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1.
The acidophilic alga Dunaliella acidophila exhibits optimalgrowth at pH 1. We have investigated the regulation of phosphateuptake by this alga using tracer techniques and by performingintracellular phosphate measurements under different growthconditions including phosphate limitation. In batch culturewith 2·2 mol m3 phosphate in the medium the uptakeof phosphate at micromolar phosphate concentrations followeda linear time dependence in the range of minutes and rates werein the range of 1 µmol phosphate mg1 chl h1,only. However, under discontinuous phosphate-limited growthconditions, tracer influx revealed a biphasic pattern at micromolarphosphate concentrations: An initial burst phase resulted ina 104-fold internal phosphate accumulation and levelled offafter about 10 s. A double reciprocal plot of the initial influxrates obtained for phosphate-limited and unlimited algae exhibitedMichaelis-Menten kinetics. Phosphate limitation caused a significantactivation of the maximum velocity of uptake, yielding Vmaxup to 1 mmol mg1 chl h1 as compared to valuesin the order of 50 µmol phosphate mg1 chl h1for the second phase (this magnitude is also representativefor non-limited batch cultures). Concomitantly the Michaelisconstant was altered from 4 mmol m3 to 0·7 mmolm3. The rapid uptake of phosphate was inhibited by arsenateand FCCP and was not stimulated by Na+. The pH dependence oftracer accumulation and measurements of the intracellular phosphatepool under different growth conditions indicate that at lowpH and low external phosphate concentrations the high protongradient present under these conditions is utilized for a H3PO4uptake or a H+/H2PO4 cotransport. However, when the externalphosphate concentration was increased to levels sufficientlyhigh for transport to be driven by the positive membrane potential(10 mol m3 phosphate), the pH dependence of phosphateuptake was more complex, but could be explained by the uptakeof H3PO4 or a H+/H2PO4-cotransport at low pH and a differenttype H2PO4-transport (with unknown type of ion coupling)at high pH-values. It is suggested that this flexible couplingof phosphate transport is of essential importance for the acidresistance of Dunaliella acidophila. Key words: Acid resistance, Dunaliella acidophila, phosphate cotransport, phosphate limitation, plasma membrane, sodium 相似文献
2.
Tobacco shoots were grown in vitro for 35 d, in MS culture mediummodified to include various sources (nitrate-N, ammonium-N ora mixture) and levels (0120 mM) of N, and in the presenceof 0180 mM NaCI or iso-osmotic concentrations of mannitol.Growth of control plantlets was significantly inhibited whenNH4+-N was the sole N source, and at high (120 mM) NO3-N supply. Under conditions of salt stress (90 and 180 mM NaCI)growth was repressed, with roots being more severely affectedthan shoots. Salinity also inhibited root emergence in vitro.The only alleviation of the salt stress by nitrate nutritionobserved in this study was on shoot growth parameters of plantletsgrown on 60 mM NO3-N and 90 mM NaCI. Although both weresignificantly inhibited by NaCI, nitrate reduc-tase activitywas more severely affected than nitrate uptake. When mannitolreplaced NaCI in the culture medium, similar Inhibition of growth,nutrient uptake and enzyme activity were recorded. These observations,together with the relatively low recorded values for Na+ andCI uptake, indicate that under in vitro salt stress conditionsthe negative effects of NaCI are primarily osmotic. Key words: Growth, nitrogen metabolism, osmotic stress, salinity 相似文献
3.
Ritchie Raymond J.; Trautman Donelle A.; Larkum A.W.D. 《Plant & cell physiology》1997,38(11):1232-1241
Phosphate uptake rates in Synechococcus R-2 in BG-11 media (anitrate-based medium, not phosphate limited) were measured usingcells grown semi-continuously and in continuous culture. Netuptake of phosphate is proportional to external concentration.Growing cells at pHo 10 have a net uptake rate of about 600pmol m2 s1 phosphate, but the isotopic flux for32P phosphate was about 4 nmol m2 s1. There appearsto be a constitutive over-capacity for phosphate uptake. TheKm and Vmax, of the saturable component were not significantlydifferent at pHo 7.5 and 10, hence the transport system probablyrecognizes both H2PO4and HPO24. The intracellularinorganic phosphate concentration is about 3 to 10 mol m3,but there is an intracellular polyphosphate store of about 400mol m3. Intracellular inorganic phosphate is 25 to 50kJ mol1 from electrochemical equilibrium in both thelight and dark and at pHo 7.5 and 10. Phosphate uptake is veryslow in the dark ( 100 pmol m2 s1) and is light-activated(pHo 7.51.3 nmol m2 s1, pHo 10600 pmol m2s1). Uptake has an irreversible requirement for Mg2+in the medium. Uptake in the light is strongly Na+-dependent.Phosphate uptake was negatively electrogenic (net negative chargetaken up when transporting phosphate) at pHo 7.5, but positivelyelectrogenic at pHo 10. This seems to exclude a sodium motiveforce driven mechanism. An ATP-driven phosphate uptake mechanismneeds to have a stoichiometry of one phosphate taken up perATP (1 PO4 in/ATP) to be thermodynamically possible under allthe conditions tested in the present study. (Received June 16, 1997; Accepted September 4, 1997) 相似文献
4.
Tuberized tap roots of Witloof chicory (Cichorium intybus L.)were forced by placing in a dark chamber in a hydroponic systemunder high RH to produce an etiolated bud, the chicon. Plantswere fed nutrient solutions with two NO3concentrations of 1·5or 18 mol m3 NO3, or demineralized water. The nutrientsolutions were labelled with 2% atom excess 15N. Although thechicon biomass increased with increasing NO3 concentration inthe nutrient solution, the chicon dry weight remained unchanged.The increased chicon biomass was, therefore, due to more waterin the chicon. The N in the chicon originated from either anendogenous source, the root, and/or an exogenous source, thenutrient solution. Organic N reserves remobilization and transferto the chicon were not been affected by NO3 supply. At the endof the forcing period 75% of the root N had been remobilized.Differences in the amount of N in the chicons of the three treatmentswere due to the uptake of exogenous N. The flux of exogenousnitrogen to the chicon in high NO3-plants was 2- to 6-fold higherthan in the low NO3-plants and, at the end of the forcing period,exogenous nitrogen contributed 30% of total chicon N in highNO3-plants and 10% in low NO3-plants. Net uptake of NO3 by chicory plants during the forcing processwas a function of N influx and N efflux. The increase in N influxwas accompanied by an increase in exogenous N flux to the chiconand probably a shift in root and/or chicon osmotic potentialwhich increased water flux to the chicon. Since NO3 did notaccumulate in either the chicon or the root, it is proposedthat osmotic solutes, such as organic acids and amino acidsmay be involved in osmotic potential changes in chicory duringthe forcing process. Key words: Cichorium intybus L., efflux, influx, nitrogen (15NO3) nutrition, remobilization 相似文献
5.
Anabaena PCC 7119 showed higher rates of phosphate uptake whencells were under P-starvation. Phosphate uptake was energy-dependentas indicated the decrease observed when assays were performedin the dark or in the presence of inhibitors of photosyntheticelectron transport, energy transfer and adenosine triphosphataseactivity. Phosphate uptake was stimulated by Na+ both in P-sufficientcells and P-starved cells. Li+ and K+ acted as partial analoguesfor Na+. The Na+-stimulation of phosphate uptake followed Michaelis-Mentenkinetics, half-saturation (K) of phosphate uptake was reachedwith a Na+ concentration of 212 µM. The absence of Na+reduced the rates of phosphate uptake at all phosphate concentrationsassayed (120 µM). The maximum uptake rates (Vmax)decreased from 658 nmol P (mg dry wt)-1 h-1 in the presenceof Na+ to 149 nmol P (mg dry wt)-1 h-1 in the absence of Na+.The absence of Na+ did not change significantly the concentrationof phosphate required to reach half-saturation (K) (3.01 µMin the presence of Na+ vs 3.21 µM in the absence of Na+).In the presence of Na+ the rate of phosphate uptake was affectedby the pH; optimal rates were observed at pH 8. In the absenceof Na+ phosphate uptake was not affected by the pH; low rateswere observed in all cases. Monensin, an ionophore which collapsesNa+-gradients, reduced the rate of phosphate uptake in Na+-supplementedcells. These results indicated the existence of a Na+-dependentphosphate uptake in Anabaena PCC 7119. (Received September 8, 1992; Accepted November 17, 1992) 相似文献
6.
Hydraulic and osmotic properties of spruce roots 总被引:9,自引:6,他引:3
Rdinger Marcus; Hallgren Stephen W.; Steudle Ernst; Schulze Ernst-Detlef 《Journal of experimental botany》1994,45(10):1413-1425
Hydraulic and osmotic properties of roots of 2-year-old Norwayspruce seedlings (Plcea abiea (L.) Karst) were investigatedusing different techniques (steady flow, pressure probe, andstop flow technique). Root pressures were measured using theroot pressure probe. Compared to roots of herbaceous plantsor deciduous trees, excised root systems of spruce did not developappreciable root pressure (-0.001 to 0.004 MPa or -10 to 40cm of water column). When hydrostatic pressure gradients wereused to drive water flows across the roots, hydraulic conductivities(Lpr) were determined in two types of experiments: (i) rootpressure relaxations (using the root pressure probe) and (ii)steady flow experiments (pneumatic pressures applied to theroot system or xylem or partial vacuum applied to the xylem).Root Lpr ranged between 0.2 and 8108m s1 MPa1(on average) depending on the conditions. In steady flow experiments,Lpr depended on the pressure applied (or on the flow acrossthe roots) and equalled (0.190.12) to (1.21.7)108m s1 MPa1 at pressures between 0.2 and 0.4 MPaand (1.51.3)108 m s1 MPa1 at appliedpressures between 0.8 and 1.0 MPa. When pressures or vacuumwere applied to the xylem, Lpr values were similar. The hydraulicconductivity measured during pressure relaxations (transientwater flows) was similar to that obtained at high pressures(and water flows). Although there was a considerable scatterin the data, there was a tendency of the hydraulic conductivityof the roots to decrease with increasing size of the root system.When osmotic gradients were used to drive water flows, Lpr valuesobtained with the root pressure probe were much smaller thanthose measured in the presence of hydrostatic gradients. Onaverage, a root Lpr=0.017108 was found for osmotic andLpr=6.4108 m s1 MPa1 in correspondinghydrostatic experiments, i.e. the two values differed by a factorwhich was as large as 380. The same hydraulic conductivity asthat obtained in osmotic experiments using the pressure probewas obtained by the 'stop flow techniquel. In this technique,the suction created by an osmoticum applied to the root wasbalanced by a vacuum applied to the xylem. Lpr values of rootsystems did not change significantly when measured for up to5 d. In osmotic experiments with different solutes (Na2S04,K2S04, Ca(NO3)2, mannitol), no passive uptake of solutes couldbe detected, i.e. the solute permeability was very low whichwas different from earlier findings on roots of herbs. Reflectioncoefficients of spruce roots (O were low for solutes for whichplant cell membranes exhibit values of virtually unity ( 相似文献
7.
Ludwig-Mller Jutta; Schubert Birgit; Pieper Kerstin 《Journal of experimental botany》1995,46(4):423-432
The rate of indole-3-butyric acid (IBA) synthesis in maize seedlingsis dependent on the culture conditions of the plants. When theseedlings were grown on filter paper soaked with different amountsof water, the activity of IBA synthetase differed strongly.High amounts of water (150 and 200 ml per bowl) inhibited IBAsynthesis completely in vitro, whereas 30 and 50 ml water perbowl increased the activity dramatically. Under conditions whereIBA synthetase was inhibited (150 ml H2O), an increase of enzymeactivity was observed when abscisic acid (ABA) was exogenouslyadded in concentrations between 5104 to 5107M. Under drought conditions (50 ml H2O per bowl)the same ABA concentrations were inhibitory. Jasmonic acid andsalicylic acid also enhanced IBA synthetase activity to someextent, whereas indole-3-acetic acid (IAA) and kinetin had noeffect. Activity could also be enhanced by osmotic stress (NaCIand sorbitol), but not under temperature stress. In accompanyinginvestigations the endogenous contents of IAA, IBA, and ABAunder the different culture conditions have been determinedas well as the energy charge of the seedlings. Similar observationshave been made with Amaranthus, wheat and pea seedlings Key words: Abscisic acid, Amaranthus paniculatus, drought stress, inole-3-butyric acid biosynthesis, Pisum sativum, Triticum aestivum, Zea mays 相似文献
8.
Sodium efflux from 22Na+-loaded root tips root tips of Hordeumvulgare L. was markedly increased by replacing 10mM Na2SO4 inthe washing solution by K2SO4 with the same electrical conductivity.This increase was inhibited by both an uncoupler and an inhibitorof oxidative phosphorylation but not by ouabain. Potassium ionsdid not enhance Na+ efflux in the presence of a rapidly absorbedcounter anion, such as Cl, instead of . Efflux of 22Na+ could also be enhanced by a low pH in theabsence of K+; this was prevented by uncouplers, but not byan inhibitor of the mitochondrial ATPase. It seems that K+ indirectly enhances Na+ efflux. It is suggestedthat metabolic K+ uptake in excess of the counter anion resultsin a proton gradient across the plasmalemma (acid outside) inducingH+/Na+ antiport. 相似文献
9.
Excised roots obtained from maize plants, previously treatedfor 24 h with different solutes (K2SO4, Na2SO4, or mannitol)at decreasing osmotic potential values, showed clearly the persistenceof osmo-saline shock in their chloride transport and in somemetabolic processes. In particular, vacuolar accumulation ofchloride was much reduced when the osmotic potential in pretreatmenthad been low; this effect was greater with solutions of electrolytes.Protein synthesis and [14C]leucine uptake were also reduced;thus there appeared to be a correlation between chloride accumulationin the vacuole and protein synthesis. By contrast, accumulationof chloride in the cytoplasm was only slightly modified or evenstimulated. Chloride influx was affected less by osmotic pretreatmentthan accumulation, except in the case of very low osmotic potentialswhere influx was depressed. Chloride efflux was slightly enhancedby osmo-saline shocks and the fluxes at the tonoplast were lessaffected in comparison with those at the plasmalemma. Malateaccumulation was stimulated by shock, but this effect was notpersistent, so that a correlation with the effects on accumulationand fluxes was not possible. Oxygen uptake was affected onlyslightly except that at osmotic potential values of 750kPa, perhaps because of a lowered viability of the roots subjectedto very low osmotic potential for 24 h. 相似文献
10.
Relationships between nitrate (NO-3) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (1530%) with increased applied NO-3concentrationfrom 0 to 520 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH+4) or 5 mMNO-3but NH+4did not affect the rate of mobilization of seedreserves. Chloride (Cl-) was similar to NO-3in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO-3or Cl-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH+4did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO-3or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO-3did not affect total seedling water or residualseed water content until 1014 DAS. The effects of seedN and NO-3on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO-3is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization 相似文献
11.
Potassium-Ammonium Uptake Interactions in Tobacco Seedlings 总被引:6,自引:0,他引:6
SCHERER HEINRICH W.; MACKOWN CHARLES T.; LEGGETT J. EVERETT 《Journal of experimental botany》1984,35(7):1060-1070
Short-term (< 12 h) uptake experiments were conducted with67-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K+ andNH4+. At equal solution concentrations (0.5 mol m3) netK+ uptake was inhibited 3035% by NH4+ and NH4+ uptakewas decreased 924%. Removal of NH4+ resulted in completerecovery in K+ uptake rate, but NH4+ uptake rate did not recoverwhen K+ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m3. The relative effectof K+-NH4+ interactions was not altered when Cl- was replacedwith SO42, but the magnitudes of the uptake rates wereless in the absence of Cl-. The Vmax for NH4+ uptake was reducedfrom 128 to 105 µmol g1 dry wt. h1 in thepresence of 0.5 mol m3 K+ and the Km for NH4+ doubledfrom 12 to 27 mmol m3 in the presence of K+. The resultsof these K+-NH4+ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K+ coupled to NH4+influx via an antiporter cannot be ruled out as contributingto the decrease in net K+ uptake. Key words: Nicotiana tabacum, K+, NH4+, Uptake interactions 相似文献
12.
Wilson Richard H.; Dever John; Harper Walter; Fry Robert 《Plant & cell physiology》1972,13(6):1103-1111
The effects of valinomycin on the respiration and volume changeshave been studied with isolated mitochondria from bean hypocotyl(Phaseolus vulgaris L.) and cauliflower bud (Brassica oleraceaL.). In the presence of 10 mM K salts of chloride, acetate,phosphate, and sulfate respiration is stimulated by valinomycinconcomitant with osmotic swelling. When swelling declines respirationwith organic acid substrates also declines. In the presenceof the K salts of acetate and PO4 but not Cl the terminationof respiration leads to contraction. The contraction in K-PO4is inhibited by addition to the external medium of between 65to 100 mM K-PO4. The results are interpreted to suggest thatvalinomycin in the presence of KCl facilitated the movementof K down an electrical gradient, with the Cl anion followingand osmotic swelling resulting. However, in a medium containingacetate or PO4 the anions are actively transported against anelectrical gradient at the expense of metabolic energy. Valinomycinfacilitates the influx of K+ with the actively transported anionand swelling follows. When respiration terminates the activelytransported anions move passively back down their electrochemicalgradient and osmotic contraction follows.
1 Present address: Department of Biology, Fort Lewis College,Durango, Colorado 81301, U.S.A. (Received July 21, 1972; ) 相似文献
13.
Transfer of excised maize root from wet sawdust to water causeda considerable reduction in the exudation rate of the root.After 1-day aging in water, the exudation rate increased about8-fold and the exudation continued for 3 days. Osmotic pressureof the exudate from the root decreased with time after excisionreaching almost zero in 2 days in spite of a high exudationrate. Concentrations of sugars, acids, Ca2+ and Mg2+ in theexudate decreased with the decrease of osmotic pressure, whilethe decrease in K+ concentration delayed and P1 concentrationincreased. The gas content of the root, especially of O2, increased duringaging in water. The accumulated O2 gas may promote water uptake,because degasification of the root by evacuation induced a decreaseof water uptake. Also, the longitudinal gradient of the O2 contentin the root coincides with the gradient of water uptake intensity. (Received February 7, 1982; Accepted July 2, 1983) 相似文献
14.
Root growth of 7-d-old wheat (Triticum aestivum cv. Gamenya)seedlings was impaired at dissolved O2 concentrations of 0.01and 0.055 mol m3 O2, while growth at 0.115 mol m3O2 was the same as that in continuously aerated controls (0.26mol m3 O2). Oxygen uptake by apical (02 mm), expanding (24mm) and expanded (1012 mm) tissues of the roots decreasedbelow 0.16, 0.09 and 0.05 mol m3 O2, respectively. Thishierarchy is consistent with the metabolic rates of these tissues.There was a small (c. 9%) inhibition of O2 uptake and some netsynthesis of ethanol and alanine in root apices at 0.115 molm3 O2. Significant amounts of anaerobic end-productsaccumulated at 0.055 mol m3 O2 and even more so at 0.01mol m3 O2, indicating that oxidative phosphorylationwas strongly inhibited. Net alanine synthesis increased in fully expanded (1016mm) tissues exposed to <0.0030.01 mol m3 O2,and this increase was accompanied either by a proportionallysmaller increase in the concentration of other free amino acidsor by a net decrease in free amino acid levels excluding alanine.This suggests that alanine was synthesized as an end-productof anaerobic catabolism and did not accumulate simply becauseof decreased net protein synthesis. Comparing the carbon flow to CO2, ethanol, lactate and alaninein roots at 0.01 mol m3 O2 with carbon loss as CO2 inaerated roots suggests that carbon flow to products of metabolismwas not greatly enhanced due to O2 deficiency. This infers,but does not prove that, in wheat, generation of energy duringperiods of O2 deficiency is not enhanced due to a Pasteur effect. Key words: Anaerobic, fermentation, oxygen, wheat 相似文献
15.
Regulation of Sulphate Transport in a Tropical Legume, Macroptilium atropurpureum, cv. Siratro 总被引:2,自引:0,他引:2
CLARKSON DAVID T.; SMITH FRANK W.; BERG PETER J. VANDEN 《Journal of experimental botany》1983,34(11):1463-1483
When young plants of Macroptilium atropurpureum, cv. Siratrowere deprived of external sulphate (-S plants) growth of shootsand roots continued at rates comparable to those in plants wellsupplied with sulphate (control) for 3 d and 5 d respectively.Dilution of internal sulphur therefore took place and redistributionof sulphur occurred between inorganic and organic forms andbetween roots and younger leaves. Even when S-deficiency limitedgrowth, plants contained 16% of their total sulphur as sulphate,but most of this was retained in old leaves and redistributedslowly to growing zones. The capacity for sulphate uptake increased in roots of Splants very soon after they were deprived of external sulphate;within 24 h the absorption from 0.25 mol m3 SO42was more than five times that of control roots. Maximum increasedcapacity was reached after 23 d stress when the Vmaxof system 1 was 1948 nmol h1g1root fr. wt. inS plants and 337 nmol h1g1root fr. wt.in controls. The Kmfor system 1 did not change significantlywith S-stress being between 58 µM in both setsof plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growthrate and the rate at which sulphate uptake capacity was enhancedby withholding sulphate from culture solutions. When S plants were replaced in sulphate-containing solutiontheir capacity for SO42 declined to the control levelwithin 24 h. Very marked repression of capacity was also foundwhen S plants were treated with L-cysteine, but therewas no immediate effect with methionine. Roots of this species appear to have a very active system fordegrading L-cysteine to sulphate, 30% of the label in 35S-cysteineabsorbed by roots was recovered in 35SO42 after 20 minor 2 h incubation. By contrast, roots had a very weak abilityto reduce sulphate. When part of the root system was in solution lacking sulphatethere was enhanced uptake of sulphate by other parts which themselveswere amply supplied with sulphate. This is seen as an exampleof compensatory absorption. The response to S-stress is specific and there were no positiveinteractions between S-stress and the absorption of phosphate,or P-stress and the uptake of sulphate. The results are discussed in relation to the close control ofsulphate uptake by internal sulphate concentration, redistributionof forms of sulphur during stress and mobility of sulphate inthe phloem. Key words: Kinetics, Amino-S, Sulpholipid, Repression;, Deficiency 相似文献
16.
L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake 总被引:1,自引:0,他引:1
The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H+ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The Km values of thethree affinity sites were 1.4 x 103 M, 1.3 x 104M, 4.3 x 105 M; the maximum velocity values were 3.3x 108, 4.5 x 109, 1.8 x 109 mol/10 min/4x 106 cells. (Received April 18, 1981; Accepted August 25, 1981) 相似文献
17.
Sagebrush (Artemisia tridentata Nutt.) roots maintain nutrient uptake capacity under water stress 总被引:2,自引:0,他引:2
Phosphorus and nitrogen uptake capacities were assessed during3658 d drying cycles to determine whether the abilityof sagebrush (Artemisia tridentata Nutt.) to absorb these nutrientschanged as the roots were subjected to increasing levels ofwater stress. Water was withheld from mature plants in large(6 I) containers and the uptake capacity of excised roots insolution was determined as soil water potentials decreased from0.03 MPa to 5.0 MPa. Phosphorus uptake rates of excised roots at given substrateconcentrations increased as preharvest soil water potentialsdecreased to 5.0 MPa. Vmax and Km also increased as soilwater potentials declined. Declining soil water potentials depressednitrogen uptake at set substrate concentrations, but uptakecapacity, calculated as the sum Vmax for both NH+4+NO3,did not change significantly with drying. The sum Vmax correlatedwith root nitrogen concentration. Root uptake capacity for nitrogen and phosphorus was extremelystable under severe water stress in this aridland shrub. Maintenanceof uptake capacity, coupled with a previously demonstrated abilityto conduct hydraulic lift, may enable A. tridentata better tomaintain nitrogen and phosphorus uptake as soil water availabilitydeclines. These mechanisms may be important in the ability ofA. tridentata to maintain growth, complete reproduction, andgain an advantage against competitors late in the season whenthe soil layers with higher nutrient availability are dry. Key words: Kinetics, nitrogen, phosphorus, roots, water stress 相似文献
18.
Ito M Haito S Furumoto M Uehata Y Sakurai A Segawa H Tatsumi S Kuwahata M Miyamoto K 《American journal of physiology. Cell physiology》2007,292(1):C526-C534
During bone resorption, a large amount of inorganic phosphate (Pi) is generated within the osteoclast hemivacuole. The mechanisms involved in the disposal of this Pi are not clear. In the present study, we investigated the efflux of Pi from osteoclast-like cells. Pi efflux was activated by acidic conditions in osteoclast-like cells derived by the treatment of RAW264.7 cells with receptor activator of nuclear factor-B ligand. Acid-induced Pi influx was not observed in renal proximal tubule-like opossum kidney cells, osteoblast-like MC3T3-E1 cells, or untreated RAW264.7 cells. Furthermore, Pi efflux was stimulated by extracellular Pi and several Pi analogs [phosphonoformic acid (PFA), phosphonoacetic acid, arsenate, and pyrophosphate]. Pi efflux was time dependent, with 50% released into the medium after 10 min. The efflux of Pi was increased by various inhibitors that block Pi uptake, and extracellular Pi did not affect the transport of [14C]PFA into the osteoclast-like cells. Preloading of cells with Pi did not stimulate Pi efflux by PFA, indicating that the effect of Pi was not due to transstimulation of Pi transport. Pi uptake was also enhanced under acidic conditions. Agents that prevent increases in cytosolic free Ca2+ concentration, including acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, 2-aminoethoxydiphenyl borate, and bongkrekic acid, significantly inhibited Pi uptake in the osteoclast-like cells, suggesting that Pi uptake is regulated by Ca2+ signaling in the endoplasmic reticulum and mitochondria of osteoclast-like cells. These results suggest that osteoclast-like cells have a unique Pi uptake/efflux system and can prevent Pi accumulation within osteoclast hemivacuoles. phosphate transporter; RAW264.7; proton dependent; acidification 相似文献
19.
Investigations on the effects of low levels of Al on P adsorption,uptake and translocation in seedlings of the indigenous grassAgrostis capillaris were undertaken. Apparent uptake and transportof H2 32PO4 from nutrient solutions containing 10 or 100mmolm3 phosphate were characterized as functions of timeand concentration. Experiments on 32P uptake and transport insolutions containing no Al (control) or Al ranging from 3.7to 185 mmol m3 at pH ranging from 4.3 to 4.6, showedthat in 10 mmol m3 P, effects of Al at 3.7 and 37 mmolm3 on the size of the initial uptake shoulder were small,but some increase in subsequent P uptake to the roots was observed,though transport to the shoots was suppressed. With 37 mmolm3 Al in nutrient solution containing 100 mmol m3P, the uptake shoulder was much increased above the control.Subsequent root uptake was stimulated but transport was unaffected.Lack of toxicity of the Al concentrations used was indicatedby a lack of significant effect on plant fresh weight. AbsorbedAl was almost totally retained in the root in all treatments.Speciation calculations showed that the major species in Alamended nutrient solution at pH 4.4 were H2PO4, AI3+and AIHPO4+, together with substantial amounts of AISO4+ andsoluble aluminium hydroxy complexes (AIOH2+, AI(OH)2+), dependingon the relative concentrations of P and Al. The effects of Al,with 10 mmol m3 P, on adsorption of complexed P werepartly accounted for in terms of preferential cell wall adsorptionof Al complexes not containing P. Conclusions were drawn aboutthe P-economy of A. capillaris plants growing on soils withlow levels of P and Al. Key words: Phosphorus, aluminium, speciation, Agrostis capillahs L 相似文献
20.
Charney AN Egnor RW Henner D Rashid H Cassai N Sidhu GS 《American journal of physiology. Cell physiology》2004,286(5):C1062-C1070
In rat ileum and colon, apical membrane exchange and net Cl- absorption are stimulated by increases in PCO2 or . Because changes in PCO2 stimulate colonic Na+ absorption, in part, by modulating vesicular trafficking of the Na+/H+ exchanger type 3 isoform to and from the apical membrane, we examined whether changes in PCO2 affect net Cl- absorption by modulating vesicular trafficking of the exchanger anion exchanger (AE)1. Cl- transport across rat distal ileum and colon was measured in the Ussing chamber, and apical membrane protein biotinylation of these segments and Western blots of recovered proteins were performed. In colonic epithelial apical membranes, AE1 protein content was greater at PCO2 70 mmHg than at PCO2 21 mmHg but was not affected by pH changes in the absence of CO2. AE1 was internalized when PCO2 was reduced and exocytosed when PCO2 was increased, and both mucosal wortmannin and methazolamide inhibited exocytosis. Wortmannin also inhibited the increase in colonic Cl- absorption caused by an increase in PCO2. Increases in PCO2 stimulated ileal Cl- absorption, but wortmannin was without effect. Ileal epithelial apical membrane AE1 content was not affected by PCO2. We conclude that CO2 modulation of colonic, but not ileal, Cl- absorption involves effects on vesicular trafficking of AE1. PCO2; ileum; colon; anion exchanger 1; Na+/H+ exchanger type 3 相似文献