The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

The effects of water hardness (9 and 220 mgl⁻¹ as CaCO₃) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1⁻¹ have been investigated using artificial soft water (<49.9 μmol Ca ^l-1, <40.1 μmol Mg 1⁻¹) and mains hard water (1671.7 μmol Ca 1⁻¹, 493.6 μmol Mg 1⁻¹) of known composition. Both hard and soft water-adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater ( P <0.001) in soft (82.9 and 6.2 μmol Zn 100 g⁻¹ h⁻¹) than in hard water (46.3 and 2.4 μmol Zn 100 g h⁻¹). Zinc efflux (- 0.2 μmol Zn 100 g⁻¹ h⁻¹) was enhanced only in hard water during the slow net influx phase.
Brown trout exposed to zinc in hard water and placed in metal-free media exhibited a greater net efflux (- 25.6 μmol Zn 100 g⁻¹ h⁻¹) of the metal than did fish in soft water (-4.2 μmol Zn 100 g⁻¹ h⁻¹) treated in the same manner. Tissue ⁶⁵Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1⁻¹) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.     

Transport of Zinc-65 at the Blood-Brain Barrier During Short Cerebrovascular Perfusion in the Rat: Its Enhancement by Histidine

Abstract: Zinc-65 transport into different regions of rat brain has been measured during short vascular perfusion of one cerebral hemisphere with an oxygenated HEPES-containing physiological saline at pH 7.40. The [Zn²⁺] was buffered with either bovine serum albumin or histidine. In each case uptake was linear with time up to 90 s. ⁶⁵Zn flux into brain in the presence of albumin followed Michaelis-Menten kinetics and for parietal cortex had a K _m of 16 n M and a V _max of 44 nmol/kg/min. Increasing concentrations of l -histidine enhanced ⁶⁵Zn flux into brain at [Zn²⁺] values between 1 and 1,000 n M . The combined effect of [histidine] and [Zn²⁺] was best accounted for by a function of [ZnHis⁺], i.e., flux = 64.4 · [ZnHis⁺]/(390 + [ZnHis⁺]) + 0.00378 · [ZnHis⁺], with concentrations being nanomolar. d -Histidine had an influence similar to that of l -histidine. ⁶⁵Zn flux in the presence of 100 µ M l -histidine was not affected by either 500 µ M l -arginine or 500 µ M l -phenylalanine. The results indicate specific transport of Zn²⁺ across the plasma membranes of brain endothelium. The enhancement due to histidine has been attributed to diffusion of ZnHis⁺ across unstirred layers "ferrying" zinc to and from transport sites.     

Transport of zinc and manganese to developing wheat grains

An understanding of the transport pathway used by Zn and Mn to enter developing grains may allow measures to increase the Zn and Mn content of wheat grain grown on Zn/Mn deficient soils. For this reason, transport of Zn and Mn into developing grains of wheat ( Triticum aestivum L. cv. Aroona) was investigated. Detached ears (18–22 days post-anthesis) were cultured for 48 h in a solution containing 185 kBq of ⁶⁵Zn and 185 kBq of ⁵⁴Mn. Transport of ⁶⁵Zn to the grain was unaffected by removal of glumes but was slightly reduced after the lemma was removed. Heat girdling the peduncle slightly reduced the amount of ⁶⁵Zn transported to the grain, whilst heat girdling the rachilla reduced transport of ⁶⁵Zn to the grain to a greater degree, suggesting phloem transport to the rachilla. The transport inhibitor CCCP (carbonyl cyanide m -chlorophenyl hydrazone) blocked ⁶⁵Zn transport to grain but not to lemma and glumes. Removing glumes and lemma and heat girdling the peduncle did not affect transport of ⁵⁴Mn, but transport was slightly affected by heat girdling the rachilla, indicating xylem transport. CCCP blocked transport of ⁵⁴Mn into the grain but not to lemma and glumes. It was concluded that xylem-to-phloem transfer of Zn occurs in the rachis and to a lesser extent in peduncle and lemma. The results suggest that the lemma may be an important site for phloem loading when the concentration of Zn within the xylem is high. The data also suggest that Mn was predominantly translocated to the spikelets in the xylem, but that transport to the grain was dependent upon membrane transport before entering the grain. Phloem loading of Mn into the grain vascular system may have occurred at the site of xylem discontinuity in the floral axis.     

Mechanism of phosphorus-induced zinc deficiency in cotton. I. Zinc deficiency-enhanced uptake rate of phosphorus

The effect of withholding Zn on the uptake, translocation and accumulation of P was studied in cotton plants ( Gossypium hirsutum L. cv. Deltapine 15/21) grown in nutrient solutions under controlled environmental conditions. The influence of P on the uptake rate, translocation and distribution of ⁶⁵Zn in the plants was also examined. Increasing the P supply resulted in severe Zn deficiency symptoms (interveinal chlorosis) as well as P toxicity symptoms, which were characterized by leaf puckering and grayish-brown marginal necrosis. Zinc deficiency markedly increased the uptake and translocation rates of P over the whole concentration range tested (5x10^-5 to 1.25x10^-3 M ). Uptake and translocation rates of P increased with both level of P and severity of Zn deficiency. This often caused P toxicity symptoms on Zn-deficient leaves. In contrast to P, the concentrations of K and Mg in the leaves were not affected by Zn deficiency. Similar results were obtained for sunflower ( Helianthus annuus L.) and buckwheat ( Fagopyrum esculentum Moench) plants. Higher P concentrations in Zn-deficient leaves or shoots could not be attributed wholly to reduced shoot growth. This was also evident when Zn deficiency was compared with other micronutrient (Fe, Mn, and Cu) deficiencies. Only Zn-deficient plants showed enhanced uptake and translocation of P. In experiments with ⁶⁵Zn, a high P supply did not depress uptake and translocation of Zn. From the results obtained it is concluded that the P-induced Zn deficiency in cotton, as well as in other species, is primarily caused by enhanced P uptake and translocation and not by inhibition of Zn uptake.     

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used ¹⁰⁹Cd and ⁶⁵Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat ( Triticum aestivum L.) and durum wheat ( Triticum turgidum L. var. durum ). Results showed that Cd²⁺ uptake was inhibited by Zn²⁺ and Zn²⁺ uptake was inhibited by Cd²⁺. Concentration-dependent uptake of both Cd²⁺ and Zn²⁺ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 µm concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd²⁺ and Zn²⁺, confirming that Cd²⁺ and Zn²⁺ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.     

The interactions of water hardness and pH with the acute toxicity of zinc to the brown trout, Salmo trutta L.

Exposure of brown trout, Salmo trutta , to zinc under continuous flow conditions over 96 h showed that both water hardness and pH exert major influences on the toxicity of the metal. 96-h LC50 values for total zinc ranged from <0.14mg 1⁻¹ in alkaline soft water (pH 8; lOmg 1⁻¹ as CaCO₃) to 3.20 mg 1⁻¹ in acidic hard water (pH 5; 204 mg 1⁻¹ as CaCO₃). A variable reduction in zinc toxicity in hard water compared with soft water over the pH range 4–9 was attributed to high external calcium. Zinc toxicity was positively correlated with decreasing acidity over the pH range 5–7, the metal being most toxic at pH 8–9 where metal complexes predominate. Below pH 5 metal toxicity also increased, irrespective of hardness. Water hardness and pH interacted with zinc toxicity in a complex manner, apparently dependent on physical and chemical transformations of the metal, and as changes in uptake. detoxification and excretion by the fish.     

Acute toxicity of copper, zinc and manganese in single and mixed salt solutions to juvenile longfin dace, Agosia chrysogaster

The acute toxicity of copper, zinc and manganese and copper-zinc and copper-manganese mixtures were determined for juvenile longfin dace, Agosia chrysogaster in hard water bioassays (mean=218 mg 1⁻¹ CaCO₃). Copper-zinc was the most lethal toxicant (96-h L.c.₅₀= 0.21 mg 1⁻¹ copper and 0.28 mg 1⁻¹ zinc) and exhibited a more than additive toxicity which was in contrast to the additive toxicity of copper-manganese mixtures (96-h L.c.₅₀= 0.45 mg 1⁻¹ copper and 64.0 mg 1⁻¹ manganese). The toxicity of copper (96-h L.c.₅₀= 0.86 mg 1⁻¹) and zinc (96-h L.c.₅₀= 0.79 mg 1⁻¹) to the fish was similar but both were considerably more lethal than manganese (96-h L.c.₅₀= 130 mg 1⁻¹).     

The ZnuABC high-affinity zinc uptake system and its regulator Zur in Escherichia coli

In Escherichia coli , lacZ operon fusions were isolated that were derepressed under iron repletion and repressed under iron depletion. Two fusions were localized in genes that formed an operon whose gene products had characteristics of a binding protein-dependent transport system. The growth defect of these mutants on TY medium containing 5 mM EGTA was compensated for by the addition of Zn²⁺. In the presence of 0.5 mM EGTA, only the parental strain was able to take up ⁶⁵Zn²⁺. This high-affinity transport was energized by ATP. The genes were named znuACB (for zinc uptake; former name yebLMI ) and localized at 42 min on the genetic map of E. coli . At high Zn²⁺ concentrations, the znu mutants took up more ⁶⁵Zn²⁺ than the parental strain. The high-affinity ⁶⁵Zn²⁺ uptake was repressed by growth in the presence of 10 μM Zn²⁺. A znuA–lacZ operon fusion was repressed by 5 μM Zn²⁺ and showed a more than 20-fold increase in β-galactosidase activity when Zn²⁺ was bound to 1.5 μM TPEN [tetrakis-(2-pyridylmethyl) ethylenediamine]. To identify the Zn²⁺-dependent regulator, constitutive mutants were isolated and tested for complementation by a gene bank of E. coli . A complementing gene, yjbK of the E. coli genome, was identified and named zur (for zinc uptake regulation). The Zur protein showed 27% sequence identity with the iron regulator Fur. High-affinity ⁶⁵Zn²⁺ transport of the constitutive zur mutant was 10-fold higher than that of the uninduced parental strain. An in vivo titration assay suggested that Zur binds to the bidirectional promoter region of znuA and znuCB .     

Suppression of ammonium uptake by nitrogen supply and its relief during nitrogen limitation

Barley ( Hordeum vulgare , cv. Triumph), wheat ( Triticum aestivum , cv. Kleiber) and oat ( Avena sativa , cv. Tarok) were grown until day 20 in nitrate-containing solutions or in nitrogen-free solutions for periods up to 8 days immediately prior to day 20. They then were exposed for 4 h to complete, nitrate-free solutions containing 0.5 or 2.0 mM ammonium (98 atom%¹⁵N). In all 3 species in 2 experiments, net ammonium uptake was low in plants grown continuously in nitrate, and increased 3 to 4-fold with increasing nitrogen deprivation. Charge balance during net ammonium uptake was largely maintained by the sum of net potassium and net proton efflux. Variations in root ammonium concentration at the time of exposure to the ammonium solutions revealed no consistent pattern with net ammonium uptake, implying that a product of ammonium assimilation may serve as a negative effector for the uptake process. In nitrogen-replete plants, and in those deprived of nitrogen for 2 days, the amounts of endogenous ¹⁴N-ammonium recovered in the ambient ¹⁵N-ammonium solution during the 4-h uptake period were greater than the initial amounts of ¹⁴N-ammonium present in the root tissue. Significant generation of ¹⁴N-ammonium from endogenous organic nitrogen sources was thus evident in all 3 species.     

Oxygen consumption by sticklebacks (Gasterosteus aculeatus L.) exposed to zinc

The rate of oxygen consumption by sticklebacks has been studied by long-term continuous-flow respirometry. Exposure to 1 p.p.m. zinc in calcium-free water causes wide variations in individual responses, but oxygen uptake tends to rise and then become extremely erratic, before declining as death approaches. Behavioural abnormalities such as increased ventilation rate, loss of balance, and long periods of inactivity alternating with spasmodic swimming also occur. Exposure to 6.5 p.p.m. zinc in high-calcium water generally causes a rise in oxygen consumption, followed by fluctuations in the rate of uptake, but no behavioural abnormalities occur and deaths are rare even after exposure for 400 h. If restored to zinc-free water after 40 h exposure to zinc, recovery is generally complete, although fluctuating rates of oxygen uptake persist. These results are discussed in relation to previous work on the effects of heavy metals on fish respiration.     

CHOLINE: HIGH AFFINITY UPTAKE IN VIVO BY RAT HIPPOCAMPUS

Abstract— [³H]Choline uptake has been measured in vivo in the rat hippocampus. Pharmacological agents and lesions which profoundly affect sodium-dependent, high-affinity [³H]choline uptake in vivo similarly affect [³H]choline uptake measured in vitro. Pentobarbital (65 mg/kg) and oxotremorine (0.75 mg kg) cause a decrease in [³H]choline uptake. Scopolamine (5 mg/kg) and iontophoretically applied extracellular potassium cause an increase in [³H]choline uptake. Septal lesions cause a decrease in [³H]choline uptake. Application of the general method may allow direct examination of presynaptic function and neural integration in the undisrupted living mammalian brain.     

Rate of 59Fe Uptake into Brain and Cerebrospinal Fluid and the Influence Thereon of Antibodies Against the Transferrin Receptor

Abstract: Uptake of ⁵⁹Fe from blood into brains of anaesthetized rats and mice has been studied by intravenous infusion of [⁵⁹Fe]ferrous ascorbate or of ⁵⁹Fe-transferrin, the results not being significantly different. Uptakes in the rat were linear with time, but increased at longer times in the mouse. Transfer constants, K _in (in ml/g/h × 10³), for cerebral hemispheres were 5.2 in the adult rat and 5.6 in the mouse. These K _in values corresponded to ⁵⁹Fe influxes of 145 and 322 pmol/g/h, respectively. ⁵⁹Fe uptake into the mouse brain occurred in the following order: cerebellum > brainstem > frontal cerebral cortex > parietal cortex > occipital cortex > hippocampus > caudate nucleus. In genetically hypotransferrinaemic mice, ⁵⁹Fe uptake into brain was 80–95 times greater than in To strain mice. Pretreatment of young rats and mice with monoclonal antibodies to transferrin receptors, i.e., the anti-rat immunoglobulin G OX 26 and the anti-mouse immunoglobulin M RI7 208, inhibited ⁵⁹Fe uptake into spleen by 94% and 98%, respectively, indicating saturation of receptors. The antibodies reduced ⁵⁹Fe uptake into rat brain by 35–60% and that into mouse brain by 65–85%. Although a major portion of iron transport across the blood-brain barrier is normally transferrin-mediated, non-transferrin-bound iron readily crosses it at low serum transferrin levels.     

Melatonin Effect on [3H]Glutamate Uptake and Release in the Golden Hamster Retina

Abstract: The effect of melatonin on [³H]glutamate uptake and release in the golden hamster retina was studied. In retinas excised in the middle of the dark phase, i.e., at 2400 h, melatonin (0.1 and 10 n M ) significantly increased [³H]glutamate uptake, and this effect persisted in a Ca²⁺-free medium. On the other hand, melatonin significantly increased [³H]glutamate release in retinas excised at 2400 h, but this effect was Ca²⁺ sensitive. Melatonin significantly increased ⁴⁵Ca²⁺ uptake by a crude synaptosomal fraction from retinas of hamsters killed at 2400 h. In retinas excised at 1200 h, melatonin had no effect on [³H]glutamate uptake, [³H]glutamate release, or ⁴⁵Ca²⁺ uptake at any concentration tested. Cyclic GMP analogues, i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'- O -dibutyrylguanosine 3',5'-cyclic monophosphate, significantly increased [³H]glutamate uptake, [³H]glutamate release, and ⁴⁵Ca²⁺ uptake by tissue removed at 1200 and 2400 h, suggesting that the effects of melatonin could correlate with a previously described effect of melatonin on cyclic GMP levels in the golden hamster retina. Taking into account the key role of glutamate in visual mechanisms, the results suggest the participation of melatonin in retinal physiology.     

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

Stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum , sharing the same diet. Atlantic salmon muscle tissues were enriched in ¹⁵N and depleted in ¹³C compared to their prey (sprat Sprattus sprattus sprattus ) and their intestinal cestode. There was no significant difference in δ¹⁵N or δ¹³C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ¹³C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in ¹⁵N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in ¹⁵N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in ¹⁵N.     

The accumulation of zinc by O-group plaice, Pleuronectes platessa (L.), from high concentrations in sea water and food

The uptake of zinc by young O-group plaice, Pleuronectes platessa was investigated at levels of high input from sea water and food ( Artemia nauplii). Zinc concentrations in the fish were strongly weight-specific and accumulation effects were distinguished by regression analysis. Sea water was estimated to contribute >10% of the total input up to levels corresponding to high environmental concentrations ( c. 0.1 μg Zn ml⁻¹) and food was the major source of zinc up to sea water concentrations of c. 0.6 μg Zn ml⁻¹. A high degree of regulation obtained along the food pathway to the fish although, in the system studied, most of this regulation was effected by the food organism. Regulation of food input explained the lack of variation in zinc levels previously observed in wild populations.     

Karyotype variability in successive generations after hybridization between the great sturgeon, Huso huso (L.), and the sterlet, Acipenser ruthenus L.

Effects of particle size, fish size and temperature on the filtration rate of silver carp were determined. When feeding at 20°C on zooplankton and spherical particles (yeast, micronic beads and pollen), 32-g silver carp filter particles larger than 70 urn at a maximum rate of 18.251 h⁻¹. For particles smaller than 70 μm, filtration rates decrease with decreasing particle size until there is no measured filtration for particles smaller than 10 μm. Filtering rates ( FR ) for particles between 10 and 50 μm are described by the equation, FR =−20.8 + 21.7 × log particle diameter. Filtration rates rise as fish size, particle size and temperature increase. Filtration rates per unit biomass, however, fall as fish size increases: FR = 1.54 W^0.713, where FR is the maximum filtration rate in ¹ h ¹ fish ¹ and W is weight of fish in grammes. The results of these trials are consistent with the hypothesis that particle selection by silver carp is a mechanical, passive function of gill raker morphology.     

Sample sizes for length and density estimation of 0+ fish when using point sampling by electrofishing

Standard lengths of cyprinids at several sites in the River Great Ouse, England, were compared between catches taken using a conventional 15 × 3 m micromesh seine (pore diameter 2 mm) and using point sampling by electrofishing (PSE). No statistically significant differences in sites were found in six out of 10 trials. In nine out of 10 trials, the difference between the mean lengths differed by <1 mm. No serial bias was found between PSE and seine netting for cyprinid fishes between 14 and 100 mm SL, although variation between size classes was high, owing to small sample sizes. The coefficient of variation in fish length with size tended to increase with the age of 0+ Rutilus rutilus . The relationship between sample size (n) and variance ( s² ) was explained by s ²=13·9 n ^−1·24. Sampling more than 30 fish resulted in little increase in precision. The relationship between the mean catch per site (̄) and the variance of the mean estimate was log s ²=1·6039 log ̄ +0·973. The number of samples required to estimate density within a given variance range was: n =9·33 ̄ ^1·6–2 CV_̄ ⁻². Given the high variance-mean ratio, great care should be taken when interpreting density data collected using PSE, and 50 point samples is the minimum required for density estimation.     

Uptake of 36Cl and 22Na by the Choroid Plexus-Cerebrospinal Fluid System: Evidence for Active Chloride Transport by the Choroidal Epithelium

Abstract: Cl and Na transport by the lateral ventricle (LVCP) and fourth ventricle (4VCP) choroid plexuses were examined by kinetic analysis of ³⁶Cl and ²²Na uptake into the choroid plexus-CSF system of the adult rat. Both radioisotopes required more than 5 h to reach steady-state distribution in the in vivo choroid plexuses and CSF after intraperitoneal injection. Whereas the LVCP and 4VCP ³⁶Cl steady-state spaces were comparable (55–56%), the 4VCP ²²Na space (39%) tended to be greater than the LVCP ²²Na space (36%). No evidence for inexchangeable Cl or Na was found for the choroid plexuses; the radioisotopic and chemical spaces were not significantly different. Choroid plexus ³⁶Cl and ²²Na uptake curves were resolved into two components, a fast component ( t _1/2 0.02–0.05 h) and a slow component ( t _1/2 0.85–1.93 h). By analysis of the distribution of [³H]inulin, [³H]mannitol, and ⁵¹Cr-tagged erythrocytes within the choroid plexuses, the fast component of ³⁶Cl and ²²Na uptake was found to represent extracellular and erythrocyte contributions to the tissue radioactivity, whereas the slow component represented isotope movement into the epithelial cell compartment. The calculated cell [Cl] of LVCP and 4VCP, 67 mmol/kg cell water, was 3.9 times greater than that predicted by the membrane potential for passive distribution. It is postulated that Cl is actively transported into the choroid epithelial cell across the basolateral membrane; the energy source for active Cl transport may be the Na electrochemical potential gradient (˜90 mV), which is twice that of the Cl electrochemical potential gradient (˜45 mV).     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

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.