Short term 22Na+ and 42K+ uptake in intact, mid-vegetative plants

Spergularia marina (L.) Griseb. is. a rapidly growing, annual, coastal halophyte. Because of its small size, it is suitable for isotope studies of ion transport well beyond the seedling stage. The purpose of this report is to establish the similarities and differences between ²²Na⁺ and ⁴²K⁺ uptake in S. marina and in more commonly used mesophytic crop species. Vegetative plants were used 18 days after transfer to solution culture. Plants were grown either on Na⁺-free medium or on 0.2 × sea water. ²²Na⁺ uptake was linear with time for several hours. The rate was relatively insensitive to external concentration between 1 and 180 mol Na⁺ m?³, particularly in Na⁺-free plants. Transport to the shoot accounted for 40 to 70% of the total uptake, dependent on salinity but largely independent of time. ⁴²K⁺ uptake decreased with increasing salinity in Na⁺-free plants and increased in 0.2 × sea water plants. Both uptake and transport to the shoot were non-linear with time, upward concavity suggesting recovery from a manipulative and/or osmotic injury. Steady state root contents were compared with predicted contents based on cortical cell electrical potentials using the Nernst equation. Reasonable agreement was found in all cases except Na⁺ content of 0.2 × sea water plants, in which active efflux was indicated. Uptake studies conducted in the presence of chemical modifiers (dicyclohexylcarbodiimide, dinitrophenol and fusicoccin) showed responses of ⁴²K⁺ uptake as expected from studies on agronomic species, and implied the presence of a similar active uptake here despite the appearance of equilibrium. Active Na⁺ uptake was suggested at low Na⁺ levels. We conclude that S. marina is a promising experimental system combining the rapid nutrient acquisition strategy of agionomically important annuals with a high degree of salt tolerance.     

Control of Na+ and K+ transport in Spergularia marina. III. Relationship between ion uptake and growth at moderate salinity

In this paper, we continue our analysis of Na⁺ and K⁺ uptake by mid-vegetative Spergularia marina (L.) Griseb. plants growing on 0.2x sea water medium, with attention to the relationship of ion uptake and growth. In the first part of the paper, growth analysis techniques are used to compare relative growth rates (RGR) and relative accumulation rates (RAR) for Na⁺ and K⁺. Under constant growth conditions, a high correlation between RGR and RAR indicated that growth and accumulation of both ions were well balanced, resulting in Na⁺ and K⁺ concentrations within the plants which were stable after adjustement to the saline medium. The analysis confirmed the existence of a Na⁺ -related growth stimulation in S. marina and an associated increase in the efficiency of K⁺ utilization for growth. When plants were subjected to more rapid salinization and step changes in the light intensity of the growth chamber, RGR and RAR were again similar, even through the discontinuities in growth conditions, suggesting that growth and ion accumulation were co-regulated rather than simply correlated. The growth analysis data were then transformed to give net uptake rates for Na⁺ and K⁺ and the results were compared to those of isotope studies under similar growth conditions. In roots, the rates estimated by the two techniques differed substantially; net uptake rates reflected primarily growth, while isotope studies indicated a substantial ion exchange rate between mature cells and the growth medium. The rates of transport of either Na⁺ or K⁺ to the shoot were very similar using the two estimation techniques. As the rates measured with isotopes were taken from studies lasting at most a few hours, this suggested a very rapid turnover of the upwardly mobile Na⁺ and K⁺ pools in the roots.     

Long-term effects of plant hormones on K+ levels and transport in young wheat plants of different K+ status

Long-term effects of 1-naphtaleneacetic acid (NAA), benzyladenine (BA), gibberellic acid (GA₃), abscisic acid (ABA) and ethylene on K⁺ levels, K⁺ uptake and translocation to the shoot were studied in young wheat plants (Triticum aesticum L. cv. Martonvásári-8) grown at different K⁺ supplies. Na⁺ levels and K⁺/Na⁺ selectivity were also investigated. Both in shoots and roots, NAA, BA and ABA decreased K⁺ and Na⁺ levels more effectively in high-K⁺ plants than in low-K⁺ plants. GA, and ethylene did not influence K⁺ and Na⁺ levels. K⁺/Na⁺ selectivity in roots of low-K⁺ plants was increased in favour of K⁺ by BA, NAA and to a lesser extent by ABA. In high-K⁺ plants only BA increased the K⁺/Na⁺ ratio, whereas the effects of the other hormones were the opposite (NAA) or less pronounced (ABA). K⁺(⁸⁶Rb) uptake was inhibited by NAA and BA in low-K⁺ plants but not in high-K⁺ plants. K⁺(⁸⁶Rb) uptake was inhibited throughout by 10 μM ABA. K⁺(⁸⁶Rb) translocation to the shoot was influenced by the hormones similarly to the uptake patterns, with the exception of ABA, which inhibited translocation in low-K⁺ plants but not in high-K⁺ plants. The results show that hormonal effects may quantitatively and qualitatively be modified by K⁺ levels in the plant and that internal K⁺ concentration may play a role in the mechanisms regulating the effects of NAA, BA and ABA but probably not in those of GA₃ or ethylene.     

Kinetic studies of a (Na++ K++ Mg2+)ATPase in sugar beet roots. III. A proposed model for the (Na++ K+) activation and its significance for field properties

A microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na⁺ and K⁺ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ratio between Na⁺ and K⁺. At low Na⁺ concentration (below 5 mM), the apparent K_m decreases with increasing concentrations of K⁺ (1–20 mM). At 5 mM Na⁺, the K⁺ level does not change the apparent K_m, while at Na⁺ levels above 10 mM, the apparent K_m between enzyme and substrate increases with increasing concentration of K⁺. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na⁺ and K⁺. The Na⁺ binding is cooperative with different K_m values and Hill coefficients (n) in the presence of low and high concentration of K⁺. At low Na⁺ level (< 5 mM). a negative cooperativity exists for Na⁺ (n_Na < 1) which is more pronounced in the presence of high [K⁺]. When the concentration of Na⁺ is raised the negative cooperativity disappears and turns into a positive one (n_Na > 1). Only K⁺ binding in the presence of low [Na⁺] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K⁺ (n_K < 1 → n_K > 1). In the presence of [Na⁺] > 10 mM, the changes in n_k are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na⁺, K⁺ and MgATP. 4. In the presence of Na⁺ (< 10 mM), K⁺ is probably bound to two K sites, one of which translocates K⁺ through the membrane by an antiport Na⁺/K⁺ mechanism. This could be connected with an elevated K⁺ uptake in the presence of Na⁺ and could therefore explain some field properties of sugar beets.     

Effects of interrupted K+ supply on growth and uptake of K+, Ca2+, Mg2+ and Na+ in spring wheat

Effects of interrupted K⁺ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K⁺ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K⁺ supply. Root structure was greatly affected by the length of the period in K⁺ -free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K⁺ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K⁺ -free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K⁺. In comparison, both higher and lower internal/external K⁺ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca²⁺, Mg²⁺ and Na⁺ uptake. Uptake of Ca²⁺ and especially Mg²⁺ significantly increased after K⁺ interruption, while Na⁺ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K⁺ in the cells and maintain electroneutrality down to a certain minimal range of K⁺ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)^?1] than in the root [20 to 30 μmol (g fresh weight)^?1]. It is suggested that the critical K⁺ values are a measure of the minimal amount of K⁺ that must be present for physiological activity in the cells. At the critical levels, K⁺ (⁸⁶Rb) influx and Ca²⁺ and Mg²⁺ concentrations were maximal. Below the critical K⁺ values, growth was reduced, and Ca²⁺ and Mg²⁺ could no longer substitute for K⁺ for electrostatic balance. In a short-term experiment, the ability of Ca²⁺ to compete with K⁺ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K⁺ status. The results indicate that K⁺, which was taken up actively and fastest at the external K⁺ concentration used (2.0 mM), partly determines the size of Ca²⁺ influx.     

Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress

Na⁺ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP- and PP_i-dependent H⁺ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na⁺/H⁺ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259–268). During a mild salt stress, Na⁺ was mainly accumulated in the roots. Under a more severe salt treatment, Na⁺ was equally distributed in shoots and roots. In contrast to what was observed with Na⁺, all the salt treatments reduced the shoot K⁺ content. Dissipation by Na⁺ of the H⁺ gradient generated by the tonoplast H⁺-ATPase, monitored as fluorescence quenching of acridine orange, was used to measure Na⁺/H⁺ exchange across tonoplast-enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na⁺/H⁺ exchange activity, which displayed saturation kinetics for Na⁺ added to the assay medium. This activity was partially inhibited by 125 μM amiloride, a competitive inhibitor of Na⁺/H⁺ antiports. No Na⁺/H⁺ exchange was detected in vesicles from control roots. The activity was specific for Na⁺. since K⁺ added to the assay medium slightly dissipated H⁺ gradients and displayed non-saturating kinetics for all salt treatments. Apparent K_m for Na⁺/H⁺ exchange in tonoplast vesicles from 150 mM NaCl-treated roots was lower than that of 75 mM NaCl-treated roots, V_max remaining unchanged. The results suggest that the existence of a specific Na⁺/H⁺ exchange activity in tonoplast-enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.     

Uptake by Yeast: Interaction of Rb+, Na+ and Phosphate

It has been shown that addition of phosphate to phosphate deficient yeast gives rise to an immediate increase in the rate of Na⁺ uptake and an immediate decrease in the rate of Rb⁺ uptake. In addition, phosphate uptake is enhanced specifically by Na ions presumably by a process with a very high affinity for phosphate with a K_m of about 2 × 10⁻⁶M at pH 7.2, whereas the K_m for phosphate uptake of the Na⁺ independent process amounts to 1.3 × 10⁻⁴M.     

Control of K+ (Rb+) influx and transport with changed internal K+ concentration and age in two varieties of barley

The influx of Rb⁺ into the roots of two barley varieties (Hordeum vulgare L. cv. Salve and cv. Ingrid) from a K⁺-free ⁸⁶Rb-labelled nutrient solution with 2.0 mM Rb⁺, was checked at intervals from day 6 to day 18. The control plants were continuously grown in complete nutrient solution containing 5.0 mM K⁺, while two other groups of plants were grown in K⁺-free nutrient solution starting on day 6 and between day 6 and day 9, respectively. The pattern of Rb⁺ influx was similar for both varieties, although their efficiencies in absorbing Rb⁺ were different. The relationship between Rb⁺ influx and K⁺ concentration of the root could be interpreted in terms of negative feedback through allosteric control of uptake across the plasmalemma of the root cells. Hill plots were bimodal, but in the opposite direction. The Hill coefficients, reflecting the minimum number of interacting allosteric binding sites for K⁺ (Rb⁺), were low (≤–3.0). It is discussed whether the threshold value, that is the breaking point in the Hill plot, is indicative of a changed efficiency of transporting units for K⁺ (Rb⁺) transport to the xylem. Moreover, feedback regulation might be involved in transport of K⁺ between root and shoot. The variation in K⁺ concentrations in the roots and shoots of control plants were cyclic but in phase opposition despite an exponential growth. The average K⁺ concentration varied only slightly with age.     

Kinetic Studies of a (Na++ K++ Mg2+) ATPase in Sugar Beet Roots

Kinetic studies of a dithiothreitol treated membrane ATPase fraction from sugar beet roots led to the following conclusions: 1) In the presence of MgATP, Na⁺ and K⁺ stimulate the ATPase activity in different ways following simple Michaelis-Menten kinetics. Thus separate sites for Na⁺ and K⁺ are suggested. 2) In the absence of K⁺, Na⁺ acts as an uncompetitive modifier raising the apparent K_m and V_max for MgATP. 3) In the absence of Na⁺, K⁺ activates non-competitively with respect to MgATP. Thus K⁺ increases V_max but does not affect the apparent affinity constant. 4) K⁺ and Na⁺ double the rate constants. 5) In the presence of Na⁺ or K⁺, Mg²⁺ in excess acts as a weak inhibitor to Na⁺ and/or K⁺ activity. 6) The temperature-activity dependence in the 5–40°C interval shows biphasic Arrhenius plots with the transition point between 15–18°C. The activation energy is lowered at temperatures > 18°C.     

Kinetics of NH4+ and K+ uptake by ectomycorrhizal fungi. Effect of NH4+ on K+ uptake

NH₄⁺ and K⁺ uptake experiments have been conducted with 3 ectomycorrhizal fungi, originating from Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) stands. At concentrations up to 250 μM, uptake of both NH₄⁺ and K⁺ follow Michaelis-Menten kinetics. Laccaria bicolor (Maire) P. D. Orton, Lactarius rufus (Scop.) Fr. and Lactarius hepaticus Plowr. ap. Boud. exhibit K_m values for NH₄⁺ uptake of 6, 35, and 55 μM, respectively, and K_m values for K⁺ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH₄⁺ raises the K_m of K⁺ uptake by L. bicolor to 35 μM, while the V_max remains unchanged. It is argued that the increase of K_m is possibly caused by depolarization of the plasma membrane. It is not due to a competitive inhibition of K⁺ by NH₄⁺ since the apparent inhibitor constant is much higher than the K_m, for NH₄⁺ uptake. The possibility that NH₄⁺ and K⁺ are taken up by the same carrier can be excluded. The K_m, values for K⁺ uptake in the two other fungi are not significantly affected by 100 μM NH₄⁺. Except for a direct effect of NH₄⁺ on influx of K⁺ into the cells, there may also be an indirect effect after prolonged incubation of the cells in the presence of 100 μM NH₄⁺.     

Sodium transport measured in plasma membrane vesicles isolated from wheat genotypes with differing K+/Na+ discrimination traits

Right-side-out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two-phase system. The purity and orientation of the vesicles were confirmed by marker enzyme analysis. Membrane potential (Ψ)-dependent ²²Na⁺ influx and sodium/proton (Na⁺/ H⁺) antiport-mediated efflux across the plasma membrane were studied using these vesicles. Membrane potentials were imposed on the vesicles using either K⁺ gradients in the presence of valinomycin or H⁺ gradients. The ΔΨ was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na⁺ into the vesicles was stimulated by a negative ΔΨ and had a K_m for extrav-esicular Na⁺ of 34.8 ± 5.9 mol m³. The ΔΨ-dependent uptake of Na⁺ was similar in vesicles from roots of hexaploid (cv. Troy) and tetraploid (cv. Langdon) wheat differing in a K⁺/Na⁺ discrimination trait, and was also unaffected by growth in 50 mol m^?3 NaCl. Inhibition of ΔΨ-dependent Na⁺ uptake by Ca²⁺ was greater in the hexaploid than in the tetraploid. Sodium/proton antiport was measured as Na⁺-dependent, amiloride-inhibited pH gradient formation in the vesicles. Acidification of the vesicle interior was measured by the uptake of ¹⁴C-methylamine. The Na⁺/H⁺ antiport had a K_m, for intravesicular Na⁺ of between 13 and 19 mol m^?3. In the hexaploid, Na⁺/H⁺ antiport activity was greater when roots were grown in the presence of 50 mol m^?3NaCl, and was also greater than the activity in salt-grown tetraploid wheat roots. Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K⁺/Na⁺ discrimination. It is concluded that neither of the transport processes measured is responsible for the Na⁺/K⁺ discrimination trait located on the 4D chromosome of wheat.     

Acclimatization of K+ uptake to changes in root temperature: experiments with oilseed rape and barley in flowing solution culture

Abstract Changes in the net uptake rate of K⁺ and in the average tissue concentration of K⁺ were measured over 14 d in response to changes in root temperature with oilseed rape (Brassica napus L. cv. Bien venu) and barley (Hordeum vulgare L. cv. Atem). Plants were grown in flowing nutrient solutions containing 2.5 mmol m^?3 K⁺ and were acclimatized over 49 d (rape) or 28 d (barley) to low root temperature (5°C) prior to steady–state treatments at root temperatures between 3 °C and 25 °C, with common air temperature. Uptake of K⁺ was monitored continuously over 14 d and nitrogen was supplied as NH₄⁺+ NO^?₃ or NH⁺₄ or NO^?₃. Unit absorption rates of K⁺ increased with time and with root temperature up to Day 4 or 5 following the change in root temperature. Thereafter they usually approached steady-state, with Q₁₀? 2.0 between 7 °C and 17°C, although rates became similar between 7 °C and 13°C. Uptake of K⁺ by rape plants was invariably greater under NO^?₃ nutrition compared with NH⁺₄. The percentage K⁺ in the plant dry matter increased with temperature from 2% at 3 °C to 4% at 25 °C in rape, but there was less effect of temperature on the average concentrations of K⁺ in the plant fresh weight or plant water content. Concentrations of K⁺ in the leaf water fraction of rape plants decreased with increasing root temperature, but in barley they increased with increasing root temperature. Concentrations of K⁺ in the root water fraction were relatively stable with respect to root temperature. The results are discussed in terms of compensatory changes in K⁺ uptake following a change in root temperature and the relationships between growth, shoot: root ratio and K⁺ composition of the plant.     

Influence of abscisic acid on unidirectional fluxes and intracellular compartmentation of K+ and Na+ in excised barley root segments

Using compartmental analysis, unidirectional fluxes of K⁺ and Na⁺ and their intracellular compartmentation in excised barley (Hordeum distichon L. cv. Kocher-perle) root segments have been measured during a steady state in the presence or absence of ABA. Almost all flux rates were altered in the presence of external ABA, in particular the xylem transport R’ and the plasmalemma influx Ø_oc (see below) were strongly inhibited in the steady state. At the same time the presence of ABA induced a strong increase in the vacuolar K⁺ and Na⁺ content Q_v and a decrease in the cytoplasmic one (Q_c). Since the fluxes of an ion and its vacuolar or, in particular, cytoplasmic concentrations are interrelated, the ratios of fluxes originating from the cytoplasm and the cytoplasmic ion content were taken into account. On this basis ABA had the following effects: a) the secretion of K⁺ or Na⁺ to the xylem vessels was drastically inhibited; b) the plasmalemma K⁺ or Na⁺ efflux Ø_co was moderately stimulated and c) the tonoplast influx Ø_cv of Na⁺ was stimulated, while the tonoplast influx of K⁺ appeared to be unchanged (the decrease in Ø_cv being due to the decreased cytoplasmic K⁺ content). By a similar argument, also the apparent inhibition of the plasmalemma influx Ø_oc of K⁺ and Na⁺ in the steady state merely is an indirect effect of ABA. It only reflects the strong ABA-induced decrease in the xylem transport, that governs the magnitude of Ø_oc in the steady state. The results are discussed with reference to possible regulatory functions of ABA. In this respect it is suggested that – in particular under conditions of stress – ABA might regulate cellular metabolic processes by changing the cytoplasmic K⁺ level.     

ACTIVE POTASSIUM TRANSPORT AND [Na++ K+]ATPase ACTIVITY IN CULTURED GLIOMA AND NEUROBLASTOMA CELLS

—The ouabain-sensitive K⁺ uptake and ATPase activities of cultured glioma and neuroblastoma cells were studied. Both cell lines showed ouabain-sensitive K⁺ uptake which correlated with the level of [Na⁺+ K⁺]ATPase activity found in the respective total cell homogenate. The glioma cells had a 2.1-fold higher rate of K⁺ uptake than neuroblastoma cells, and a 2.4-fold higher [Na⁺+ K⁺]ATPase activity. In the presence of ouabain neuroblastoma cells released K⁺ and took up Na⁺ in a 1:1 ratio. These results are compared and contrasted with similar studies on brain tissue and isolated cells. It is suggested that the cultured cell lines may serve as good models for the cation transport properties of their tissue counterparts.     

RAPID EFFECTS OF VERATRIDINE, TETRODOTOXIN, GRAMICIDIN D, VALINOMYCIN AND NaCN ON THE NA+, K+ AND ATP CONTENTS OF SYNAPTOSOMES

Abstract— The effects of brief exposures of a number of depolarizing agents on ²⁴Na⁺ influx and on the Na⁺, K⁺ and ATP contents of synaptosomes were studied using a Millipore filtration technique to terminate the reaction. When synaptosomes were incubated in normal medium, there was a rapid influx of ²⁴Na⁺ and a gain in Na’contents; neither the ²⁴Na⁺ influx nor the Na⁺ gain were blocked by tetrodotoxin suggesting that this Na⁺ entry did not involve Na⁺-channels. Veratridine markedly increased the rate of ²⁴Na⁺ influx into synaptosomes and also increased the Na⁺ content and decreased the K⁺ content of synaptosomes within the first 10s of exposure. The normal ion contents were reversed by 1 min. The effects of veratridine on Na⁺ influx and on synaptosomal ion contents were prevented by tetrodotoxin and required Na⁺ in the medium. The ionophores gramicidin D and valinomycin also rapidly reversed the Na⁺ and K⁺ contents of synaptosomes, but these effects could not be blocked by tetrodotoxin. The reducing effect of gramicidin D on synaptosomal K⁺ content required Na’in the medium, whereas valinomycin caused a fall in the K⁺ content of synaptosomes in a Na⁺-free medium. Veratridine and gramicidin D, at concentrations known to reverse the synaptosomal ion contents, did not affect synaptosomal ATP levels. In contrast, valinomycin and NaCN caused an abrupt fall in synaptosomal ATP levels. The above findings suggest that veratridine quickly alters synaptosomal Na⁺ and K⁺ contents by opening Na ⁺-channels in the presynaptic membrane, and provide direct evidence for the existence of Na⁺-channels in synaptosomes. In contrast, gramicidin D and valinomycin appear to act independently of Na ⁺-channels, possibly by their ionophoric effects and, in the case of valinomycin, by diminishing synaptosomal ATP contents and hence diminishing Na⁺-pump activity. The rapid reversals of Na⁺ and K⁺ contents by these drugs could affect the resting membrane potentials, Na⁺-Ca²⁺ exchange across the synaptosomal membrane, and the release, synthesis and uptake of neurotransmitters by synaptosomes.     

Mechanism for Cd2+ inhibition of (K++ Mg2+)ATPase activity and K+ (86Rb+) uptake join roots of sugar beet (Beta vulgaris)

Steady state kinetics were used to examine the influence of Cd²⁺ both on K⁺ stimulation of a membrane-bound ATPase from sugar beet roots (Beta vulgaris L. cv. Monohill) and on K⁺(⁸⁶Rb⁺) uptake in intact or excised beet roots. The in vitro effect of Cd²⁺ was studied both on a 12000–25000 g root fraction of the (Na⁺+K⁺+Mg²⁺)ATPase and on the ATPase when further purified by an aqueous polymer two-phase system. The observed data can be summarized as follows: 1) Cd²⁺ at high concentrations (>100 μM) inhibits the MgATPase activity in a competitive way, probably by forming a complex with ATP. 2) Cd²⁺ at concentrations <100 μM inhibits the specific K⁺ activation at both high and low affinity sites for K⁺. The inhibition pattern appears to be the same in the two ATPase preparations of different purity. In the presence of the substrate MgATP, and at K⁺ <5 mM, the inhibition by Cd²⁺ with respect to K⁺ is uncompetitive. In the presence of MgATP and K⁺ >10 μM, the inhibition by Cd²⁺ is competitive. 3) At the low concentrations of K⁺, Cd²⁺ also inhibits the 2,4-dinitrophenol(DNP)-sensitive (metabolic) K⁺(⁸⁶Rb⁺) uptake uncompetitively both in excised roots and in roots of intact plants. 4) The DNP-insensitive (non metabolic) K⁺(⁸⁶Rb⁺) uptake is little influenced by Cd²⁺. As Cd²⁺ inhibits the metabolic uptake of K⁺(⁸⁶Rb⁺) and the K⁺ activation of the ATPase in the same way at low concentrations of K⁺, the same binding site is probably involved. Therefore, under field conditions, when the concentration of K⁺ is low, the presence of Cd²⁺ could be disadvantageous.     

长苞香蒲对人工盐碱湿地Na+和K+的吸收与转运特征

为揭示长苞香蒲(Typha domingensis)对盐生湿地生态系统中Na⁺和K⁺的吸收与转运特征,探讨长苞香蒲对盐生湿地的生态修复效果,该研究采用人工模拟盐生湿地的方法,设置CK(对照)、T1(浇灌100 mmol·L^-1盐水)、T2(浇灌200 mmol·L^-1盐水)及T3(浇灌300 mmol·L^-1盐水)4种不同盐浓度的人工湿地生态系统,并分别于5月5日(开始盐胁迫处理,S0)、5月30日(S1)、6月30日(S2)和7月30日(S3)测量其株高和干重、植株地上与地下部分Na⁺和K⁺的含量以及底泥和水体中Na⁺和K⁺的含量以分析长苞香蒲对盐碱湿地的脱盐作用。结果表明：(1)各处理的长苞香蒲的株高和干重随着处理时间的延长呈增加趋势,但与CK相比,各处理生长量随盐浓度升高出现下降趋势。(2)高浓度盐处理(T3)使长苞香蒲的地上部分和地下部分的Na⁺分别增加了2.5...     

Differential effects of Na+, Mg2+, K+, Ca2+ and osmotic stress on the wild type and the NaCl-tolerant mutants stl1 and stl2 of Ceratopteris richardii

In order to identify physiological components that contribute to salinity tolerance, we compared the effects of Na⁺, Mg²⁺ and K⁺ salts (NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl and K₂SO₄), Ca₂₊ (CaSO₄), mannitol and melibiose on the wild type and the single-gene NaCl-tolerant mutants stl1 and stl2 of Ceratopteris richardii. Compared with gametophytic growth of the wild type, stl2 showed a low level of tolerance that was restricted to Na⁺ salts and osmotic stress. stl2 exhibited high tolerance to both Na⁺ and Mg²⁺ salts, as well as to osmotic stress. In response to short-term exposure (3 d) to NaCl, accumulation of K⁺ and Na⁺ was similar in the wild type and stl1. In contrast, stl2 accumulated higher levels of K⁺ and lower levels of Na⁺. Ca²⁺ supplementation (1.0 mol m^?3) ameliorated growth inhibition by Na⁺ and Mg²⁺ stress in wild type and stll, but not in stl2. In addition, under Na⁺ stress (175 mol m^?3) wild-type, stll and stl2 gametopbytes maintained higher tissue levels of K⁺ and lower levels of Na⁺ when supplemented with Ca²⁺ (1.0 mol m^?3). stl2 gametophytes were extremely sensitive to K⁺ supplementation. Growth of stl2 was greater than or equal to that of the wild type at trace concentrations of K⁺ but decreased substantially with increasing K⁺ concentration. Supplementation with K⁺ from 0 to 1.85 mol m^?3 alleviated some of the inhibition by 75 mol m^?3 NaCl in the wild type and in stl1. In stl2, growth at 75 mol m^?3 NaCl was similar at 0 and 1.85 mol m^?3 K⁺ supplementation. Although K⁺ supplementation above 1.85 mol m^?3 did not alleviate inhibition of growth by Na⁺ in any genotype, stl2 maintained greater relative tolerance to NaCl at all K⁺ concentrations tested.     

Involvement of cation channels and NH4+-sensitive K+ transporters in Na+ uptake by cowpea roots under salinity

Na⁺ accumulation was investigated in the roots of 11-d-old cowpea [Vigna unguiculata (L.) Walp.] plants. The relative contribution of different membrane transporters on Na⁺ uptake was estimated by applying Ca²⁺, K⁺, NH₄ ⁺, and pharmacological inhibitors. Na⁺ accumulation into the root symplast was decreased by half in the presence of 1 mM Ca²⁺ and it was almost abolished by 100 mM K⁺. The inhibitory effect of external NH⁴⁺ on Na⁺ accumulation was more pronounced in the roots of NH₄ ⁺-free growing plants. Na⁺ accumulation was reduced about 73 % by 0.1 mM flufenamate and it was almost blocked by 2 mM quinine. In addition, 20 mM tetraethylammonium and 1.0 mM Cs⁺ decreased Na⁺ accumulation by 28 and 30 %, respectively. These results evidenced that low-affinity Na⁺ uptake by cowpea roots depends on Ca²⁺-sensitive and Ca²⁺-insensitive pathways. The Ca²⁺-sensitive pathway is probably mediated by nonselective cation channels and the Ca²⁺-insensitive one may involve K⁺ channels and to a lesser extent NH₄ ⁺-sensitive K⁺ transporters.     

短期NaCl胁迫对不同小麦品种幼苗K+吸收和Na+、K+积累的影响

为研究盐胁迫下小麦幼苗生长及Na⁺、K⁺的吸收和积累规律,以中国春、洲元9369和长武134等3种耐盐性不同小麦品种为材料,采用非损伤微测技术检测盐胁迫2 d后的根系K⁺离子流变化,并对植株体内的Na⁺、K⁺含量进行测定。结果表明:短期(2d)盐胁迫对小麦生长有抑制作用,且对根系的抑制大于地上部,耐盐品种下降幅度小于盐敏感品种。盐胁迫下,小麦根际的 K⁺大量外流,盐敏感品种中国春K⁺流速显著高于耐盐品种长武134,最高可达15倍。小麦幼苗地上部分和根系均表现为Na⁺积累增加,K⁺积累减少,Na⁺/K⁺比随盐浓度增加而上升。中国春限Na⁺能力显著低于长武134,Na⁺/K⁺则显著高于长武134。综上所述,盐胁迫下造成小麦组织器官中Na⁺/K⁺比上升的主要原因是根系K⁺大量外流和Na⁺的过量积累,耐盐性不同的小麦品种间差异显著,并认为根系对K⁺的保有能力可能是作物耐盐性评价的一个重要指标。