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.

Effect of different nitrogen forms on the toxicity of Zn in wheat seedling root: a modeling analysis.

Heavy metal stress in culture media is always rhizotoxic. Our study aims to investigate the role of negative potential (ψ 0) at root cell membrane surface (CMs) on modeling Zn2+ toxicity to wheat seedling roots and to examine the effects of different nitrogen forms (NH4+ and NO3-) on ψ 0 and Zn rhizotoxicity. Solution culture experiments were conducted to measure the root elongation and Zn accumulation under Zn2+ exposure. The role of two nitrogen forms in affecting Zn2+ toxicity was compared, giving particular consideration to ψ 0 and Zn2+ activities at CMs ({Zn2+}0). Results showed that NH4+ alleviates Zn2+ rhizotoxicity and NO3- increases Zn2+ rhizotoxicity. In modeling the rhizotoxicity, root length correlated better with {Zn2+}0 than {Zn2+}b, and the predictive accuracy (r 2) of NH4+ treatment increased from 0.748 to 0.917 when incorporation of {Zn2+}0 and {Ca2+}0 into analysis. Oppositely, ψ 0 played a limited role in modeling Zn2+ rhizotoxicity and bioavailability in NO3- treated medium (r 2 = 0.609). Moreover, higher concentration of Zn in roots was found in NO3- treatment, compared with the NH4+ treatment. ψ 0 rather than the rhizotoxicity data correlated better with Zn accumulation especially in the NO3- treatment (r 2 > 0.7), which meant the electrical driving force at CMs playing a dominant role in modeling the metal accumulation. In conclusion, the alleviatory role of NH4+ on Zn toxicity and uptake was well explained and modeled by electrostatic effects at CMs. Though our data do not explore mechanisms for the NO3--Zn2+ interactions, we propose that ψ 0 worked better in affecting the driving force for root Zn uptake, than influencing metal bioavailability at CMs.

Pilot-scale electrokinetic treatment of a Cu contaminated red soil.

A pilot-scale experiment for electrokinetic treatment of 700 kg of copper contaminated red soil was conducted using a constant voltage of 80 V. Dynamic removal percentages of Cu from the soil and energy consumption during the treatment were evaluated together with changes of soil pH, electrical conductivity and soil microbial functional diversity before and after the electrokinetic treatment. The results indicate that 76% of Cu was successfully removed from the soil after 140 d of treatment when lactic acid was used as enhancing reagent for adjusting the catholyte pH and dissolving soil Cu by complexation, and the pilot-scale electrokinetic experiment consumed electric energy of 224 kW h t-1 soil. The post-treatment soil pH values decreased about 0.1-1.6 units compared with the initial value (pH 4.8), and soil electrical conductivities in most of soil sections also significantly decreased. Soil microbial functional diversity varied after the electrokinetic treatment, particularly the increase of substrate richness index, which is possibly due to the stimulation of lactic acid that was introduced into the soil column during the experiment.

Cosorption of zinc and glyphosate on two soils with different characteristics.

Agricultural application of large amounts of glyphosate [N-(phosphonomethyl)-glycine] may affect soil metal behaviors to some extend, because glyphosate can react with many kinds of metals to form metal complexes. Cosorption of Zn and glyphosate on a Red soil (RS, Udic Ferrosols) and a Wushan soil (WS, Anthrosol) was studied. In comparison with the WS, the RS has less adsorption capacity for Zn and higher for glyphosate. The presence of glyphosate decreased Zn adsorption on the two soils, which are resulted from the decreased equilibrium solution pH caused by the added glyphosate, and also the formation of water-soluble complexes of glyphosate with solution Zn(2+) that had lower affinity to soil surface in comparison with Zn(2+) itself. Such effect is more significant on the RS than on the WS, mainly because of the less adsorption quantity of Zn on the former one. On the contrary, the presence of Zn increased the adsorption quantities of glyphosate on the RS and WS, which is resulted from the decreasing pH value of the equilibrium solution caused by Zn(2+) exchange with H(+) ions of soil surface. Such results suggest that glyphosate in field may increase the mobility and bioavailability of Zn and correspondingly increase its environmental risk.

Copper and Zn uptake by radish and pakchoi as affected by application of livestock and poultry manures.

Environmental safety of agricultural utilization of livestock and poultry manures from intensive farming is attracting great attention because the manures often contain high concentrations of heavy metals and organic pollutants. Pot experiments, in which a pig manure (PM), a chicken manure (CM) and a commercial organic manure (OM) with different concentrations of Cu and Zn to simulate soil metal accumulation by manure application for different times were utilized in a garden soil at a rate of 2% (W/W), were conducted to study the effect of application of these livestock and poultry manures on growth of radish (Raphanus sativus L.) and pakchoi (Brassica chinensis L.) as well as their Cu and Zn uptake. The results exhibit that the manures except the PM improved the growth of radish and pakchoi. The difference of biomass among the same manure treatments containing different concentrations of Cu and Zn, however, was insignificant. In addition, application of the livestock and poultry manures significantly increased soil pHs and electric conductivities (EC) compared with the control, which is ascribed that these manures had high pH and contained large amounts of inorganic ions. The available soil Zn concentrations in the PM were higher than that in the CM and OM, and the extractable soil Cu concentrations in the three manures were almost the same after radish growth in the garden soil but were different after pakchoi growth. Zinc and Cu concentrations in the radish and pakchoi tissues increased when the soil Zn and Cu concentrations increased by manures application, but were still within a safe value. An except is the treatment PM4 in which the Zn concentration of the above-ground part of radish was 28.7 mg kg-1, exceeding the Chinese Food Hygiene Standard of 20 mg kg-1 based on fresh weight. Good correlation was obtained between the extractable soil Zn (or Cu) concentrations extracted by 1.0 mol l-1 NH4NO3 and the Zn (or Cu) concentrations in radish and pakchoi tissues, which was expected to be effective in forecasting Cu and Zn availability to radish and pakchoi in manure agronomic utilization.

Adsorption and cosorption of cadmium and glyphosate on two soils with different characteristics.

Glyphosate [N-(phosphonomethyl)glycine] (GPS; H3G) is a widely used pesticide throughout the world. It affects metal behaviors in soil-plant system due to its functional groups, which react with metal ions to form metal complexes. Adsorption and cosorption of cadmium and glyphosate on a Wushan soil (WS soil, Anthrosol) and a Zhuanhong soil (ZH soil, Udic Ferrisol) as affect by solution pH were studied by means of batch adsorption experiments. It indicated that the adsorption quantity of Cd or glyphosate was highly relevant to soil characteristics. The WS soil had higher adsorption capacity of Cd than the ZH soil, due to its high organic matter content and cation exchange capacity (CEC). In contrast, the adsorption quantity of glyphosate on the WS soil was less than that on the ZH soil, because the WS soil has lower iron and aluminum oxides content but higher pH than the ZH soil. The herbicide glyphosate affected Cd adsorption on the two soils when they coexisted in a same soil solution, which was attributed to a glyphosate-induced pH-decrease and the corresponding decline in negative surface charges of the soil. Beside that, glyphosate reacted with solution Cd to form the water-soluble complexes that had lower affinity to soil surface in comparison with Cd itself. On the other hand, the presence of Cd in the soil solution also affected the adsorption of glyphosate on the soils. The presence of Cd increased adsorption quantity of glyphosate on the WS and ZH soils, which was resulted from the decrease of equilibrium solution pH caused by Cd2+ exchange with H+ ions of soil surface. In addition to that, glyphosate adsorption possibly takes place on sites where Cd was previously adsorbed and acted as a bridge between the soil and glyphosate.

Speciation and fractionation of heavy metals in soil experimentally contaminated with Pb, Cd, Cu and Zn together and effects on soil negative surface charge.

Speciation and fractionation of heavy metals in soil subsamples experimentally loaded with Pb, Cd, Cu and Zn in orthogonal design was investigated by sequential extraction, and operationally defined as water-soluble and exchangeable(SE), weakly specific adsorbed(WSA), Fe and Mn oxides-bound(OX) and organic-bound(ORG). The results showed that fractions of heavy metals in the soil subsamples depended on their speciation. About 90% of Cd and 75% of Zn existed in soil subsamples in the SE fraction. Lead and Cu existed in soil subsamples as SE, WSA and OX fractions simultaneously, although SE was still the major fraction. Organic-bound heavy metals were not clearly apparent in all the soil subsamples. The concentration of some heavy metal fractions in soil subsamples showed the good correlation with ionic impulsion of soil, especially for the SE fraction. Continuous saturation of soil subsamples with 0.20 mol/L NH4Cl, which is the first step for determination of the negative surface charge of soil by the ion retention method, resulted in desorption of certain heavy metals from the soil. It was found that the percentage desorption of heavy metals from soil subsamples depended greatly on pH, the composition and original heavy metal content of the soil subsamples. However, most of the heavy metals in the soil subsamples were still be retained after multiple saturation. Compared with the parent soil, the negative surface charge of soil subsamples loaded with heavy metals did not show difference significantly from that of the parent one by statistical analysis. Heavy metals existed in the soil subsamples mainly as exchangeable and precipitated simultaneously.

Assessment of the Zn-Co mixtures rhizotoxicity under Ca deficiency: using two conventional mixture models based on the cell membrane surface potential.

Toxicity assessment of Zn-Co mixtures involves multiple ions interactions. The negative potential (ψ0) at the cell membrane surface (CMs) concentrated cationic toxicants (denoted {M(2+)}0) and influenced the rhizotoxicity of Co(2+) or Zn(2+). The single and joint rhizotoxicity of Co(2+) and Zn(2+) to wheat (Triticum aestivum L.) were examined, coupled with different Ca(2+) levels. Joint effects of Zn(2+), Co(2+) and Ca(2+) were estimated by the linearly extended concentration addition (CA) and response addition (RA) models. Incorporation of Ca(2+) in single metal toxicity assessment significantly enhanced the prediction accuracy (r(2) increased from 0.948 to 0.550 for Zn(2+) and from 0.903 to 0.611 for Co(2+), respectively). ψ0 affected the multiple metals toxicity in both conventional mixture models (r(2)=0.814 for CA model and 0.820 for RA model). Concretely, {Zn(2+)}0 alleviated the toxicity of {Co(2+)}0, while {Co(2+)}0 had non-significant effect on {Zn(2+)}0 toxicity. Growth responses to {Ca(2+)}0 were substantially affected by {Zn(2+)}0 and {Co(2+)}0. Ca addition in medium decreased the {M(2+)}0 by reducing the ψ0 negativity, moreover this addition alleviated Ca deficiency at CMs induced by Zn(2+) (or Co(2+)). These consistent results from both extended CA and RA models indicated that ψ0 provided a novel sight for understanding the rhizotoxicity of multiple metals.