Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Sulphate supply and its regulation of transport in roots of a tropical legume Macroptilium atropurpureum cv. Sirato

During a period of sulphate deprivation, roots of Macroptiliumatropurpureum responded by increasing their uptake capacityat the plasma membrane. This effect was apparent both in intactplants and in tissues excised prior to uptake. In experiments using excised root systems previousy labelledwith ³⁵SO₄^2- the rate of tracer transport to the xylem was muchgreater in roots subsequently deprived of external sulphatethan in those supplied with unlabelled sulphate. Removing theexternal sulphate to the external solution. Additionally, compartmentalanalysis of tracer exchange kinetics showed that the flux ofsulphate from the cytoplasm to the xylem(     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

RESPIRATORY METABOLISM IN THE PHLOEM, XYLEM AND CAMBIUM OF CARROT ROOT

Comparisons of respiratory metabolism among the phloem, cambiumand xylem of mature carrot root were carried out and the followingresults were obtained. 1) The activity of O₂ uptake on a dry weight basis in the cambiumwas higher than that in the xylem. The phloem showed the lowestactivity. 2) RQ was close to unity in all the three tissues.3) The inhibition rate of O₂ uptake by malonate was considerablyhigh in the three tissues. The highest inhibition was observedin the cambium. 4) In the phloem and xylem the malonic inhibitionrate of ¹⁴CO₂ release from G-U-¹⁴C was not so high as comparedwith that of O₂ uptake. In the cambium, however, those of O₂uptake and ¹⁴CO₂ output were comparable. 5) Malonate treatmentdid not cause any significant change in RQ. 6) The C₆/C₁ ratiowas highest in the cambium and lowest in the phloem. From these it is concluded that the participation of the TCAcycle in respiration is great in all the three tissues of carrotroot, and among the three the cambium has the highest activityof the cycle. As for the PP pathway, the xylem and phloem havea higher activity than the cambium. (Received April 11, 1967; )     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

Chemical Changes In Young Tea Plant (Camellia sinensis L.) Tissues Following Application of Fertilizer Nitrogen

The uptake of ammonium sulphate by 14-month-old potted tea plantsgrown in a glasshouse was studied over the 11-week period followingapplication. Concurrent changes in the starch of root-wood,amino acids of xylem sap, and total nitrogen of leaves, stems,root-wood, and feeder roots were determined. Depletion of nitrogenfrom the soil at different depths and transformation of NH₄⁺to NO₃^– was also followed. The results show that the uptake of nitrogen commences within2 days of application as indicated by a marked increase in theamino-acid content of the xylem sap. Glutamine and, to a lesserextent, theanine were quantitatively the most important aminoacids in the sap. The amino-acid content of the sap was a maximumat about the time rapid depletion of the ammonium of the soiltook place. An interesting feature of the work is the reciprocalrelationship between the changes in the starch of root-woodand amino acids in the sap a few days after fertilizer application.Studies on the ammonium and nitrate levels of the soil at differentdepths showed that transformation of NH₄⁺ to NO₃^– occurredin the soil. The response of the various tissues to applied fertilizer nitrogenand increase in the fresh weight of the shoot system showedsimilar trends and may be correlated with the depletion of ammonium-nitrogenfrom the soil.     

Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration andcontents of total C, NO₃-N and reduced N were followed throughoutthe life of leaf 6 of nitrate-dependent plants of castor beanexposed to moderate salinity stress (71 mol m^–3 NaCl).Salt treatment was applied for measuring mineral flows in aparallel study (Jeschke and Pate, 1991b). Concurrent measurementswere made of solute composition and C: N molar ratios and concentrationsof reduced N and collected NO₃-N in phloem sap bleeding fromshallow incisions in the top and at the base of petioles andin xylem exudates from flaps of proximal leaf midribs followingpressurization of the root system. The resulting data were usedto construct empirical models of the respective economies ofC, total N, NO₃ and reduced N for a sequence of defined phasesof leaf life. Water use efficiency increased 3-fold from emergenceto a maximum of 1·5 mmol CO₂ mol^–1 H₂O before decliningto 0·5 mmol CO₂ mol^–1 H₂O at senescence. Xylemmolar ratios of C:N varied from 1·2–2·8,with nitrate always a smaller component than reduced N. Phloemsap C:N increased from 10–40 with leaf expansion and wasthen maintained in the range of 40–50 until falling steeplyto 20 at leaf senescence. Nitrate comprised less than 1% oftotal N in all phloem sap samples. The models of C uptake, flow,and utilization showed a major role of phloem import and thenincreasingly of laminar photosynthesis in providing C for leafgrowth. The carbon budget was thereafter characterized by ratesof phloem export closely matched to net rates of CO₂ fixationby the lamina. Corresponding data for total N depicted an earlymajor role of both xylem and phloem import, but the eventualdominance of xylem import as the N source for leaf growth. Cyclingof N by xylem to phloem exchange commenced before the leaf hadachieved maximum N content, and was the major contributor tophloem export until leaf senescence when mobilized N providedmost exported N. The nitrate economy of the leaf was characterizedby early establishment of tissue pools of the ion in the petioleand to a lesser extent in the lamina, continued high rates ofnitrate reduction in the lamina but negligible assimilationin the petiole, and a release through xylem of previously accumulatedNO₃ from petiole to lamina. Related data for reduced N illustratedthe much greater importance of this form of N than nitrate intransport, storage and cycling of N at all stages of leaf andpetiole life. Xylem to phloem interchanges of reduced N in petiolewere minimal in comparison with cycling through the lamina.The ratio of CO₂ reduction to NO₃ reduction in the lamina wasat first low (57 mol mol^–1) increasing to a peak valueof 294 during mature leaf functioning before declining to 190during the presenescence phase of leaf development. This patternreflected age-related effects on water use efficiency, changesin NO₃ levels in the xylem stream entering the lamina, and therelatively low photosynthetic performances of very young andsenescent laminae. Key words: Ricinus communis, leaf development, phloem transport, xylem transport, carbon, nitrogen, nitrate, reduced nitrogen, nitrate reduction, partitioning     

Characterization of transport mechanisms and determinants critical for Na+-dependent Pi symport of the PiT family paralogs human PiT1 and PiT2

The general phosphate need in mammalian cells is accommodated by members of the P_i transport (PiT) family (SLC20), which use either Na⁺ or H⁺ to mediate inorganic phosphate (P_i) symport. The mammalian PiT paralogs PiT1 and PiT2 are Na⁺-dependent P_i (NaP_i) transporters and are exploited by a group of retroviruses for cell entry. Human PiT1 and PiT2 were characterized by expression in Xenopus laevis oocytes with ³²P_i as a traceable P_i source. For PiT1, the Michaelis-Menten constant for P_i was determined as 322.5 ± 124.5 µM. PiT2 was analyzed for the first time and showed positive cooperativity in P_i uptake with a half-maximal activity constant for P_i of 163.5 ± 39.8 µM. PiT1- and PiT2-mediated Na⁺-dependent P_i uptake functions were not significantly affected by acidic and alkaline pH and displayed similar Na⁺ dependency patterns. However, only PiT2 was capable of Na⁺-independent P_i transport at acidic pH. Study of the impact of divalent cations Ca²⁺ and Mg²⁺ revealed that Ca²⁺ was important, but not critical, for NaP_i transport function of PiT proteins. To gain insight into the NaP_i cotransport function, we analyzed PiT2 and a PiT2 P_i transport knockout mutant using ²²Na⁺ as a traceable Na⁺ source. Na⁺ was transported by PiT2 even without P_i in the uptake medium and also when P_i transport function was knocked out. This is the first time decoupling of P_i from Na⁺ transport has been demonstrated for a PiT family member. Moreover, the results imply that putative transmembrane amino acids E⁵⁵ and E⁵⁷⁵ are responsible for linking P_i import to Na⁺ transport in PiT2. inorganic phosphate transport; retroviral receptor; SLC20     

Metabolic Adaption in Mature Roots of Salt-stressed Zea mays

Respiratory gas exchange and incorporation of ¹⁴C-leucine intoprotein were studied in proximal root segments from 25-day-oldmaize plants grown for the last ten days in 50 mM Na₂SO₄. ¹⁴C-leucineincorporation, and oxygen uptake in the presence of glucose,were as large in Na₂SO₄-grown tissues tested under saline conditionsas in tissues exposed to non-saline solutions throughout Thisadaptation was attributed to an increased metabolic capacityof Na₂SO₄-treated tissues, because these tissues, when returnedto non-saline solutions, evolved oxygen and incorporated ¹⁴C-leucinefaster than tissues exposed continuously to non-saline solutions. These changes are interpreted as a ‘compensation’for the inhibitory effects found when non-adapted tissues wereexposed to 50 mM Na₂SO₄. Moreover, we have related them to ultrastructuralchanges observed previously in xylem parenchyma cells of thesetissues, and to the possible involvement of these xylem parenchymacells in the re-absorption of sodium from the ascending xylemfluid Zea mays L., maize, salt-stress, respiration, protein synthesis     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

Diurnal regulation of NO3- uptake in soybean plants III. Implication of the Dijkshoorn-Ben Zioni model in relation with the diurnal changes in NO3- assimilation

According to the Dijkshoorn-Ben Zioni model, NO₃^– uptakein the roots is stimulated by NO₃^– assimilation in theshoots, through downward phloem transport of malate synthesizedin response to reduction of NO₂^– to NH³. In this paper,one hypothesis resulting from this model was tested, i.e. thatthe diurnal changes in NO₃^– uptake are due to the lightdependence of NO₃^– reduction in the leaves. This dependencewas studied in detached leaves transferred to deionized wateror supplied via the transpiration stream with similar amountsof ¹⁵NO₃^– in light or darkness. In the dark, the reductionof previously stored NO₃^– or xylem-borne ¹⁵NO₃^–was generally about 40–50% of that measured in the light.Glucose supply to the detached leaves stimulated NO₃^–reduction in the dark, but not enough to increase it up to thesame rate as in the light. Nitrite reduction in detached leaveswas much less affected by darkness, and could be maintainedat a high level by exogenous supply of substrate. Advantagewas taken from this last observation to sustain NO₂^–reductionin attached darkened shoots at the same rate as in the light,by ensuring an appropriate delivery of NO₂^– from the xylem.Although this was assumed to restore the light level of theassociated synthesis of malate, it led to a marked inhibitionof NO₃^– uptake. In addition, the direct supply of malateto the shoots or to the roots failed to prevent the decreaseof NO₃^– uptake in darkness. Thus, our conclusion is thatthe mechanisms evoked in the Dijkshoorn-Ben Zioni model do notplay an important role in the diurnal variations of NO₃^–uptake in soybean plants. Key words: Glycine max, light/dark cycle, malate synthesis, NO₃^– reduction, NO₃^– uptake     

Influence of the Form of Nitrogen Supply on Root Uptake and Translocation of Cations in the Xylem Exudate of Maize (Zea mays L)

Concentrations of inorganic cations are often lower in plantssupplied with NH₄⁺ as compared with NO₃^–. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m^–3 NO₃^–or NH₄⁺ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO₃^– and NH₄⁺ declined by more than 50%Compared with NO₃^– supply, NH₄⁺ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH₄⁺- than in NO₃^–-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH₄⁺-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH₄⁺ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH₄⁺-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation     

Relationships between Adsorption, Chemical State and Fluxes of Cadmium Applied as Cd(NO3)2 in Isolated Xylem Cell Walls of Tomato

Wolterbeek, H. Th. 1987. Relationships between adsorption, chemicalstate and fluxes of cadmium applied as Cd(NO₃)₂ in isolatedxylem cell walls of tomato.—J. exp. Bot. 38: 419–432. Isolated xylem cell wall pieces were applied as membranes inion diffusion experiments. The cell walls were isolated fromtomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim)and sealed in a two-compartment diffusion system. In flux andadsorption calculations, the cell wall was regarded as a leakymembrane with parallel fluxes through Donnan Free Space (DFS)and Water Free Space (WFS). During the experiments absorptioninto and diffusion across the walls was determined of Cd^{2 +}, applied as ¹¹⁵Cd(NO₃)₂. Flux experiments with ⁸²Br^–indicated that excluded volume effects and path tortuosity resultedin apparent WFS diffusion coefficients in the walls which were0·012 times as high as in water. The free proton concentration in the DFS was shown to be relatedto a complex formation between fixed charges and Cd^{2 +}. Thecell wall permeability for Cd^{2 +} and NO₃^– varied withapplied and absorbed concentrations, and the Cd^{2 +} flux curveshowed an inflexion point coinciding with a buffered degreeof dissociation of fixed charges in the DFS. The necessary couplingof fluxes of opposite charges resulted in relatively high NO₃^–and small Cd^{2 +} permeability of the DFS for strongly dilutedsolutions (P = 10^–4 m s^–1 and 10^–11 m s^–1for NO₃^– and Cd^{2 +} respectively). The results demonstratethe possible regulatory effects of the cell wall in processesof ion transfer from xylem vessels, or ion uptake in plant tissues. Key words: Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate     

The Transport and Metabolism of Urea in Chara australis: I. PASSIVE DIFFUSION, SPECIFIC TRANSPORT AND METABOLISM OF UREA, THIOUREA AND METHYLUREA

Most of the urea entering Chara australis cells is rapidly metabolizedto produce CO₂, which diffuses out of the cells into the surroundingmedium. A simple and convenient apparatus to measure both the¹⁴C-urea retained by cells and the ¹⁴CO₂ released into the mediumwas developed and used in a study of urea transport in Chara.The permeability coefficient for urea in the Chara plasmalemmawas estimated from the slope of an uptake versus concentrationfunction as 85 nm s^-1. Computer modelling of urea uptake andmetabolism suggests that this could be a 20% underestimate ofthe true value.The corresponding permeability coefficients forthiourea and N-methyl-urea were estimated in the same way as34 and 35 nm s^-1, respectively. These permeabilities are muchgreater than expected on the basis of either/water partitioncoefficients for the solutes and are consistent with the diffusionof urea and its similarly-sized analogues through aqueous poresin the plasmalemma.At external concentrations of urea less than20 mmol m^-3, the bulk of the uptake is effected by a specifictransport mechanism with an apparent K_m for urea of less than1.0 mmol m^-3. This transport system operates most rapidly withexternal pH in the range 6.5–7.5 and is influenced bythe nitrogen status of the cell.Evidence is produced here suggestingthat the specific transport of urea may be an active process. Key words: Chara, urea uptake, metabolism, diffusion, specific transport     

Water Transport during Acid-Induced Growth of Excised Hypocotyl Sections of Vigna unguiculata under Xylem Perfusion

Elongation growth of abraded hypocotyl sections of Vigna unguiculataunder xylem perfusion was markedly promoted a few minutes afterthe application of an acid aerosol generated from a solutionof HCl. At the beginning of the acid-induced growth, intracellularpressure (Pⁱ) began to decrease and the membrane potential betweenthe symplast and the xylem apoplast (V^px) began to depolarize.Subsequently, Pⁱ and V^px remained at a reduced level and a depolarizedlevel, respectively, while the promotion of elongation growthcontinued for more than 4 hours. The electrogenic componentof the xylem membrane potential (V^px_act) gradually increasedto about twice that before acid treatment. There was a closecorrelation between the enhanced growth and the decrease inintracellular pressure within 30 min after application of acidbut little correltion after 60 min. By contrast, there was littlecorrelation between the promotion of growth and the activityof the xylem pump after 30 min while a close correlation wasobserved after 60 min. It is inferred that the acid-induced activation of water uptakeconsists of two major processes, in series, that are drivenby different forces: the rapid uptake of water for more than30 min, driven by hydrostatic force generated by loosening ofcell walls; and a long-lasting enhancement of water uptake forat least 4 h, which is driven by osmotic force that is generatedby the canal system within the xylem. (Received October 17, 1994; Accepted January 23, 1995)     

Phosphate uptake and transport in Agrostis capillaris L Effects of non-toxic levels of aluminium and the significance of P and Al speciation

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 100mmolm^–3 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 m^–3 at pH ranging from 4.3 to 4.6, showedthat in 10 mmol m^–3 P, effects of Al at 3.7 and 37 mmolm^–3 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 mmolm^–3 Al in nutrient solution containing 100 mmol m^–3P, 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 m^–3 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     

Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

Thecharacteristics of L-lactic acid transport across thetrophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake ofL-[¹⁴C]lactic acid into basal membranevesicles was Na⁺ independent, and an uphill transport wasobserved in the presence of a pH gradient([H⁺]_out > [H⁺]_in).L-[¹⁴C]lactic acid uptake exhibitedsaturation kinetics with a K_m value of 5.89 ± 0.68 mM in the presence of a pH gradient.p-Chloromercuribenzenesulfonate and-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4'-diisothiocyanostilbene-2,2'-disulfonate did not.Mono- and dicarboxylic acids suppressed the initial uptake. Inconclusion, L-lactic acid transport in the basal membraneis H⁺ dependent and Na⁺ independent, as is alsothe case for the brush-border membrane transport, and itscharacteristics resemble those of monocarboxylic acid transporters.However, there were several differences in the effects of inhibitorsbetween basal and brush-border membrane vesicles, suggesting that thetransporter(s) involved in L-lactic acid transport in thebasal membrane of placental trophoblast may differ from those in thebrush-border membrane.

     

Characterization of L-glutamine transport by a human neuroblastoma cell line

This study characterized theNa⁺-dependent transport of L-glutamine by ahuman neuroblastoma cell line, SK-N-SH. The Na⁺-dependentcomponent represented >95% of the total glutamine uptake. Kineticstudies showed a single saturable high-affinity carrier with aMichaelis constant (K_m) of 163 ± 23 µMand a maximum transport velocity (V_max) of13,713 ± 803 pmol · mgprotein¹ · min¹. Glutamine uptakewas markedly inhibited in the presence of L-alanine, L-asparagine, and L-serine. Li⁺ didnot substitute for Na⁺. These data show thatL-glutamine is predominantly taken up through systemASC. Glutamine deprivation resulted in the decrease of glutamine transport by a mechanism that decreasedV_max without affectingK_m. The expression of the system ASC subtypeASCT2 decreased in the glutamine-deprived group, whereas glutaminedeprivation did not induce changes in system ASC subtype ASCT1 mRNAexpression. Adaptive increases in Na⁺-dependent glutamate,Na⁺-dependent 2-(methylamino)isobutyric acid, andNa⁺-independent leucine transport were observed underglutamine-deprived conditions, which were completely blocked byactinomycin D and cycloheximide. These mechanisms may allow cells tosurvive and even grow under nutrient-deprived conditions.