Cell Membrane Surface Potential ( c 0 ) Plays a Dominant Role in the Phytotoxicity of Copper and Arsenate 1[W] Peng Wang, Dongmei Zhou*, Thomas B. Kinraide, Xiaosan Luo, Lianzhen Li, Dandan Li, and Hailin Zhang State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China (P.W., D.Z., X.L., L.L., D.L.); Graduate School of Chinese Academy of Sciences, Beijing 100049, China (P.W., X.L., L.L., D.L.); Agricultural Research Service, United States Department of Agriculture, Beaver, West Virginia 25813–9423 (T.B.K.); and Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Oklahoma 74078 (H.Z.) Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (c 0 ) that affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The z 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 c 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 c 0 . The order of effectiveness of the ions in reducing the negativity of c 0 was H + . Ca 2+ » Mg 2+ . Na + » K + . The calculated values of c 0 were proportional to the measured z 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 c 0 became less negative and the intrinsic toxicity of As(V) appeared to be reduced. The c 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. Current environmental quality criteria and risk as- sessment procedures for metals and metalloids are predominantly based on total or dissolved metal con- centrations (De Schamphelaere and Janssen, 2002). However, it is widely recognized that total or dis- solved metal concentrations are sometimes poor predictors of bioavailability and toxicity. The physico- chemical characteristics of soil and water, such as the contents of common cations and organic matter, have important effects on the bioavailability and toxicity of metals. Therefore, modifying factors for the bioavail- ability and toxicity should be taken into account in the regulatory frameworks (Peijnenburg et al., 1997). The biotic ligand model (BLM; Di Toro et al., 2001; De Schamphelaere and Janssen, 2002), as a useful construct for assessing the effects of metals on organ- isms, has gained increasing attention from both aca- demic scientists and regulators. Recently, the U.S. Environmental Protection Agency (EPA) has incorpo- rated the BLM into its regulatory framework, and some other countries are considering the implications of following suit (Slaveykova and Wilkinson, 2005). The most important assumption of the BLM is that metal toxicity occurs as a result of the reaction of a free metal ion (or other reactive metal species) with bind- ing sites (biotic ligand [BL]) at the cell membrane surface (CMS; Di Toro et al., 2001; Santore et al., 2001). The magnitude of the toxic effect is proportional to the concentration of metal-BL complex. However, the ac- tual toxic lesion may not be the interaction of the toxicant and BL. For example, the BL may be a binding site in a transport channel such that influx of the toxicant is proportional to the number of sites occu- pied. The actual lesion may occur intracellularly. In fact, the mechanisms of metal intoxication are gener- ally very poorly understood. Decades of investigation have not revealed the principal mechanisms by which Cu 2+ and Al 3+ inhibit root elongation, for example, despite several known effects ranging from the in- duced synthesis of reactive oxygen species to altera- tions of cell membrane structure (effects that often require greater concentrations than those required to inhibit root elongation; Murphy et al., 1999). The BLM provides a possible mechanism of ionic alleviation of toxicity. The ameliorative effectiveness of major cations, such as H + , Ca 2+ , Mg 2+ , Na + , and K + , on 1 This work was supported by the National Natural Science Foundation (grant no. 40671095), the Knowledge Innovative Pro- gram of the Chinese Academy of Sciences (grant no. KXCX3–SW– 435), and the Chinese Academy of Sciences Research Program on Soil Biosystems and Agro-Product Safety (grant no. CXTD– Z2005–4–1). * Corresponding author; e-mail dmzhou@issas.ac.cn. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Dongmei Zhou (dmzhou@issas.ac.cn). 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