Received August 1, 2008
Accepted September 26, 2008
Cell Membrane Surface Potential (
0) Plays a Dominant Role in the Phytotoxicity of Copper and Arsenate
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; Agricultural Research Service, United States Department of Agriculture, Beaver, West Virginia 25813-9423, USA; Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, Ok 74078, USA; Graduate School of Chinese Academy of Sciences, Beijing 100049, China
* Corresponding author; email: dmzhou{at}issas.ac.cn.
Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (
0) which affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The zeta (
) 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 (BM) 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 BM 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.
