Thomas B. Kinraide

Identity of the rhizotoxic aluminium species

The aluminium (III) released from soil minerals to the soil solution under acid conditions may appear as hexaaquaaluminium (Al(H2O)63+, or Al3+ for convenience) or may react with available ligands to form additional chemical species. That one or more of these species is rhizotoxic (inhibitory to root elongation) has been known for many decades, but the identity of the toxic species remains problematical for the following reasons. 1. Several Al species coexist in solution so individual species cannot be investigated in isolation, even in artificial culture media. 2. The activities of individual species must be calculated from equilibrium data that may be uncertain. 3. The unexpected or undetected appearance of the extremely toxic triskaidekaaluminium (AlO4Al12(OH)24(H2O)127+ or Al13) may cause misatribution of toxicity to other species, especially to mononuclear hydroxy-Al. 4. If H+ ameliorates Al3+ toxicity, or vice versa, then mononuclear hydroxy-Al may appear to be toxic when it is not. 5. The identity and activities of the Al species contacting the cell surfaces are uncertain because of the H+ currents through the root surface and because of surface charges. This article considers the implications of these problems for good experimental designs and critically evaluates current information regarding the relative toxicities of selected Al species. It is concluded that polycationic Al (charge >2) is rhizotoxic as are other polyvalent cations.

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.

Use of a Gouy-Chapman-Stern Model for Membrane-Surface Electrical Potential to Interpret Some Features of Mineral Rhizotoxicity

A consideration of mineral toxicity to roots only in terms of ion activities in the rooting medium can be misleading. A Gouy-Chapman-Stern model, by which relative ion activities at cell-membrane surfaces may be estimated, has been applied to problems of mineral rhizotoxicity, including the toxicity of Al3+, La3+, H+, Na+, and SeO42-, to wheat (Triticum aestivum L.) roots. The Gouy-Chapman portion of the model is expressed in the Grahame equation, which relates the charge density ([sigma]) and electrical potential (E0) at the surface of a membrane to the concentrations of ions in a contracting bulk solution. The Stern modification of the theory takes into account changes in [sigma] caused by ion binding at the membrane surface. Several theoretical problems with the model and its use are considered, including the fact that previous authors have usually related the physiological effects of an ion at a membrane surface to the computed concentration (Ci0) of the unbound ion rather than its computed activity (ai0). This practice implies the false assumption that Ci0 is proportional to ai0. It is demonstrated here that ai0, computed from external activities (ai[infinity symbol]) by a Nernst equation [ai0 = ai[infinity symbol]exp([mdash]ZiFE0/RT), where Zi is the charge on the ion, F is the Faraday constant, R is the gas constant, and T is the temperature], correlates well with ion toxicity and that Ci0 sometimes correlates poorly. These conclusions also apply to issues of mineral nutrition.

Fine root diameters can change in response to changes in nutrient concentrations

Plant roots function in the critical role of water and nutrient uptake. Although extensive data exist on functioning of seedling roots, little is known of the actual functionality of the fine roots of mature plants. Although this class of root represents 90% or more of the total root length of a given mature plant, their small size has inhibited detailed studies. Commonly, the critical metrics for studies of root function are root length and total weight, expressed as Specific Root Length. The metric that classifies “fine roots,” root diameter, is rarely a focus except as average diameter, even though this is the primary characteristic from which accurate estimates of surface area and volume can be calculated. Using data from several preliminary experiments, this study shows consistent changes in measured fine root diameter with changes in concentration of some nutrients. Twelve different species demonstrated concentration dependent diameter increases, or decreases, in response to increasing concentrations of nitrate, phosphorus, aluminum or tannic acid. On the other hand, Cacao (Theobroma cacao L) fine roots changed diameter in response to changes in nitrate concentration, but not ammonium. Clearly pattern of diameter change in response to nutrient concentration is dependent on nutrient, species and their interaction. It is suggested that the routine assessment of fine root diameter will be essential to understanding nutrient uptake dynamics.

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

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.

Al3+-Ca2+ interactions in aluminum rhizotoxicity: I. Inhibition of root growth is not caused by reduction of calcium uptake

The cation A13 + is toxic to plants at micromo- lar concentrations and can severely inhibit root growth in solution experimentsJ The inhibition of Ca 2+ uptake in roots by A13+ has been proposed as a possible mecha- nism for A13+ toxicity, and in this study the hypothesis was tested directly. Root growth and Ca 2+ uptake were measured in 5-d-old seedlings of wheat (Triticum aestivum L. Thell) during exposure to A13+ in a low-Ca 2+ basal medium, and to A13§ in the presence of added cations. Uptake of Ca 2+ in whole roots and translocation to the shoot were measured using 45Ca2+, and localized mea- surements of net Ca 2+ flux were also made at the root apex using the technique of microelectrode ion-flux esti- mation. Treatment with 2.64 ~tM A1C13 in 226 ~tM CaC12, at pH 4.5, severely inhibited root growth without affect- ing Ca 2§ uptake. Addition of 30 mM Na +, 3 mM Mg 2+ or 50 gM tris(ethylenediamine)cobalt(III) to this A13+ treatment restored root growth but significantly reduced Ca 2+ uptake measured over the entire root system and at the root apex. The A13+ and Ca e+ concentrations were adjusted so that the activities of the A13+ and Ca 2+ ions were constant in all solutions (1.5 ~tM and 200 pM, re- spectively). Root growth can be severely inhibited by A13-- concentrations that do not affect Ca 2+ uptake, while the addition of ameliorating cations depresses Ca 2+ uptake. These results argue against the hypothesis that A13-- inhibits root growth by reducing Ca 2+ uptake.

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 Al3+, Ca2+, Mg2+, H+, and Na+) on the uptake and toxicity of these and other ions (such as Cu2+, Zn2+, Ni2+, Cd2+, and H2AsO4−) 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 (ψ0o). This reduction decreased the activities of metal cations at the PM surface and increased the activities of anions such as H2AsO4−. Furthermore, the reduced ψ0o 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 ψ0o (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.

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

Abstract. 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−+IZ⇌PIZ−1 in which P− represents a negatively charged PM ligand, located in an estimated area of 540 Å2, and IZ 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.

Cell Membrane Surface Potential (ψ0) Plays a Dominant Role in the Phytotoxicity of Copper and Arsenate

Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (ψ0) that affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The ζ 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 ψ0 calculated with a Gouy-Chapman-Stern model. The mechanisms for the effects of cations (H+, Ca2+, Mg2+, Na+, and K+) on the acute toxicity of Cu2+ and As(V) to wheat were studied in terms of ψ0. The order of effectiveness of the ions in reducing the negativity of ψ0 was H+ > Ca2+ ≈ Mg2+ > Na+ ≈ K+. The calculated values of ψ0 were proportional to the measured ζ potentials (r2 = 0.93). Increasing Ca2+ or Mg2+ activities in bulk-phase media resulted in decreased CMS activities of Cu2+ ({Cu2+}0) and increased CMS activities of As(V) ({As(V)}0). The 48-h EA50{Cu2+}b ({Cu2+} 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 Cu2+ (toxicity expressed in terms of {Cu2+}0) appeared to be enhanced as ψ0 became less negative and the intrinsic toxicity of As(V) appeared to be reduced. The ψ0 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 Cu2+ and As(V) to wheat.

Electrical Potentials of Plant Cell Walls in Response to the Ionic Environment

Electrical potentials in cell walls (ψWall) and at plasma membrane surfaces (ψPM) are determinants of ion activities in these phases. The ψPM plays a demonstrated role in ion uptake and intoxication, but a comprehensive electrostatic theory of plant-ion interactions will require further understanding of ψWall. ψWall from potato (Solanum tuberosum) tubers and wheat (Triticum aestivum) roots was monitored in response to ionic changes by placing glass microelectrodes against cell surfaces. Cations reduced the negativity of ψWall with effectiveness in the order Al3+ > La3+ > H+ > Cu2+ > Ni2+ > Ca2+ > Co2+ > Cd2+ > Mg2+ > Zn2+ > hexamethonium2+ > Rb+ > K+ > Cs+ > Na+. This order resembles substantially the order of plant-root intoxicating effectiveness and indicates a role for both ion charge and size. Our measurements were combined with the few published measurements of ψWall, and all were considered in terms of a model composed of Donnan theory and ion binding. Measured and model-computed values for ψWall were in close agreement, usually, and we consider ψWall to be at least proportional to the actual Donnan potentials. ψWall and ψPM display similar trends in their responses to ionic solutes, but ions appear to bind more strongly to plasma membrane sites than to readily accessible cell wall sites. ψWall is involved in swelling and extension capabilities of the cell wall lattice and thus may play a role in pectin bonding, texture, and intercellular adhesion.