Replacement of potassium ions by ammonium ions in different micro-organisms grown in potassium-limited chemostat culture

The biomass concentration extant in potassiumlimited cultures of either Klebsiella pneumoniae or Bacillus stearothermophilus (when growing at a fixed temperature and dilution rate in a glucose/ammonium salts medium) increased progressively as the medium pH value was raised step-wise from 7.0 to 8.5. Because the macromolecular composition of the organisms did not vary significantly, this increase in biomass could not be attributed to an accumulation of storage-type polymers but appeared to reflect a pH-dependent decrease in the cells' minimum K⁺ requirement. Significantly, this effect of pH was not eviden with cultures in which no ammonium salts were present and in which either glutamate or nitrate was added as the sole nitrogen source; however, it was again manifest when various concentrations of NH₄Cl were added to the glutamate-containing medium. This suggested a functional replacement of K⁺ by NH ₄ ⁺ , a proposition consistent with the close similarity of the ionic radii of the potassium ion (1.33 Å) and the ammonium ion (1.43 Å). At pH 8.0, and with a medium containing both glutamate (30 mM) and NH₄Cl (100 mM), cultures of B. stearothermophilus would grow without added potassium at a maximum rate of 0.7 h^-1. Under these conditions the cells contained maximally 0.1% (w/w) potassium (derived from contaminating amounts of this element in the medium constituents), a value which should be compared with one of 1.4% (w/w) for cells growing in a potassiumlimited medium containing initially 0.5 mM K⁺. Qualitatively similar findings were made with cultures of K. pneumoniae; and whereas one may not conclude that NH ₄ ⁺ can totally replace K⁺ in the growth of these bacteria, it can clearly do so very extensively.     

The Influence of Ammonium Nitrate,pH and Indole Butyric Acid on Root Induction and Survival in Soil of Micropropagated <Emphasis Type="Italic">Eucalyptus globulus</Emphasis>

Rooting of Eucalyptus globulus shoots was influenced by the concentration of the indole butyric acid (IBA) and NH₄ ⁺ in the root-induction medium. Optimum plantlet vigor and survival were achieved using low concentrations (1 – 2.5 μM) of IBA and when NH₄NO₃ was removed. Removal of NH₄ ⁺ also had a significant effect on medium pH, its presence caused a decrease in pH as the culture period proceeded. When different nitrate compounds (excluding NH₄NO₃) were used as the nitrogen source, the medium pH was more stable and this was associated with higher root production. The higher root production, in association with appropriate IBA concentrations, produced plantlets with higher survival and better growth on transfer to soil. This revised version was published online in July 2006 with corrections to the Cover Date.     

A critical experimental evaluation of methods for determination of NH4+ in plant tissue, xylem sap and apoplastic fluid

Ammonium (NH₄⁺) is a central intermediate in the N metabolism of plants, but the quantitative importance of NH₄⁺ in transporting N from root to shoot and the capability of plants to store NH₄⁺ in leaves are still matters of substantial controversy. This paper shows that some of these controversies have to be related to the use of inadequate analytical procedures used for extraction and quantification of NH₄⁺ in plants. The most frequently used methods for determination of NH₄⁺, viz. colorimetric methods based on the classical Berthelot reaction, suffered severely from interference caused by amino acids, amines, amides and proteins. For some of these metabolites the interference was positive, while for others it was negative, making correction impossible. Consequently, colorimetric analysis is inapplicable for determination of NH₄⁺ in plants. Results obtained by ion chromatography may overestimate the NH₄⁺ concentration due to co‐elution of NH₄⁺ with amines like methylamine, ethylamine, ethanolamine and the non‐protein amino acid Γ‐aminobutyric acid. Derivatization of NH₄⁺ with o‐phthalaldehyde at alkaline pH and subsequent quantification of NH₄⁺ by fluorescence spectroscopy was also associated with interference. However, when pH was lowered to 6.8 during derivatization and 2‐mercaptoethanol was used as reductant, NH₄⁺ could be determined with a high selectivity and sensitivity down to a detection limit of 3.3 μM in a 10‐μl sample volume. Derivatization was performed on‐line using a column‐less HPLC system, enabling rapid quantification of NH₄⁺ in a few minutes. Flow injection analysis with on‐line gas dialysis was, likewise, free from interference, except when applied on highly senescent plant material containing volatile amines. Labile N metabolites in leaf tissue extract, xylem sap and apoplastic fluid were degraded to NH₄⁺ during extraction and subsequent instrumental analysis if the samples were not stabilised. A simple and efficient stabilisation could be obtained by addition of 10 mM ice‐cold HCOOH to the plant extraction medium or to the samples of apoplastic fluid or xylem sap. We conclude that significant concentrations of NH₄⁺, exceeding 1 mM, may occur in xylem sap, leaf apoplastic fluid and leaf tissue water of nitrate‐grown tomato and oilseed rape plants. The measured NH₄⁺ concentrations were not a result of excessive N supplies, as even plants grown under mildly N‐deficient conditions contained NH₄⁺.     

Acid-base regulation during ammonium assimilation in Hydrodictyon africanum

Abstract The acid-base balance during ammonium (used to mean NH ₄⁺ and/or NH₃) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m^?3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H⁺ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H⁺ accumulate in the medium per organic N synthesized. Granted a permeability (PNH₃) of some 10^?3 cm s^?1, and a finite [NH₃] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH ₄⁺. This means that most of the H ⁺ appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H⁺ accumulates in the medium equivalent to any NH₃ entry by requilibration from exogenous NH ₄⁺. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO₃⁺grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO₃⁺in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO₃⁺-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm^?3) the net influx was all attributable to NH₄⁺influx via a uniporter, probably partly short-circuited by a passive NH₃ efflux due to intrinsic membrane permeability to NH₃. The net ammonium influx was in all cases associated with H⁺ accumulation in the medium. (1.3-1.7 H ⁺ per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H⁺ fluxes.     

Effects of pH on ammonium uptake by Typha latifolia L.

The effects of solution pH on NH₄⁺ uptake kinetics and net H⁺ extrusion by Typha latifolia L. were studied during short-term (days) and long-term (weeks) exposure to pH in the range of pH 3.5–8.0. The NH₄⁺ uptake kinetics were estimated from depletion curves using a modified Michaelis-Menten model. T. latifolia was able to grow in solution culture with NH₄⁺ as the sole N source and to withstand a low medium pH for short periods (days). With prolonged exposure (weeks) to pH 3.5, however, the plants showed severe symptoms of stress and stopped growing. The solution pH affected NH₄⁺ uptake kinetics. The affinity for NH₄⁺, as quantified by the half saturation constant (K_1/2) and C_min (the NH₄⁺ concentration at which uptake ceases), decreased with pH. K_1/2 was increased from 7.1 to 19.2 mmol m^?3 and C_min from 2.0 to 5.7 mmol m^?3 by lowering the pH in steps from 8.0 to 3.5. V_max was, however, largely unaffected by pH (～22 μmol h^?1 g^?1 root dry weight). Under prolonged exposure to constant pH, growth rates were highest at PH 5.0 and 6.5. At pH 8.0 growth was slightly depressed and at pH 3.5 growth completely stopped. NH₄⁺ uptake kinetics were similar at pH 5.0, 6.5 and 8.0 whereas at pH 3.5 NH₄⁺ uptake almost completely stopped. The ratio between net H⁺ extrusion and NH₄⁺ uptake decreased significantly at low pH. The adverse effects of low pH on NH₄⁺ uptake kinetics are probably a consequence of a reduced H⁺-ATPase activity and/or an increased re-entry of H⁺ at low pH, and the associated decrease in the electrochemical gradient across the plasma membranes of the root cells.     

Optimization of culture conditions for the production of rifamycin oxidase by Curvularia lunata

Maximum activity (8.9 IU/ml) of rifamycin oxidase in Curvularia lunata, grown in shake-flask culture at 28°C and pH 6.5, was after 96 h. Nearly all the glucose was used in 72 h. An initial culture pH of 6.5 and 28°C were optimum for the growth and enzyme production. Among various carbon and organic nitrogen sources, carboxymethylcellulose and peptone were the most effective for enzyme yield. The rate of enzyme production was enhanced when yeast extract was also added to the medium. The optimum medium for the production of rifamycin oxidase contained 10 g each of yeast extract, peptone and carboxymethylcellulose/l and 0.04% (NH₄)₂SO₄.The author is with the Biochemical Engineering Research and Process Development Centre, Institute of Microbial Technology, Post Box 1304, Sector 39-A, Chandigarh 160 014, India     

Effect of nickel on certain physiological and biochemical behaviors of an acid tolerantChlorella vulgaris

This study concerns the inhibitory effects of acid pH and nickel on growth, nutrient (NO₃ ^- and NH₄ ⁺) uptake, carbon fixation, O₂ evolution, electron transport chain and enzyme (nitrate reductase and ATPase) activities of acid tolerant and wild-type strains of Chlorella vulgaris. Though a general reduction in all these variables was noticed with decreasing pH, the tolerant strain was found to be metabolically more active than the wild-type. A reduced cation (NH₄ ⁺, Na⁺, K⁺ and Ca²⁺) uptake, coupled with a facilitated influx of anions (NH₄ ⁺, PO₄ ^3- and HCO₃ ^-), suggested the development of a positive membrane potential in acid tolerant Chlorella. Nevertheless, a tremendous increase in ATPase activity at decreasing pH revealed the involvement of superactive ATPase in exporting H⁺ ions and keeping the internal pH neutral. A difference in Na⁺ and K⁺ efflux of the two strains at decreasing pH suggests there is a difference in membrane permeability. The low toxicity of Ni in the acid tolerant strain may be due to the low Ni uptake brought about by a change in membrane potential as well as in permeability. Hence, the development of superactive ATPase and a change in both membrane potential and permeability not only offers protection against acidity, but also co-tolerance to metals.     

Optimum conditions for the biological production of lactic acid by a newly isolated lactic acid bacterium,Lactobacillus sp. RKY2

Lactic acid is a green chemical that can be used as a raw material for biodegradable polymer. To produce lactic acid through microbial fermentation, we previously screened a novel lactic acid bacterium. In this work, we optimized lactic acid fermentation using a newly isolated and homofermentative lactic acid bacterium. The optimum medium components were found to be glucose, yeast extract, (NH₄)₂HPO₄, and MnSO₄. The optimum pH and temperature for a batch culture ofLactobacillus sp. RKY2 was found to be 6.0 and 36°C, respectively. Under the optimized culture conditions, the maximum lactic acid concentration (153.9 g/L) was obtained from 200 g/L of glucose and 15 g/L of yeast extract, and maximum lactic acid productivity (6.21 gL⁻¹h⁻¹) was obtained from 100 g/L of glucose and 20 g/L of yeast extract. In all cases, the lactic acid yields were found to be above 0.91 g/g. This article provides the optimized conditions for a batch culture ofLactobacillus sp. RKY2, which resulted in highest productivity of lactic acid.     

Cultivation of the hyperthermophilic archaeonSulfolobus solfataricus in low-salt media

Two low-salt complex media, bactopeptone and desalted yeast extract, were used for high density cultivation of the hyperthermophilic archaeonSulfolobus solfataricus (DSM 1617). Bactopeptone, which has low mineral ion content among various complex media, was good for cell growth in batch cultures; the maximal cell density in bactopeptone was comparable to that in yeast extract. Howver, cell growth was rather poor when bactopeptone was added by the fed-batch procedure. Since several vitamins are deficient in bactopeptone, the effect of vitamins on cell growth was examined. Among the vitamins tested, pyridoxine was found to improve the growth rate ofS. solfataricus. To reduce the growth inhibition caused by mineral ions, yeast extract was dialyzed against distilled water and then fed-batch cultures were carried out using a feed medium containing desalted yeast extract. Although the concentrations of mineral ions in yeast extract were significantly lowered by the dialysis procedure, fed-batch cultivation with desalted yeast extract was unsatisfactory. To examine whether low molecular weight solutes in yeast extract are crucial for cell growth, we investigated the effect of trehalose, a most abundant compatible solute in yeast extract, on the growth pattern. Cell densities were increased and the length of the lag phase was markedly shortened by the presence of trehalose, indicating that trehalose plays an important role in the growth ofS. solfataricus.     

Morphogenesis in callus tissue ofMedicago sativa: The role of ammonium ion in somatic embryogenesis

Exogenously supplied ammonium ion is critical to alfalfa morphogenesis in vitro. In alfalfa, the ability to induce the formation of either roots or somatic embryos provided an opportunity to examine the effects of ammonium ion on each pattern of morphogenesis. Somatic embryo formation required a minimum of 12.5 mM NH ₄ ⁺ in regeneration medium for optimal expression. Root formation was inhibited by NH ₄ ⁺ levels of 50 mM and above, and occurred in the absence of exogenous NH₄ ⁺. At high levels of NH ₄ ⁺ somatic embryos were formed from cells exposed to the root-inducing combination of hormones. This observation suggests that the growth regulators and exogenously supplied ammonium ion comprise an interactive system controlling the in vitro pattern of alfalfa morphogenesis.     

Variation inRhizobium leguminosarum response to short term application of NH4NO3 to nodulatedPisum sativum L.

Summary This study was conducted to determine the effect of short term application of NH₄NO₃ on nodule function and to determine whether the rhizobial isolate used was a significant factor in this effect. Pea plants were inoculated with 10 differentRhizobium leguminosarum isolates and grown for 3 weeks in N-free medium before addition of 0, 1, 2 or 5 mM NH₄NO₃ for 2 to 7 days. Acetylene reduction and leghemoglobin content decreased with increasing exposure time to NH₄NO₃ and with increasing concentration of NH₄NO₃. NH ₄ ⁺ and NO ₃ ⁻ depletion from the nutrient medium were assayed in plants exposed to 5 mM NH₄NO₃ and mean uptake rates were similar for each ion. There were significant differences among isolates in the rate of decrease of C₂H₂ reduction with increasing NH₄NO₃ concentration (C₂H₂ reduction responsiveness to NH₄NO₃) 4 and 7 days after addition of NH₄NO₃ but no differences after 2 days of exposure to NH₄NO₃. There were significant differences among isolates in NH ₄ ⁺ depletion from the nutrient medium but these differences were not correlated with the differences observed in C₂H₂ reduction. Ranking of the isolates for C₂H₂ reduction responsiveness to NH₄NO₃ applied to plants with nodules was different from that obtained when NH₄NO₃ was applied at seeding. Isolates with varying sensitivity to NH₄NO₃ may be useful tools for determining the mechanisms responsible for inhibition of symbiotic N₂ fixation by combined nitrogen. NRCC paper no. 25863.     

Intracellular measurement of ammonium in Chara corallina using ion-selective microelectrodes

The study of ammonium (NH₄ ⁺) transport across plant cell membranes requires accurate measurement of NH₄ ⁺ gradients across subcellular gradients. We have developed an ammonium-selective microelectrode based on the ionophore nonactin. This electrode can detect NH₄ ⁺ activities (a_NH4) in vivo in the millimolar range in the presence of cytosolic levels of potassium, the main interfering ion. The electrode was used to measure intracellular a_NH4 in internodal cells of the giant alga Chara corallina. Results from cells incubated in media supplemented with 1 mM NH₄ ⁺ produced two populations, with means of 7.3 and 30.8 mM, respectively. HPLC analysis of vacuolar sap suggests the higher population represents vacuolar impalements, and the lower population can thus be assumed to be cytosolic. These results suggest a four-fold accumulation of NH₄ ⁺ in the vacuolar compartment of Chara. This revised version was published online in June 2006 with corrections to the Cover Date.     

Analysis of feeding mechanism in a pitcher ofNepenthes hybrida

The process of digestion of captured feeds in a pitcher, an insect-trapping organ, ofNepenthes was studied. Changes in bacterial population, pH and NH₄ ⁺ concentrations in pitcher juice were examined. Strong activities of both acid- and alkaline phosphatase, phosphoamidase, esterase C4 and esterase C8 were found in the pitcher juice. Optimum pH of proteases in the juice and those secreted from bacteria showed pH 3.0 and pH 8.0–9.0, respectively. Twenty six strains of bacteria were isolated from 4 pitchers: 10 strains were gram positive, 16 strains were gram negative (10 strains had casein hydrolase activity). A proton excretion was induced by NH₄ ⁺ released from the added solutions, and accordingly, the pH of the solutions fell. As a simulation model of the digestion process of feeds in pitcher juice and polypeptone solution was added into the washed pitcher. A good correlation was found among the NH₄ ⁺ concentration, pH and bacterial cell titer.     

Production of Alcaligenes xylosoxydans EMS33 in a Bench-scale Fermenter

Summary Optimization of medium composition and pH for chitinase production by the Alcaligenes xylosoxydans mutant EMS33 was carried out in the present study and the optimized medium composition and conditions were evaluated in a fermenter. The medium components screened initially using Plackett–Burman design were (NH₄)₂SO₄, MgSO₄ 7H₂O, KH₂PO₄, yeast extract, Tween 20 and chitin in shake flask experiments. The significant medium components identified by the Plackett–Burman method were MgSO₄ 7H₂O, Tween 20 and chitin. Central composite response surface methodology was applied to further optimize chitinase production. The optimized values of MgSO₄ 7H₂O, Tween 20, chitin and pH were found to be 0.6 g/l, 0.05 g/l, 11.5 g/l and 8.0, respectively. Chitinase and biomass production of Alcaligenes xylosoxydans EMS33, was studied in a 2-l fermenter containing (g/l): chitin, 11.5; yeast extract, 0.5; (NH₄)₂SO₄, 1; MgSO₄ 7H₂O, 0.6; KH₂PO₄, 1.36 and Tween 20, 0.05. The highest chitinase production was 54 units/ml at 60 h and pH 8.0 when the dissolved O₂ concentration was 60%, whereas the highest biomass production was achieved at 36 h and pH 7.5 without any dissolved O₂ control.     

Development of a multivariable auxostat

Summary Auxostat operation was extended to two set points in an aerobic yeast fermentation of Saccharomyces cerevisiae. The concentration of NH₄ ⁺ was controlled directly with an on-line NH₃ probe and control of pH indirectly fixed the glucose concentration, thus demonstrating an NH₄ ⁺:pH-auxostat. Multivariable control in the auxostat was superior to a chemostat for studying nutrient interaction and exploring multisubstrate limitations on metabolism.     

NH4 + transport system of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1

NH₄ ⁺ transport system of a psychrophilic marine bacterium Vibrio sp. strain ABE-1 (Vibrio ABE-1) was examined by measuring the uptake of [¹⁴C]methylammonium ion (¹⁴CH₃NH^{3 +}) into the intact cells. ¹⁴CH₃NH₃ ⁺ uptake was detected in cells grown in medium containing glutamate as the sole nitrogen source, but not in those grown in medium containing NH₄Cl instead of glutamate. Vibrio ABE-1 did not utilize CH₃NH₃ ⁺ as a carbon or nitrogen source. NH₄Cl and nonradiolabeled CH₃NH₃ ⁺ completely inhibited ¹⁴CH₃NH₃ ⁺ uptake. These results indicate that ¹⁴CH₃NH₃ ⁺ uptake in this bacterium is mediated via an NH₄ ⁺ transport system and not by a specific carrier for CH₃NH₃ ⁺. The respiratory substrate succinate was required to drive ¹⁴CH₃NH₃ ⁺ uptake and the uptake was completely inhibited by KCN, indicating that the uptake was energy dependent. The electrochemical potentials of H⁺ and/or Na⁺ across membranes were suggested to be the driving forces for the transport system because the ionophores carbonylcyanide m-chlorophenylhydrazone and monensin strongly inhibited uptake activities at pH 6.5 and 8.5, respectively. Furthermore, KCl activated ¹⁴CH₃NH₃ ⁺ uptake. The ¹⁴CH₃NH₃ ⁺ uptake activity of Vibrio ABE-1 was markedly high at temperatures between 0° and 15°C, and the apparent K _m value for CH₃NH₃ ⁺ of the uptake did not change significantly over the temperature range from 0° to 25°C. Thus, the NH₄ ⁺ transport system of this bacterium was highly active at low temperatures. Received: August 1, 1998 / Accepted: October 8, 1998     

Optimization of submerged culture conditions for exopolysaccharide production in <Emphasis Type="Italic">Sarcodon aspratus</Emphasis> (Berk) S.lto TG-3

The effect of medium components (carbon, nitrogen, and mineral sources) and environmental factors (initial pH and temperature) for mycelial growth and exopolysaccharide (EPS) production in Sarcodon aspratus(Berk) S.lto TG-3 was investigated. The optimal temperature (25°C) and initial pH (5.0) for the EPS production in shake flask cultures of S. aspratus were determined using the two-dimensional contour plot. The most suitable carbon, nitrogen, and mineral sources for EPS production were glucose, yeast extract, CaCl₂ and KH₂PO₄, respectively. Notably, the EPS production was significantly enhanced by supplementation of calcium ion. Subsequently, the optimum concentration of glucose (30gl^–1), yeast extract (15gl^–1), CaCl₂ (1.1gl^–1), and KH₂PO₄ (1.2gl^–1) were determined using the orthogonal matrix method. The effects of nutritional requirement on the mycelial growth of S.aspratuswere in regular sequence of glucose>KH₂PO₄>yeast extract>CaCl₂, and those on EPS production were in the order of glucose>yeast extract>CaCl₂>KH₂PO₄. Under the optimal culture conditions, the maximum EPS concentration in a 5-l stirred-tank reactor was 2.68gl^–1 after 4days of fermentation, which was 6-fold higher than that at a basal medium. The two-dimensional contour plot and orthogonal matrix method allowed us to find the relationship between environmental factors and nutritional requirement by determining optimal operating conditions for maximum EPS production in S.asparatus. The statistical experiments used in this work can be useful strategies for optimization of submerged culture processes for other mushrooms.     

Extracellular and intracellular chemical environments in relation to embryogenesisin vitro

Summary Tissue culture isolated from the root ofDaucus carota readily produced embryos when they were grown on a medium (M_s) containing relatively large amounts of NH₄ ⁺ and nitrate. Embryos were never formed on a second medium (M_w) which contained only nitrate in a low concentration. The cells of M_s-cultures always had a high level of NH₄ ⁺ while it was scarcely detected in M_w-cultures. Addition of either nitrate or glutamate to M_w resulted in the formation of embryos and also in the accumulation of a detectable amount of NH₄ ⁺ in the cells of the tissues. Although the occurrence of NH₄ ⁺ in the medium is not necessary for embryo formationin vitro it appears that a certain level of intracellular NH₄ ⁺ is a prerequisite for this process. Since there is a positive correlation between embryo formation and the contents in the cultures of both soluble and insoluble organic nitrogen, it is probable that NH₄ ⁺ is important for embryo formation because it is an essential substrate for the synthesis of organic nitrogen compounds such as amino acids and proteins. The ability to synthesize organic nitrogen from NH₄ ⁺ decreased conspicuously in the tissues which were cultured on M_s for long periods of time and had lost the capacity to form embryos. A certain level of K⁺ in culture media is necessary for optimal production of embryos on media with NH₄ ⁺, while Na⁺ has no effect on embryo formation.The authors are indebted to Mr. E.Mahn, Miss S.Semenoff, and Mrs. E.Bock for their technical assistances.     

The effect of urea on growth of moulds and biomass yield in solid-state cultivation

Urea, added at 2 to 20 mg/g in solid bran medium supporting growth of Aspergillus oryzae and Rhizopus oligosporus, led to a higher concentration of NH₄ ⁺ and pH but a decrease in biomass production.The authors are with the Department of Food Science and Technology, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 61000 Ljubljana, Slovenia.     

Effects of N source on pH and nutrient exchange of extramatrical mycelium in a mycorrhizal Ri T-DNA transformed root system

 The influence of different N sources on medium pH variation and the effect of the external mycelium of arbuscular mycorrhizal fungi on nutrient dynamics were studied using a two-compartment, aseptic Petri plate system. VA mycorrhizal, transformed roots of carrot (Daucus carota L.) were cultured in the proximal compartment and external mycorrhizal mycelium in the distal compartment. The medium in the distal compartament contained N either as NO₃ ^– or as NH₄ ⁺. The pH and the anion and cation concentrations were measured every 15 days in filtrates prepared from the distal compartments. Thirteen weeks after colonization, there was a significant basification or a light acidification of the NO₃ ^– and NH₄ ⁺ medium, respectively. There was no change in NO₃ ^– concentration but a significant decrease in NH₄ ⁺ concentration. Treatments containing N as NO₃ ^– showed no variation in cations such asCa²⁺ and Mg²⁺ or anions such as PO₄ ^2–, and SO₄ ^2– but showed significant increases in the concentration of K⁺. Treatments containing N as NH₄ ⁺ showed no variation in cations or anions, except for increases in the concentrations of K⁺ and Cl^–. Accepted: 7 March 1996