Absence Of NH Research Articles

High-affinity potassium transport in barley roots. Ammonium-sensitive and -insensitive pathways.

In an attempt to understand the process mediating K(+) transport into roots, we examined the contribution of the NH(4)(+)-sensitive and NH(4)(+)-insensitive components of Rb(+) transport to the uptake of Rb(+) in barley (Hordeum vulgare L.) plants grown in different ionic environments. We found that at low external Rb(+) concentrations, an NH(4)(+)-sensitive component dominates Rb(+) uptake in plants grown in the absence of NH(4)(+), while Rb(+) uptake preferentially occurs through an NH(4)(+)-insensitive pathway in plants grown at high external NH(4)(+) concentrations. A comparison of the Rb(+)-uptake properties observed in roots with those found in heterologous studies with yeast cells indicated that the recently cloned HvHAK1 K(+) transporter may provide a major route for the NH(4)(+)-sensitive component. HvHAK1 failed to complement the growth of a yeast strain defective in NH(4)(+) transport, suggesting that it could not act as an NH(4)(+) transporter. Heterologous studies also showed that the HKT1 K(+)/Na(+)-cotransporter may act as a pathway for high-affinity Rb(+) transport sensitive to NH(4)(+). However, we found no evidence of an enhancement of Rb(+) uptake into roots due to Na(+) addition. The possible identity of the systems contributing to the NH(4)(+)-insensitive component in barley plants is discussed.

Investigations of ion absorption during NH4+ exposure I. Relationship between H+ efflux and NO3- absorption

Root NO 3 - , absorption was examined under steady-state conditions in the presence and absence of NH 4 + using intact tomato plants (Lycopersicon esculentum cv. T-5). Plants grown under a low-salt regime showed much higher rates of NO 3 - , absorption than plants grown under a high-salt regime, but the presence of NH 4 + at concentrations less than 200 μM increased the capacity for net NO 3 - , uptake for both the low- and high-salt conditions. Simultaneous changes in net NO 3 - , K + , and H + exchanges were continuously monitored for 3 h prior to and up to 7 h following exposure to NH 4 + . Upon first exposure to 50 or 100 μM NH 4 + , NO 3 - , absorption remained constant; but during the subsequent 6 to 7h, NO 3 - , absorption continually increased. Net K + absorption decreased immediately following its first exposure to NH 4 + , but gradually recovered during the 7 h following first exposure. Changes in K + absorption were not correlated with changes in NO 3 - absorption. Proton efflux gradually increased under NH 4 + exposure and was significantly correlated with the observed increase in NO 3 - absorption. When roots absorbing NO 3 - were exposed to 5000 μM NH 4 + , NO 3 - absorption declined throughout the entire observation period.

Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings

The effect of exogenous NH4+ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8‐day‐old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3+ in the presence of 0, 1 or 10 mM NH4+, Exogenous NH4+ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO3+, In contrast, the induction of NiRA was inhibited by NH4+ at all NO3+ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH4+ Pre‐treatment with NH4+ had no effect on the subsequent induction of NRA in the absence of additional NH4+ whereas the induction of NiRA in NH4+‐pretreated roots was inhibited in the absence of NH4+ At 10 mM NO3+ L‐methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L‐methionine sulfoximine irrespective of NH4+ supply. During the postinduction phase, exogenous NH4+ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH3+ whereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NRA in roots by limiting the availability of NO3+. Conversely, it has a direct effect, independent of the availability of NO3+, on the induction of NiRA. The lack of an NH4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.

Influence de la pression partielle de NH 3 sur l'activité d'un catalyseur commercial d'hydrocraquage en hydroconversion du n-heptane

The hydroconversion of n-heptane has been studied either in the presence or absence of NH 3 on a hydrocracking catalyst prealably deactivated by coke in an industrial unit. When NH 3 is added, a loss of activity is observed (deactivation) but is totally reversible (inhibition). This loss is all the more important as long as the NH 3 partial pressure increases. An equation linking the hydroconversion rate of n-heptane and the pressure of ammonia has been elaborated. It is based on a classical bifunctional mechanism assuming that the limiting step is the isomerisation of intermediate n-heptanic carbenium ions. In industrial units, this equation could be used to ajust the operating conditions each time the feed is changed.

Metabolism of Melamine by Klebsiella terragena

Experiments were conducted to determine the pathway of melamine metabolism by Klebsiella terragena (strain DRS-1) and the effect of added NH(inf4)(sup+) on the rates and extent of melamine metabolism. In the absence of added NH(inf4)(sup+), 1 mM melamine was metabolized concomitantly with growth. Ammeline, ammelide, cyanuric acid, and NH(inf4)(sup+) accumulated transiently in the culture medium to maximal concentrations of 0.012 mM, 0.39 mM, trace levels, and 0.61 mM, respectively. In separate incubations, in which cells were grown on either ammeline or ammelide (in the absence of NH(inf4)(sup+)), ammeline was metabolized without a lag while ammelide metabolism was observed only after 3 h. In the presence of 6 mM added NH(inf4)(sup+) (enriched with 5% (sup15)N), ammeline, ammelide, and cyanuric acid accumulated transiently to maximal concentrations of 0.002 mM, 0.47 mM, and trace levels, respectively, indicating that the added NH(inf4)(sup+) had little effect on the relative rates of triazine metabolism. These data suggest that the primary mode of melamine metabolism by K. terragena is hydrolytic, resulting in successive deaminations of the triazine ring. Use of (sup15)N-enriched NH(inf4)(sup+) allowed estimates of rates of triazine-N mineralization and assimilation of NH(inf4)(sup+)-N versus triazine-N into biomass. A decrease in the percent (sup15)N in the external NH(inf4)(sup+) pool, in conjunction with the accumulation of ammelide and/or triazine-derived NH(inf4)(sup+) in the culture medium, suggests that the initial reactions in the melamine metabolic pathway may occur outside the cytoplasmic membrane.

K +/NH 4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla

The mechanism of NH 4 + transport in inner medulla is not known. The purpose of these experiments was to study the process that is involved in ammonium (NH 4 +) transport in cultured inner medullary collecting duct (mIMCD-3) cells. Cells grown on coverslips were exposed to NH 4 + and monitored for pH i changes by the use of the pH-sensitive dye BCECF. The rate of cell acidification following the initial cell alkalinization was measured as an index of NH 4 + transport. The rate of NH 4 + transport was the same in the presence or absence of sodium in the media (0.052±0.003 vs 0.048±0.004 pH/min, P>0.05), indicating that NH 4 + entry into the cells was independent of sodium. The presence of ouabain, bumetanide, amiloride, barium, or 4,4′-di-isothiocyanostilbene-2-2′-disulfonic acid (DIDS) did not block the NH 4 +-induced cell acidification, indicating lack of involvement of Na +:K +-ATPase, Na +:K +:2 Cl − transport, Na +:H + exchange, K + channel, or Cl −/base exchange, respectively, in NH 4 + transport. The NH 4 +-induced cell acidification was significantly inhibited in the presence of high external [K +] as compared to low external [K +] (0.018±0.001 vs. 0.049±0.003 pH/min for 140 mM K + vs. 1.8 mM K + in the media, respectively, P<0.001). Inducing K + efflux by imposing an outward K + gradient caused intracellular acidification by ∼0.3 pH unit in the presence but not the absence of NH 4 +. This K + efflux-induced NH 4 + entry increased by extracellular NH 4 + in a saturable manner with a Km of ∼5 mM, blocked by increasing extracellular K + and was not inhibited by barium. The K + efflux-coupled NH 4 + entry was electroneutral as monitored by the use of cell membrane potential probe 3,3′-dipropylthiadicarbocyanine. These results are consistent with the exchange of internal K + with external NH 4 + in a 1:1 ratio. The K +-NH 4 + antiporter was inhibited by verapamil and Schering 28080 in a dose-dependent manner, was able to work in reverse mode, and did not show any affinity for H + as a substrate, indicating that it is distinct from other NH 4 +-carrying transporters. We conclude that a unique transporter, a potassium-ammonium (K +/NH 4 +) antiport, is responsible for NH 4 + transport in renal inner medullary collecting duct cells. This antiporter is sensitive to verapamil and Schering 28080, is electroneutral, and is selective for NH 4 + and K + as substrates. The K +/NH 4 + antiporter may play a significant role in acid-base regulation by excretion of ammonium and elimination of acid.

Regulation of the expression of ferredoxin-nitrite reductase in synchronous cultures of Chlamydomonas reinhardtii

The regulation of ferredoxin-nitrite reductase — the second enzyme involved in the nitrate assimilatory pathway — in synchronous cultures of C. reinhardtii has been studied both at the activity and protein levels using specific antibodies. During a cycle of 12 h light/12 h dark (12L:12D), ferredoxin-nitrite reductase activity shows a 24-h fluctuation with a maximum in the middle of the light period. The increase of activity during the first few hours of the light phase is due to de novo synthesis of the enzyme. This synthesis occurs in the absence of NH 4 + and it is highly induced by either nitrate or nitrite, but it does not require light so long as carbon skeleton s are available. The decrease of ferredoxin-nitrite reductase activity during the last hours of the light period and during the dark phase is suggested to be due to protein degradation, although this process is slow because of the high stability of the enzyme. The changes in the level of ferredoxin-nitrite reductase seem to be related to events in the cell cycle under the illumination conditions used. Thus, synthesis of the enzyme correlates to growth periods within the cell cycle, and it does not seem to be under the control of a circadian rhythm.

New VLA Observations of NH 3 in S140

The S140 molecular cloud was observed in the NH<SUB>3</SUB> (1, 1) and (2, 2) lines with 5" resolution. Compared with the previous VLA observations, the new observations have 3 times better flux sensitivity, 4 times better spatial resolution, and 2.5 times better velocity resolution. These advantages allowed the detection of 80%-85% of the single-dish flux in the two NH<SUB>3</SUB> lines. The NH<SUB>3</SUB>emission region consists of a long filament perpendicular to the optical bright rim and an arclike structure along the edge of the bright rim. Within 15" of IRS 1, there is an absence of NH<SUB>3</SUB>emission which is probably caused by a true decrease in NH<SUB>3</SUB> column density, not by missing short spacings or changes in the partition function due to a temperature increase. Comparison with a multitransition analysis of CS observations shows that this low NH<SUB>3</SUB> column density indicates a low NH<SUB>3</SUB> abundance in the dense gas near the infrared sources rather than an absence of molecular gas. The gas temperature is derived from the NH<SUB>3</SUB> data and can be accounted for mostly by heating from IRS 1-IRS 3. We also detected a compact continuum source at 2.7 mm with the Owens Valley Millimeter Interferometer. This continuum source coincides with the peak of low-J CS emission and an NH<SUB>3</SUB> emission peak. The NH<SUB>3</SUB> lines at this position are unusually wide, and the profiles are consistent with models of gravitational collapse.

DRIFT study of the oxidation and the ammoxidation of toluene over a TiO 2(B)-supported vanadia catalyst

A DRIFT study of the adsorption and the (amm) oxidation of toluene was carried out at 150–300°C on a TiO 2(B)-supported vanadia catalyst with a loading corresponding to 1.9 Mg V 2O 5 per m 2 of the support surface area. In the absence of NH 3 and gaseous oxygen, the adsorption of toluene at 150°C produces benzyl and benzoate intermediates. After one hour at the same temperature, further conversion of benzyl species, probably benzyloxy, into benzoate species is observed. Upon introduction of NH 3 into the reaction cell, some benzonitrile is formed together with intermediates which possibly are (C 6H 5)CH(NH 2)O- and (C 6H 5)C(NH 2)(O-) 2 species. Subsequent heat treatment at 300°C causes an increase in the amounts of benzonitrile and CO 2 formed and the simultaneous disappearance of the precursor intermediates. On the basis of identified intermediates and observed transformations, a mechanism for the partial (amm)oxidation of toluene is proposed. The formation of CO 2 is believed to pass over intermediates, which are identified as benzoquinone and butenolide species.

Sulfur dioxide oxidation in laboratory clouds

SO 2 oxidation in the presence of NH 3 was studied in a mixing-type continuous-flow cloud chamber. NaCl and soot particles (∼5–15 μ m −3) were used as cloud condensation nuclei. Cloud liquid water content was varied between 0.2 and 3 g m −3. SO 2 and NH 3 concentrations were 0.6 and 1.1 ppm, respectively. The contact time between the SO 2 and the cloud drops was varied from 8 s to 3 min. Up to 80% of the input SO 2 can be oxidized within short contact times in the presence of NH 3 and when the water is in the condensed cloud-drop phase. Negligible sulfate formation was observed in the absence of the liquid phase regardless of the presence or absence of NH 3. No significant dependence of the oxidation on the cloud condensation nuclei type nor the contact time was found. This in-cloud SO 2 oxidation is much faster than predicted by S(IV) oxidation by molecular oxygen measured in bulk solutions.

Effect of ammonium ion and extracellular pH on hybridoma cell metabolism and antibody production

The effects of NH(4)Cl addition on batch hybridoma cell growth at different external pH values (pH(e)) were investigated in a bioreactor at constant pH and dissolved oxygen concentration. In agreement with measurements in flasks, changes in pH(e) over the range 6.8-7.6 had minor effects on growth. Addition of 3 mM NH(4)Cl had little effect on cell growth while 10 mM NH(4)Cl caused a substantial growth inhibition, Measurements of the effects of pH(e) and NH(4)Cl concentration on cell metabolism gave similar results for cells grown in flasks in an incubator and in the bioreactor. As pH(e) decreases, the integral cell yield on glucose increases. There is a correlation between the effects of pH(e) on glycolysis and previous measurements of its effects on intracellular pH (pH(i)). Increases in NH(4)Cl concentration were previously determined to decrease pH(i) and are shown here to decrease the integral cell yield on glucose. At all pH(e) values in the absence of NH(4)Cl, glutamine is depleted at the time the maximum cell density is reached. Both pH(e) decreases and NH(4)Cl concentration increases lead to decreases in the integral cell yield on glutamine. Changes in pH(e) and in the NH(4)Cl concentration that cause growth inhibition have no effect on the specific antibody production rate for cells grown in flasks in an incubator or in the bioreactor. Changes in the NH(4)Cl concentration have no effect on the quality of the antibody produced, to a first level of characterization.

Ammonium-dependent transports of amino acids and glucose in a facultatively anaerobic alkalophile

It was found that the growth of a facultatively anaerobic alkalophile depends on the presence of NH + 4. The poor growth in the absence of NH + 4 did not seem to be ascribed to a lack of nitrogen sources. The bacterium was suggested to require specifically NH + 4 for the acceleration of transports of amino acids and glucose, which is likely to contribute to the optimum growth.

Hydrogenation reactions with RuCl 2(L-L) 2 (L-L = bidentate phosphine) Part II

The compounds with general formula RuCl 2(L-L) 2 (L-L bidentate phosphine, Ph 2P(CH 2) n PPh 2, n = 4, 5, 6) catalytically hydrogenate unsaturated organic substrates such as olefins, dienes, aldehydes and ketones under moderate reaction conditions in polar solvents, even in the absence of NH 4 + salts. In general the catalytic activity follows the order n = 4 > n = 6 > n = 5. Small percentages of isomerization products of the terminal olefin are observed, which increase with the value of n. With 1,5-cyclooctadiene, the hydrogenation to the monoene is faster than the second hydrogenation, and follows the order n = 4 > n = 5 > n = 6.

Comparison of the in vitro metabolisms and mutagenicities of dibenzo[a, c]anthracene, dibenzo[a, j]anthracene and dibenzo[a,j]anthracene: influence of norharman

The comparison of the behaviour of three dibenzoanthracene (DBA) isomers, dibenzo[a,c]anthracene (DB[a,c]A), dibenzo[a,h]anthracene (DB[a,h]A) and dibenzo[a,j]anthracene (DB[a,j]A), polycyclic aromatic hydrocarbons (PAHs), whose carcinogenicity varies from very potent to apparently inactive, has been carried out. Influence of norharman (NH; 9H-pyrido[3,4-b]indol) was investigated for mutagenicity (reversion of histidine prototrophy) on Salmonella typhimurium TA 100, using 3-methylcholanthrene (3-MC)-induced rat liver microsomes or S9 (post-mitochondrial fractions). A correlation with its influence, on the in vitro metabolism of radiolabelled molecules by the same enzymatic systems, was carried out. NH enhances the mutagenicities of DB[a,c]A and DB[a,h]A which are very well known mutagenic and carcinogenic PAHs. Contrary to its two isomers, the mutagenic potency of DB[a,j]A, which is considered as a weak mutagen and not a carcinogen, is strongly inhibited by NH. The balance sheets of the in vitro metabolism by microsomal enzymes, where the conjugation is excluded, were reported with or without NH. In the presence of the latter, the amounts of remaining DBAs slightly decreased while the metabolites covalently bound to microsomal proteins strongly decreased and the amount of hydrophobic metabolites highly increased. At the same time, the HPLC elution profiles of the metabolism pathways of the three DBAs are found to be modified in a similar way by NH: some of the metabolites are highly enhanced, and for all three DBAs, a tetraol, not detectable in the absence of NH, emerges. The results are discussed with regard to possible effects of NH.

A Two-Translocator Model for the Transport of 2-Oxoglutarate and Glutamate in Chloroplasts during Ammonia Assimilation in the Light

This study examines the transport of 2-oxoglutarate (2-OG) and other dicarboxylates during ammonia assimilation in illuminated spinach chloroplasts. The transport of all dicarboxylates examined was strongly inhibited by NH(4)Cl preincubation in the light. Treatment with NH(4)Cl caused a rapid depletion of the endogenous glutamate pool and a corresponding increase in endogenous glutamine content. The inhibition of transport activity by NH(4)Cl was apparently linked to its metabolism in the light because inhibition of glutamine synthetase activity by the addition of l-methionine sulfoximine or carbonylcyanide-m-chlorophenylhydrazone abolished this affect. Measurements of endogenous metabolite pools showed that malate was most rapidly exchanged during the uptake of all exogenous dicarboxylates examined. Depending on the exogenous substrates used, the apparent half-times of efflux measured for endogenous malate, aspartate and glutamate were 10, 10 to 30, and 15 to 240 seconds, respectively. The transport of 2-OG was also inhibited by malate. But chloroplasts preincubated with malate in the presence or absence of NH(4)Cl were found to have high transport activity similar to untreated chloroplasts. A two-translocator model is proposed to explain the stimulation of 2-OG transport as well as the stimulation of (NH(3), 2-OG)-dependent O(2) evolution by malate (KC Woo, CB Osmond 1982 Plant Physiol 69: 591-596) in isolated chloroplasts. In this model the transport of 2-OG on the 2-OG translocator and glutamate on the dicarboxylate translocator is coupled to malate counter-exchange in a cascade-like manner. This results in a net 2-OG/glutamate exchange with no net malate transport. Thus, during NH(3) assimilation the transport of 2-OG into and the export of glutamate out of the chloroplast occurs via the 2-OG and the dicarboxylate translocators, respectively.

Effect of carbohydrate source on alfalfa somatic embryogenesis

Cultures of alfalfa ( Medicago sativa L. clone RA-3) were screened for their response to a wide variety of carbohydrate sources in the presence or absence of sucrose. Maltose, maltotriose, and soluble starch all act to improve the morphology and eventual conversion to plantlets of somatic embryos. Glucose, sucrose, and other carbohydrates do not have a similar effect. Additional studies were carried out with maltose since this carbohydrate also gives the highest embryo yield. The concentration optimum for maltose is about 4% (w/v). Maltose acts independently of sucrose and in a way which suggests that it serves as a nutritional, rather than as an osmotic, effector of embryogenesis. The effect of maltose on embryogenesis is dependent on the presence of NH 4 + whose optimum is approximately 15 mM. Embryogenesis on maltose will not occur in the absence of NH 4 +. A highly effective regeneration medium can be achieved by including NH 4 + and amino acids, especially proline, in a maltose-containing regeneration medium. Maltose is also useful in increasing embryo formation in genotypes which show low regeneration.

The Influence of Ammonium and Chloride on Potassium and Nitrate Absorption by Barley Roots Depends on Time of Exposure and Cultivar

Net uptakes of K(+) and NO(3) (-) were monitored simultaneously and continuously for two barley (Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K(+) and NO(3) (-) uptake in the absence of NH(4) (+) or Cl(-). Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH(4) (+), K(+), Cl(-), or NO(3) (-) affected the cultivars very differently: (a) the presence of NH(4) (+) as NH(4)Cl stimulated net NO(3) (-) uptake in Prato barley but inhibited net NO(3) (-) uptake in Olli barley; (b) Cl(-) inhibited net NO(3) (-) uptake in Prato but had little effect in Olli; and (c) NH(4) (+) as (NH(4))(2)SO(4) inhibited net K(+) uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl(-) initially inhibited net K(+) uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH(4) (+) and Cl(-) uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption.

Inactivation of Serine:Glyoxylate and Glutamate:Glyoxylate Aminotransferases from Tobacco Leaves by Glyoxylate in the Presence of Ammonium Ion

Serine:glyoxylate and glutamate:glyoxylate aminotransferases (SGAT and GGAT), which catalyze the formation of glycine from glyoxylate during photorespiration, have been purified >300-fold from tobacco leaf extracts. Incubation with glyoxylate in the absence of amino acid substrate resulted in the time- and concentration-dependent inhibition of the partially purified enzymes. The second order rate constants for glyoxylate inhibition were 1.25 and 0.175 per millimolar per minute for SGAT and GGAT, respectively. The enzymes of highest specific activity were not inhibited by 5 millimolar glyoxylate alone but when 1 millimolar NH(4) (+) was added to 1 millimolar glyoxylate for 10 min in the absence of amino donor, SGAT was irreversibly inhibited more than 50%. GGAT was inhibited 50% in 10 minutes by 15 millimolar NH(4) (+) and 1 millimolar glyoxylate. By itself, NH(4) (+) was a reversible inhibitor; SGAT and GGAT were inhibited 50% at 6 millimolar and 50 millimolar, respectively. The irreversible inhibition occurred only with glyoxylate and NH(4) (+) added together; oxalate, formate, acetaldehyde, pyruvate, hydroxypyruvate, and alpha-ketoglutarate did not inhibit either in the presence or absence of NH(4) (+). Glyoxylate and NH(4) (+) could form a carbinolamine which is a serine analog and might bind irreversibly to the enzymes. Under conditions in vivo in which reassimilation of NH(4) (+) is reduced or blocked, the activity of SGAT may be inhibited and if glyoxylate is available, as in leaf peroxisomes, irreversible inhibition may occur.

Magnesium (Mg) and rubidium (Rb) absorption by tall fescue

Abstract Kinetic experiments were conducted to examine Mg+2 [28Mg] and K+ [86Rb] absorption by 2‐week‐old tall fescue (Lolium multiflorum Lam. X Festuca arundinacea Schreb.) seedlings. Magnesium absorption was linear for 180 min and was approximately 3.5 ueq Mg+2 g dry weight‐1.h‐1 from a 0.0625 mM Mg+2 solution. Shoot accumulation of Mg was reduced by increased solution K+. A curvilinear concentration absorption isotherm for roots was obtained for Mg+2. The resultant hyperbolic Hofstee plot could be separated into two linear components with Km values of 0.833 and 0.0117 mM, suggesting that two systems are operable in Mg+2 absorption. Increased solution pH increased Mg+2 absorption both in the presence and absence of NH+ 4. At pH 6.5 NH+ 4 increased Mg+2 absorption 75%; at pH 3.5 the increase was more than 6‐fold. Potassium absorption was linear through 60 min and was decreased by increased Mg2+ levels. Shoot accumulation of [86Rb] was decreased by the first increment of Mg+2, whereas further increases in...

Relationships Between Proton Extrusion and Fluxes of Ammonium Ions and Organic Acids in Penicillium cyclopium

Acidification observed in cultures of Penicillium cyclopium was mostly due to the extrusion of protons into the medium. In the absence of NH+ 4 protons were extruded together with citrate. The citric acid thus produced underwent a dynamic equilibrium of continuous efflux and uptake, which was shifted towards the latter at decreasing glucose concentrations. In cultures supplied with NH+ 4 and glucose a stoichiometric coupling of H+ excretion with NH+ 4 uptake was observed. If the protons excreted were not absorbed by external buffers (e.g. tartrate ions), the pH of the medium dropped to below 2, thereby exceeding the capacity of the cells to maintain their internal pH. The decrease of intracellular pH was accompanied by the cessation of NH+ 4 uptake, H+ extrusion and growth. In the presence of tartrate ions the NH+ 4/H+ exchange proceeded until the glucose concentration of the medium dropped below 25 mm. Here, the external pH was kept above 3, without seriously affecting the intracellular pH. After termination of H+ extrusion an appreciable amount of NH+ 4 was taken up together with hydrogentartrate anions, thereby causing a further acidification of the medium, which was finally reversed due to the uptake of tartaric acid in the late phase of growth.