Vulkan, R., Agricultural Research Organization, Gilat Research Center, D.N. Negev 2 85280, Israel
Yermiyahu, U., Agricultural Research Organization, Gilat Research Center, D.N. Negev 2 85280, Israel
Mingelgrin, U., Agricultural Research Organization, Israel P.O. Box 6 50250, Bet Dagan, Israel
Rytwo, G., Dept. of Biotech. and Environ. Sci., Tel Hai Academic College, Upper Galilee 12210, Israel
Kinraide, T.B., Appalachian Farming Syst. Res. Ctr., Agricultural Research Service, U. States Department of Agriculture, Beaver, WV 25813-9423, United States

Sorption of Cu2+ and Zn2+ to the plasma membrane (PM) of wheat root (Triticum aestivum L cv. Scout 66) vesicles was measured at different pH values and in the presence of organic acids and other metals. The results were analyzed using a Gouy-Chapman-Stem model for competitive sorption (binding and electrostatic attraction) to a negative binding site. The binding constants for the two investigated cations as evaluated from the sorption experiments were 5 M-1 for Zn2+ and 400 M-1 for Cu2+. Thus, the sorption affinity of Cu2+ to the PM is considerably larger than that of Ca2+, Mg2+ or Zn 2+. The greater binding affinity of Cu2+ was confirmed by experiments in which competition with La3+ for sorption sites was followed. The amount of sorbed Cu2+ decreased with increasing K +, Ca2+, or La3+ concentrations, suggesting that all these cations competed with Cu2+ for sorption at the PM binding sites, albeit with considerable differences among these cations in effectiveness as competitors with Cu2+. The sorption of Cu 2+ and Zn2+ to the PM decreased in the presence of citric acid or malic acid. Citric acid (as well as pH) affected the sorption of Cu 2+ or Zn2+ to PM more strongly then did malic acid. © Springer Science+Business Media, Inc. 2004.