. The Vmax of this system was 0.1 [mu]mol g-1 h-1, and the Km was approximately 15 [mu]M. Following exposure to NO3- for 3 d, this CHATS activity was increased approximately 3-fold, with no change of Km. In addition, a NO3--inducible high-affinity system became apparent with a Km of approximately 100[mu]M. The combined Vmax for these discrete saturable components was 0.7 [mu]mol g-1 h-1. In both uninduced and induced plants a linear low-affinity system, additive to CHATS and an NO3--inducible high-affinity system, operated at [NO3-]o [greater than or equal to] 1 mM. The time taken to achieve maximal rates of uptake (full induction) was 2 d from 1.5 mM [NO3-]o and 3 d from 200 [mu]M [NO3-]o.

Kinetics of NO3- Influx in Spruce

Influxes of 13NO3- across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Three kinetically distinct uptake systems for NO3- were identified. In seedlings not previously exposed to external NO3-, a single Michaelis-Menten-type constitutive high-affinity transport system (CHATS) operated in a 2.5 to 500 [mu]M range of external NO3- [NO3-]o. The Vmax of this system was 0.1 [mu]mol g-1 h-1, and the Km was approximately 15 [mu]M. Following exposure to NO3- for 3 d, this CHATS activity was increased approximately 3-fold, with no change of Km. In addition, a NO3--inducible high-affinity system became apparent with a Km of approximately 100[mu]M. The combined Vmax for these discrete saturable components was 0.7 [mu]mol g-1 h-1. In both uninduced and induced plants a linear low-affinity system, additive to CHATS and an NO3--inducible high-affinity system, operated at [NO3-]o [greater than or equal to] 1 mM. The time taken to achieve maximal rates of uptake (full induction) was 2 d from 1.5 mM [NO3-]o and 3 d from 200 [mu]M [NO3-]o.

Ammonium Uptake by Rice Roots (II. Kinetics of 13NH4+ Influx across the Plasmalemma)

Short-term influxes of 13NH4+ were measured in intact roots of 3-week-old rice (Oryza sativa L. cv M202) seedlings that were hydroponically grown at 2, 100, or 1000 [mu]M NH4+. Below 1 mM external concentration ([NH4+]0), influx was saturable and due to a high-affinity transport system (HATS). For the HATS, Vmax values were negatively correlated and Km values were positively correlated with NH4+ provision during growth and root [NH4+]. Between 1 and 40 mM [NH4+]0, 13NH4+ influx showed a linear response due to a low-affinity transport system (LATS). The 13NH4+ influxes by the HATS, and to a lesser extent the LATS, are energy-dependent processes. Selected metabolic inhibitors reduced influx of the HATS by 50 to 80%, but of the LATS by only 31 to 51%. Estimated values for Q10 (the ratio of rates at temperatures differing by 10[deg]C) for HATS were greater than 2.4 at root temperatures from 5 to 10[deg]C and were constant at approximately 1.5 between 5 and 30[deg]C for the LATS. Influx of 13NH4+ by the HATS was insensitive to external pH in the range from 4.5 to 9.0, but influx by the LATS declined significantly beyond pH 6.0. The data presented are discussed in the context of the kinetics, energy dependence, and the regulation of ammonium influx.

Ammonium Uptake by Rice Roots (I. Fluxes and Subcellular Distribution of 13NH4+)

The time course of 13NH4+ uptake and the distribution of 13NH4+ among plant parts and subcellular compartments was determined for 3-week-old rice (Oryza sativa L. cv M202) plants grown hydroponically in modified Johnson's nutrient solution containing 2,100, or 1000 [mu]M NH4+ (referred to hereafter as G2, G100, or G1000 plants, respectively). At steady state, the influx of 13NH4+ was determined to be 1.31, 5.78, and 10.11 [mu]mol g-1 fresh weight h-1, respectively, for G2, G100, and G1000 plants; efflux was 11, 20, and 29%, respectively, of influx. The NH4+ flux to the vacuole was calculated to be between 1 and 1.4 [mu]mol g-1 fresh weight h-1. By means of 13NH4+ efflux analysis, three kinetically distinct phases (superficial, cell wall, and cytoplasm) were identified, with t1/2 for 13NH4+ exchange of approximately 3 s and 1 and 8 min, respectively. Cytoplasmic [NH4+] was estimated to be 3.72, 20.55, and 38.08 mM for G2, G100, and G1000 plants, respectively. These concentrations were higher than vacuolar [NH4+], yet 72 to 92% of total root NH4+ was located in the vacuole. Distributions of newly absorbed 13NH4+ between plant parts and among the compartments were also examined. During a 30-min period G100 plants metabolized 19% of the influxed 13NH4+. The remainder (81%) was partitioned among the vacuole (20%), cytoplasm (41%), and efflux (20%). Of the metabolized 13N, roughly one-half was translocated to the shoots.

Nitrate and ammonium uptake by maize: adaptation during relief from nitrogen suppression

Initial (0-1 h) net rates of nitrate and ammonium uptake from 200 μM NH₄ NO₃ were progressively increased as 8-d-old maize (Zea mays L.) seedlings, grown on 5 mM nitrate, were exposed to nitrogen-free solutions for up to 48 h. Further nitrogen deprivation to 72 h resulted in a decline in the nitrate uptake rate. Nitrate uptake rates of plants at all stages of nitrogen deprivation increased steadily during an 8 h exposure to 200 μM NH₄ NO₃ . The pattern of the response of ammonium uptake during the 8 h adaptation period was considerably different. In nitrogen-replete plants the ammonium uptake rate increased steadily, but deprivation of nitrogen for 12 h and longer resulted in complex responses in which the initial rate was followed by a decline, a subsequent increase, and another decline. The responses of the nitrate uptake system are considered to reflect a lifting of the suppressive effects of nitrate and a product of nitrate assimilation during nitrogen deprivation, a concomitant degradation of an induced component of the nitrate uptake system during that time, and reinduction of the uptake system during the adaptation period. The responses of the ammonium uptake system are considered to reflect the interplay of suppression by a product of ammonium assimilation, the accumulation of root ammonium and associated ammonium efflux, and a stimulation by ammonium of its own uptake. As a consequence of the differential responses of the two uptake systems, nitrate and ammonium uptake rates were positively correlated, largely independent, or negatively correlated as the plants progressed through the 8 h adaptation period.

Nitrate use by tobacco cells in response to N-stress and ammonium nutrition

Characterization of NO 3 (-) use by suspension cultured tobacco cells during a culture cycle is needed to take advantage of cell cultures for further study of the biochemical regulation of NO 3 (-) uptake induction and decay processes. Tobacco (Nicotiana tabacum L., cv. Ky14) cells were cultured with media containing different N sources. Cells cultured with a mixture of NO 3 (-) and NH 4 (+) (40 mM NO 3 (-) plus 20 mM NH 4 (+) , in Murashige and Skoog media) initially grew slightly faster but attained the same maximum cell culture density as those cultured with 40 mM NO 3 (-) only. Cells subcultured with N-free media grew at a similar rate for the first 3 d as those cells grown with N, then ceased further growth. The cessation of growth of cells subcultured with N-free media coincided with depletion of cell NO 3 (-) . The NO 3 (-) influx of cells subcultured with N-free media increased eleven-fold and those grown with N increased four- to five-fold before declining. Maximal NO 3 (-) influx rates occurred at the onset of the stationary growth phase for N-stressed cells, while cells grown with N reached maximums prior to the stationary phase of cell growth. Cells grown with a mixture of NO 3 (-) and NH 4 (+) had lower NO 3 (-) reductase (NR) activity and higher cell NO 3 (-) levels than those of cells grown with NO 3 (-) only. The NR activity of cells subcultured with N-free media peaked within 1 d after subculture before declining to a constitutive level when cell NO 3 (-) was depleted. The level of cell NO 3 (-) plays a critical role in the expression of the NO 3 (-) uptake and reduction processes. The transitions in the expression of NO 3 (-) uptake and reduction activities of tobacco cell suspension cultures should prove valuable for further study of the biochemical and molecular basis for the regulation of these processes.

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.

Effects of Exposure to Ammonium and Transplant Shock upon the Induction of Nitrate Absorption

In barley (Hordeum vulgare L. cv Steptoe) seedlings, the time course for induction of root nitrate absorption varied significantly with pretreatment. Net nitrate uptake of nitrogen-deprived plants more than doubled during the 12 hours after first exposure to nitrate. For these plants, gentle physical disturbance of the roots inhibited net nitrate absorption for more than 6 hours and potassium absorption for 2 hours. Pretreatment with ammonium appeared sufficient to induce nitrate absorption; plants either grown for 2 weeks on or exposed for only 10 hours to a medium containing ammonium as a sole nitrogen source showed high rates of net nitrate uptake when first shifted to a medium containing nitrate. Gentle physical manipulation of these plants inhibited nitrate absorption for 2 hours and potassium absorption for more than 12 hours. These results indicate (a) that experimental protocols should avoid physical manipulation of the roots when-ever possible and (b) that ammonium or a product of ammonium assimilation can induce nitrate absorption.

Studies of the Regulation of Nitrate Influx by Barley Seedlings Using NO(3)

Using (13)NO(3) (-), effects of various NO(3) (-) pretreatments upon NO(3) (-) influx were studied in intact roots of barley (Hordeum vulgare L. cv Klondike). Prior exposure of roots to NO(3) (-) increased NO(3) (-) influx and net NO(3) (-) uptake. This ;induction' of NO(3) (-) uptake was dependent both on time and external NO(3) (-) concentration ([NO(3) (-)]). During induction influx was positively correlated with root [NO(3) (-)]. In the postinduction period, however, NO(3) (-) influx declined as root [NO(3) (-)] increased. It is suggested that induction and negative feedback regulation are independent processes: Induction appears to depend upon some critical cytoplasmic [NO(3) (-)]; removal of external NO(3) (-) caused a reduction of (13)NO(3) (-) influx even though mean root [NO(3) (-)] remained high. It is proposed that cytoplasmic [NO(3) (-)] is depleted rapidly under these conditions resulting in ;deinduction' of the NO(3) (-) transport system. Beyond 50 micromoles per gram [NO(3) (-)], (13)NO(3) (-) influx was negatively correlated with root [NO(3) (-)]. However, it is unclear whether root [NO(3) (-)] per se or some product(s) of NO(3) (-) assimilation are responsible for the negative feedback effects.

Nitrate transport is independent of NADH and NAD(P)H nitrate reductases in barley seedlings

Barley (Hordeum vulgare L.) has NADH-specific and NAD(P)H-bispecific nitrate reductase isozymes. Four isogenic lines with different nitrate reductase isozyme combinations were used to determine the role of NADH and NAD(P)H nitrate reductases on nitrate transport and assimilation in barley seedlings. Both nitrate reductase isozymes were induced by nitrate and were required for maximum nitrate assimilation in barley seedlings. Genotypes lacking the NADH isozyme (Az12) or the NAD(P)H isozyme (Az70) assimilated 65 or 85%, respectively, as much nitrate as the wild type. Nitrate assimilation by genotype (Az12;Az70) which is deficient in both nitrate reductases, was only 13% of the wild type indicating that the NADH and NAD(P)H nitrate reductase isozymes are responsible for most of the nitrate reduction in barley seedlings. For all genotypes, nitrate assimilation rates in the dark were about 55% of the rates in light. Hypotheses that nitrate reductase has direct or indirect roles in nitrate uptake were not supported by this study. Induction of nitrate transporters and the kinetics of net nitrate uptake were the same for all four genotypes indicating that neither nitrate reductase isozyme has a direct role in nitrate uptake in barley seedlings.

Development of accelerated net nitrate uptake : effects of nitrate concentration and exposure time

Upon initial nitrate exposure, net nitrate uptake rates in roots of a wide variety of plants accelerate within 6 to 8 hours to substantially greater rates. Effects of solution nitrate concentrations and short pulses of nitrate (</=1 hour) upon ;nitrate-induced' acceleration of nitrate uptake in maize (Zea mays L.) were determined. Root cultures of dark-grown seedlings, grown without nitrate, were exposed to 250 micromolar nitrate for 0.25 to 1 hour or to various solution nitrate concentrations (10-250 micromolar) for 1 hour before returning them to a nitrate-free solution. Net nitrate uptake rates were assayed at various periods following nitrate exposure and compared to rates of roots grown either in the absence of nitrate (CaSO(4)-grown) or with continuous nitrate for at least 20 hours. Three hours after initial nitrate exposure, nitrate pulse treatments increased nitrate uptake rates three- to four-fold compared to the rates of CaSO(4)-grown roots. When cycloheximide (5 micrograms per milliliter) was included during a 1-hour pulse with 250 micromolar nitrate, development of the accelerated nitrate uptake state was delayed. Otherwise, nitrate uptake rates reached maximum values within 6 hours before declining. Maximum rates, however, were significantly less than those of roots exposed continuously for 20, 32, or 44 hours. Pulsing for only 0.25 hour with 250 micromolar nitrate and for 1 hour with 10 micromolar caused acceleration of nitrate uptake, but the rates attained were either less than or not sustained for a duration comparable to those of roots pulsed for 1 hour with 250 micromolar nitrate. These results indicate that substantial development of the nitrate-induced accelerated nitrate uptake state can be achieved by small endogenous accumulations of nitrate, which appear to moderate the activity or level of root nitrate uptake.

Short-term studies of NO 3 (-) uptake in Pisum using (13)NO 3 (-)

Influx, efflux and net uptake of NO 3 (-) was studied in Pisum sativum L. cv. Marma in short-term experiments where (13)NO 3 (-) was used to trace influx. The influx rate in N-limited plants was similar both during net uptake at external concentrations of around 50 μM, and at low external NO 3 (-) concentrations (4-6 μM) when net uptake was practically zero. Efflux could be inferred from discrepancies between influx and net uptake but was never very high in the N-limited plants during net uptake. Close to the threshold concentration for not NO 3 (-) uptake, efflux was high and equalled influx. Thus, the threshold concentration can be regarded as a NO 3 (-) compensation point. The inclusion of NH 4 (+) in the outer medium decreased influx by about 40% but did not significantly affect efflux. The roles of NO 3 (-) fluxes and nitrate-reductase activity in regulating/limiting NO 3 (-) utilization are discussed.

The uptake of NO3-, NO2-, and NH4+ by intact wheat (Triticum aestivum) seedlings. I. Induction and kinetics of transport systems

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.

Effect of ammonium on nitrate utilization by roots of dwarf bean

The effect of exogenous NH(4) (+) on NO(3) (-) uptake and in vivo NO(3) (-) reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO(3) (-) uptake. Pretreatment with NH(4)Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO(3) (-) uptake.When NH(4) (+) was given at the start of NO(3) (-) nutrition, the time pattern of NO(3) (-) uptake was the same as in plants receiving no NH(4) (+). After 6 hours, however, the NO(3) (-) uptake rate (NUR) and root NRA were inhibited by NH(4) (+) to a maximum of 45% and 60%, respectively.The response of the NUR of NO(3) (-)-induced plants depended on the NH(4)Cl concentration. Below 1 millimolar NH(4) (+), the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH(4) (+) at 2 millimolar and above caused a rapid and transient stimulation of NO(3) (-) uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO(3) (-)-induced plants, root NRA responded less and more slowly to NH(4) (+) than did NUR.Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH(4) (+) inhibition of the NUR of NO(3) (-)-induced plants. We conclude that repression of the NUR by NH(4) (+) depends on NH(4) (+) assimilation. The repression by NH(4) (+) was least at the lowest and highest NH(4) (+) levels tested (0.04 and 25 millimolar).

Utilization of nitrite and nitrate by dwarf bean

The uptake and conversion of NO 2 (-) and the effect of NO 2 (-) on the uptake and reduction of NO 3 (-) were examined in N-depleted Phaseolus vulgaris L. Nitrite uptake at 0.1 mmol dm(-3) was against an electrochemical gradient and became constant after one or two initial phases. Steadystate uptake declined with increasing ambient NO 2 (-) concentration (0-0.7 mmol dm(-3)). In this concentration range root oxygen consumption was unaffected by NO 2 (-) , indicating that the decrease of NO 2 (-) uptake was not related to respiration. After 6 h NO 2 (-) supply, about one-third of the absorbed NO 2 (-) had accumulated, mainly in the root system. Oxidation of NO 2 (-) to NO 3 (-) was not observed. The apparent induction period for NO 3 (-) uptake was about 6 h in control plants and 3.5 h in plants that were pretreated for 18 h with NO 2 (-) . In contrast, the time course of NO 2 (-) uptake was unaffected by pretreatment with NO 3 (-) . Steadystate NO 3 (-) uptake was less affected by NO 2 (-) than was steady-state NO 2 (-) uptake by NO 3 (-) . Nitrate reductase activity (NRA) in leaves and roots was induced by both NO 3 (-) and NO 2 (-) . In roots, induction with NO 2 (-) was faster than with NO 3 (-) , but there was no difference in NRA after 5 h. Nitrite inhibited NRA in the roots of NO 3 (-) -induced plants and thus seems to stimulate the induction, but not the activity of induced nitrate reductase. In view of the observed differences in time course and mutual competition, a common uptake mechanism for NO 2 (-) and NO 3 (-) seems unlikely. Expression of the NO 2 (-) effect on the induction of NO 3 (-) uptake required more time than the induction itself. We therefore conclude that NO 2 (-) is not the physiological inducer of NO 3 (-) uptake.