Effect of glutamine on the induction of nitrate reductase

Nitrate reductase (NR. EC 1.6.6.1/2) is a substrate inducible enzyme that could be repressed by its end product glutamine or amino acids. To test this hypothesis, 6-day-old maize seedlings ( Zea mays cv. W64A × W182E) were grown hydroponically in a 1/10 strength Hougland's salt solution modified to contain no nitrogen. Previous experiments had established that after a 24-h induction with NO₃⁻ (5 mM KNO₃⁻) the level of NR activity and protein had reached a constant level. In the present experiments when glutamine (5 mM) was included together with NO₃⁻, there was a significant reduction in NR activity (34% of the control values). NR protein and NR mRNA accumulation in the root. In the shoot, on the other hand, glutamine additions had little or no effect on the levels of either NR activity (81% of control) or NR protein. Inhibition of glutamine synthetase by methionine sulfoximine (MSX) resulted in reduced levels of glutamine in both root and shoot tissues. Contrary in our prediction, however, it had no effect on NR activity and mRNA content in roots. In the shoot, on the other hand, there was a marked reduction of NR activity (34% of the control value) and NR protein, but no apparent effect on NR mRNA. When detached shoots were treated with MSX and other inhibitors of glutamine synthetase (tabtoxinine-β-lactam or phosphinothricin) the induction of NR activity by NO₃⁻ was also inhibited. Glutamine additions 15 or 50 mM to detached shoots had essentially no effect on the induction of NR activity (90% of control). These results demonstrate that the influence of glutamine and MSX on the induction of NR in maize root and shoot tissues, respectively, is very different.     

Induction of nitrate reductase in needles of Scots pine seedlings by NOX and NO3−

The possibility to induce nitrate reductase (NR; EC 1.6.6.2) in needles of Scots pine ( Pinus sylvestris L.) seedlings was studied. The NR activity was measured by an in vivo assay. Although increased NR activities were found in the roots after application of NO₃⁻, no such increase could be detected in the needles. Detached seedlings placed in NO₃⁻ solution showed increasing NR activities with increasing NO₃⁻ concentrations. Exposure of seedlings to NO_x (70–80 ppb NO₂ and 8–12ppb NO) resulted in an increase of the NR activity from 10–20 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹ to about 400 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. This level was reached after 2–4 days of exposure, thereafter the NR activity decreased to about 200 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. Analyses of free amino acids showed low concentrations of arginine and glutamine in NO_x-fumigated seedlings compared to corresponding controls.     

Effect of nitrate on nitrogen fixation and nodule carbohydrate and organic acid concentrations in pea mutants deficient in nitrate reductase

Nitrate inhibits symbiotic N₂ fixation and a number of hypotheses concerned with NO₃⁻ assimilation have been suggested to explain this inhibition. These hypotheses were tested using a pea ( Pisum sativum L. cv. Juneau) with normal nitrate reductase NR; (EC 1,6,6,4) activity and two mutants of cv. Juneau, A317 and A334, with impaired NR activity. The plants were inoculated with three strains of Rhizobium leguminosarum and grown for 3 weeks in N-free medium, followed by 1 week in medium supplemented with 0, 5 or 10 m M KNO₃ before harvesting. NO₃ was taken up at comparable rates by the parent and the mutants and accumulated in leaf and stem tissue of the latter. Acetylene reduction rates were inhibited similarly in both the parent and mutants in the presence of KNO₃ but there were differences among rhizobial strains. Starch concentration of the nodules decreased by 46% in the presence of KNO₃ and there were differences among rhizobial strains but not among pea genotypes. Malate and succinate accumulated in nodules in the presence of KNO₃. These data are not consistent with the photosynthate deprivation hypothesis as a primary mechanism for NO₃ inhibition of N₂ fixation since NO₃ affected the nodule carbohydrate composition of all three pea genotypes in a similar manner. The lack of correlation between NR activity and NO₃ inhibition of N₂ fixation suggests that NO₃ assimilation may be only indirectly involved in the inhibition phenomenon.     

Carbon and nitrogen partitioning in young nodulated pea (wild type and nitrate reductase-deficient mutant) plants exposed to NH4NO3

Carbon and nitrogen partitioning was examined in a wild-type and a nitrate reductase-deficient mutant (A317) of Pisum sativum L. (ev. Juneau), effectively inoculated with two strains of Rhizobium leguminosarum (128C23 and 128C54) and grown hydroponically in medium without nitrogen for 21 days, followed by a further 7 days in medium without and with 5 mM NH₄NO₃. In wild-type symbioses the application of NH₄NO₃ significantly reduced nodule growth, nitrogenase (EC 1.7.99.2) activity, nodule carbohydrates (soluble sugars and starch) and allocation of [¹⁴C]-labelled (NO₃⁻, NH₄⁺, amino acids) in roots. In nodules, there was a decline in amino acids together with an increase in inorganic nitrogen concentration. In contrast, symbioses involving A317 exhibited no change in nitrogenase activity or nodule carbohydrates, and the concentrations of all nitrogenous solutes measured (including asparagine) in roots and nodules were enhanced. Photosynthate allocation to the nodule was reduced in the 128C23 symbiosis. Nitrite accumulation was not detected in any case. These data cannot be wholly explained by either the carbohydrate deprivation hypothesis or the nitrite hypothesis for the inhibition of symbiotic nitrogen fixation by combined nitrogen. Our result with A317 also provided evidence against the hypothesis that NO₃⁻ and NH₄⁺ or its assimilation products exert a direct effect on nitrogenase activity. It is concluded that more than one legume host and Rhizobium strain must be studied before generalizations about Rhizobium /legume interactions are made.     

Nitrate metabolism in roots and nodules of Vicia faba in response to exogenous nitrate

Activities of nitrate reduction enzymes, nitrate reductase activity (NRA) and nitrite reductase activity (NiRA) from roots and nodules of 5 mutant genotypes and one commercial cultivar (Alameda) of faba bean ( Vicia faba L. var. minor) grown in the presence of N₂ alone or with additional NO⁻₃ in the medium have been studied. A naturally occurring mutant (VFM109) with impaired ability to reduce nitrate in its nodules is described. All the other cultivars of V. faba showed nodule NRA, although the range was very wide, from almost negligible (VFM72) up to 2 μmol h⁻¹ (g FW)⁻¹. This activity was entirely of plant origin. Root NRA also ranged widely accross cultivars. However, the level of activity expressed as well as the response of NRA to nitrate followed a pattern opposite to that observed in nodules. Roots and nodules of all cultivars showed very high rates of NiRA, respectively 50 and 150-fold higher than NRA, thus precluding accumulation of nitrite in these tissues. Root enzymes were significantly stimulated by nitrate while negative (NRA) or little effect (NiRA) was found for nodules. Nitrate and nitrite reduction are carried out by inducible enzymes in roots of V. faba and by constitutive enzymes in nodules, indicating that there may be different forms of these enzymes in each tissue. Differences in the plant genotype were a major cause of the variability in nitrate and nitrite reduction by nodulated root systems of V. faba .     

Reduction of nitrate in the perennial tissues of aerial parts of Alnus glutinosa

In vivo nitrate reductase (NR, EC 1.6.6.1.) activity was measured in leaves, branches and trunk of field-grown Alnus glutinosa (L.) Gaertn. All of the assayed tissues enzymatically reduced nitrate with a decreasing activity [μmol NO₂⁻ (g dry weight)⁻¹ h⁻¹] in the order: leaves > branch bark > inner branch tissues > trunk xylem. The NR activity of the various tissues of excised branches was inhibited by tungstate added to the transpiration stream. Part of the nitrate added to the feeding solution (0.2, 0.5 or 1 m M KNO₃) of excised branches disappeared during its transport via the transpiration stream in the perennial tissues. This disappearance was enzymatic since it was decreased by tungstate.
No evidence was obtained for the presence of nitrate in natural xylem sap nor for a significant correlation between nitrate content of soil and leaf NR activity. These results indicate that in the field-grown black alder, the nitrate not reduced in the roots could be reduced in the perennial tissues of aerial parts. Since the leaf NR activity does not reflect the actual in situ nitrate reduction, the existence of a constitutive NR activity in Alnus leaves is suggested.     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Control of Nitrate Uptake by Phloem-Translocated Glutamine in Zea mays L. Seedlings

Abstract: The putative role of glutamine, exported from leaves to roots, as a negative feedback signal for nitrate uptake was investigated in Zea mays L. seedlings. Glutamine (Gln) was supplied by immersion of the tip-cut leaves in a concentrated solution. Nitrate (NO₃⁻) uptake was measured by its depletion in amino acid-free medium. The treatment with Gln resulted in a strong inhibition of nitrate uptake rate, accompanied by a significant enrichment of amino compounds in root tissue. The effect of N-availability on NO₃⁻ uptake was determined in split-root cultures. The plants were subjected to complete or localized N supply. Inducible NO₃⁻ uptake systems were also induced in N-deprived roots when the opposite side of the root system was supplied with KNO₃. The inhibitory effect of Gln was unaffected by localized N supply on one side of the split-root. The potential role of Gln in the shoot-to-root control of NO₃⁻ uptake is discussed.     

Biochemical characterization of nitrate and nitrite reduction in the wild-type and a nitrate reductase mutant of soybean

Enzyme activities involved in nitrate assimilation were analyzed from crude leaf extracts of wild-type (cv. Williams) and mutant ( nr₁ ) soybean [ Glycine max (L.) Merr.] plants lacking constitutive nitrate reductase (NR) activity. The nr₁ soybean mutant (formerly LNR-2), had decreased NADH-NR, FMNH₂-NR and cytochrome c reductase activities, all of which were associated with the loss of constitutive NR activity. Measurement of FMNH₂-NR activity, by nitrite determination, was accurate since nitrite reductase could not use FMNH₂ as a reductant source. Nitrite reductase activity was normal in the nr₁ plant type in the presence of reduced methyl viologen. Assuming that constitutive NR is similar in structure to nitrate reductases from other plants, presence of xanthine dehydrogenase activity and loss of cytochrome c reductase activity indicated that the apoprotein and not the molybdenum cofactor had been affected in the constitutive enzyme of the mutant. Constitutive NR from urea-grown wild-type plants had 1) greater ability to use FMNH₂ as an electron donor, 2) a lower pH optimum, and 3) decreased ability to distinguish between NO₃⁻ and HCO₃⁻, compared with inducible NR from NO₃⁻-grown nr₁ plants. The presence in soybean leaves of a nitrate reductase with a pH optimum of 7.5 is contrary to previous reports and indicates that soybean is not an exception among higher plants for this activity.     

Soluble and membrane-associated nitrate reductases in the dinoflagellate Peridinium gatunense

An improved method of cell fractionation allowed the extraction of soluble (sNR) and membrane-associated (mNR) forms of nitrate reductase (NR) from a dinoflagellate, even though in previous studies only mNR had been found in these algae. Both activities were assayed in cell-free extracts of Peridinium gatunense from Lake Kinneret, Israel, after disruption of the cells and differential centrifugation. In the cultures used, sNR showed much higher NO₃⁻-reducing activity. Only a low proportion, 2.5–3% of NR activity, was found to be associated with mNR. Moreover, mNR comprised two forms as indicated by protein solubilization: a tightly membrane-bound and a more weakly attached NR. Ascorbate inhibited all NR activities, but that of mNR recovered after its removal. Polyvinyl pyrrolidone (PVP) and DTT also diminished sNR and mNR activities. For both enzymes, pH optima (7.65) and temperature optima (13–25°C) were similar, and agreed with those for optimum growth of P. gatunense both in culture and in the lake. The most efficient electron donor was NADH, though NADPH sustained low NR activities. Carboxylic anions such as succinate and malate did not support any reduction of NO₃⁻, nor did they cause any stimulation of sNR or mNR activities. Both forms of NR showed a high affinity for their substrates: K _m was c. 10 μM for NO₃⁻ and c. 5 μM for NADH. The high efficiency of NO₃⁻ assimilation by Peridinium seems to be limited mainly by energy under otherwise optimal nutritional conditions and, at low nitrate concentrations, the low K _m may be one of the main reasons for the high competitivity of this alga in Lake Kinneret.     

Regulation of K+ and NO3− fluxes in roots of sunflower (Helianthus annuus) after changes in light intensity

Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K⁺ and NO₃⁻ uptake by roots, both uptake of K⁺ and NO₃⁻ and secretion into the xylem of young sunflower plants ( Helianthus annuus L.) were measured after changes in light intensity.
No close correlation was observed between the uptake of NO₃⁻ and that of K⁺; an increase in light intensity produced a much greater stimulation of NO₃⁻ uptake than of K⁺ uptake. On the other hand, secretion of NO₃⁻ into the xylem was tightly coupled to that of K⁺, and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K₂-malate shuttle on the regulation by the shoot of K⁺ and NO₃⁻ secretion in the xylem, which is linked to NO₃⁻ uptake, while K⁺ uptake is independent of this regulation mechanism.     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

Nitrate and nitrite reduction in the plant fraction of alfalfa root nodules

The plant fraction of alfalfa ( Medicago sativa L. cv. Aragon) nodules contained both nitrate reductase (NR) and nitrite reductase (NiR). Specific activity of NADH-NR from the cytosol of nodules not treated with NO₃- was about 30 nmol (mg protein)^-1-h^-1 and was not basically affected by NO₃ addition. In contrast, typical specific activity for cytosolic NiR was 1.5 umol (mg protein)^-1h^-1 using methyl viologen as electron donor. This activity strongly increased with NO₃ concentration, probably due to substrate induction. Maximal activity was 3.5 μmol (mg protein)^-1h^-1 at 50 to 200 mM NO₃.
Estimates indicate that the contribution of cytosol to the overall NR and NiR activities of alfalfa nodules is distinctly different: less than 10% and about 70%, respectively. The increasing amounts of NO₂ accumulating in the cytosol upon NO₃, supply, and the different response to NO₃ of bacteroid and cytosolic NRs support the concept that most of this NO₂ comes from the bacteroids.     

Soybean root and nodule nitrate reductase

Nitrate reductase (NR) activity was followed in root and nodule from Glycine max (L.) Merr. (Cv. Tracy) inoculated with Rhizobium japonicum . Initially, a plus NO^3- in vivo assay was used. When chlorate-resistant mutants were used as inoculum, nodule NR activity was reduced by about 90%. indicating that the bacteroid accounts for much of the normal nodule's NR. With plants 3 to 15 weeks of age nodule NR activity (g fresh weight)^-1 was highest in young plants and root activity highest in old plants. Root and nodule total NR activity increased with plant age and were often not greatly different. Root NR activity correlated with plant NO^3- supply and increased from 0.8 to 11.4 μmol plant^-1 h^-1 as NO^3- was increased from 0 to 3 m M . In contrast, nodule NR activity was high in plants grown without NO^3- and did not appear to increase as nitrate supply to the plant was increased. Nodule activity was 6 to 14 μmol NO^2- plant^-1 h^-1. Use of a minus NO^3- in vivo assay had little affect on root NR activity, but greatly reduced nodule activity. Root tissue was found to have 5 to 38 times more NO^3- than nodule tissue. It is concluded that low nitrate levels within the nodule limit NR activity and that it is improbable that the nodule is a major site of plant nitrate reduction.     

Impact of gaseous nitrogen deposition on plant functioning

Dry deposition of NH₃ and NO_x (NO and NO₂) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH₃ dissolves to form NH₄⁺, whereas NO₂ dissolves to form NO₃⁻ and NO₂⁻. The latter compound can also be formed after exposure to NO. There is evidence that NH₃-N and NO_x-N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO₃⁻ and NO₂⁻ can be reduced, and NH₄⁺ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH₃ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH₃ and in vitro GDH activity. The contribution of atmospheric NH₃ and NO₂ deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates ( RGRs ). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

Methane-dependent nitrate production by a microbial consortium enriched from a cultivated humisol

Abstract A consortium was enriched from a humisol incubated with 3.6 kPa CH₄ and NH₄⁺. This consortium oxidized NH₄⁺ to NO₂⁻ and NO₃⁻ (NO₃⁻/NO₂⁻ ratio about 20) with smaller amounts of N₂O. This oxidation stopped in the stationary phase after depletion of CH₄. CH₃OH or CO₂ did not support oxidation. Growth and resting cell experiments suggested that nitrification was associated with methanotrophic activity and that chemoautotrophic nitrifiers were absent.     

Different effects of supplied ammonium on glutamine synthetase activity in mustard (Sinapis alba) and pine (Pinus sylvestris) seedlings

The activity of glutamine synthetase (GS) in mustard ( Sinapis alba L.) and Scots pine ( Pinus sylvestris L.) seedlings was used as an index to evaluate the capacity to cope with excessive ammonium supply. In these 2 species GS activity was differently affected by the application of nitrogen compounds (NH₄⁺ or NO₃⁻). Mustard seedlings older than 5 days showed a considerable increase in GS activity after NH₄⁺ or NO₃⁻ application. This response was independent of the energy flux, but GS activity in general was positively affected by light. Endogenous NH₄⁺ did not accumulate greatly after nitrogen supply. In contrast, seedlings of Scots pine accumulated NH₄⁺ in cotyledons and roots and showed no stimulation of GS activity after the application of ammonium. In addition, root growth was drastically reduced. Thus, the pine seedlings seem to have insufficient capacity to assimilate exogenously supplied ammonium. NO₃⁻, however, did not lead to any harmful effects.     

Anion uptake in maize roots: Interactions between chlorate and nitrate

Effects of nitrate, chloride and chlorate ions upon nitrate and chlorate uptake by roots of maize ( Zea mays L., cv. B73) seedlings were examined. Net nitrate uptake, ³⁶ClO₃⁻ influx and ³⁶Cl⁻ influx (the latter two in a background of 0.5 m M KNO₃) displayed similar pH profiles with optima at pH 5.5 and below. External, non-labeled chloride had little effect on the accumulation of ³⁶ClO₃⁻ (both in 5 h and 20 min uptake assays), while nitrate and chlorate had almost identical, marked inhibitory effects. Nitrate pretreatment caused an apparent induction of both ³⁶ClO₃⁻ and ¹⁵NO₃⁻ uptake activities. After 5 h of treatment in nitrate, the uptake activities of chloride- and chlorate-pretreated plants increased to that of nitrate-pretreated plants. During 6 h exposure to chlorate, ³⁶ClO₃⁻ uptake activity of nitrate-pretreated plants decreased to that of chlorate- and chloride-pretreated plants. The results support the existence of a shared nitrate/chlorate transport system in maize roots which is not inhibited by external chloride, and which is induced by nitrate, but not by chlorate or chloride. The suggestion is made that selection of chlorate-resistant mutants of maize can identify nitrate uptake as well as nitrate reductase mutants.