Molecular and physiological analysis of Al³âº and H⁺ rhizotoxicities at moderately acidic conditions.

Al³âº and H⁺ toxicities predicted to occur at moderately acidic conditions (pH [water] = 5-5.5) in low-Ca soils were characterized by the combined approaches of computational modeling of electrostatic interactions of ions at the root plasma membrane (PM) surface and molecular/physiological analyses in Arabidopsis (Arabidopsis thaliana). Root growth inhibition in known hypersensitive mutants was correlated with computed {Al³âº} at the PM surface ({Al³âº}(PM)); inhibition was alleviated by increased Ca, which also reduced {Al³âº}(PM) and correlated with cellular Al responses based on expression analysis of genes that are markers for Al stress. The Al-inducible Al tolerance genes ALUMINUM-ACTIVATED MALATE TRANSPORTER1 and ALUMINUM SENSITIVE3 were induced by levels of {Al³âº}(PM) too low to inhibit root growth in tolerant genotypes, indicating that protective responses are triggered when {Al³âº}(PM) was below levels that can initiate injury. Modeling of the H⁺ sensitivity of the SENSITIVE TO PROTON RHIZOTOXICITY1 knockout mutant identified a Ca alleviation mechanism of H⁺ rhizotoxicity, possibly involving stabilization of the cell wall. The phosphatidate phosphohydrolase1 (pah1) pah2 double mutant showed enhanced Al susceptibility under low-P conditions, where greater levels of negatively charged phospholipids in the PM occur, which increases {Al³âº}(PM) through increased PM surface negativity compared with wild-type plants. Finally, we found that the nonalkalinizing Ca fertilizer gypsum improved the tolerance of the sensitive genotypes in moderately acidic soils. These findings fit our modeling predictions that root toxicity to Al³âº and H⁺ in moderately acidic soils involves interactions between both toxic ions in relation to Ca alleviation.

Surface Electrical Potentials of Root Cell Plasma Membranes: Implications for Ion Interactions, Rhizotoxicity, and Uptake.

Many crop plants are exposed to heavy metals and other metals that may intoxicate the crop plants themselves or consumers of the plants. The rhizotoxicity of heavy metals is influenced strongly by the root cell plasma membrane (PM) surface's electrical potential (ψ0). The usually negative ψ0 is created by negatively charged constituents of the PM. Cations in the rooting medium are attracted to the PM surface and anions are repelled. Addition of ameliorating cations (e.g., Ca2+ and Mg2+) to the rooting medium reduces the effectiveness of cationic toxicants (e.g., Cu2+ and Pb2+) and increases the effectiveness of anionic toxicants (e.g., SeO42- and H2AsO4-). Root growth responses to ions are better correlated with ion activities at PM surfaces ({IZ}0) than with activities in the bulk-phase medium ({IZ}b) (IZ denotes an ion with charge Z). Therefore, electrostatic effects play a role in heavy metal toxicity that may exceed the role of site-specific competition between toxicants and ameliorants. Furthermore, ψ0 controls the transport of ions across the PM by influencing both {IZ}0 and the electrical potential difference across the PM from the outer surface to the inner surface (Em,surf). Em,surf is a component of the driving force for ion fluxes across the PM and controls ion-channel voltage gating. Incorporation of {IZ}0 and Em,surf into quantitative models for root metal toxicity and uptake improves risk assessments of toxic metals in the environment. These risk assessments will improve further with future research on the application of electrostatic theory to heavy metal phytotoxicity in natural soils and aquatic environments.

Modeling rhizotoxicity and uptake of Zn and Co singly and in binary mixture in wheat in terms of the cell membrane surface electrical potential.

The usually negative, but variable electrical potential (ψ0) at the cell membrane (CM) surface influences the surface activities of free ions and the electrical driving force for the transport of ions across the CM. The rhizotoxic effects and uptake of Zn(2+) and Co(2+) singly and in binary mixture in wheat ( Triticum aestivum L.) at three pH values (4.5, 5.5, or 6.1) were examined in terms of the free ion activities of Zn(2+), Co(2+), and H(+) at the CM surface (these ions are denoted {M(n+)}(0)). Toxicity and uptake of Zn(2+) or Co(2+) singly to roots were better correlated with {M(2+)}(0) than with their bulk-phase activities. Studies of toxicant interactions using the electrostatic approach and a response-multiplication model for toxicant mixtures indicated that {Co(2+)}(0) significantly enhanced the toxicity of {Zn(2+)}(0), but {Zn(2+)}(0) did not significantly affect the toxicity of {Co(2+)}(0). {H(+)}(0) substantially enhanced the toxicity of both metal ions. Taking ψo into account improved the correspondence (denoted r(2)) between observed and predicted uptake of both Zn(2+) and Co(2+), and each inhibited the uptake of the other. Results showed that r(2) increased from 0.776 to 0.936 for Zn uptake and improved from 0.805 to 0.951 for Co uptake. Thus electrostatic models for metal toxicity and uptake proved superior to models incorporating only bulk-phase activities of ions.

Plasma membrane surface potential: dual effects upon ion uptake and toxicity.

Electrical properties of plasma membranes (PMs), partially controlled by the ionic composition of the exposure medium, play significant roles in the distribution of ions at the exterior surface of PMs and in the transport of ions across PMs. The effects of coexisting cations (commonly Al(3+), Ca(2+), Mg(2+), H(+), and Na(+)) on the uptake and toxicity of these and other ions (such as Cu(2+), Zn(2+), Ni(2+), Cd(2+), and H(2)AsO(4)(-)) to plants were studied in terms of the electrical properties of PMs. Increased concentrations of cations or decreased pH in rooting media, whether in solution culture or in soils, reduced the negativity of the electrical potential at the PM exterior surface (ψ(0)(o)). This reduction decreased the activities of metal cations at the PM surface and increased the activities of anions such as H(2)AsO(4)(-). Furthermore, the reduced ψ(0)(o) negativity increased the surface-to-surface transmembrane potential difference, thus increasing the electrical driving force for cation uptake and decreasing the driving force for anion uptake across PMs. Analysis of measured uptake and toxicity of ions using electrostatic models provides evidence that uptake and toxicity are functions of the dual effects of ψ(0)(o) (i.e. altered PM surface ion activity and surface-to-surface transmembrane potential difference gradient). This study provides novel insights into the mechanisms of plant-ion interactions and extends current theory to evaluate ion bioavailability and toxicity, indicating its potential utility in risk assessment of metal(loid)s in natural waters and soils.

Separating multiple, short-term, deleterious effects of saline solutions on the growth of cowpea seedlings.

• Reductions in plant growth as a result of salinity are of global importance in natural and agricultural landscapes. • Short-term (48-h) solution culture experiments studied 404 treatments with seedlings of cowpea (Vigna unguiculata cv Caloona) to examine the multiple deleterious effects of calcium (Ca), magnesium (Mg), sodium (Na) or potassium (K). • Growth was poorly related to the ion activities in the bulk solution, but was closely related to the calculated activities at the outer surface of the plasma membrane, {I(z)}0°. The addition of Mg, Na or K may induce Ca deficiency in roots by driving {Ca²+}0° to < 1.6 mM. Shoots were more sensitive than roots to osmolarity. Specific ion toxicities reduced root elongation in the order Ca²+ > Mg²+ > Na+ > K+. The addition of K and, to a lesser extent, Ca alleviated the toxic effects of Na. Thus, Ca is essential but may also be intoxicating or ameliorative. • The data demonstrate that the short-term growth of cowpea seedlings in saline solutions may be limited by Ca deficiency, osmotic effects and specific ion toxicities, and K and Ca alleviate Na toxicity. A multiple regression model related root growth to osmolarity and {I(z)}0° (R²=0.924), allowing the quantification of their effects.

Alleviation of Cu and Pb rhizotoxicities in cowpea (Vigna unguiculata) as related to ion activities at root-cell plasma membrane surface.

Cations, such as Ca and Mg, are generally thought to alleviate toxicities of trace metals through site-specific competition (as incorporated in the biotic ligand model, BLM). Short-term experiments were conducted with cowpea (Vigna unguiculata L. Walp.) seedlings in simple nutrient solutions to examine the alleviation of Cu and Pb toxicities by Al, Ca, H, Mg, and Na. For Cu, the cations depolarized the plasma membrane (PM) and reduced the negativity of ψ(0)(o) (electrical potential at the outer surface of the PM) and thereby decreased {Cu(2+)}(0)(o) (activity of Cu(2+) at the outer surface of the PM). For Pb, root elongation was generally better correlated to the activity of Pb(2+) in the bulk solution than to {Pb(2+)}(0)(o). However, we propose that the addition of cations resulted in a decrease in {Pb(2+)}(0)(o) but a simultaneous increase in the rate of Pb uptake (due to an increase in the negativity of E(m,surf), the difference in potential between the inner and outer surfaces of the PM) thus offsetting the decrease in {Pb(2+)}(0)(o). In addition, Ca was found to alleviate Pb toxicity through a specific effect. Although our data do not preclude site-specific competition (as incorporated in the BLM), we suggest that electrostatic effects have an important role.

The surface charge density of plant cell membranes (sigma): an attempt to resolve conflicting values for intrinsic sigma.

The electrical potentials at membrane surfaces (psi0) strongly influence the physiological responses to ions. Ion activities at membrane surfaces may be computed from psi0, and physiological responses to ions are better interpreted with surface activities than with bulk-phase activities. psi0 influences the gating of ion channels and the driving force for ion fluxes across membranes. psi0 may be computed with electrostatic models incorporating the intrinsic surface charge density of the membrane (sigma0), the ion composition of the bathing medium, and ion binding to the membrane. Some of the parameter values needed for the models are well established: the equilibrium constants for ion binding were confirmed for several ions using multiple approaches, and a method is proposed for the computation of other binding constants. sigma0 is less well established, although it has been estimated by several methods, including computation from the near-surface electrical potentials [zeta (zeta) potentials] measured by electrophoreses. Computation from zeta potentials yields values in the range -2 mC m(-2) to -8 mC m(-2), but other methods yield values in the range -15 mC m(-2) to -40 mC m(-2). A systematic discrepancy between measured and computed zeta potentials was noted. The preponderance of evidence supports the suitability of sigma0=-30 mC m(-2). A proposed, fully paramatized Gouy-Chapman-Stern model appears to be suitable for the interpretation of many plant responses to the ionic environment.

Interactive intoxicating and ameliorating effects of tannic acid, aluminum (Al), copper (Cu), and selenate (SeO) in wheat roots: a descriptive and mathematical assessment.

Tannic acids and tannins are produced by plants and are important components of soil and water organic matter. These polyphenolic compounds form complexes with proteins, metals and soil particulate matter and perform several physiological and ecological functions. The tannic acid (TA) used in our study was a mixture of gallic acid and galloyl glucoses ranging up to nonagalloyl glucose. TA inhibited root elongation in wheat seedlings (Triticum aestivum L. cv. Scout 66) at concentrations >4 mg l(-1); but TA alleviated the toxicity of Al(3+), Cu(2+) and SeO(4)(2-); and Al(3+) and SeO(4)(2-) alleviated the toxicity of TA. The interactions of Al(3+) and TA (each toxic but each alleviating the toxicity of the other) were stoichiometric. Growth was affected as though 1 kg TA bound 2.76 mol Al so strongly that if (mol Al)/(kg TA) <2.76, then free Al approximately 0, and if (mol Al)/(kg TA) >2.76, then free TA approximately 0. This stoichiometry is consistent with one mole of galloyl groups binding approximately 0.5 mol Al. Using this binding scheme, growth was modeled successfully on the basis of free TA and free Al. TA enhanced the negativity of root surfaces and enhanced the binding of Al and Cu there without enhancing their toxicity. These and other interactions among TA, Al(3+), Cu(2+), SeO(4)(2-), Ca(2+), Na(+) and H(+) were quantified with a comprehensive non-linear equation with statistically significant coefficients.

Improved scales for metal ion softness and toxicity.

Ten scales relating to chemical hardness or softness of metal ions were compiled. These included eight published scales such as those of Pearson, Ahrland, Klopman, and Misono. Another scale consisted of the logs of the solubility products of metal sulfides, and yet another was a consensus scale constructed from -log K values for metal ion binding to seven soft ligands. These 10 scales were normalized and averaged. The resulting consensus scale for softness (sigma(Con)) appeared to be superior to any of the 10 scales used in its construction based on correlations among the scales. Other possible indicators of softness were examined, including the standard electrode potential (E(0)) and the bulk metal density (rho(Metal)), both of which were also superior to most of the 10 scales just mentioned. Vales for sigma(Con) may be computed from E(0), rho(Metal), and the first ionization potential (I(P)), R(2) = 0.867, for the equation sigma(Con) = aE(0)I(P) + brho(Metal). A consensus scale for toxicity (T(Con)) derived from studies with many different taxa correlated well (R(2) = 0.807) with sigma(Con) computed from the preceding equation, but incorporation of ion charge (Z) into the following equation, T(Con) = asigma(Con) + bsigma(Con)Z + cZ, increased R(2) to 0.923. Substitution of other softness scales for sigma(Con) into equations to predict T(Con) reduced the value of R(2). Thus, sigma(Con) appears to be a superior scale for metal ion softness and toxicity, the latter being an interactive function of both softness and charge.

A scale of metal ion binding strengths correlating with ionic charge, Pauling electronegativity, toxicity, and other physiological effects.

Equilibrium constants for binding to plant plasma membranes have been reported for several metal ions, based upon adsorption studies and zeta-potential measurements. LogK values for the ions are these: Al(3+), 4.30; La(3+), 3.34; Cu(2+), 2.60; Ca(2+) and Mg(2+), 1.48; Na(+) and K(+), 0 M(-1). These values correlate well with logK values for ion binding to many organic and inorganic ligands. LogK values for metal ion binding to 12 ligands were normalized and averaged to produce a scale for the binding of 49 ions. The scale correlates well with the values presented above (R(2)=0.998) and with ion binding to cell walls and other biomass. The scale is closely related to the charge (Z) and Pauling electronegativity (PE) of 48 ions (all but Hg(2+)); R(2)=0.969 for the equation (Scale values)=-1.68+Z(1.22+0.444PE). Minimum rhizotoxicity of metal ions appears to be determined by binding strengths: log a(PM,M)=1.60-2.41exp[0.238(Scale values)] determines the value of ion activities at the plasma membrane surface (a(PM,M)) that will ensure inhibition of root elongation. Additional toxicity appears to be related to softness, accounting for the great toxicity of Ag(+), for example. These binding-strength values correlate with additional physiological effects and are suitable for the computation of cell-surface electrical potentials.

Plasma membrane surface potential (psiPM) as a determinant of ion bioavailability: A critical analysis of new and published toxicological studies and a simplified method for the computation of plant psiPM.

Plasma membranes (PMs) are negatively charged, and this creates a negative PM surface electrical potential (psiPM) that is also controlled by the ionic composition of the bathing medium. The psiPM controls the distribution of ions between the PM surface and the medium so that negative potentials increase the surface activity of cations and decrease the surface activity of anions. All cations reduce the negativity of psiPM, and these common ions are effective in the following order: Al3+ > H+ > Cu2+ > Ca2+ = Mg2+ > Na+ = K+. These ions, especially H+, Ca2+, and Mg2+, are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects on anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H+, Ca2+, and Mg2+) as competitions for binding sites at a PM surface ligand. The psiPM is rarely considered in this biotic ligand model, which incorporates the free ion activity model. The thesis of this article is that psiPM effects are likely to be more important to bioavailability than site-specific competition. Furthermore, psiPM effects could give the false appearance of competition even when it does not occur. The electrostatic approach can account for the bioavailability of anions, whereas the biotic ligand model cannot, and it can account for interactions among cations when competition does not occur. Finally, a simplified procedure is presented for the computation of psiPM for plants, and the possible use of psiPM in a general assessment of the bioavailability of ions is considered.

Aluminum enhancement of plant growth in acid rooting media. A case of reciprocal alleviation of toxicity by two toxic cations.

The generally rhizotoxic ion Al3+ often enhances root growth at low concentrations. The hypothesis that Al3+ enhances growth by relieving H+ toxicity was tested with wheat seedlings (Triticum aestivum L.). Growth enhancement by Al3+ only occurred under acidic conditions that reduced root elongation. Al3+ increased cell membrane electrical polarity and stimulated H+ extrusion. Previous investigations have shown that Al3+ decreases solute leakage at low p H and that the alleviation of H+ toxicity by cations appears to be a general phenomenon with effectiveness dependent upon charge (C3+ >C2+ >Cl+ ). Alleviation of one cation toxicity by another toxic cation appears to be reciprocal so that Al3+ toxicity is relieved by H+ . It has been argued previously that this latter phenomenon accounts for the apparent toxicity of ALOH2+ and Al(OH)+2 . Reduction of cell-surface electrical potential by the ameliorative cation may reduce the cell-surface activity of the toxic cation.

The rhizotoxicity of metal cations is related to their strength of binding to hard ligands.

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix. Specifically, the authors propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations such as Al(3+) and Hg(2+), which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations such as Ca(2+), Na(+), Mn(2+), and Zn(2+) , which bind weakly to hard ligands, bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl(+), Ag(+), Cs(+), and Cu(2+)) are sometimes more toxic than expected through binding to hard ligands. The data presented in the present study demonstrate the importance of strength of binding to hard ligands in influencing a range of important physiological processes within roots through nonspecific mechanisms.

Delineating ion-ion interactions by electrostatic modeling for predicting rhizotoxicity of metal mixtures to lettuce Lactuca sativa.

Effects of ion-ion interactions on metal toxicity to lettuce Lactuca sativa were studied based on the electrical potential at the plasma membrane surface (ψ0 ). Surface interactions at the proximate outside of the membrane influenced ion activities at the plasma membrane surface ({M(n+)}0). At a given free Cu(2+) activity in the bulk medium ({Cu(2+)}b), additions of Na(+), K(+), Ca(2+), and Mg(2+) resulted in substantial decreases in {Cu(2+)}0. Additions of Zn(2+) led to declines in {Cu(2+)}0, but Cu(2+) and Ag(+) at the exposure levels tested had negligible effects on the plasma membrane surface activity of each other. Metal toxicity was expressed by the {M(n+)}0 -based strength coefficient, indicating a decrease of toxicity in the order: Ag(+) > Cu(2+) > Zn(2+). Adsorbed Na(+), K(+), Ca(2+), and Mg(2+) had significant and dose-dependent effects on Cu(2+) toxicity in terms of osmolarity. Internal interactions between Cu(2+) and Zn(2+) and between Cu(2+) and Ag(+) were modeled by expanding the strength coefficients in concentration addition and response multiplication models. These extended models consistently indicated that Zn(2+) significantly alleviated Cu(2+) toxicity. According to the extended concentration addition model, Ag(+) significantly enhanced Cu(2+) toxicity whereas Cu(2+) reduced Ag(+) toxicity. By contrast, the response multiplication model predicted insignificant effects of adsorbed Cu(2+) and Ag(+) on the toxicity of each other. These interactions were interpreted using ψ0, demonstrating its influence on metal toxicity.

Modelling metal-metal interactions and metal toxicity to lettuce Lactuca sativa following mixture exposure (Cu²âº-Zn²âº and Cu²âº-Ag⁺).

Metal toxicity to lettuce Lactuca sativa was determined following mixture exposure based on the concepts of concentration addition (CA) and response addition (RA). On the basis of conventional models assuming no interaction between mixture components, Ag(+) was the most toxic, followed by Cu(2+) and Zn(2+). Furthermore, ion-ion interactions were included in quantitatively estimating toxicity of interactive mixtures of Cu(2+)-Zn(2+) and Cu(2+)-Ag(+) by linearly expanding the CA and RA models. About 80-92% of the variability in the root growth could be explained by this approach. Estimates by the extended models indicate significant alleviative effects of Zn(2+) on Cu(2+) toxicity whereas Cu(2+) did not significantly affect Zn(2+) toxicity. According to the extended CA model, Cu(2+) significantly reduced Ag(+) toxicity while Ag(+) enhanced Cu(2+) toxicity. Similar effects were not found by the extended RA model. These interactions might result from their individual uptake mechanisms and toxic actions as published in literature.

The standard electrode potential (Eθ) predicts the prooxidant activity and the acute toxicity of metal ions.

The standard electrode potential (E(θ)) has been known for many decades to predict the toxicity of metal ions. We have compiled acute toxicity data from fifteen studies and find that the toxicity of thirty metal ions correlates with E(θ) at r(2)=0.868 when toxicity is expressed as log concentration of comparably effective doses. We have discovered an even stronger relationship between the prooxidant activity (PA) of metal ions and E(θ) (and electronegativity, χ). Data compiled from thirty-four studies demonstrate that the PA of twenty-five metal ions correlates with E(θ) at r(2)=0.983 (and χ at r(2)=0.968). PA was commonly measured as metal-induced peroxidation of cell membranes or accumulation of reactive oxygen species. None of the redox metals (capable of Fenton-like reactions) in our studies (i.e., Mn, Fe, Co, Ni, and Cu) was prooxidative or toxic beyond what was expected from E(θ) or χ. We propose that the formation of superoxide-metal ion complexes is greater at greater E(θ) or χ values and that these complexes, whether free or enzyme-bound, function in PA without redox cycling of the complexed ion.

Rhizotoxic effects of silver in cowpea seedlings.

Silver (Ag) is highly toxic to aquatic organisms, including algae, invertebrate animals, and fish, but little information exists on Ag rhizotoxicity in higher plants. In two solution culture experiments with approximately 1,000 microM Ca(NO3)2 and 5 microM H3BO3 (pH 5.4), 20 to 80% of added Ag (< or =2 microM) was lost from solution within approximately 30 min, with a further decrease after 48 h root growth. Using measured Ag concentrations at the start of the experiments, the median effective concentration (EC50) for root elongation rate of cowpea (Vigna unguiculata [L.] Walp. cv. Caloona) was 0.010 microM Ag in the first 4 h of exposure (0.021 microM in the first 8 h). This demonstrates that Ag (as Ag+) is rapidly rhizotoxic to cowpea seedlings at concentrations similar to those that are toxic to freshwater biota. Rupturing of rhizodermal and outer cortical layers was evident after 48 h with 0.13 to 0.57 microM Ag initially in solution, being most severe at 0.13 or 0.25 microM Ag. An additional experiment showed that ruptures were first evident after 20 h exposure to 0.17 microM Ag, with increased severity of rupturing over time. The rhizotoxic effects of Ag are similar to those of some other trace metals (e.g., Cu, Al, La) that bind strongly to hard ligands and weakly to soft ligands. The similarity of rupturing effects, despite the difference in strong binding to soft ligands by Ag and to hard ligands by the other metals, suggests a distinctive metabolic effect of Ag that binds only weakly to hard ligands.

Cell membrane surface potential (psi0) plays a dominant role in the phytotoxicity of copper and arsenate.

Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (psi(0)) that affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The zeta potentials of root protoplasts of wheat (Triticum aestivum), as affected by the ionic environment of the solution, were measured and compared with the values of psi(0) calculated with a Gouy-Chapman-Stern model. The mechanisms for the effects of cations (H(+), Ca(2+), Mg(2+), Na(+), and K(+)) on the acute toxicity of Cu(2+) and As(V) to wheat were studied in terms of psi(0). The order of effectiveness of the ions in reducing the negativity of psi(0) was H(+) > Ca(2+) approximately Mg(2+) > Na(+) approximately K(+). The calculated values of psi(0) were proportional to the measured zeta potentials (r(2) = 0.93). Increasing Ca(2+) or Mg(2+) activities in bulk-phase media resulted in decreased CMS activities of Cu(2+) ({Cu(2+)}(0)) and increased CMS activities of As(V) ({As(V)}(0)). The 48-h EA50{Cu(2+)}(b) ({Cu(2+)} in bulk-phase media accounting for 50% inhibition of root elongation over 48 h) increased initially and then declined, whereas the 48-h EA50{As(V)}(b) decreased linearly. However, the intrinsic toxicity of Cu(2+) (toxicity expressed in terms of {Cu(2+)}(0)) appeared to be enhanced as psi(0) became less negative and the intrinsic toxicity of As(V) appeared to be reduced. The psi(0) effects, rather than site-specific competitions among ions at the CMS (invoked by the biotic ligand model), may play the dominant role in the phytotoxicities of Cu(2+) and As(V) to wheat.

Organic acid secretion as a mechanism of aluminium resistance: a model incorporating the root cortex, epidermis, and the external unstirred layer.

The resistance of some plants to Al (aluminium or aluminum) has been attributed to the secretion of Al(3+)-binding organic acid (OA) anions from the Al-sensitive root tips. Evidence for the 'OA secretion hypothesis' of resistance is substantial, but the mode of action remains unknown because the OA secretion appears to be too small to reduce adequately the activity of Al(3+) at the root surface. In this study a mechanism for the reduction of Al(3+) at the root surface and just beneath the epidermis by complexation with secreted OA(2-) is considered. According to our computations, Al(3+) activity is insufficiently reduced at the surface of the root tips to account for the Al resistance of Triticum aestivum L. cv. Atlas 66, a malate-secreting wheat. Experimental treatments to decrease the thickness of the unstirred layer (increased aeration and removal of root-tip mucilage) failed to enhance sensitivity to Al(3+). On the basis of additional modelling, the observed spatial distribution of Al in roots, and the anatomical responses to Al, it is proposed that the epidermis is an essential component of the diffusion pathway for both OA and Al. We suggest that Al(3+) in the cortex must be reduced to small concentrations in order substantially to alleviate the inhibition of root elongation and so that the outer surface of the epidermis can tolerate relatively large concentrations of Al(3+). If OA secretion is required for reducing Al(3+) mainly beneath the root surface, rather than in the rhizosphere, then the metabolic cost to plants will be greatly reduced.

Possible influence of cell walls upon ion concentrations at plasma membrane surfaces. Toward a comprehensive view of cell-surface electrical effects upon ion uptake, intoxication, and amelioration.

Plant uptake of ions, intoxication by ions, and the alleviation of intoxication by other ions often correlate poorly with ion concentrations in the rooting medium. By contrast, uptake, intoxication, and alleviation correlate well with ion concentrations at the plasma membrane (PM) surface computed as though the PM were bathed directly in the rooting medium with no effect from the cell wall (CW). According to two separate lines of analysis, a close association of CWs and PMs results in a slight increase in cation concentrations and a slight decrease in anion concentrations at the PM surface compared with concentrations when the CW is separated or has no effect. Although slightly different, the ion concentrations at the PM surface computed with and without close association with the CW are highly correlated. Altogether, the CW would appear to have a small effect upon ion uptake by the PM or upon intoxication or alleviation of intoxication originating at the PM surface. These analyses have been enabled by the recent evaluation of parameters required for the electrostatic models (Gouy-Chapman-Stern and Donnan-plus-binding) used to compute electrical potentials and ion concentrations in CWs and at PM surfaces.