title: Surface Electrical Potentials of Root Cell Plasma Membranes: Implications for Ion Interactions, Rhizotoxicity, and Uptake.
abstract: Many crop plants are exposed to heavy metals and other metals that may intoxicate the crop plants themselves or consumers of the plants. The rhizotoxicity of heavy metals is influenced strongly by the root cell plasma membrane (PM) surface's electrical potential (ψ0). The usually negative ψ0is created by negatively charged constituents of the PM. Cations in the rooting medium are attracted to the PM surface and anions are repelled. Addition of ameliorating cations (e.g., Ca2+ and Mg2+) to the rooting medium reduces the effectiveness of cationic toxicants (e.g., Cu2+ and Pb2+) and increases the effectiveness of anionic toxicants (e.g., SeO42- and H2AsO4-). Root growth responses to ions are better correlated with ion activities at PM surfaces ({IZ}0) than with activities in the bulk-phase medium ({IZ}b) (IZ denotes an ion with charge Z). Therefore, electrostatic effects play a role in heavy metal toxicity that may exceed the role of site-specific competition between toxicants and ameliorants. Furthermore, ψ0controls the transport of ions across the PM by influencing both {IZ}0and the electrical potential difference across the PM from the outer surface to the inner surface (Em,surf). Em,surfis a component of the driving force for ion fluxes across the PM and controls ion-channel voltage gating. Incorporation of {IZ}0and Em,surfinto quantitative models for root metal toxicity and uptake improves risk assessments of toxic metals in the environment. These risk assessments will improve further with future research on the application of electrostatic theory to heavy metal phytotoxicity in natural soils and aquatic environments.
authors.affiliation: Key Laboratory of Soil Environment & Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China. wangym@issas.ac.cn.
authors.forename: Yi-Min
authors.initials: YM
authors.lastname: Wang
authors.affiliation: Agricultural Research Service, United States Department of Agriculture, Collaborating Scientist, Plant Science Research Unit, Raleigh, NC 27695, USA. tom@kinraide.net.
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Kinraide
authors.affiliation: The University of Queensland, School of Agriculture and Food Sciences, St. Lucia, Queensland 4072, Australia. p.wang3@uq.edu.au.
authors.forename: Peng
authors.initials: P
authors.lastname: Wang
authors.affiliation: Key Laboratory of Soil Environment & Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China. xzhao@issas.ac.cn.
authors.forename: Xiu-Zhen
authors.initials: XZ
authors.lastname: Hao
authors.affiliation: Key Laboratory of Soil Environment & Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China. dmzhou@issas.ac.cn.
authors.forename: Dong-Mei
authors.initials: DM
authors.lastname: Zhou