Nitrate Uptake by Dark-grown Corn Seedlings: Some Characteristics of Apparent Induction

Five-or six-day old seedlings of corn (Zea mays L.) were exposed to 0.25 mm Ca(NO₃)₂, 1.0 mm sodium 2-[N-morpholino]-ethanesulfonate, 5 μg Mo per liter and 50 μg of chloramphenicol per ml at pH 6. Nitrate uptake was determined from depletion of the ambient solution. The pattern of nitrate uptake was characterized, after the first 20 minutes, by a low rate which increased steadily to a maximal rate by 3 to 4 hours. Transfer of nitrate to the xylem did not totally account for the increase. Development of the maximal accelerated rate did not occur at 3 C with excised roots nor with seedlings whose endosperm had been removed. Use of CaCl₂ rather than Ca(NO₃)₂ resulted in a linear rate of chloride uptake during the first 4 hours, and chloride uptake was not as restricted by endosperm removal as was nitrate uptake.     

Nitrite Uptake Patterns in Wheat Seedlings as Influenced by Nitrate and Ammonium

Nitrite and nitrate uptake by wheat (Triticum vulgare) from 0.5 mM potassium solutions both showed an apparent induction pattern characterized by a slow initial rate followed by an accelerated rate. The accelerated phase was more rapid for nitrate uptake, was initiated earlier, and was seriously restricted by the presence of equimolar nitrite. The accelerated phase of nitrite uptake was restricted by nitrate to a lesser extent. The two anions seem not to be absorbed by identical mechanisms. Ammonium pretreatments or prior growth with ammonium had relatively little influence on the pattern of nitrite uptake. However, prior growth with nitrate eliminated the slow initial phase and induced development of the accelerated phase of nitrite uptake. A beneficial effect was noted after 3 h nitrate pretreatment and full development had occurred by 12 h nitrate pretreatment. The evidence suggests that a small amount of tissue nitrite, which could be supplied either by absorption or by nitrate reduction, was specifically required for induction of the accelerated phase of nitrite uptake. Cycloheximide (2 μg ml^?1) seriously restricted development of the accelerated phase of nitrite uptake, but its effect was not as severe when it was added after the accelerated phase had been induced by prior exposure to nitrite or nitrate. However, translocation of ¹⁵N from the absorbed nitrite was sharply decreased under the latter conditions, indicating a difference in sensitivity of the uptake and translocation processes to cycloheximide. Potassium uptake was greater from KNO₃ than from KNO₂ and in both instances it was enhanced during the early stages of the accelerated phase of anion uptake. Moreover, addition of NaNO₃ to KNO₂ substantially increased potassium uptake. A coupling between anion and potassium uptake was therefore evident, but the coupling was not obligatory because the accelerated phase of nitrite uptake could occur in absence of rapid potassium uptake.     

Uptake of ammonium and nitrate by carob (Ceratonia siliqua) as affected by root temperature and inhibitors

Seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were grown in nutrient solution culture for 5 weeks, with or without nitrogen at different root temperatures (10, 16, 22, 30, 35 or 40deg;C) and with the air temperature kept between 20 and 24°C. The nitrogen was given as either ammonium or nitrate. At all root temperatures studied, nitrogen-depleted plants developed higher net uptake rates for nitrogen than plants grown in the presence of nitrogen. Temperature affected the kinetic parameters of nitrate uptake more than those of ammonium uptake. With increasing root temperature, the K_m of ammonium uptake decreased, but to a lesser extent than the K_m for nitrate. The increase in V_max of ammonium uptake with temperature was also less noticeable than that for nitrate uptake. Ammonium and nitrate uptakes were inhibited in a similar way by respiratory or protein synthesis inhibitors. It may be noted that ammonium uptake in the presence of inhibitors at 40°C was higher than uptake at 10°C without inhibitors. Some similarities between the transport mechanisms for nitrate and ammonium are underlined in the present work. Components of both transport systems displayed saturation kinetics and depended on protein synthesis and energy. The following components of nitrate uptake were distinguished: (a) a passive net influx into the apparent free space; (b) a constitutive active uptake and (c) active uptake dependent on protein synthesis. We may similarly define three ammonium uptake systems: (a) a passive influx into the apparent free space; (b) passive diffusion uptake at high temperature and (c) active uptake dependent on protein synthesis. The possible role of the ratio between mechanism (c) and mechanism (b) as determinant of ammonium sensitivity is discussed.     

Amino acid uptake by Ricinus communis roots: characterization and physiological significance

Abstract Roots of sterile-grown, intact 6-day-old seedlings of Ricinus communis possess at least two independent active amino acid uptake systems, one for neutral and one for basic amino acids. The kinetics of uptake of L-proline and L-arginine, which were taken as representative substrates for the two systems, are biphasic. At low concentrations (0.01–0.5 mol m^?3) Michaelis -Menten kinetics prevail, changing to a linear concentration dependence at higher substrate concentrations (1–50 mol m^?3). L-glutamate uptake velocity is linear over the whole substrate concentration range. For comparison the uptake kinetics of nitrate and ammonium were determined as well as interactions among the different nitrogen sources. The K_m value for nitrate uptake was 0.4 mol m^?3, and for ammonium 0.1 mol m^?3. The uptake capacity for nitrate or ammonium was approximately the same as for amino acids. The interaction between the uptake systems for organic and inorganic nitrogen is small. Two hypotheses for the physiological significance of amino acid uptake by roots were considered: (i) Uptake of amino acids from the soil-determination of amino acids in soil and in soil water indicates that they might contribute 15–25% to the nitrogen nutrition of the plant. (ii) Amino acid uptake systems of root cells serve primarily as retrieval of amino acids delivered from the phloem- it was found that ¹⁴C L-glutamine, which was delivered to the cotyledon and transported to the root via the phloem, was not lost by the roots, whereas it appeared in the bathing medium if L-glutamine was applied externally to the root to compete for the uptake sites; this suggests that an apoplastic pool of amino acids in the root exists due to their efflux from the phloem.     

Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

The rate of influx of nitrate into the roots of intact barleyplants was measured over a period of 3–5 min from externalnitrate concentrations of 1–150 mmol m^–3, using¹³N-labelled nitrate as tracer. Ammonium at external concentrationsof 0.005–50 mol m^–3 inhibited nitrate influx ina manner which did not conform to a simple kinetic model butincreased approximately as the logarithm of the ammonium concentration.At any particular ammonium concentration, inhibition of nitrateinflux reached its full extent within 3 min of the ammoniumbeing supplied and was not made more severe by up to 17 minpre-treatment with ammonium. On removing the external ammonium,nitrate influx returned to its original rate within about 3min. Potassium at 0.005–50 mol m^–3 did not reproducethe rapid effect of ammonium on nitrate influx. Net uptake of nitrate also decreased when ammonium was supplied,over a similar timescale and to a similar extent as nitrateinflux. The decrease in nitrate influx caused by ammonium wassufficient to account for the observed reduction in net uptake,without necessitating any acceleration of nitrate efflux. Key words: Hordeum vulgare, roots, ion transport, short-lived isotopes, ¹³N     

Removal of Nitrate from Groundwater by Cyanobacteria: Quantitative Assessment of Factors Influencing Nitrate Uptake

The feasibility of biologically removing nitrate from groundwater was tested by using cyanobacterial cultures in batch mode under laboratory conditions. Results demonstrated that nitrate-contaminated groundwater, when supplemented with phosphate and some trace elements, can be used as growth medium supporting vigorous growth of several strains of cyanobacteria. As cyanobacteria grew, nitrate was removed from the water. Of three species tested, Synechococcus sp. strain PCC 7942 displayed the highest nitrate uptake rate, but all species showed rapid removal of nitrate from groundwater. The nitrate uptake rate increased proportionally with increasing light intensity up to 100 μmol of photons m⁻² s⁻¹, which parallels photosynthetic activity. The nitrate uptake rate was affected by inoculum size (i.e., cell density), fixed-nitrogen level in the cells in the inoculum, and aeration rate, with vigorously aerated, nitrate-sufficient cells in mid-logarithmic phase having the highest long-term nitrate uptake rate. Average nitrate uptake rates up to 0.05 mM NO₃⁻ h⁻¹ could be achieved at a culture optical density at 730 nm of 0.5 to 1.0 over a 2-day culture period. This result compares favorably with those reported for nitrate removal by other cyanobacteria and algae, and therefore effective nitrate removal from groundwater using this organism could be anticipated on large-scale operations.     

In Vitro Sugar Transport in Zea mays L. Kernels : I. Characteristics of Sugar Absorption and Metabolism by Developing Maize Endosperm

Short-term transport studies were conducted using excised whole Zea mays kernels incubated in buffered solutions containing radiolabeled sugars. Following incubation, endosperms were removed and rates of net ¹⁴C-sugar uptake were determined. Endogenous sugar gradients of the kernel were estimated by measuring sugar concentrations in cell sap collected from the pedicel and endosperm. A sugar concentration gradient from the pedicel to the endosperm was found. Uptake rates of ¹⁴C-labeled glucose, fructose, and sucrose were linear over the concentration range of 2 to 200 millimolar. At sugar concentrations greater than 50 millimolar, hexose uptake exceeded sucrose uptake. Metabolic inhibitor studies using carbonylcyanide-m-chlorophenylhydrazone, sodium cyanide, and dinitrophenol and estimates of Q₁₀ suggest that the transport of sugars into the developing maize endosperm is a passive process. Sucrose was hydrolyzed to glucose and fructose during uptake and in the endosperm was either reconverted to sucrose or incorporated into insoluble matter. These data suggest that the conversion of sucrose to glucose and fructose may play a role in sugar absorption by endosperm. Our data do not indicate that sugars are absorbed actively. Sugar uptake by the endosperm may be regulated by the capacity for sugar utilization (i.e. starch synthesis).     

Rapid and slow modulation of nitrate reductase activity in the red macroalga <Emphasis Type="Italic">Gracilaria chilensis</Emphasis> (Gracilariales,Rhodophyta): influence of different nitrogen sources

Nitrate is one of the most important stimuli in nitrate reductase (NR) induction, while ammonium is usually an inhibitor. We evaluated the influence of nitrate, ammonium or urea as nitrogen sources on NR activity of the agarophyte Gracilaria chilensis. The addition of nitrate rapidly (2 min) induced NR activity, suggesting a fast post-translational regulation. In contrast, nitrate addition to starved algae stimulated rapid nitrate uptake without a concomitant induction of NR activity. These results show that in the absence of nitrate, NR activity is negatively affected, while the nitrate uptake system is active and ready to operate as soon as nitrate is available in the external medium, indicating that nitrate uptake and assimilation are differentially regulated. The addition of ammonium or urea as nitrogen sources stimulated NR activity after 24 h, different from that observed for other algae. However, a decrease in NR activity was observed after the third day under ammonium or urea. During the dark phase, G. chilensis NR activity was low when compared to the light phase. A light pulse of 15 min during the dark phase induced NR activity 1.5-fold suggesting also fast post-translational regulation. Nitrate reductase regulation by phosphorylation and dephosphorylation, and by protein synthesis and degradation, were evaluated using inhibitors. The results obtained for G. chilensis show a post-translational regulation as a rapid response mechanism by phosphorylation and dephosphorylation, and a slower mechanism by regulation of RNA synthesis coupled to de novo NR protein synthesis.     

High Nitrogen and Phosphorous Acquisition by Belowground Parts of Caulerpa prolifera (Chlorophyta) Contribute to the Species' Rapid Spread in Ria Formosa Lagoon,Southern Portugal

Despite worldwide proliferation of the genus Caulerpa and subsequent effects on benthic communities, little is known about the nutritional physiology of the Caulerpales. Here, we investigated the uptake rates of ammonium, nitrate, amino acids, and phosphate through the fronds and rhizoids + stolon, the internal translocation of nitrogen, and developed a nitrogen budget for the rapidly spreading Caulerpa prolifera in Ria Formosa lagoon, southern Portugal. Caulerpa prolifera acquired nutrients by both aboveground and belowground parts at similar rates, except nitrate, for which fronds showed 2-fold higher uptake rates. Ammonium was the preferential nitrogen source (81% of the total nitrogen acquisition), and amino acids, which accounted for a significant fraction of total N acquisition (19%), were taken up at faster rates than nitrate. Basipetal translocation of ¹⁵N incorporated as ammonium was nearly 3-fold higher than acropetal translocation, whereas ¹⁵N translocation as nitrate and amino acids was smaller but equal in either direction. The estimated total nitrogen acquisition by C. prolifera was 689 μmol · m⁻² · h⁻¹, whereas the total nitrogen requirement for growth was 672 μmol · m⁻² · h⁻¹. The uptake of ammonium and amino acids by belowground parts accounted for the larger fraction of the total nitrogen acquisition of C. prolifera and is sufficient to satisfy the species nitrogen requirements for growth. This may be one reason explaining the fast spreading of the seaweed in the bare sediments of Ria Formosa where it does not have any macrophyte competitors and the concentration of nutrients is high.     

Distinctive Light and CO(2)-Fixation Requirements of Nitrate and Ammonium Utilization by the Cyanobacterium Anacystis nidulans

The effect of light intensity on the rates of ammonium and nitrate uptake and of CO₂ fixation has been determined in intact Anacystis nidulans cells. Ammonium uptake became saturated at photon flux values of about 60 microeinsteins per square meter per second, whereas both nitrate uptake and CO₂ fixation reached saturation at about 250 microeinsteins per square meter per second, the rates of the two latter processes being tightly correlated at any light intensity assayed. Inhibition of ammonium assimilation resulted in the loss of correlation between CO₂ fixation and nitrate uptake, the latter process exhibiting then a reduced light requirement. The results establish a clear distinction between ammonium utilization and nitrate utilization with regard to their light requirement and to the nature of their dependence upon CO₂ fixation.     

Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was measured as depletion from solutions by roots of intact 11 week old solution-cultured seedlings. Uptake rates of ammonium were consistently higher than those of nitrate in all experiments. Uptake rates for ammonium were 200% higher at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of ammonium and nitrate were both reduced to a similar extent (70%) with a decrease in temperature from 20 °C to 10 °C. For ammonium uptake, there was rapid (<24 hr) adaptation to a reduction in root temperature. The apparent preference shown here for ammonium over nitrate could be indicative of E. nitens growing in cold, acidic forest soils where ammonium is commonly more available than nitrate. These results suggest that N uptake rates of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate translocation by detopped corn seedlings

Five-day-old seedlings of corn (Zealpha mays L.) grown without nitrate were decapitated and exposed to 0.5 mm KNO(3) or 0.5 mm KCl in aerated solutions at 30 C. Uptake of nitrate, chloride, and potassium was determined by replacing solutions hourly and measuring their depletion. Translocation of these ions and of organic nitrogen was determined by hourly analysis of the vascular exudate. Nitrate reduction was estimated by the difference between nitrate uptake and nitrate recovered in the tissue and exudate. Nitrate uptake exhibited its usual pattern of apparent induction resulting in the development of an accelerated uptake phase. Chloride uptake remained fairly constant throughout the experimental period. Translocation of nitrate increased progressively for at least 7 hours whereas chloride translocation reached a maximum about the 3d hour and then declined to a lower rate than nitrate translocation. Nitrate uptake and translocation were restricted by anaerobiosis, by 20 and 40 C relative to 30 C, and by 0.05 mm 6-methylpurine, an RNA-synthesis inhibitor. Accumulation, reduction and translocation of nitrate had different sensitivities to all these factors. The effect of 0.05 mm 6-methylpurine was more detrimental to nitrate translocation and nitrate reduction than to nitrate uptake.Ambient nitrate, relative to chloride, enhanced the exudation volume and the translocation of organic nitrogen within 4 hours from initiation of the experiments. Translocation of nitrate and organic nitrogen decreased shortly after removal of external nitrate. The higher rates of organic nitrogen translocation which occurred during nitrate uptake indicates either (a) rapid translocation of amino acids synthesized from the entering nitrate, or (b) an accelerated rate of protein turnover and a resulting enhancement in translocation of endogenous amino acids.     

Urea Uptake and Carbon Fixation by Marine Pelagic Bacteria and Archaea during the Arctic Summer and Winter Seasons

How Arctic climate change might translate into alterations of biogeochemical cycles of carbon (C) and nitrogen (N) with respect to inorganic and organic N utilization is not well understood. This study combined ¹⁵N uptake rate measurements for ammonium, nitrate, and urea with ¹⁵N- and ¹³C-based DNA stable-isotope probing (SIP). The objective was to identify active bacterial and archeal plankton and their role in N and C uptake during the Arctic summer and winter seasons. We hypothesized that bacteria and archaea would successfully compete for nitrate and urea during the Arctic winter but not during the summer, when phytoplankton dominate the uptake of these nitrogen sources. Samples were collected at a coastal station near Barrow, AK, during August and January. During both seasons, ammonium uptake rates were greater than those for nitrate or urea, and nitrate uptake rates remained lower than those for ammonium or urea. SIP experiments indicated a strong seasonal shift of bacterial and archaeal N utilization from ammonium during the summer to urea during the winter but did not support a similar seasonal pattern of nitrate utilization. Analysis of 16S rRNA gene sequences obtained from each SIP fraction implicated marine group I Crenarchaeota (MGIC) as well as Betaproteobacteria, Firmicutes, SAR11, and SAR324 in N uptake from urea during the winter. Similarly, ¹³C SIP data suggested dark carbon fixation for MGIC, as well as for several proteobacterial lineages and the Firmicutes. These data are consistent with urea-fueled nitrification by polar archaea and bacteria, which may be advantageous under dark conditions.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

Uptake of Nitrate by Mutants of Arabidopsis thaliana, Disturbed in Uptake or Reduction of Nitrate

The uptake of nitrate by wildtype plants and chlorate-resistant mutants of Arabidopsis thaliana (L.) Heynh. was studied by intermittent or continuous measurement of the nitrate concentration of the ambient solution. The uptake rate in the wildtype and the nitrate reductase-less mutant B 25 showed a dual-phase relation ship with concentration. Each phase showed Michaelis-Menten kinetics although multiphasic patterns within each phase could not be excluded. A dual-phase relationship was also found in the uptake mutant B I. Here, however, phase II did not follow Michaelis-Menten kinetics and uptake rate of nitrate in the phase II concentration range was considerably lower in the B 1 mutant than in the wildtype. It is concluded that the mutation in B I has disturbed phase II of the nitrate uptake, without affecting phase I, which leads to the suggestion that uptake of nitrate in Arabidopsis is mediated by at least two independent uptake mechanisms. The nitrate uptake rate showed an optimum at pH 8, and it was not stimulated by the presence of calcium. Ammonium had different effects on nitrate uptake: a direct effect, when it was present during the uptake of nitrate, resulting in a release of nitrate and a reduced rate of uptake, and an indirect inhibitory effect, possibly caused by assimilation products of ammonium, which is most pronounced after growth on ammonium as the sole nitrogen source or in long-lasting uptake experiments in the presence of ammonium. Chlorate also showed a multiple effect, an inhibiting one which proved to be competitive and, at very low concentrations of chlorate, a stimulating one. Evidence was obtained that chlorate and nitrate arc taken up by the same carrier.     

The Utilization of Endosperm Reserves During Early Growth of Barley Cultivars and the Effect of Time of Application of Nitrogen

The cultivars Akka and Hiproly with high grain nitrogen andJulia and Foma with low grain nitrogen and Proctor containinghigh or low nitrogen were compared. The high nitrogen forms germinated more rapidly and by day 6had a higher axis dry weight than the low nitrogen types. Byday 6 dry weight of tops and roots was similar in the high nitrogencultivars but that of the tops was less in the low nitrogenforms. Analyses of the partition of nitrogen from endospermto axes confirmed that for all types this was maximal after48 h from planting and occurred at a similar relative rate sothat 50 per cent of the endosperm nitrogen was translocatedby 84–96 h from planting. Loss of endosperm dry weightwas much slower and here high nitrogen forms translocated drymatter at a significantly faster relative rate than the lownitrogen types. Nitrate supplied at planting or on day 2 increased the rateof endosperm depletion in the low nitrogen types and resultedin their achieving similar axis dry weights to high nitrogenforms which were largely unaffected by nitrate supply. Delayin nitrate application had progressively less effect on endospermbreakdown. There was a greater uptake of nitrate by the lownitrogen forms and rate of uptake was rapid. Uptake of nitrateby high nitrogen types was initially rapid but was followedby a much slower phase. The results indicate that endogenous and exogenous sources ofnitrogen are equally suitable for seedling growth. The significanceof grain nitrogen content in relation to endosperm breakdownis discussed as is the strategy of partition of endosperm reservesbetween root and shoot.     

Planktonic nitrogen transformations during a declining cyanobacteria bloom in the Baltic Sea

The uptake of ¹⁵N-labelled nitrogen nutrients (ammonium, urea,nitrate) was studied during the decline of a bloom of nitrogen-fixingcyanobacteria in the Baltic Sea. This was done by sampling anorth-south transect of stations, representing different stagesof the bloom. Comparison with nitrogen fixation data showedthat this process was of minor importance, and that the nitrogenuptake was dominated by regenerated nitrogen, mainly ammonium.From time series incubations for studying nutrient uptake, itappears that the regeneration of ammonium was substantial, butthat the production of urea or nitrate was slow. The integrateddaily uptake was calculated for the 0–15 m interval atfour stations and values ranged between 6 and 21 mmol N m^–2day^–1, of which the regenerated nutrients, ammonium andurea, constituted 71–93%. Nitrate was of minor importanceand the highest nitrate uptake rates were found close to thethermocline (at 15 m) and in the southern part of the Baltic.Comparison with carbon fixation data reported from simultaneousmeasurements at two stations gave C/N uptake ratios of 4.9 and2.1 for integrated daily uptake. Contrary to earlier findings,the concentration of DON increased with increasing salinity(from 15 to 17 µmol l^–1). This was correlated withthe declination of the bloom and is suggested to be a resultof a gradual release of less easily utilized DON from the degradationof cyanobacteria. The C/N ratio of DOM was high, 21–23.     

Organic and inorganic nitrogen uptake in lichens

In order to learn more about nitrogen (N) acquisition in lichens, and to see whether different lichens differ in their affinity to various N sources, N uptake was measured in 14 various lichen associations (species). These species represented various morphologies (fruticose or foliose), contrasting microhabitat preferences (epiphytic or terricolous), and had green algal, cyanobacterial or both forms of photobionts. N was supplied under non-limiting conditions as an amino acid mixture, ammonium, or nitrate, using ¹⁵N to quantify uptake. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) was used to separate active and passive uptake. Thallus N, amino acids, soluble polyol concentrations, and the biont-specific markers chlorophyll a and ergosterol were quantified, aiming to test if these metabolites or markers were correlated with N uptake capacity. Ammonium uptake was significantly greater and to a higher extent passive, relative to the other two N sources. Nitrate uptake differed among lichen photobiont groups, cyanobacterial lichens having a lower uptake rate. All lichens had the capacity to assimilate amino acids, in many species at rates equal to nitrate uptake or even higher, suggesting that organic N compounds could potentially have an important role in the N nutrition of these organisms. There were no clear correlations between N uptake rates and any of the measured metabolites or markers. The relative uptake rates of ammonium, nitrate and amino acids were not related to morphology or microhabitat.Abbreviations CCCP Carbonyl cyanide m-chlorophenylhydrazone - Chl Chlorophyll - N Nitrogen     

Ammonium regulation of urate uptake in Chlamydomonas reinhardtii

Urate was taken up at a negligible rate by Chlamydomonas reinhardtii cells grown on ammonium and transferred to media containing urate plus ammonium or urate plus chloral hydrate or cycloheximide. Addition of ammonium to cells actively consuming urate produced a rapid inhibition of urate uptake whereas the intracellular oxidation of urate was unaffected. Methylammonium but not glutamine or glutamate inhibited urate uptake. Addition of l-methionine-dl-sulfoximine to cells actively consuming urate provoked ammonium excretion, which was accompanied by a rapid inhibition of urate uptake. In cells growing on urate and exhibiting noticeable levels of nitrite-reductase activity, nitrite caused a sudden inhibition of urate uptake whereas nitrate required a time to induce nitrate reductase and to exert its inhibitory effect on uptake. The urate-uptake system did not require urate for induction since the urate-uptake capacity appeared in nitrogen-starved cells. From these results it is concluded that, in Chlamydomonas reinhardtii, ammonium inhibits urate uptake and also acts as co-repressor of the uptake system.     

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.