Al3+-Ca2+ interactions in aluminum rhizotoxicity

Several mineral rhizotoxicities, including those induced by Al³⁺, H⁺, and Na⁺, can be relieved by elevated Ca²⁺ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca²⁺ from transport channels or surface ligands that must be occupied by Ca²⁺ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al³⁺, that (i) Ca²⁺, Mg²⁺, and Sr²⁺ are equally ameliorative, (ii) that root elongation does not increase as Ca²⁺ replaces Mg²⁺ or Sr²⁺ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al³⁺ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca²⁺ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC³⁺), in contrast to Al³⁺, was specifically relieved by Ca²⁺ at the membrane surface. The rhizotoxicity induced by H⁺ exhibited a weak specific response to Ca²⁺ at the membrane surface. We conclude that the Ca²⁺-displacement hypothesis fails in the case of Al³⁺ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al³⁺. However, TEC³⁺, but not Al³⁺, may be toxic because it inhibits Ca²⁺ uptake. The nature of the specific H⁺-Ca²⁺ interaction is uncertain.Abbreviations {Al³⁺ }₀ chemical activity of Al³⁺ at the root-cell membrane surface - {Al³⁺ }_E chemical activity of Al³⁺ in the external rooting medium - E₀ electrical potential at the root-cell membrane surface - HXM²⁺ hexamethonium ion - TEC³⁺ tris(ethylenediamine)cobaltic ion     

Cation amelioration of aluminum toxicity in wheat

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practical interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing 0.4 millimolar CaCl₂ at pH 4.3 variously supplemented with AlCl₃ and additional amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 {Al³⁺}/({Al³⁺} + K_m + α{C}^β) where {Al³⁺} is the activity of Al³⁺ expressed in micromolar, {C} is the activity of the added cation expressed in millimolar, and K_m (= 1.2 micromolar) is the {Al³⁺} required for 50% inhibition in the absence of added salt. For Ca²⁺, Mg²⁺, and Na⁺ the values of α were 2.4, 1.6, and 0.011, respectively, and the values for β were 1.5, 1.5, and 1.8, respectively. With regard to relative ameliorative effectiveness, Ca²⁺ > Mg²⁺ ≈ Sr²⁺ K⁺ ≈ Na⁺. Other cations were tested, but La³⁺, Sc³⁺, Li⁺, Rb⁺, and Cs⁺ were toxic at potentially ameliorative levels. The salt amelioration is not solely attributable to reductions in {Al³⁺} caused by increases in ionic strength. Competition between the cation and Al for external binding sites may account for most of the amelioration.     

Functional reconstitution of the mitochondrial Ca2+/H+ antiporter Letm1

The leucine zipper, EF hand–containing transmembrane protein 1 (Letm1) gene encodes a mitochondrial inner membrane protein, whose depletion severely perturbs mitochondrial Ca²⁺ and K⁺ homeostasis. Here we expressed, purified, and reconstituted human Letm1 protein in liposomes. Using Ca²⁺ fluorophore and ⁴⁵Ca²⁺-based assays, we demonstrate directly that Letm1 is a Ca²⁺ transporter, with apparent affinities of cations in the sequence of Ca²⁺ ≈ Mn²⁺ > Gd³⁺ ≈ La³⁺ > Sr²⁺ >> Ba²⁺, Mg²⁺, K⁺, Na⁺. Kinetic analysis yields a Letm1 turnover rate of 2 Ca²⁺/s and a K_m of ∼25 µM. Further experiments show that Letm1 mediates electroneutral 1 Ca²⁺/2 H⁺ antiport. Letm1 is insensitive to ruthenium red, an inhibitor of the mitochondrial calcium uniporter, and CGP-37157, an inhibitor of the mitochondrial Na⁺/Ca²⁺ exchanger. Functional properties of Letm1 described here are remarkably similar to those of the H⁺-dependent Ca²⁺ transport mechanism identified in intact mitochondria.     

Characterization of a NO(3)-Sensitive H-ATPase from Corn Roots

When assayed in the presence of azide, NO₃⁻ was shown to be a specific inhibitor of a proton-translocating ATPase present in corn (Zea mays L. cv WF9 × M017) root microsomal membranes. The distribution of the NO₃⁻-sensitive ATPase on sucrose gradients and its general characteristics are similar to those previously reported for the anion-stimulated H⁺-ATPase of corn roots believed to be of tonoplast origin. An ATPase inhibited by 20 μm vanadate and insensitive to molybdate was also identified in corn root microsomal membranes which could be largely separated from the NO₃⁻-sensitive ATPase on sucrose gradients and is believed to be of plasma membrane origin. Inasmuch as both ATPase most likely catalyze the efflux of H⁺ from the cytoplasm, our objective was to characterize and compare the properties of both ATPases under identical experimental conditions. The vanadate-sensitive ATPase was stimulated by cations (K⁺ > NH₄⁺ > Rb⁺ > Cs⁺ > Li⁺ > Na⁺ > choline⁺) whereas the NO₃⁻-sensitive ATPase was stimulated by anions (Cl⁻ > Br⁻ > C₂H₃O₂⁻ > SO₄²⁻ > I⁻ > HCO₃⁻ > SCN⁻). Both ATPases required divalent cations. However, the order of preference for the NO₃⁻-sensitive ATPase (Mn²⁺ > Mg²⁺ > Co²⁺ > Ca²⁺ > Zn²⁺) differed from that of the vanadate-sensitive ATPase (Co²⁺ > Mg²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺). The vanadate-sensitive ATPase required higher concentrations of Mg:ATP for full activity than did the NO₃⁻-sensitive ATPase. The kinetics for Mg:ATP were complex for the vanadate-sensitive ATPase, indicating positive cooperativity, but were simple for the NO₃⁻-sensitive ATPase. Both ATPases exhibited similar temperature and pH optima (pH 6.5). The NO₃⁻-sensitive ATPase was stimulated by gramicidin and was associated with NO₃⁻-inhibitable H⁺ transport measured as quenching of quinacrine fluorescence. It was insensitive to molybdate, azide, and vanadate, but exhibited slight sensitivity to ethyl-3-(3-dimethylaminopropyl carbodiimide) and mersalyl. Overall, these results indicate several properties which distinguish these two ATPases and suggest that under defined conditions NO₃⁻-sensitive ATPase activity may be used as a quantitative marker for those membranes identified tentatively as tonoplast in mixed or nonpurified membrane fractions. We feel that NO₃⁻ sensitivity is a better criterion by which to identify this ATPase than either Cl⁻ stimulation or H⁺ transport because it is less ambiguous. It is also useful in identifying the enzyme following solubilization.     

Calcium uptake by excised maize roots and interactions with alkali cations

Ca²⁺ uptake was studied in short-term experiments using 5-day-old excised maize roots. This tissue readily absorbs Ca²⁺, and inhibition by dinitrophenol and low temperature shows that the process is metabolically mediated. The uptake of Ca²⁺, like that of other cations, is influenced by the counter ion, the pH and concentration of the ambient solution, and the presence of other cations. The rate of uptake from various salts decreases in the following order: NO₃⁻ > Cl⁻ = Br⁻ > SO₄²⁻. K⁺ and H⁺ greatly interfere with Ca²⁺ absorption, while Li⁺ and Na⁺ have only slight effects.     

Buffered,phosphate-containing media suitable for aluminum toxicity studies

Studies of Al rhizotoxicity sometimes require the use of well-defined rooting media. For that reason, buffers and phosphate are often omitted from Al solutions for which species composition must be determined precisely. Homopipes and succinate appear to be suitable buffers for short-term studies with seedlings of an Al-sensitive wheat (Triticum aestivum L. cv. Scout 66) and white clover (Trifoliau repens L. cv. Huia). In the case of homopipes (homopiperazine-N,N-bis-2-[ethane-sulfonic acid]), a slight inhibition of root elongation must be taken into account, but no binding of Al³⁺ was observed. In the case of succinate, no inhibition of root elongation was observed, but Al³⁺ binding must be considered. Phosphate-containing media remain free of solid-phase or polynuclear species whenever {Al³⁺}²{HPO₄ ^2-}{OH^–}³ < 10^–47.0 (or {Al³⁺}{HPO₄ ^2-}{OH^–}< 10^–22.7) and when {Al³⁺}³ / {H⁺}³ < 10^8.8. These ion activity products, that define stable Al solutions in the laboratory, appear to apply in soils also, according to an analysis of published data. The published equilibrium values {AlH₂PO₄ ²⁺} / ({Al³⁺}{H₂PO₄ ^–}) = 10^3.0, {AlHPO₄ ⁺} / ({Al³⁺}{HPO₄ ^2-}) = 10^7.0, and {Alsuccinate⁺} / ({Al³⁺}{succinate ^2-}) = 10^4.62 appear to be suitable, because solution toxicity could be accounted for entirely on the basis of computed Al³⁺ even in solutions containing high levels of Alsuccinate⁺ and AlHPO₄ ⁺ (in every case {AlHPO₄ ⁺}>> {AlH₂PO₄ ²⁺}). Thus, AlHPO₄ ⁺ and Alsuccinate⁺ were not toxic at achieved concentrations.     

Binding and Electrostatic Attraction of Lanthanum (La3+) and Aluminum (Al3+) to Wheat Root Plasma Membranes

A general model for the sorption of trivalent cations to wheat-root (Triticum aestivum L cv. Scout 66) plasma membranes (PM) has been developed and includes the first published coefficients for La³⁺ and Al³⁺ binding to a biological membrane. Both ions are rhizotoxic, and the latter ion is the principal contributor to the toxicity of acidic soils around the world. The model takes into account both the electrostatic attraction and the binding of cations to the negatively charged PM surface. Ion binding is modeled as the reaction P ⁻+I ^Z⇌PI ^{Z −1} in which P ⁻ represents a negatively charged PM ligand, located in an estimated area of 540 ?², and I ^Z represents an ion of charge Z. Binding constants for the reaction were assigned for K⁺ (1 m ⁻¹) and Ca²⁺ (30 m ⁻¹) and evaluated experimentally for La³⁺ (2200 m ⁻¹) and H⁺ (21,500 m ⁻¹). Al sorption is complicated by Al³⁺ hydrolysis that yields hydroxoaluminum species that are also sorbed. Binding constants of 30 and 1 m ⁻¹ were assigned for AlOH²⁺ and Al(OH)⁺ ₂, respectively, then a constant for Al³⁺ (20,000 m ⁻¹) was evaluated experimentally using the previously obtained values for K⁺, Ca²⁺ and H⁺ binding. Electrostatic attraction was modeled according to Gouy-Chapman theory. Evaluation of parameters was based upon the sorption of ions to PM vesicles suspended in solutions containing variable concentrations of H⁺, Ca²⁺ and La³⁺ or Al³⁺. Use of small volumes, and improved assay techniques, allowed the measurement of concentration depletions caused by sorption to vesicles. Some independent confirmation of our model is provided by substantial agreement between our computations and two published reports of La³⁺ effects upon zeta potentials of plant protoplasts. The single published report concerning the electrostatic effects of Al on cell membranes is in essential agreement with the model. Received: 6 January 1997/Revised: 6 June 1997     

Cation Penetration through Isolated Leaf Cuticles

The rates of penetration of various cations through isolated apricot Prunus armeniaca L. leaf cuticles were determined. Steady state rates were measured by using a specially constructed flow-through diffusion cell. The penetration rates of the monovalent cations in group IA followed a normal lyotropic series, i.e., CS⁺ ≥ Rb⁺ > K⁺ > Na⁺ > Li⁺. The divalent cations all penetrated through the cuticle more slowly than the monovalent cations. Comparison of the relative values of k (permeability coefficient) and D (diffusion coefficient) indicates that the penetration of ions through isolated cuticles took place by diffusion and was impeded by charge interactions between the solute and charge sites in the penetration pathway. Cuticular penetration rates of K⁺ and H₂O at pH above 9 were of similar magnitude. At pH 5.5 H₂O penetration was not affected but that of K⁺ was greatly reduced. From this observation and from data on cuticle titration and ion adsorption studies, we hypothesize that cuticular pores are lined with a substance (perhaps a protein) which has exposed positively charged sites.     

Identifying the species of copper that are toxic to plant roots in alkaline nutrient solutions

Background and aims

The pH of the growth medium influences Cu speciation in solution, the negativity of plasma membrane (PM) surface potential, and hence the rhizotoxicity of Cu.

Methods

Solution culture experiments were conducted with wheat (Triticum aestivum L.) seedlings to examine the toxicity of various Cu species at pH values ranging from 4.50 to 8.25. The toxic species of Cu was identified, giving particular consideration to the electrical properties at the plant cell membrane and ion activities at the PM surface.

Results

The solution culture studies showed that at pH?<?6.60 (i.e., free Cu²⁺ >95 % of total Cu), the addition of cations (Ca²⁺ or H⁺) decreased the toxic effects of Cu by decreasing the negativity of the PM surface potential (and hence decreasing the activity of Cu²⁺ at the PM surface). For solutions with pH values from 7.50 to 8.25 (CuCO ⁰₃ >50 % of total Cu), an increase in pH significantly enhanced the toxicity of Cu, whilst the addition of Ca had negligible influence on toxicity.

Conclusions

Root growth in solution cultures was influenced primarily by the surface activities of free Cu²⁺ and CuCO ⁰₃ . Across all experiments, the data indicate that it was CuCO ⁰₃ , rather than CuOH⁺, that contributed Cu toxicity over pH?>?7.00. Although our data do not explore the mechanism of toxicity, we propose that CuCO ⁰₃ has an important role in Cu rhizotoxicity in alkaline growth media.

     

Effects of monovalent cations and anions on ADP-induced aggregation of bovine platelets, and mechanism thereof

The effects of monovalent cations - inorganic alakali metal cations and organic quanternary ammonium cations - and monovalent inorganic anions on ADP-induced aggregation of bovine platelets were investigated. In the presence of K⁺, Rb⁺, Cs⁺, choline or tetramethylammonium, aggeregation proceeded. However, aggregation was markedly restricted in media containing Li⁺, Na⁺, tetrabutylammonium or dimethyldibenzylammonium. With anions, aggregation proceeded in the order Cl⁻ > Br⁻ > I⁻ > Clo₄⁻ > SCN⁻. The effects of cations significantly depended on Ca²⁺ concentration, whereas those of the anions depended little of Ca²⁺. Anions such as SCN⁻ and ClO₄⁻ markedly decreased the fluorescence of the surface charge probe 2-p-tuluidinylnaphthalene-6-sulfonate, whereas cations had less pronouced effects. The relative effects of the anions on the fluorescence were consistent with their relative inhibitory effects on aggregation. These results suggest that inhibition of platelet aggregation by the anions is due to a change in the surface change of the platelet plasma membrane. On the other hand, kinetic analysis suggests that the effects of monovalent cations on platelet aggregation are due to their competition with Ca²⁺ during the process of aggregation.     

Dissipation of the Membrane Potential in Susceptible Corn Mitochondria by the Toxin of Helminthosporium maydis, Race T, and Toxin Analogs

We have tested directly the effect of Helminthosporium maydis T (Hmt) toxin and various analogs on the membrane potential formed in mitochondria isolated from a Texas (T) cytoplasmic male-sterile and a normal (N) corn. ATP, malate or succinate generated a membrane potential (negative inside) as monitored by the absorbance change of a cationic dye, safranine. The relative membrane potential (Δψ) could also be detected indirectly as ⁴⁵Ca²⁺ uptake. Hmt toxin added to T mitochondria dissipated the steady state Δψ similar to addition of a protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Toxin analogs (Cpd XIII: C₄₁H₆₈O₁₂ and Cpd IV: C₂₅H₄₄O₆), reduced native toxin (RT2C: C₄₁H₈₄O₁₃) and Pm toxin (band A: C₃₃H₆₀O₈, produced by the fungus, Phyllosticta maydis) were effective in dissipating Δψ and decreasing Ca²⁺ uptake with the following order: Pm (100) » HmT (23-30) > Cpd XIII (11-25) » RT2C (0-4−1.8) > Cpd IV (0.2−1.0). In contrast, the toxins and analogs had no effect on Δψ formed in N mitochondria. The striking similarities of the HmT toxin (band 1: C₄₁H₆₈O₁₃) and Cpd XIII on T mitochondrial activities provide strong evidence supporting the correctness of the polyketol structure assigned to the native toxin. Since the Δψ in energized mitochondria is caused mainly by the electrogenic extrusion of H⁺, the results support the idea that HmT toxin increases membrane permeability of T mitochondria to H⁺. The host specificity of the toxin suggests that an interaction with unique target site(s) on the inner mitochondrial membrane of T corn causes H⁺ leakage.     

Theoretical study on the aromaticity from d-AOs in cationic X 3 + (X = Sc,Y, La) clusters

The stable structures and aromatic characters for three cationic X₃⁺ (X = Sc, Y, and La) and three relevant neutral X₃Cl (X = Sc, Y, La) clusters are investigated at the DFT and post HF level of theory. The calculated results show that the X₃⁺ cations each has two stable structures: the regular trigon (D_3h) and the line (D_￥h {{\hbox{D}}_{\infty {\rm{h}}}} ) with the regular trigon (D_3h) being the ground state, while for three neutral X₃Cl clusters, Sc₃Cl has three stable isomers: the trigon-pyramidal (C_3v), bidentate (C_2v-1), and C_2v-2 structures, Y₃Cl and La₃Cl each has only two stable isomers: the trigon-pyramidal (C_3v) and bidentate (C_2v-1) structures. The ground states for three X₃Cl species are all the bidentate (C_2v-1) isomers. The calculations of the resonance energy (RE) and NICS show that trigonal X₃⁺ isomers exhibit higher degree of aromaticity. The detailed molecular orbital analyzes reveal that the isolated trigonal Sc₃⁺ and Y₃⁺ cations each has one delocalized π-type MO and shows single π-aromaticity, while the isolated trigonal La₃⁺ cation has one delocalized σ-type MO and shows single σ-aromaticity. The single π- or σ-aromaticity for X₃⁺ are attributed to the contributions mainly from the d AOs of the corresponding transition metal X atoms. However, when a singly negatively charged counterion Cl^- is added to Sc₃⁺, Y₃⁺, and La₃⁺ cations respectively, the aromatic type for the two Sc₃⁺, Y₃⁺ units in the corresponding neutral Sc₃Cl, Y₃Cl complexes are changed from π-aromaticity into σ-aromaticity, whereas the σ-aromaticity of the La₃⁺ units in the La₃Cl complex keeps unchanged in this process. Thus three Sc₃⁺, Y₃⁺, La₃⁺ units in the corresponding X₃Cl complexes all have only one σ-type MO and exhibit single σ-aromaticity.     

Adenosine triphosphatase from soybean callus and root cells

The ATPase activity of a membrane fraction from soybean (Glycine max L.) root and callus cells, presumed to be enriched in plasma membrane, has been characterized with respect to ion stimulation, pH requirement, and nucleotide specificity. The enzyme from both sources was activated by divalent cations (Mg²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺ > Sr²⁺) and further stimulated by monovalent salts. Preparations from root cells were stimulated by monovalent ions according to the sequence: K⁺ > Rb⁺ > Choline⁺ > Na⁺ > Li⁺ > NH₄⁺ > Cs⁺ > tris⁺. Membrane preparations from callus cells showed similar stimulatory patterns except for a slight preference for Na⁺ over K⁺. No synergism between K⁺ and Na⁺ was found with preparations from either cell source.     

Stimulation of plasmalemma adenosine triphosphatase from oat roots by inorganic and organic cations: concentration-dependence, selectivity, and sites

K⁺-stimulated ATPase activity of a plasmalemma-enriched fraction from excised roots of oat was triphasic in the range 5 to 80 millimolar KCl. The phases obeyed Michaelis-Menten kinetics and were separated from each other by jumps or sharp breaks at about 10 and 20 millimolar. Stimulation by alkali cations was in the order K⁺ > Rb⁺ > Na⁺ > Cs⁺ > Li⁺ or in a closely related sequence. The specificity reflected differences in V_max, not in affinity (K_m⁻¹). Stimulation by the organic cations ethanolamine and choline in the interval 11 to 80 millimolar appeared monophasic rather than biphasic. Substitution on the quaternary nitrogen of the amino alcohols decreased their effectiveness, as did extension and branching of the chain. Stimulation was maximal at about pH 7 both for K⁺ and choline.     

Altered Ionic Selectivity of the Sodium Channel Revealed by Cysteine Mutations within the Pore

To explore the role of pore-lining amino acids in Na⁺ channel ion-selectivity, pore residues were  replaced serially with cysteine in cloned rat skeletal muscle Na⁺ channels. Ionic selectivity was determined by measuring permeability and ionic current ratios of whole-cell currents in Xenopus oocytes. The rSkM1 channels displayed an ionic selectivity sequence Na⁺>Li⁺>NH₄ ⁺>>K⁺>>Cs⁺ and were impermeable to divalent cations.  Replacement of residues in domain IV showed significantly enhanced current and permeability ratios of NH₄ ⁺ and K⁺, and negative shifts in the reversal potentials recorded in the presence of external Na⁺ solutions when compared to cysteine mutants in domains I, II, and III (except K1237C). Mutants in domain IV showed altered selectivity sequences: W1531C (NH₄ ⁺>K⁺>Na⁺≥Li⁺≈Cs⁺), D1532C, and G1533C (Na⁺>Li⁺≥NH₄ ⁺>K⁺>Cs⁺). Conservative replacement of the aromatic residue in domain IV (W1531) with phenylalanine or tyrosine retained Na⁺ selectivity of the channel while the alanine mutant (W1531A) reduced ion selectivity. A single mutation within the third pore forming region (K1237C) dramatically altered the selectivity sequence of the rSkM1 channel (NH₄ ⁺>K⁺>Na⁺≥Li⁺≈Cs⁺) and was permeable to divalent cations having the selectivity sequence Ca²⁺≥Sr²⁺>Mg²⁺>Ba²⁺. Sulfhydryl modification of K1237C, W1531C or D1532C with methanethiosulfonate derivatives that introduce a positively charged ammonium group, large trimethylammonium moiety, or a negatively charged sulfonate group within the pore was ineffective in restoring Na⁺ selectivity to these channels. Selectivity of D1532C mutants could be largely restored by increasing extracellular pH suggesting altering the ionized state at this position influences selectivity. These data suggest that K1237 in domain III and W1531, D1532, and G1533 in domain IV play a critical role in determining the ionic selectivity of the Na⁺ channel.     

Kinetics of Ca/H Antiport in Isolated Tonoplast Vesicles from Storage Tissue of Beta vulgaris L

Artificial pH gradients across tonoplast vesicles isolated from storage tissue of red beet (Beta vulgaris L.) were used to study the kinetics of a Ca²⁺/H⁺ antiport across this membrane. Ca²⁺-dependent H⁺ fluxes were measured by the pH-dependent fluorescence quenching of acridine orange. ΔpH-dependent Ca²⁺ influx was measured radiometrically. Both H⁺ efflux and Ca²⁺ influx displayed saturation kinetics and an identical dependence on external calcium with apparent K_m values of 43.9 and 41.7 micromolar, respectively. Calcium influx was unaffected by an excess of Mg²⁺ but was inhibited by La³⁺ > Mn²⁺ > Cd²⁺. The apparent K_m for external calcium was greatly affected (5-fold) by internal pH in the range of 6.0 to 6.5 and a transmembrane effect of internal proton binding on the affinity for external calcium is suggested.     

Selectivity of the Ca2+-activated and Light-dependent K+ Channels for Monovalent Cations

The ionic selectivity of the Ca²⁺-activated K⁺ channel of Aplysia neurons and of the light-dependent K⁺ channel of Pecten photoreceptors to metal and organic cations was studied. The selectivity sequence determined from reversal potential measurements is T1⁺ K⁺ > Rb⁺ > NH⁺₄ > Cs⁺ > Na⁺, Li⁺ and is identical to the sequence determined previously for voltage-dependent K⁺ channels in a variety of tissues. Our results suggest that some physical aspect of the K⁺ channel is conserved in phyllogenetically different tissues and cells.     

Three Mechanisms for the Calcium Alleviation of Mineral Toxicities

Ca²⁺ in rooting medium is essential for root elongation, even in the absence of added toxicants. In the presence of rhizotoxic levels of Al³⁺, H⁺, or Na⁺ (or other cationic toxicants), supplementation of the medium with higher levels of Ca²⁺ alleviates growth inhibition. Experiments to determine the mechanisms of alleviation entailed measurements of root elongation in wheat (Triticum aestivum L. cv Scout 66) seedlings in controlled medium. A Gouy-Chapman-Stern model was used to compute the electrical potentials and the activities of ions at the root-cell plasma membrane surfaces. Analysis of root elongation relative to the computed surface activities of ions revealed three separate mechanisms of Ca²⁺ alleviation. Mechanism I is the displacement of cell-surface toxicant by the Ca²⁺-induced reduction in cell-surface negativity. Mechanism II is the restoration of Ca²⁺ at the cell surface if the surface Ca²⁺ has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca²⁺ beyond mechanisms I and II, and may involve Ca²⁺-toxicant interactions at the cell surface other than the displacement interactions of mechanisms I and II. Mechanism I operated in the alleviation of all of the tested toxicities; mechanism II was generally a minor component of alleviation; and mechanism III was toxicant specific and operated strongly in the alleviation of Na⁺ toxicity, moderately in the alleviation of H⁺ toxicity, and not at all in the alleviation of Al³⁺ toxicity.     

The Disastrous Effects of Salt Dust Deposition on Cotton Leaf Photosynthesis and the Cell Physiological Properties in the Ebinur Basin in Northwest China

Salt dust in rump lake areas in arid regions has long been considered an extreme stressor for both native plants and crops. In recent years, research on the harmful effects of salt dust on native plants has been published by many scholars, but the effect on crops has been little studied. In this work, in order to determine the impact of salt dust storms on cotton, we simulated salt dust exposure of cotton leaves in Ebinur Basin in Northwest China, and measured the particle sizes and salt ions in the dust, and the photosynthesis, the structure and the cell physiological properties of the cotton leaves. (1) Analysis found that the salt ions and particle sizes in the salt dust used in the experiments were consistent with the natural salt dust and modeled the salt dust deposition on cotton leaves in this region. (2) The main salt cations on the surface and inside the cotton leaves were Na⁺, Ca²⁺, Cl^- and SO₄^2-, while the amounts of CO₃^- and HCO₃^- were low. From the analysis, we can order the quantity of the salt cations and anions ions present on the surface and inside the cotton leaves as Na⁺>Ca²⁺>Mg²⁺>K⁺ and Cl^->SO₄^2->HCO₃^->CO₃^-, respectively. Furthermore, the five salt dust treatment groups in terms of the total salt ions on both the surface and inside the cotton leaves were A(500g.m^-2)>B(400g.m^-2)>C(300g.m^-2)>D(200g.m^-2)>E(100g.m^-2)>F(0g.m^-2). (3)The salt dust that landed on the surface of the cotton leaves can significantly influence the photosynthetic traits of P_n, PE, C_i, T_i, G_s, T_r, WUE, L_s, φ, A_max, k and R_ady of the cotton leaves. (4)Salt dust can significantly damage the physiological functions of the cotton leaves, resulting in a decrease in leaf chlorophyll and carotenoid content, and increasing cytoplasmic membrane permeability and malondialdehyde (MDA) content by increasing the soluble sugar and proline to adjust for the loss of the cell cytosol. This increases the activity of antioxidant enzymes to eliminate harmful materials, such as the intracellular reactive oxygen and MDA, thus reducing the damage caused by the salt dust and maintaining normal physiological functioning. Overall, this work found that the salt dust deposition was a problem for the crop and the salt dust could significantly influence the physiological and biochemical processes of the cotton leaves. This will eventually damage the leaves and reduce the cotton production, leading to agricultural economic loss. Therefore, attention should be paid to salt dust storms in the Ebinur Basin and efficient measures should be undertaken to protect the environment.     

Characterization of a partially purified adenosine triphosphatase from a corn root plasma membrane fraction

The (K⁺,Mg²⁺)-ATPase was partially purified from a plasma membrane fraction from corn roots (WF9 × Mol7) and stored in liquid N₂ without loss of activity. Specific activity was increased 4-fold over that of the plasma membrane fraction. ATPase activity resembled that of the plasma membrane fraction with certain alterations in cation sensitivity. The enzyme required a divalent cation for activity (Co²⁺ > Mg²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺) when assayed at 3 millimolar ATP and 3 millimolar divalent cation at pH 6.3. When assayed in the presence of 3 millimolar Mg²⁺, the enzyme was further activated by monovalent cations (K⁺, NH₄⁺, Rb⁺ Na⁺, Cs⁺, Li⁺). The pH optima were 6.5 and 6.3 in the absence and presence of 50 millimolar KCl, respectively. The enzyme showed simple Michaelis-Menten kinetics for the substrate ATP-Mg, with a K_m of 1.3 millimolar in the absence and 0.7 millimolar in the presence of 50 millimolar KCl. Stimulation by K⁺ approached simple Michaelis-Menten kinetics, with a K_m of approximately 4 millimolar KCl. ATPase activity was inhibited by sodium orthovanadate. Half-maximal inhibition was at 150 and 35 micromolar in the absence and presence of 50 millimolar KCl. The enzyme required the substrate ATP. The rate of hydrolysis of other substrates, except UDP, IDP, and GDP, was less than 20% of ATP hydrolysis. Nucleoside diphosphatase activity was less than 30% of ATPase activity, was not inhibited by vanadate, was not stimulated by K⁺, and preferred Mn²⁺ to Mg²⁺. The results demonstrate that the (K⁺,Mg²⁺)-ATPase can be clearly distinguished from nonspecific phosphohydrolase and nucleoside diphosphatase activities of plasma membrane fractions prepared from corn roots.