The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

NH4 + and NO3 − requirement for wheat somatic embryogenesis

The influence of three nitrogen salts: NH₄NO₃, KNO₃ and NH₄Cl on wheat in vitro cultures was investigated. Both NO ₃ ⁻ and NH ₄ ⁺ ions were indispensable for proliferation of embryogenic calli and development of wheat somatic embryos. It is possible to obtain wheat somatic embryos when the medium is enriched with NH₄NO₃ only as a source of inorganic nitrogen. The results of the statistical analysis showed that the level of NH₄NO₃ and KNO₃ in the medium had a great influence on the efficiency of somatic embryogenesis. We observed tendency that calli on media containing 50 mM NH₄NO₃ and 0 to 20 mM KNO₃ turned out to be more embryogenic than on control MS medium. High concentrations of KNO₃- 100 mM inhibited somatic embryogenesis, while 100 mM NH₄NO₃ did not. The level of total N did not have significant influence on wheat somatic embryogenesis. Ratio NO ₃ ⁻ :NH ₄ ⁺ also turned out to be not substantial. We observed that mutual connection of concentration levels between NH₄NO₃ and KNO₃ and between NH₄Cl and KNO₃ was more important. The efficiency of somatic embriogenesis obtained in the experiment with NH₄Cl and KNO₃ was significantly lower than in experiment with NH₄NO₃ and KNO₃.     

Composition of the xylem sap of tomato seedlings cultivated on media with HCO3 − and nitrogen source as NO3 − or NH4 +

The results of the experiments discussed here present changes in the chemical composition of xylem sap of tomato seedlings cultivated in hydroponics on media containing 5 mmol HCO₃ ^– and an N-source given as NO₃ ^–, NH₄ ⁺ or these two forms in different proportions. The occurrence of free NH₄ ⁺ in the xylem sap of NH₄ ⁺-seedlings and in NO₃ ^–-seedlings indicates that the process of N-assimilation was not only confined to roots. The application of HCO₃ ^– to the medium effected a decrease in the concentration of NH₄ ⁺ in the xylem sap of NH₄ ⁺-seedlings, having no effect on changes in the concentration of NO₃ ^– or NH₄ ⁺ in NO₃ ^–-seedlings. Malate, citrate, fumarate, and succinate were identified in the xylem sap. The concentration of carboxylates in NO₃ ^–-seedlings exceeded by about 50% that recorded in NH₄ ⁺-seedlings. The highest concentration of malate constituting from 80% to 93.5% of this fraction, was determined in this group of compounds. The enrichment of the medium with HCO₃ ^– ions induced an increase in the content of carboxylates, chiefly of malate. In these experimental conditions an increase in the malate concentration in the xylem sap of NO₃ ^– and NH₄ ⁺-seedlings reached relative values of 100% and 36%, respectively. The total concentration of amides and amino acids was about 2.6 times higher in the xylem sap of NH₄ ⁺-seedlings than in NO₃ ^–-seedlings. Amide glutamine was the main component of this fraction in xylem sap and its total concentration was about 3.3 times higher in NH₄ ⁺-seedlings than that determined in NO₃ ^–-seedlings. Glutamine, glutamate, aspargine, and aspartate constituted from 69% to 77% of this fraction. The concentration of the remaining amino acids varied from 0.6% to 7%. The enrichment of the medium with HCO₃ ^– ions also effected an increase in the concentration of amides and amino acids in the xylem sap by about 17% and 56% in the case of NO₃ ^– and NH₄ ⁺-seedlings, respectively, in comparison with the respective controls (without HCO₃ ^–). Abbreviations: DAG – days after germination; DIC – dissolved inorganic carbon; GOGAT – glutamine:2-oxoglutarate aminotransferase; GS – glutamine synthetase; PAR – photosynthetically active radiation; PEPc – phosphoenolpyruvate carboxylase     

Effects of clipping and shading on 15NO3− and 15NH4+ recovery by plants in grazed and ungrazed temperate grasslands

Plant and Soil - In natural ecosystems, plants generally promote the acquisition of nitrogen (N) through the input of carbon (C) into the soil. The present study aimed to clarify how changes in C...     

Effects of NH4 +, NO3 − and HCO3 − on apoplast pH in the outer cortex of root zones of maize, as measured by the fluorescence ratio of fluorescein boronic acid

A fluorimetric ratio technique was elaborated to measure apoplastic pH in the outer root cortex of maize (Zea mays L.) grown hydroponically. A newly synthesized fluorescent probe, fluorescein boronic acid (pK_a = 5.48), which covalently binds to the cell wall of the outer cell layers, was used. Under conditions of saturating ion concentrations the apoplastic pH was determined along the root axis ranging from 1 to 30 mm behind the root tip. Apoplastic pH was recorded for root segment areas (1 mm²), and pH values of high statistical significance were obtained. With an external solution of pH 5, the apoplastic pH was about pH 5.1 in the division zone, between pH 4.8 and 4.9 in the elongation region and about pH 4.9 in the root hair zone. At an external pH of 8.6, the difference between the external pH and the apoplastic pH was considerably more, with a pH of 5.2–5.3 in all root zones. Addition of 1 mM NH₄ ⁺ caused a small apoplastic pH decrease (0.05 of a pH unit) in all root zones. Apoplastic alkalization upon application of 6 mM NO₃ ⁻ was highest (0.3 of a pH unit) in the zone where root hairs emerge; in the division and early elongation zones, apoplastic pH increased only transiently. In the presence of 10 mM HCO₃ ⁻, NO₃ ⁻ elicited a higher and persistent alkalization (0.06–0.25 of a pH unit) in all root zones. Application of fusicoccin reduced apoplastic pH from 4.85 to 4.75 in the elongation zone, while inhibition of the H⁺-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences along the root axis upon differential N supply and application of HCO₃ ⁻ provide evidence that this new pH technique is a useful tool with which to measure apoplastic pH, and in future may permit measurements at microsites at the cell level by use of microscope imaging. Received: 26 August 1998 / Accepted: 4 May 1999     

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

Influx, efflux and net uptake of NO ₃ ^– was studied in Pisum sativum L. cv. Marma in short-term experiments where ¹³NO 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 M, and at low external NO ₃ ^– concentrations (4–6 M) 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.Abbreviations DW dry weight - FW fresh weight - R_N relative nitrogen addition rate     

Alteration in enrichment of NO 3 − and reduced-N in xylem exudate during and after extended plant exposure to15NO 3 −

Summary Distinctly different patterns of¹⁵N enrichment were observed in the nitrate and reduced-N fractions of xylem exudate from soybean plants during and after 5 to 6 days of exposure to¹⁵NO ₃ ^– . Within 1 d after changes in solution NO ₃ ^– label, more than 90% of the exudate NO ₃ ^– originated from the exogenous supply. Alterations in the enrichment of exudate reduced-N were much slower, however, and the enrichment reached only 40% even after 5 d of continuous exposure to¹⁵NO ₃ ^– . Taking into account possible reduction of endogenous NO ₃ ^– and delayed translocation of NO ₃ ^– reduction products, it was concluded that root reduction could have contributed only 30 to 42% of the reduced-N found in the exudate.     

Effects of Al on nitrogen (NH 4 + and NO 3 − ) uptake,nitrate reductase activity and proton release in two sorghum cultivars differing in Al tolerance

After growth for 17 to 36 days on nutrient solutions with NH₄NO₃ as nitrogen source (pH 4.2) dry matter of sorghum genotype SC0283 was much less affected by Al (1.5 and 3.0 ppm) than that of genotype NB9040. In the absence of Al both cultivars released protons into the nutrient solution as a result of an excess of cationic nutrients taken up. When Al was present, this proton efflux per unit dry weight increased drastically, especially with the sensitive genotype NB9040. Chemical analysis of plant material and continuous analyses of NO ₃ ⁻ and NH ₄ ⁺ in the nutrient solution indicated, that the Al-induced shift in H⁺-balance of both genotypes could almost completely be attributed to a decreased NO ₃ ⁻ /NH ₄ ⁺ uptake ratio. In vivo nitrate reductase activity (NRA) was reduced in the shoot of NB9040 and to a lesser degree in SC0283. Al-induced decrease in NRA was accompanied by similar percentual decreases in NO ₃ ⁻ tissue concentrations. Therefore this decrease is interpreted as being indirect,i.e., the consequence of the reduced NO ₃ ⁻ uptake of the plants. A direct repression of NRA by Al seems also unlikely because nitrate reductase activity of the roots (where cellular Al-concentrations should be higher than in shoots) was not affected in Al-treated plants of either genotype.     

Effect of NH4+/NO3− availability on nitrate reductase activity and nitrogen accumulation in wetland helophytes Phragmites australis and Glyceria maxima

The effect of NH₄⁺/NO₃⁻ availability on nitrate reductase (NR) activity in Phragmites australis and Glyceria maxima was studied in sand and water cultures with the goal to characterise the relationship between NR activity and NO₃⁻ availability in the rhizosphere and to describe the extent to which NH₄⁺ suppresses the utilization of NO₃⁻ in wetland plants.The NR activity data showed that both wetland helophytes are able to utilize NO₃⁻. This finding was further supported by sufficient growth observed under the strict NO₃⁻ nutrition. The distribution of NR activity within plant tissues differed between species. Phragmites was proved to be preferential leaf NO₃⁻ reducer with high NR activity in leaves (NR_max > 6.5 μmol NO₂⁻ g dry wt⁻¹ h⁻¹) under all N treatments, and therefore Phragmites seems to be good indicator of NO₃⁻ availability in flooded sediment. In the case of Glyceria the contribution of roots to plant NO₃⁻ reduction was higher, especially in sand culture. Glyceria also tended to accumulate NO₃⁻ in non-reduced form, showing generally lower leaf NR activity levels. Thus, the NR activity does not necessarily correspond with plant ability to take up NO₃⁻ and grow under NO₃⁻-N source. Moreover, the species differed significantly in the content of compounds interfering with NR activity estimation. Glyceria, but not Phragmites, contained cyanogenic glycosides releasing cyanide, the potent NR inhibitor. It clearly shows that the use of NR activity as a marker of NO₃⁻ utilization in individual plant species is impossible without the precise method optimisation.     

The role of gross and net N transformation processes and NH4 + and NO3 − immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO₃ ^? leaching. On the other hand, despite this elevated N deposition, the dominant fate of NO₃ ^? and NH₄ ⁺ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha^?1 yr^?1, but at the same time shows low NO₃ ^? concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory ¹⁵N pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0–10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 m inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg^?1 d^?1, the net N mineralization averaged 6.1 and 6.8 mg N kg^?1 d^?1 and NH₄ ⁺ immobilization rates averaged 3.8 and 3.6 mg N kg^?1 d^?1. In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg^?1 d^?1, the average net nitrification was ?25.2 and ?31.3 mg N kg^?1 d^?1 and the NO₃ ^? immobilization rates averaged 29.0 and 35.9 mg N kg^?1 d^?1. For the mineral (0–10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH₄ ⁺ pool and not significantly different from zero for the NO₃ ^? pool. Except for the turnover of the NH₄ ⁺ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH₄ ⁺ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO₃ ^? immobilisation to gross nitrification led to the following observations. The NH₄ ⁺ pool of the forest soil was controlled by N mineralization and NO₃ ^? immobilization was importantly controlling the forest NO₃ ^? pool. Therefore it was concluded that this process is most probably responsible for the limited NO₃ ^? leaching from the forest ecosystem, despite the chronically high N deposition rates.     

Growth and Major Nutrient Concentrations in Brassica campestris Supplied with Different NH4^＋/NO3 Ratios

In the present study, we investigated whether growth and main nutrient ion concentrations of cabbage （Brassica campestris L.） could be increased when plants were subjected to different NH4^＋/NO3- ratios. Cabbage seedlings were grown in a greenhouse in nutrient solutions with five NH4^＋/NO3- ratios （1：0; 0.75：0.25; 0.5：0.5; 0.25：0.75; and 0：1）. The results showed that cabbage growth was reduced by 87% when the proportion of NH4^＋-N in the nutrient solution was more than 75% compared with a ratio NH4^＋/NO3- of 0.5：0.5 35 d after transplanting, suggesting a possible toxicity due to the accumulation of a large amount of free ammonia in the leaves. When the NH4＋/NO3- ratio was 0.5：0.5, fresh seedling weight, root length, and H2PO4- （P）, K^＋, Ca^2＋, and Mg^2＋ concentrations were all higher than those in plants grown under other NH4^＋/NO3- ratios. The nitrate concentration in the leaves was the lowest in plants grown at 0.5： 0.5 NH4^＋/NO3-. The present results indicate that an appropriate NH4^＋/NO3- ratio improves the absorption of other nutrients and maintains a suitable proportion of N assimilation and storage that should benefit plant growth and the quality of cabbage as a vegetable.     

Enhancement of rhizosphere citric acid and decrease of NO3−/NH4+ ratio by root interactions facilitate N fixation and transfer

Plant and Soil - Plant growth is often limited by low soil phosphorus (P) availability, soil nitrogen (N) availability may affect plants’ responses to P supply. We studied the growth and...     

Hg2+-induced leakage of electrolytes and inhibition of NO3 − utilization inNostoc calcicola

Summary The effect of mercury (Hg²⁺) in the absence and presence of methylmercury (CH₃Hg⁺), cadmium (Cd²⁺), copper (Cu²⁺), nickel (Ni²⁺) and calcium (Ca²⁺) on Nostoc calcicola Bréb. has been studied in terms of electrolyte leakage, NO₃ ^– uptake and in vivo nitrate reductase (NR) activity to discover any possible correlation among such parameters under Hg²⁺ stress. Leakage of electrolytes from Hg²⁺-treated cyanobacterial cells was directly proportional to Hg²⁺ concentrations and exposure time. In comparison to NO₃ ^– uptake, an about 60-fold slower rate of NR activity was observed in the untreated cultures, the former being five times more Hg²⁺-sensitive. A non-competitive synergistic interaction of Hg²⁺ with CH₃Hg⁺ or Cd²⁺ and antagonistic with that of Ni²⁺ or Ca²⁺ has been observed for both the processes of NO₃ ^– utilization. The antagonistic interaction of Cu²⁺ with Hg²⁺ in terms of NO₃ ^– uptake and synergistic with respect to NR activity, has been attributed to the dual bonding preference of Cu²⁺ for cellular ligands. These findings suggest that (a) a statistically significant correlation exists among such parameters; (b) Hg²⁺ predominantly attacks the cyanobacterial cell membrane; (c) Hg²⁺ inhibits NO₃ ^– utilization; (d) the presence of other cations increases or decreases the inhibitory actions of Hg²⁺.     

Influence of nitrogen species (NH4 + and NO3 −) on the dynamics of P in water–sediment–Salvinia herzogii systems

Water – Salvinia herzogii – sediment systems were exposed to different phosphorus and nitrogen combinations in outdoor experiments. The aim was to estimate the amounts of P immobilized in macrophytes and sediments, as well as to elucidate whether or not the presence of N affects the retention of P. The following components were added: o-P, o-P + NH₄ ⁺, o-P + NO₃ ^– + NH₄ ⁺, o-P + NO₃ ^–. The concentration of nutrients was periodically determined throughout the experiment (28 days). The concentrations of P and N in plant tissues and sediments were determined at the beginning and the end of the experiment. Sequential extractions of P-fractions in sediment were performed using the EDTA method (Golterman, 1996). The removal efficiency of P in water was 95–99%. The removal of NH₄ ⁺ (97–98%) was more effective than that of NO₃ ^– (44–86%). The presence of nitrogen species increased the removal velocity of o-P from water, NH₄ ⁺ was the most effective species. Sediments not only had higher P removal rates than macrophytes but, in the control treatment without macrophytes, they reached the values obtained by macrophytes plus sediments in the other treatments. The adsorption of P takes place at the surface layer of the sediment (1 cm). Most of the P incorporated into the sediment during the experiment was sorbed by the fraction Fe(OOH)P. The addition of nutrients to water modified the leaves/lacinias weight ratio.     

Influence of nitrogen form and NH4 +-N/:NO3 −-N ratios on adventitious shoot formation from pear (Pyrus communis) leaf explants in vitro

The effect of various basal salts media, containing different nitrogen levels on in vitro adventitious shoot regeneration from leaf explants of Louise Bonne Panachee and Seckel pear (Pyrus communis L.) were investigated. Among the different basal salt formulae tested, Nitsch (1969) gave significantly better regeneration in most of the experiments. Shoot regeneration was altered with different NH₄ ⁺-N/NO₃ ^–-N ratios. The best regeneration was obtained when NH₄ ⁺:NO₃ ^– was either 1:2 or 1:3 regardless of overall N concentration. In addition, these data show that NH₄ ⁺ was essential for adventitious shoot regeneration from pear leaf explants on White's (1943) medium.     

Cell pH and H+ Secretion by S3 Segment of Mammalian Kidney: Role of H+-ATPase and Cl−

The role of H⁺-ATPase in proximal tubule cell pH regulation was studied by microperfusion techniques and by confocal microscopy. In a first series of experiments, proximal S3 segments of rabbit kidney were perfused ``in vitro' while their cell pH was measured by fluorescence microscopy after loading with BCECF. In Na⁺- and Cl⁻-free medium, cell pH fell by a mean of 0.37 ± 0.051 pH units, but after a few minutes started to rise again slowly. This rise was of 0.17 ± 0.022 pH units per min, and was significantly reduced by bafilomycin and by the Cl⁻ channel blocker NPPB, but not by DIDS. In a second series of experiments, subcellular vesicles of proximal tubule cells of S3 segments of mouse kidney were studied by confocal microscopy after visualization by acridine orange or by Lucifer yellow. After superfusion with low Na⁺ solution, which is expected to cause cell acidification, vesicles originally disposed in the basolateral and perinuclear cell areas, moved toward the apical area, as detected by changes in fluorescence density measured by the NIH Image program. The variation of apical to basolateral fluorescence ratios during superfusion with NaCl Ringer with time was 0.0018 ± 0.0021 min⁻¹, not significantly different from zero (P > 0.42). For superfusion with Na⁺0 Ringer, this variation was 0.081 ± 0.015 min⁻¹, P < 0.001 against 0. These slopes were markedly reduced by the Cl⁻ channel blocker NPPB, and by vanadate at a concentration that has been shown to disrupt cytoskeleton function. These data show that the delayed alkalinization of proximal tubule cells in Na⁺-free medium is probably due to a vacuolar H⁺-ATPase, whose activity is stimulated in the presence of Cl⁻, and dependent on apical insertion of subcellular vesicles. The movement of these vesicles is also dependent on Cl⁻ and on the integrity of the cytoskeleton. Received: 11 April 2000/Revised: 14 August 2000