Article

Dual Mechanisms of Ion Uptake in Relation to Vacuolation in Corn Roots

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Abstract

Absorption isotherms for chloride and rubidium ions have been determined through a wide concentration range for nonvacuolate root tips, and for vacuolate subapical sections of corn root. In the range 0 to 0.5 mm, chloride absorption is hyperbolic with concentration in both tips and proximal sections. At high concentrations, 1 to 50 mm, a second multiple-hyperbolic isotherm for chloride is noted in vacuolate tissue, while the isotherm for nonvacuolate tips rises exponentially. A linear to exponentially rising isotherm is taken to signify diffusive permeation.The same distinction between tip and subapical tissue characterizes Rb absorption. Rb uptake is indifferent to the nature of the counterion at all concentrations in the tip, while the counterion exerts a predictable influence on Rb absorption in proximal tissue. The effect of a poorly absorbable anion on Rb uptake is greater in the high concentration range. Evidence is presented for the metabolic nature of ion transport into nonvacuolate root tips. Verification is offered that ion uptake is mediated by dual mechanisms, and the thesis is developed that the high-affinity (low K(s)) system mediates ion passage through the plasma membrane while the low-affinity (high K(s)) system implements transport through the tonoplast.

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... If it is possible to estimate the size of the initial influx to the cell, then this can be compared with the influx to the vacuole. Despite the expected drawbacks, it seemed worth while to use the method as a rapid and fairly unambiguous, although only semiquantitative, way of seeing whether the plasmalemma influx is greater than the influx to the vacuole, and thus checking some of the results obtained by other methods (Torii and Laties 1966;Cram 1968aCram , 1968b. ...
... Nitella, MacRobbie 1969), then this straight-through path saturates at 20-30 mM, and there is no movement inwards from the bulk of the cytoplasm to the vacuole across the tonoplast. However the chloride arrives at the vacuole, it is a transport process within the cell and not at the plasmalemma which is saturated at high external solution concentrations, in confirmation of the conclusions of Torii and Laties (1966) and Cram (1968a). Pitman, Courtice, and Lee (1968) show that after this treatment the roots would probably have contained 90-100 fL-equiv. ...
... The quasi-steady influx to plant cells has been found by many authors to show some complexity in relation to the external concentration (reviews ~ Epstein 1966;Laties 1969). From a comparison of vacuolate and predominantly non-vacuolate maize root cells, Torii and Laties (1966) concluded that the influx to the vacuole at low external concentrations (below 1 mM) is limited by the plasmalemma influx, but that at higher external concentrations (above 5-10 mM) the observed "saturation" of influx to the vacuole is determined by the saturation of the tonoplast transport process. This implies that the plasmalemma influx rises to large values at high concentrations, although showing some properties of saturation between about 0·5 and 1 mM (cf. ...
Article
Chloride influx measured with 36Cl over a 10 min loading period followed by a 5 min wash in inactive solution is used as an estimate of influx across the plasmalemma in barley roots. Chloride influx measured over a 60 min loading period followed by a 30 min wash is used as an estimate of the quasi-steady influx from the external solution to the vacuole
... Indeed, molecular analyses in rice and Arabidopsis have revealed that nitrate transporters mainly involved in nitrate uptake such as NRT1 . 1 and NRT2 . 1 are inserted in parallel and not in series in the different cell layers of the mature root (Guo et al., 2001(Guo et al., , 2002Nazoa et al., 2003;Remans et al., 2006a, b;Girin et al., 2007;Feng et al., 2011). This location contradicts the one-root-membrane hypothesis with nitrate transporters inserted in series proposed by Epstein (Epstein et al., 1963;Epstein, 1966Epstein, , 1972Crawford and Glass, 1998) or inserted in series and in parallel in plasma membrane and tonoplast of the epidermal layer proposed by Torii and Laties (1966). Secondly, in Brassica napus plants, K 15 NO 3 labelling for nitrate influx rate measurements (5 min) revealed that more than 40-60 % of 15 N is translocated to the shoot throughout a daily period whatever the nitrate concentrations used, 100 mM or 5 mM (Supplementary Data Fig. ...
... Generally, these reference isotherms are established in specific experimental conditions of temperature, pH, pressure and photosynthetically active radiation (PAR) (and determined in plants of specific age and N status). Alternatively, it is well established that variations of temperature, for example, or the combination of duration and concentrations of nitrate pretreatment induced variations in nitrate influx kinetics (Torii and Laties, 1966;Siddiqi et al., 1989Siddiqi et al., , 1990Laîné et al., 1994;Macduff et al., 1997). Accordingly, the level of influx given by the root catalytic structure for a given nitrate external concentration is not fixed but fluctuates according to the variations of exogenous and in planta factors. ...
Article
Background and AimsIn spite of major breakthroughs in the last three decades in the identification of root nitrate uptake transporters in plants and the associated regulation of nitrate transport activities, a simplified and operational modelling approach for nitrate uptake is still lacking. This is due mainly to the difficulty in linking the various regulations of nitrate transport that act at different levels of time and on different spatial scales.MethodsA cross-combination of a Flow-Force approach applied to nitrate influx isotherms and experimentally determined environmental and in planta regulation is used to model nitrate in oilseed rape, Brassica napus. In contrast to 'Enzyme-Substrate' interpretations, a Flow-Force modelling approach considers the root as a single catalytic structure and does not infer hypothetical cellular processes among nitrate transporter activities across cellular layers in the mature roots. In addition, this approach accounts for the driving force on ion transport based on the gradient of electrochemical potential, which is more appropriate from a thermodynamic viewpoint.Key Results and Conclusions Use of a Flow-Force formalism on nitrate influx isotherms leads to the development of a new conceptual mechanistic basis to model more accurately N uptake by a winter oilseed rape crop under field conditions during the whole growth cycle. This forms the functional component of a proposed new structure-function mechanistic model of N uptake.
... Indeed, molecular analyses in rice and Arabidopsis have revealed that nitrate transporters mainly involved in nitrate uptake such as NRT1 . 1 and NRT2 . 1 are inserted in parallel and not in series in the different cell layers of the mature root (Guo et al., 2001(Guo et al., , 2002Nazoa et al., 2003;Remans et al., 2006a, b;Girin et al., 2007;Feng et al., 2011). This location contradicts the one-root-membrane hypothesis with nitrate transporters inserted in series proposed by Epstein (Epstein et al., 1963;Epstein, 1966Epstein, , 1972Crawford and Glass, 1998) or inserted in series and in parallel in plasma membrane and tonoplast of the epidermal layer proposed by Torii and Laties (1966). Secondly, in Brassica napus plants, K 15 NO 3 labelling for nitrate influx rate measurements (5 min) revealed that more than 40-60 % of 15 N is translocated to the shoot throughout a daily period whatever the nitrate concentrations used, 100 mM or 5 mM (Supplementary Data Fig. ...
... Generally, these reference isotherms are established in specific experimental conditions of temperature, pH, pressure and photosynthetically active radiation (PAR) (and determined in plants of specific age and N status). Alternatively, it is well established that variations of temperature, for example, or the combination of duration and concentrations of nitrate pretreatment induced variations in nitrate influx kinetics (Torii and Laties, 1966;Siddiqi et al., 1989Siddiqi et al., , 1990Laîné et al., 1994;Macduff et al., 1997). Accordingly, the level of influx given by the root catalytic structure for a given nitrate external concentration is not fixed but fluctuates according to the variations of exogenous and in planta factors. ...
Article
Full-text available
Background The top-down analysis of nitrate influx isotherms through the Enzyme-Substrate interpretation has not withstood recent molecular and histochemical analyses of nitrate transporters. Indeed, at least four families of nitrate transporters operating at both high and/or low external nitrate concentrations, and which are located in series and/or parallel in the different cellular layers of the mature root, are involved in nitrate uptake. Accordingly, the top-down analysis of the root catalytic structure for ion transport from the Enzyme-Substrate interpretation of nitrate influx isotherms is inadequate. Moreover, the use of the Enzyme-Substrate velocity equation as a single reference in agronomic models is not suitable in its formalism to account for variations in N uptake under fluctuating environmental conditions. Therefore, a conceptual paradigm shift is required to improve the mechanistic modelling of N uptake in agronomic models.
... At the root level, to explain the dual-phase of ion isotherms it was proposed that the carriers responsible for mechanisms I and II were located in parallel on the plasmalemma and tonoplast of epidermal cells, respectively (Torii and Laties, 1966). However, a study in barley and corn roots that used mutual interaction between K + and Na + demonstrated that mechanisms I and II of K + absorption operate in parallel across the epidermal plasma- lemna (Welch and Epstein, 1968). ...
Article
Background and AimsAn updated version of a mechanistic structural-functional model was developed to predict nitrogen (N) uptake throughout the growth cycle by a crop of winter oilseed rape, Brassica napus, grown under field conditions.Methods The functional component of the model derives from a revisited conceptual framework that combines the thermodynamic Flow-Force interpretation of nitrate uptake isotherms and environmental and in planta effects on nitrate influx. Estimation of the root biomass (structural component) is based upon a combination of root mapping along the soil depth profile in the field and a relationship between the specific root length and external nitrate concentration. The root biomass contributing actively to N uptake was determined by introduction of an integrated root system age that allows assignment of a root absorption capacity at a specific age of the root.Key ResultsSimulations were well matched to measured data of N taken up under field conditions for three levels of N fertilization. The model outputs indicated that the two topsoil layers (0-30 and 30-60 cm) contained 75-88 % of the total root length and biomass, and accounted for 90-95 % of N taken up at harvest.Conclusions This conceptual framework provides a model of nitrate uptake that is able to respond to external nitrate fluctuations at both functional and structural levels.
... Whatever the nature of the coupling process, the control of rate of accumulation seems to be located inwards from the plasmalemma (if not at the tonoplast) in barley root cells. Uptake to these cells from 10 mM solutions is largely phase II uptake, which Torii and Laties (1966) have shown to be present in vacuolated cells but not in non-vacuolated cells. Another reason is that organic acids taken up during salt accumulation are formed in the cytoplasm (Osmond and Laties 1968). ...
Article
Measurements are described of the uptake of sodium and potassium by barley roots at low salt status. It was found that initially there was little preference for potassium in the salt accumulated, but about 5 hr after the start of salt accumula· tion net uptake of sodium decreased markedly, and thereafter much more potassium was taken up than sodium. The roots reached salt saturation in about 24 hr. The decreased sodium uptake was due to a reduction in sodium influx.
... This mechanism 2 has a much lower affinity for potassium, so that it does not measurably contribute to potassium absorption at concentrations of about 1·5 mM and below. Mechanism 2 of alkali cation absorption has been characterized in several other tissues in considerable detail (Epstein, Rains, and Elzam 1963a;Epstein and Rains 1965;Epstein 1965, 1967;Elzam 1966;Torii and Laties 1966). ...
Article
One aspect of the salt tolerance of the mangrove, Avicennia marina, was investigated: the preferential absorption of potassium by leaf tissue in the presence of high concentrations of salt (sodium chloride). The rate of absorption of potassium, over the concentration range 0·02 to 1·5 mM, follows the Michaelis-Menten relation, approaching the theoretical maximum, V max, at 1· 5 mM. The apparent Michaelis constant is 0·20 mM. At higher concentrations, up to 50 mM potassium, the rate of potassium absorption reaches values several times higher than the theoretical maximum calculated on the basis of the relation applying over the low range of concentrations, indicating the operation of a second mechanism of absorption. At both low (1 mM) and high (10 mM) concentrations of potassium, its absorption was little affected by sodium chloride concentrations up to 200 mM. At 10 mM potassium, sodium chloride at concentrations up to and including 500 mM (more than its concentration in sea.water) failed to interfere with the absorption of potassium.
Chapter
Plants are sprayed and dusted with chemicals for numerous reasons. Some of the chemicals applied to plants exert their influence simply by remaining on the foliage surface; others must be absorbed and perhaps translocated before their activity is realized. Many of the compounds used have a mammalian toxicity less than that of table salt, whereas others exceed strychnine in toxicity. Likewise, many are relatively non-phytotoxic but others kill plants readily. Possible foliar residues which may be left by various pesticides are of course influenced by both absorption characteristics and metabolic breakdown. The various plant environmental, and chemical factors which affect absorption are examined in this review. A variety of pesticides is considered, as well as certain growth regulators and other substances which are not, strictly speaking, “pesticides.” A limited consideration is also given to recent findings relating to penetration of inorganic ions, where the mechanisms involved may bear certain similarities to the penetration of some organic substances which also ionize or hydrolyze during the course of their penetration. Because of this broad coverage, space precludes detailed discussions of all aspects of absorption and of associated structural relationships. These subjects are developed quite fully in many of the reviews listed below and in the introductions to some of the specific sections in this report. Also, only foliar absorption and movement within the leaf mesophyll and veins are considered. Long distance transport in the vascular system of the stems and roots is not included, since several excellent reviews and monographs on this subject have recently been published or are in preparation.
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Borstlap (1983) has alleged that (a) there are no abrupt changes in curves for the concentration dependence of solute uptake in plants, and (b) many such uptake isotherms may be described by the sum of two Michaelis-Menten terms and a linear term. These claims are considered in detail in connection with the recent finding (Nandi, Pant & Nissen, 1987) that phosphate uptake by corn roots increased more rapidly within the higher phases, i.e. at high external phosphate concentrations, but also levelled off faster than predicted from Michaelis-Menten kinetics. Similar deviations are, in retrospect, also found for uptake of other solutes and result in fewer phases at high external solute concentrations. The simplified and strikingly similar multiphasic patterns in the present paper show that (a) the abrupt changes in published isotherms are not due to error in the data, and (b) uptake isotherms cannot be adequately represented by the sum of two Michaelis-Menten terms and a linear term, or by similar continuous functions, if sufficiently detailed and precise data are used. These findings are not consistent with the existence of multiple uptake mechanisms, including free diffusion, in the plasmalemma. Uptake occurs instead by a single, multiphasic mechanism for each solute or group of related solutes. The similarities in the multiphasic patterns indicate, furthermore, that influx of the various solutes may be coupled.
Article
Experiments with detached maize roots confirmed the inhibitory effect of DNP on the uptake of orthophosphate ions (Pi) from nutrient solutions containing ≤ 0.5 mM phosphate. In contrast, the covalent compound bis-[dimethylamido]-phosphoric acid amide [(CH3)2N]2P(0)NH2 (BAPA) was taken up by a process not inhibited by DNP. Moreover, BAPA is more rapid exchanged in root tissue than Pi (efflux studies, indirect compartmental analysis). Pot experiments with soils showed a significant negative correlation between the relative atmospheric humidity and uptake of BAPA. Therefore, this compound seems to be taken up by a manner independent on respiration but dependent on transpiration.
Article
Specific uptake (S.U.) of α-aminoisobutyric acid ([1-14C]AIB), a non-metabolizable neutral amino acid analog, by dwarf bush bean plants (Phaseolus vulgaris cv Top Crop) demonstrated wide differences in active transport between various plant organs. The kinetic and timed uptake data reported were expressed as S.U. because this corrects for the diffusion of AIB which is part of the total AIB uptake process. Roots accumulated AIB to concentrations up to 18 times and leaf disks to twice those of the incubation medium. Stem tissue showed very little uptake, if any, that could not be accounted for by simple diffusion or water free space. Although initial rate kinetic studies demonstrated the presence of a normal transport system, timed uptake studies revealed greatly decreased transport by etiolated plants, suggesting a relationship between active transport and the lack of photosynthate. The reproducibility of the AIB uptake pattern by mature roots strongly supports the concept that the transport of neutral amino acids is biphasic and suggested one or more carrier systems are inducible by either low intracellular concentrations or repressed by high intracellular concentrations of the amino acid.
It has been demonstrated that root pressure exudation from excised primary roots of Zea mays is adequately described by a standing gradient osmotic flow model. The differential equations set out by Diamond & Bossart (1967) to describe isotonic water transport across animal epithelia have been modified to render them more appropriate to the excised root, and solved by an analogue programme. A good fit of the experimentally determined values of water flux, salt flux and exudate salt concentration has been obtained providing the osmotic permeability of the root is allowed to vary with distance from the root tip. The anatomical identity of the exudate channel is discussed, but no firm assignment can be made.
Chapter
Rates of uptake of silicic acid as a function of its concentration by the roots of various plants have been interpreted by their authors in the framework of a top-down approach based on the theoretical model of Michaelis and Menten. Although a hyperbola was fitted to the set of experimental points, this does not prove that all the simplifying assumptions underlying the Michaelis–Menten approach to transport were warranted. In consequence, the Vjm-V_j^m- and Kjm-K_j^m-values, which were inferred from these experiments, may not characterize the carrier molecules involved in the uptake process in the way that K m - and V m -values characterize classical enzymes (Vjm =V_j^m = uptake rate when the carriers are saturated by the transported substrate, 1/Kjm =1/K_j^m = carrier affinity for this substrate). Since the experimental measurements have dealt only with solute uptake by entire biological systems (plant roots in the present case), here we adopt a “top-top” approach. The idea is to find logical parameters that characterize these entire systems rather than look for key molecular constituents (e.g. carriers). By using the fact that the equations relating flows to forces always have a linear approximation when the system is close to equilibrium, we introduce two parameters to describe the uptake features of plant roots in the system of coordinates {lncje,Jj(cej)}:°cej=\{{\rm ln}c_j^e,J_j(c^e_j)\}:{^\circ}c^e_j= the particular value of the concentration cjec_j^e of the substrate, S j , in the uptake medium for which the net flow, Jj(cej)J_j(c^e_j), of S j exchange between medium and plant samples is zero and Lj = the overall conductance of the sample for S j -uptake (= slope of the linear part of the curve {lncej,Jj(cej)})\{{\rm ln}c^e_j,J_j(c^e_j)\}). We find that (1) the °cej{^\circ}c^e_j values are of the order of magnitude expected in the experimental conditions used and (2) the greater the ability of a plant to accumulate silicon, the greater the L j -value for the radial absorption of silicon by this plant. The flow/force approach to uptake is applicable to the absorption of any type of substrate by any type of plant. In the flow/force approach, °cej{^\circ}c^e_j must equal the concentration of S j in the growth solution of the plants prior to the experiments, which can be easily checked. It is possible to complement the interpretation of the uptake kinetics by using (1) a “symmetry criterion” to test whether uptake is active or passive and (2) an Arrhenius plot to check whether a modification of L j corresponds to a modification of the uptake mechanism that is quantitative (e.g. number of carriers) or qualitative (e.g. post-translational modification of carriers).
Article
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Article
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Article
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Article
Uptake and Translocation of Diethyl Phosphoric acid Ester in Comparison to the Orthophosphate..The uptake of the diethyl phosphoric acid ester (DEP) through intact adventive roots of maize plants and its further translocation to the shoots was studied over a wide range of concentrations (0.001–50.0 mM P/L) in comparison to the orthophosphate (MP). The effect of the relative humidity (r.h.) and the metabolic inhibitor Na N3 on both processes, uptake and translocation, was also investigated.At low concentrations the plants tended to retain a greater part of the MP intaked in the absorbing roots. Depression of the transpiration streaming influenced relatively the phosphate translocation to the shoots. The higher the MP-outside-concentration was, the greater the effect of the transpiration streaming was on its translocation. Treatment with 10-4 MNaN3 inhibited MP-accumulation but not its translocation from the treated roots.The results showed that passive processes are mainly responsible for the DEP-uptake and translocation by the maize plants. Both processes were dependent to a great extent on the transpiration streaming.The r.h. affected the translocation patterns of both DEP and MP dissimilarly at low concentration, though, they come close to each other when their outside concentrations were increased.The results are discussed on the basis of the symplasm theory (2) and the dual mechanisms of ion uptake (28).
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The importance of water for plant growth is indicated by the fact that the distribution of land plants is more nearly controlled by the availability of water than by any other single environmental factor. Even in temperate, humid climates, crop yield is reduced more often by lack of water than by any other environmental factor. The agricultural and ecological importance of water indicates its important role in the physiological processes of plants. Although a great deal is known about cell structure and cell water relations, the answers to some important questions remain in doubt. The data on the hydraulic conductance of the apoplast and the symplast are inadequate to establish the principal pathway for water movement is across masses of cells. The long held view that turgor pressure is the cause of cell enlargement is being questioned as evidence accumulates that cell enlargement in growing tissue is more closely related to water potential than to turgor pressure.
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Plant cells exhibit unique features of ion transport. Historically, transport investigations have been confined largely to the characterization of the kinetics of the passage of a plethora of substances through biological membranes. This chapter discusses the transport of ions in Neurospora, describes the asexual part of the growth cycle of Neurospora, and identifies the developmental stages that have been used for transport studies. The chapter also discusses the properties of a variety of transport systems characterized in Neurospora. The characteristics of ion transport in roots include its competence in coping with spatial and temporal variations, high affinity for nutrient ions, great selectivity, and maintenance of a state of nonequilibrium with an ambient medium. Evidence for passive, diffusional movements of ions across uninjured plasmalemmas of the cells of higher plants in a state of active absorption is neither extensive nor firm; this favors the view that active transport agents or carriers must mediate the movement of ions across these membranes. The chapter presents evidence, mainly kinetic, for carrier-mediated transport to deduce certain properties of active agents or carriers, especially those related to the specificity or selectivity of ion transport.
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The mechanisms of action by which aluminium affects phosphate absorption were evaluated in Stylosanthes guianensis (aluminium‐sensitive) and Stylosanthes macrocephala (aluminium‐tolerant), through kinetic studies with aluminium in pre‐ and co‐treatment. Multiphasic and cooperative models of ions absorption were tested. In the absence of aluminium, S. guianensis, a specie which normaly requires more phosphorus, showed lower rates of phosphate absorption, which was attributed to slower turnover of the carrier. In S. guianensis, the higher rates of absorption may be related to higher turnover and/or higher concentration of the carrier in the roots. Both species showed negative correlation in the absorption of phosphate, but it was more pronounced in S. guianensis. The effect of aluminium seems to be related to the decreased capacity of the carrier to associate with the ion. There may also be a reduction in the turnover of the carrier, which appears to be more pronounced in the tolerant species, reducing phosphate absorption. In the presence of aluminium, the association becomes positive or approaches Michaelis‐Menten kinetics in both species. However, the absorption of phosphate is not increased since there is also a reduction in the capacity of the carrier to bind to the ion.
Article
Effect of NaCl and Na2SO4 on dry matter production, mineral content and organic compounds of spinach and lettuce. In water culture experiments the effect of 2,5 meq and 25 meq NaCl and Na2SO4 respectively on dry matter production and content of mineral elements, chloroplasts pigments and carbohydrates in lettuce (salt sensitive) and spinach (salt tolerant) has been studied. With increasing Na-supply the dry matter production was decreased in lettuce and increased in spinach. With increasing Na-supply in both species the content of K, Mg and Ca in the leaves decreased. This decrease was more pronounced with sulfate as accompanying anion (Na2SO4) and induced already deficiency in Ca and Mg. This induced deficiency of Mg was reflected especially in lettuce in lower contents of chloroplasts pigments. In both plant species there was no effect of the Na salt treatments on the content of phosphorus or nitrogen in roots or leaves. The carbohydrate content in both species was strongly affected by the Na salt treatments. Irrespectively of the accompanying anion this effect occured already at the low Na supply and before the dry matter production was influenced. In leaves and roots of lettuce the contents of glucose, fructose and sucrose was considerably decreased; this decrease was less expressed in the starch content. In spinach the Na supply only decreased the carbohydrate content in the roots whereas in the leaves especially the sucrose content was increased. This different effect of Na on carbohydrate content in spinach and lettuce could be an indication of different action of Na on carbohydrate metabolism, namely inhibited synthesis in lettuce and inhibited translocation in spinach. The results demonstrate that in studies on the effect of increasing Na salt concentrations besides the osmotic effects also the ion specific effects have to be carfully considered. These ion specific effects are competition of Na+ with other cations during uptake and the influence of Na on the cell metabolism, especially on the pathway of carbohydrates. The authors thanks Mrs. Hwie Juen Tjandraatmadja for her engaged help in various laboratory works.
Article
The question of the site of the dual mechanisms of K absorption was investigated by following the effects of anions and of temperature on absorption and long-distance transport of K in KCl-pretreated barley plants. In such plants, fluxes at the tonoplasts of root cells should be minimal and unlikely to influence long-distance transport of K from roots to shoots. Anions had the same effect on transport of K to shoots of KCl-pretreated plants as on absorption of K. Changing the counter-anion from S04 to Cl had no effect at 0.5 mM K but nearly doubled absorption and transport at 20 mM K. NO, stimulated absorption and transport of K above that in Cl at both 0.5 and 20 mM K. Increasing the temperature from 15 to 25°C increased to the same extent the rates of both absorption and transport of K from solutions of both 0.5 and 20 mM KC1. Values for Q10 varied in different experiments from 1.5 to 2.1. The present results suggest that the factor responsible for the differential effects of Cl and S04 on K absorption at high ambient K concentrations is located at the plasmalemma of root cells. This supports the view that both reported mechanisms of K absorption are located at the plasmalemma; however, the possibility of passive salt diffusion across the plasmalemma at high concentrations cannot be discounted.
Article
Die Kinetik der Ionenaufnahme durch junge und alte Sprosse von Mnium cuspidatum wurde untersucht. Die verschieden alten Sprosse unterscheiden sich vor allem durch das Vorhandensein einer aktiven Gipfelknospe bei den jungen Gametophyten, die bei den alten offenbar ihre Tätigkeit eingestellt hat. Im niedrigen Konzentrationsbereich (0–0,5 mM) haben jungen und alte Sprosse hyperbolische Isothermen der Ionenaufnahme, die sich etwas hinsichtlich der apparenten Michaeliskonstanten und der Maximalgeschwindigkeit unterscheiden. Im hohen Konzentrationsbereich (1–10 mM) ist der Unterschied qualitativ. Mit jungen Sprossen erhält man eine lineare oder exponentielle Isotherme, mit alten Sprossen eine hyperbolische Kurve. Der vermutete Einfluß der Gipfelknospe kann mit Effekten von Wuchsstoffen auf den Stofftransport zusammenhängen und läßt einen Einfluß dieser Regulationssysteme auf die Membranfunktion vermuten.
Article
The flux of water, \(f_{{\text{H}}_{\text{2}} {\text{O}}}\), to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, f K, to be studied. The relationship between \(f_{{\text{H}}_{\text{2}} {\text{O}}}\) and f K thus obtained was investigated, for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent f K both of which varied with Cm. The water dependent f K shows a parabolic relationship with Cm for Cm values <1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent f K shows an hyperbolic relationship over the same range of Cm values. Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent f K. It is postulated that the water independent f K is contributed by that portion of f K arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent f K is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.
Chapter
On the basis of a highly significant correlation between the rate of Rb+ uptake and RbCl-stimulated ATPase activity, FISHER et al. (1970) postulated that ion-stimulated ATPases were involved in plant ion uptake. HODGES et al. (1972) found that the highest specific activity of KCl-stimulated ATPase from oat roots was associated with a fraction which equilibrated at high density on a sucrose gradient and was estimated to contain 75% plasma-lemma. Later LEONARD and HODGES (1973) showed that the kinetics of the KCl stimulation of this enzyme closely resembled those of ion uptake by oat roots and concluded that the dual absorption isotherm observed during ion uptake (LATIES, 1969) is the result of the activity of this plasmalemma enzyme.
Article
1. The uptake of monosaccharides by isolated maize root tips was found to be a function of substrate concentration. It shows typical “dual isotherm” as they are well known with regard to the kinetics of ion uptake by plant tissues.
Chapter
Im zweiten Kapitel haben wir vorwiegend ein einfaches, zweikompartimentelles Außen-Innen-Modell betrachtet und gesehen, wie nützlich es für die Untersuchung grundlegender Probleme des Membrantransportes ist. Lebende Zellen sind aber durch Membranen in zahlreiche Kompartimente gegliedert. Die Mannigfaltigkeit der Transportprozesse an den Grenzen dieser Kompartimente ist noch um ein Vielfaches größer, weil an jeder Membranbarriere die verschiedensten Stoffe transportiert werden müssen. Man kann versuchen, möglichst viele dieser Membrantransporte zu analysieren und sich überlegen, wie sie zusammen oder gegeneinander wirken.
Chapter
In the 57 years of research reviewed in this essay, transport functions were studied in a variety of plant systems. Processes of membrane transport are essential in the operation of various glands, such as nectary glands, the glands of carnivorous plants, and the salt glands of halophytes. In the photosynthetic mode of crassulacean acid metabolism (CAM), a central feature is nocturnal accumulation of organic acids in the vacuoles. Thus, CAM poses a transport problem, which was resolved by the identification of the complement of an H+-transporting ATPase, a malate channel, and a passive diffusion of non-dissociated malic acid at the tonoplast. The free running endogenous rhythm of CAM is operated by a biochemical-biophysical oscillator where the tonoplast acts as a master switch. The paths of transport with apoplastic and symplastic transport and diffusion in the gas phase of aerenchymas couple and integrate cells within tissues. The energization of membrane transport is linked to the multicomponent network of energy metabolism. Transport in roots and leaves was investigated to show this. All these features of plant biology indirectly or directly bear relations to physiological ecology of field performance. CAM is an ecophysiological adaptation to limited water supply which was studied intensively in the field in various tropical environments with respect to physiological autecology and ecosystem-relevant synecology. Whole-plant physiology shows that transport is the basis of the functioning of entire plants. Transport is the pathway for interaction and integration creating plant’s individuality as unitary organisms. The integration of modules via transport leads to the emergence of holistic systems across a large range of scalar levels from compartments within cells to cells and eventually the whole biosphere. Comprehending emergence of holism leads to understanding life beyond mechanistic modularity.
Chapter
In den letzten Jahren, deren Literatur hier besprochen werden soll, (ungefähr zurückreichend bis 1963/64), erschienen vielbeachtete elektronenmikroskopische Arbeiten zum Problem der Membranstruktur (z.B. Sjøstrand, 1963, 1964; Fernandez-Moran, 1963, 1964; Mühlethaler, 1966 a u. b; Branton, 1966), noch kennzeichnender für diesen Zeitraum war aber, daß auch Physiologen und Biochemiker versuchten, ihre Befunde über funktionelle Eigenschaften von Membranen nicht nur festzustellen, sondern auch auf das Bild von der Membranstruktur zu projizieren.
Chapter
In relation to ions, plant roots have two main functions. At the surface of the root ions are absorbed from the soil or solution; from within the root, ions are transported to other parts of the plant. Active processes maintain concentrations of ions in the vacuole and cytoplasm of cortical cells as well as participating in the general movement across the root to the stele.
Chapter
Transport is an important feature of the contact between plants and their environment. In many lower plants, the plant body, or the population of cells is predominantly of a single type and this cell type mediates all the exchanges between the plant and its environment. This is partly the reason why certain algal cells (Part A, Chap. 6) and fungal hyphae (Part A, Chap. 7) have become standard systems for study of transport processes (e.g. Chara, Chlorella, Neurospora).
Chapter
Since the shapes of influx isotherms are being accepted as evidence for two (or more) transport processes involved in influx to plant cells, and since parameters of curves fitted to isotherms are being used as fundamental characteristics of transport systems, it is becoming important to examine critically whether these interpretations of influx data are valid. The necessity for such a reappraisal is most strikingly apparent when one considers the possible ways of identifying isolated transport machinery. If the shape of the isotherm is not sufficient to distinguish or characterise transport processes it will not be profitable to attempt to identify transport machinery amongst microsomal fractions by a response to changing concentrations of the transported substance.
Article
This chapter focuses on ion absorption by cells and other tissues—such as storage roots, leaves, algae—and in organelles such as mitochondria and chloroplasts. Ion absorption by cells, including the flux of ions across both the plasma membrane and tonoplast is presented. The chapter provides an overview of nutrient absorption by excised roots. Excised roots usually function for several hours, at least with regard to ion absorption, as if they had never been removed from the shoot. The value of using excised roots that are low in salt content for studying nutrient absorption is determined. These roots accumulate salts in a short time and the increase in ion content can be measured easily either chemically or by using the radioisotope of a specific element. The chapter discusses the kinetics and selectivity of ion absorption and proposes a model for ion absorption by roots. This model consists of two main features: a cation-activated ATPase in the plasma membrane and an anion carrier that brings about the exchange of anions across the plasma membrane.
Article
Detopped root systems of Ricinus communis plants were used for the study of the effects of temperature and DNP on the uptake of K and Na ions supplied as KNO3 and NaNO3. When K and Na ions were offered together in equivalent concentrations, the steady state uptake rates for K+ and Na+ at 23 to 25° gave a K+/Na+ ratio of 3. Increasing the Na+ concentration relative to K+ 3-fold did not alter the preferential uptake of K+. The uptake of K+ was more sensitive to temperature in the range 10 to 40° and to the application of DNP at 1.5x10-4 M than was the uptake of Na+. When NaNO3 was the only salt supplied Na+ uptake became more sensitive to DNP than when both K+ and Na+ nitrates were supplied. Prolonged application of DNP led to net K+ efflux from the roots, even when no K+ was being supplied to the roots. Net Na+ efflux under the influence of DNP occurred only in roots previously grown on Na-containing nutrient medium. The different responses of the K+ and Na+ uptake processes to temperature and DNP suggest the operation of different uptake mechanisms for K+ and Na+ These results have been considered in relation to the recent concept of dual mechanisms for the absorption of alkali cations by plant tissues.
Article
By means of a transport kinetic concept the uptake of a given ion by plants can be expressed as a function of the concentration of the ion in soil solution, isolated from the soil in situ in the soil-plant system at any given time during the growth period without disturbance to growing plants. The kinetic model rests on the assumption that the rate determining step in ion absorption at concentrations of 1 m M or less, is located in the tissues between epidermis and xylem and in all probability in the plasmalemma of the cortical cells. As the rate of ion transport through the cell membrane may be expressed by a kinetics, analogous to that proposed by Michaelis and Menten for enzymecatalyzed reactions, the rate of uptake (v) of ion M at any time may be expressed by the differential equation {Mathematical expression} The uptake by plants of M during a time interval to-tn is expressed by the integral of above equation. One solution is submitted and experimental conditions, necessary for a verification of the proposed kinetic concept, are discussed.
Article
Uptake of inorganic ions by higher plants can be represented by single, multiphasic isotherms, i.e. by a series of phases following Michaelis-Menten kinetics and separated by discontinuous transitions (NISSEN, 1971, 1973a-d, 1974). These kinetics indicate mediation by a structure which changes characteristics at certain discrete salt concentrations. Uptake of glucose and 3-O-methylglucose (LINASK and LATIES, 1973; NISSEN, 1974) and at least some amino acids (NISSEN, 1974; SHTARKSHALL and REINHOLD, 1974) are similarly mediated by multiphasic mechanisms.
Article
Nach wie vor nehmen Messungen der Membranpotentiale und die daraus entwickelten Vorstellungen über Influx- und Effluxpumpen einen breiten Raum ein (JESCHKE, 2; SADDLER; MACKLON u. HIGINBOTHAM; BOWLING u. ANSARI). Wenn damit auch viele Spekulationen, z.B. über Existenz von H+-Effluxpumpen (SMITH), verbunden sind, so haben diese Arbeiten doch allgemein zu einer besseren begrifflichen Trennung zwischen aktivem Transport durch Membranen und stoffwechselabhängiger Aufnahme geführt und auch erreicht, daß z.B. der Ladung der Zellmembranen für die Ionenaufnahme größere Bedeutung geschenkt wird (THELLIER).
Article
We have previously brought forward evidence (SHTARKSHALL et al., 1970; REINHOLD et al., 1970) for the operation of an amino acid transport system in leaf cells, and demonstrated the usefulness of the unnatural analogue α-aminoisobutyric acid (AIB) in characterizing it. In the present communication we report on further studies on this system: a more extensive examination of the uptake versus concentration curve and its possible explanation; the sensitivity of the system to pH; and its structural specificity.
Article
The influence of the counter-ion on K uptake was investigated in excised barley roots. The effect of the counter-ion on K uptake is restricted to the first three to four hours of the experimental period. The results suggest that the counter-ion influences only the binding of K ions in the cytoplasm and not the accumulation to the vacuole; and fit into the previously developed general scheme of cation uptake, according to which the rate of vacuolar accumulation of K is limited at the tonoplast.
Article
Zusammenfassung Transport-ATPasen sind in pflanzlichen Geweben nur vereinzelt nachgewiesen (z.B. MacRobbie 1965, Cram 1968). Dennoch scheint ATP für viele metabolische Transportprozesse als energiereicher Cofaktor zu fungieren (Marré et al. 1963, Weigl 1964a, 1967a, Jeschke 1967, Rains 1968, Penth und Weigl 1969). Die Apofaktoren spezifischer Transportsysteme müssen unter Beteiligung von tertiär strukturierten Proteinen konstruiert sein. Diese müssen in der Diffusionssperre (Abb. 3c und d), nicht nur neben der Diffusionssperre angenommen werden (Weigl 1963, 1964b, 1967b). Über die molekulare Architektur der Proteine und Lipoide in Zellmembranen wissen wir aus elektronenmikroskopischen und anderen Untersuchungen nichts zugunsten des Konzepts der Abbildungen 3a und b. Anelektrolyte permeieren auch durch eine, Lipoproteinmembran (Abb. 3c bis d) nach dem Öl/Wasser-Verteilungskoeffizienten. Die Abweichungen von dieser Regel sind bei derartigen Membranen, nicht jedoch bei Membran 3a verständlich. Lipoproteinmembranen sind also nicht nur zu vielfältigen spezifischen Vermittlungs- und Beschleunigungseffekten fähig, sondern sie sind auch, semipermeabel”. Das Verhältnis der in der Proteinstruktur als inter- oder intramolekular anzusprechenden Lipoide (Abb. 3c bis d) ist ebenso unbekannt wie überhaupt das Mengenverhältnis der Proteine und Lipoide in den verschiedenen Zellmembranen der Pflanzen.
Article
Ion absorption and transport by intact roots, isolated cortex and isolated stele were compared shortly after tissue isolation and after aging. Absorption isotherms in the low and in the high concentration range show that in stripped-stele, which absorbs at a very low rate immediately after isolation, the capacity of system 1 but not system 2 is built up with aging. In agreement with this result analysis of individual fluxes across plasmamembrane and tonoplast reveals that only the influx from the medium into the cytoplasm increases considerably with aging of stele. Changes observed in aging excised roots and in isolated cortex are much less significant. In spite of the increase of absorption with aging by isolated stele, long distance transport, which is essentially passive through freshly stripped stele, decreases with aging. The reported results reflect the marked permeability of the plasmamembrane of fresh isolated stele, and demonstrate the importance of the cortex as a tissue "collecting" ions for long distance transport. New evidence for the theory of symplasmatic transport of ions into the xylem vessels is thus provided.
Article
The effects of Ca²⁺ and K⁺ on the ATPase activities of microsomal fractions prepared from mature roots, cell suspension and callus cultures and redifferentiated roots of rice (Oryza sativa L. cv. Dunghan Shali) exhibit correlations with the effects of Ca²⁺ on the K⁺ influx rates. ATPase stimulated by Ca²⁺ and K⁺ and the stimulatory effect of Ca²⁺ on K⁺ influx are associated in mature roots of low‐salt status. Roots of high‐salt status also possess ATPase of this kind, but K⁺ influx is depressed by Ca²⁺. Cell suspension and callus tissue cultures do not possess K⁺ influx with a positive Ca²⁺‐response and the ATPase prepared from them is insensitive to the effects of Ca²⁺ and K⁺. Redifferentiated roots show similar transport characteristics and enzyme pattern as mature roots of high salt status. The specific transport function of the membrane, and the ATPase stimulated by Ca²⁺ and K⁺ are suggested to develop during diffrentiation.
Article
The efflux of (36)Cl and (86)Rb and the fluxes of these ions into the xylem were investigated using the device shown in Fig. 1.Efflux of (36)Cl is stimulated by external KCl while transport into the xylem is inhibited. Stimulation of the efflux appears to be stronger than inhibition of the transport.The stimulation of the efflux of (36)Cl was also observed with roots of intact seedlings.Assuming that the mode of transfer of Cl(-) into the xylem (flux 3, Fig. 8) is diffusion exhibiting a linear isotherm (LUTTGE and LATIES, 1966), these results suggest that the primary action of external salts is on the efflux across the plasma-lemma (WEIGL, 1967, 1968). We were unable, however, to find a linear relationship between concentration and rate of chloride transport to the shoots of intact seedlings.With respect to the mode of ion transfer to the xylem (WEIGL and LüTTGE, 1965; LUTTGE and LATIES, 1966) we have to be aware of the following facts:A linear isotherm cannot be taken to signify diffusive permeation (TORII and LATIES, 1966; LUTTGE and LATIES, 1966). If the Michaelis constant is extremely high relative to the ion concentration, the relationship between the ion concentration and the rate of a metabolic or mediated transport approaches linearity.The isotherm of the transport into the xylem may primarily reflect the difference of two large fluxes (4 and 5; Fig. 8).The transport data of LUTTGE and LATIES (1966) need not be presented as a straight line (Fig. 6).If at high external ion concentrations the ratio of the ion concentration in the exudation sap to the external ion concentration approaches unity, diffusive permeation into the stele is still not proved to be the mode of migration, since at high stelar ion concentration flux 6 tends to become equal to flux 3.Considerations on radial ion transfer into the xylem depend on contemporary knowledge of the location of transport systems. Cl(-)-uptake into root tips (2 mm) from solutions of 1-10 mM KCl did not exhibit a linear isotherm. These results are unpublished since the discrepancy to the results of TORII and LATIES (1966) may be due to a higher content of vacuoles in our root tips. We feel it unlikely, however, that a linear isotherm of Cl(-)-uptake into root tips is adequately explained by assuming that it is due to a lack of vacuoles while the sensibility to inhibitors is assumed to be due to the presence of vacuoles in root tips.Transport of Cl(-) into the xylem is susceptible to inhibitors of oxydative phosphorylation, suggesting that this process, even at high external ion concentrations, is dependent on metabolic energy in contrast to the passive efflux from the cortical cells across the plasmalemma into the environment of the root. The precise location of the metabolic step(s) on the pathway of ions from the environment of the root to the xylem is unknown.The observed effects of Ca(++), EDTA and IAA may be considered in relation to the theory that auxin exerts its influence on growth by altering the diffusion potential across cell membranes (BRAUNER and DIEMER, 1967). Growth is susceptible to the effect of Ca(++) and EDTA (ADAMSON, 1962; SETTERFIELD, 1963; THIMANN, 1963). Nevertheless, since IAA exerts no influence on ion fluxes in corn roots, it is not clear whether IAA really exerts its influence on growth by altering the diffusion potential across plant cell membranes. We might be dealing with occasional effects of secondary importance.
Article
Protoplasts wereenzymically isolated fromsuspension cultured cells of Nicodana glutnoss L.andaspects oftransport selectivity andkinetics werestudied. Inthepresence ofCa2O, transport wasselective forK+ (8Rb) overNa+.36CI- transport wasinhibited byBr-orI-butnotby H2PO4-. Te kinetic dataforshort term(30minutes) K+influx overthe rangeof0.05to100milmlar KCIwerecomplex butsimilar tothose observed inother plant tissues. Incontrast, thekinetic dataforCl-and H232P04- overthesameconcentration range weredifferent fromthose observed forK+,andcould beaccounted forbyasingle isotherm inthe range of0.05 to4miHimolar andbyanalmost linear increase ininflux rate above 4millimolar. Thekinetic dataforCl-transport into intact cultured cells wereidentical incharacter tothose observed forisolated protoplasts. Theresults support theviewthat enzymic removal ofthecell wail produced nosignificant alteration inthetransport properties oftheprotoplast. Inthepreceding paper(7), we showedthatitisfeasible touse protoplasts isolated fromcultured tobacco cellstostudy iontrans- port.Transport ofK+(6Rb), 36CF-, andH232P04- intoisolated tobacco protoplasts was tightly coupled toenergyproduction by aerobic respiration. Influx ofthese ionsresponded tochanges in pHortosubstances suchasCa21 orfusicoccin, inasimilar manner ashasbeenreported forintact plant cellsandtissues. Therewas no indication thatthetransport properties oftheprotoplast was significantly altered byenzymic removal ofthecell wall. Inthis paperwe characterize further thetransport properties of isolated tobacco protoplasts byconsidering aspects oftheselectiv- ityandkinetics oftransport inthese cells. MATERIALS AND METHODS CellCulture andProtoplast Isolation. Experiments were con- ducted withprotoplasts isolated fromcultured cellsofNicotiana glutinosa L.asdescribed intheaccompanying paper(7). Briefly, cells weremixedwith1%(w/v) Cellulysin and0.2%Macerase in 0.7M mannitol andincubated for4hrtodigest thecellwalland release theprotoplast. Theprotoplasts were filtered, andthen collected andwashedbyrepeated centrifugation in0.7 M mannitol. Thefinal protoplast pellet (2-3mlpacked volumefrom20gfresh weight ofcultured cells) was suspended in5 volumes of0.7M mannitol.
Article
We have examined the kinetics of chloride absorption by barley root tissue in experiments similar to those in which we demonstrated the duality of potassium absorption mechanisms in this tissue ( and ). In those experiments, rates of potassium absorption were found to be a function of the external concentration of KCl in accord with Michaelis-Menten kinetics over a low concentration range, approaching the theoretical maximal velocity at an external concentration of 0.2 mM KCl. At higher concentrations (0.5 to 50 mM), another mechanism of potassium absorption was shown to come into play. In the present experiments, we measured une rate of chloride absorption from solutions of KCl ranging in concentration from 0.005 mM to 50 mM.
Article
The purpose of this investigation is the study of the properties of the cell with regard to potassium absorption as the cell expands. Serial sections each 1·5 mm. in length have been cut along the course of maize roots over the first 9·0 mm. from the tip. The sections have been immersed in 0·01 N. solution of potassium chloride in water and in 2 per cent. fructose. Absorption from these solutions has been measured over a period of 48–72 hours. It is shown that absorption by the first section which consists almost entirely of meristematic cells is abnormally low, and this is attributed to the absence of a tonoplast surrounding a central vacuole in the cells of this section. The different initial rates with the different sections indicate that after vacuolation in the root has occurred the rate per unit area of surface decreases as the cell expands. When the data are reduced to a unit cell basis, however, they show that, per cell, absorption increases as expansion occurs in the root. This is attributed to an increasing protein content, and the decrease in rate per unit area to a corresponding decrease in protein per unit area. At the same time it is shown that during cell growth in the root the cytoplasm differentiates in such a way that absorption per unit protein increases. After excision, whether growth occurs or not, the protein content does not increase. The final internal concentrations in the different sections vary according to the growth that occurs. When vacuolation has occurred and growth is limited the final internal concentration is greater than it is at the beginning of the experiment. When growth occurs, on the other hand, the final internal concentration may be lower. This is taken to indicate that absorption depends on an inward secretion into the vacuole which is independent of surface area and an outward diffusion into the medium which increases with increase in surface area. It is suggested that the results obtained with sugar support this interpretation. When sugar is provided the rate of absorption is always stimulated and when growth does not occur the final internal concentration is enhanced. When growth occurs, however, sugar not only stimulates absorption but also the expansion of the cell, and the latter effect leads to a final concentration which may be lower than that given in the absence of sugar.
Article
Plant nutrients in the soil reach the root by root extension, mass-flow, and diffusion. Plant roots grow to less than 3% of the available nutrients in the soil. They may grow to much of the plant's calcium and magnesium requirement but very little of the plant's need for nitrogen, phosphorus, and potassium. Mass-flow may supply the root with much of the plant's need for calcium, magnesium, and nitrogen, but this process does not supply much of the requirement for potassium or phosphorus on many soils. Usually most of the phosphorus and potassium must reach the root by diffusion. Nutrient absorption by the root lowers the phosphorus and potassium content in the soil at the root-soil interface, and a concentration gradient is established along which diffusion occurs. Diffusion is very slow in soil as compared to water.
Article
The distribution of K and Na between root and shoot of intact maize seedlings after absorption periods of increasing length up to 24 hours was determined in three series of experiments. In two additional series the redistribution between root and shoot of K and Na absorbed previously during a period of 12 hours was followed up during the next 12 hours. Concentrations were such that either the specific or both the specific and non-specific mechanism of K-absorption present in this tissue were operative. A high degree of specificity in the transfer to the shoot became apparent, the amount of Na transported to the shoot within 24 hours being negligible in comparison to the amount of K transferred within the same period. For the most part this specificity appeared to be due to the fact that the specific mechanism of K-absorption is especially involved in the supply of K for transfer to the shoot. However, non-specifically absorbed K is also liable to transport to the aerial parts at a much higher rate than Na and, in contrast to Na, the translocation of K continues after the plants have been transferred to K- and Na-free media. A scheme is proposed to explain the phenomena described.
Article
The relationship between the rates of absorption of K and Rb by barley roots and the concentration of these ions in the external solution, over the range 0.002 to 50 mM, is predictable on the assumption that two carrier sites bind and transport the ions. One of these operates at half-maximal velocity at a concentration of about 0.018 mM, with very low affinity for Na, the second at about 16 mM, with severe competition by Na. The latter but not the former mechanism is inhibited when SO/sub 4/ is the anion instead of Cl. (auth)
Article
Themechanisms whereby plant cells absorbin- organic ionsfromtheexternal mediaareselective: theydiscriminate amongdifferent ions.Particularly intriguing isthediscrimination betweenpotassium andsodiumions.Mostplant cells accumulate much higherconcentrations ofpotassium thanofsodium ions, evenfrommediainwhichthesodiumconcentra- tiongreatly exceeds thatofpotassium. Perhaps thebest waytoinvestigate these specifici- tiesistostudy themannerinwhichdifferent ions affect eachother's absorption. Ina previous paper (4)experiments weredescribed demonstrating that inexcised barley roots potassium andrubidium com- petewitheachother inanidentical absorption mech- anism.Atmoderate concentrations, neither sodium norlithium ionscompeted forthepotassium-rubidium transport mechanism. Thededuction wasthat there arespecific ionbinding sites oncarrier molecules whicheffect thetransport oftheionsacross cellular membranes. Potassium andrubidium ionsarebound andtransported bysites whichfail todiscriminate between them,hencethese ionscompete witheach other foridentical carrier sites. Sodiumandlithium arenotboundbythepotassium-rubidium sites, hence
Factors affecti -the fluixes of potassiumn1 and cl oride(l loios ini Vill
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The plhosplhate absorption by sugarcane. Verslag 13e Vergadering Vereeniiging Proefstations Personeel
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Cationis and(I anlioins: Inlhibitionls and(i initeractionis iii miIetabolism and in enzyme activity
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A comi1parison1 of clhloriide anid potassium fluxes in red beet tissule. Phv siol
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The essential role of calciumni inl selective cation transport by planit cells
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Catioinanion balanice duiring potassium anid sodiuim absorption by barley roots
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The determinatioll of the salt relationis of the cy tolplasmic phase of beetroot tissue
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Ioii1c relations of Vil, I/a trans1cltcn
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Effects of Ca upon metabolic anid noiinmetabolic uptake of Na anid RI) by root segments of Zca 7mavs
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Carbon dioxide fixationi anid ioIn absorption in barley-roots
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Or-ianic acidl sv ntihesis in respionse to excess cationi absorption in vaccuolate anld nonvacuiolate section.s of corii aiid, barley roots
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Metabolic and(I nionimetabolic uptake of sodiumii ii rioots of Zca ways
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The role of calciuim in the absorption of aniions and( cations by excised barley roots
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Carrier-m11edi- ated cationi transl)ort in barley roots: Kinetic evi(lence for a sp)ectrum of active sites
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Absorption of cationis bl roots. Effects of hydrogen ions and essential role of calciuimil
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