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Title: CELL MEMBRANE SURFACE POTENTIAL (PSI 0) PLAYS A DOMINANT ROLE IN THE PHYTOTOXICITY OF COPPER AND ARSENATE

Author
item WANG, PENG - CHINESE ACADEMY OF SCI
item ZHOU, DONGMEI - CHINESE ACADEMY OF SCI
item Kinraide, Thomas
item LUO, XIAOSAN - CHINESE ACADEMY OF SCI
item LI, LIANZHEN - CHINESE ACADEMY OF SCI
item ZHANG, HAILIN - OKLAHOMA STATE UNIV

Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2008
Publication Date: 11/20/2008
Citation: Wang, P., Zhou, D., Kinraide, T.B., Luo, X., Li, L., Zhang, H. 2008. CELL MEMBRANE SURFACE POTENTIAL (PSI 0) PLAYS A DOMINANT ROLE IN THE PHYTOTOXICITY OF COPPER AND ARSENATE. Journal of Plant Physiology. 148:2134-2143.

Interpretive Summary: Toxicology is an important discipline in agricultural, environmental, and health studies. Toxicants are always present, and some toxicants are also essential nutrients for plants and animals. Examples are copper and fluoride, each of which is deadly in excessive amounts, but essential in smaller amounts. The bioavailability of toxicants is determined by the environment, and toxicologists have long sought to understand these determinants. For example, copper in ‘hard’ waters (high concentration of calcium and magnesium) is much less available (and less intoxicating) than copper in ‘soft’ waters. Toxicologists have adopted the Biotic Ligand Model (BLM) to interpret and predict toxicant bioavailability. The model states that ameliorative ions (e.g., calcium) alleviate toxicity by competing with toxicants (e.g., copper) for a cell-surface binding site, but that is often not the case. The contribution of present article is to correct this and some other deficiencies in the BLM and to present specific toxicological data on copper and arsenic. In particular, the BLM must be extended to take into account cell-surface electrical properties, which can have the effect of attracting or repelling toxicants, which are themselves often electrically charged.

Technical Abstract: Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (Psi 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 Psi 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 Psi 0. The order of effectiveness of the ions in reducing the negativity of Psi 0 was H+ > Ca 2+ ˜ Mg 2+ > Na + ˜ K +. The calculated values of Psi 0 were proportional to the measured zeta potentials (r2 = 0.93). Increasing Ca 2+ or Mg 2+ activities in bulk-phase media (BM) resulted in decreased CMS activities of Cu 2+ and increased CMS activities of As(V). 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+} at the CMS) appeared to be enhanced as Psi 0 became less negative, and the intrinsic toxicity of As(V) appeared to be reduced. The Psi 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).