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

Abstract

a11 distances from the membrane surface. This article presents a consideration of this probably false assumption and the possibilities of a revision of the Grahame equation.The Stem modification of the theory considers ion binding at the membrane surface (Nir et al., 1978;Lau et al., 1981). ms b i n h g the surface charge density to be variable and dependent on the concentration and identities of the various ions. Use of the Gouy-Chapman-Stem model requires the detennination Or assumption Of binding mechaIlisms and the Of binding constants for each ion and each QPe of membrane-binding site.artide Presents a consideration of the suitability of a generalized set of parameters, whose values were selected after evaluation of the literature and without reference to the present data.Finally, data are presented to evaluate the suitability of a A consideration of mineral toxicity to roots only in terms of ion adivities in the rooting medium can be misleading. A CouyChapman-Stern model, by which relative ion activities at cellmembrane surfaces may be estimated, has been applied to problems of mineral rhizotoxicity, including the toxicity of AI3+, La3+, H+, Na+, and SeO, ' -, to wheat (Jriticum aesfivum 1.) roots. l h e Couy-Chapman portion of the model is expressed in the Crahame equation, which relates the charge density ((I) and eledrical potentia1 (4) at the surface of a membrane to the concentrations of ions in a contading bulk solution. l h e Stern modification of thetheory takes into account changes in u caused by ion binding at the membrane surface. Severa1 theoretical problems with the model and its use are considered, including the f a d that previous authors have usually related the physiological effeds of an ion at a membrane surface to the computed concentration (Cio) of the unbound ion rather than its computed adivity (aio). This pradice implies the false assumption that Cio is proportional to aio. It is demonstrated here that aio, computed from externa1 adivities (ai,) by a Nernst equation [a,,, = ai,exp(-ZiFEo/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 Ci o sometimes correlates poorly. These conclusions also apply to issues of mineral nutrition.generahed Gouy-Chapman-Stem mode1 to interpret Some features of mineral rh~otoficity. THEORY Chemical Adivity of lonsChemical activity is the thermodynamic property of solutes that determines equilibrium position in chemical reactions (Lindsay, 1979;Nobel, 1991). The activity of solute i (ai) is equal to the concentration (Ci) times an activity coefficient (ri). In ordinary solutions, r i is a function of the charge on solution and may be computed by the vi^^ extension of the Debye-Hückel equation ( h d s a y , 1979). ~~t i v i t y is aiso a d e t e h n a n t in the eqfibrium partitioning of a ion bemeen mo phases (0 and differing in electrical potential, a c c o r~n g to the Nemst equation (Nobel, 1991),ne physiological effects of ions have sometim...

Sorption of Aluminum to Plasma Membrane Vesicles Isolated from Roots of Scout 66 and Atlas 66 Cultivars of Wheat

“…Previous studies demonstrate high negative correlations between root elongation and the computed activity of toxicant ions at the PM surface (Kinraide, 1994;Kinraide et al, 1994). Treatments that reduced surface negativity reduced the toxic effects of cationic toxicants and increased the toxic effect of the anionic toxicant SeO,'-.…”

“…Treatments that reduce cell-surface negativity reduce the sensitivity of roots to cationic toxicants (Kinraide et al, 1992;Kinraide, 1994) and increase the sensitivity to at least one anionic toxicant, Se0,'- (Kinraide, 1994). Consequently, it was proposed decades ago that negative surface-charge densities may be lower in AI3+-tolerant genotypes (Vose and Randall, 1962).…”

Apoplastic Binding of Aluminum Is Involved in Silicon-Induced Amelioration of Aluminum Toxicity in Maize

“…Therefore, it can be expected that part of these fractions were recovered in the symplastic1 fraction during the extraction/centrifugation steps, which then is overestimated at the expense of the apoplastic fractions. The mobile apoplastic fractions are expected to determine the Al activity at the plasma membrane and thus Al toxicity (Kinraide, 1994) as revealed by enhanced callose formation in the presence of Al (Fig. 1B).…”

Arbuscular mycorrhizal colonisation increases copper binding capacity of root cell walls of Oryza sativa L. and reduces copper uptake

“…3b. Metal binding capacity is correlated with the density of acidic polysaccharides whose dissociation exposes anionic side groups capable of complexing cations, or simply reducing their mobility via electrostatic attraction (Kinraide, 1994). Various studies have identified uronic and galacturonic groups as the dominant binding moieties for Cu and other cations in cell walls (Zheng et al, 2004;Konno et al, 2005).…”