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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Cell Membrane Surface Potential (₀) Plays a Dominant Role in the Phytotoxicity of Copper and Arsenate^1,[W]

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 (₀) that affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The 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 ₀ 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 ₀. The order of effectiveness of the ions in reducing the negativity of ₀ was H⁺ > Ca²⁺ Mg²⁺ > Na⁺ K⁺. The calculated values of ₀ were proportional to the measured 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 ₀ became less negative and the intrinsic toxicity of As(V) appeared to be reduced. The 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 Cu²⁺ and As(V) to wheat.

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{at}issas.ac.cn).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.127464

^* Corresponding author; e-mail dmzhou{at}issas.ac.cn.

Received August 1, 2008; accepted September 26, 2008; published October 1, 2008.