Role of Binding to the Root Surface and Electrostatic Attraction in the Uptake of Heavy Metal by Plants
Authors: Yermiyahu Uri , Kinraide Thomas , Mingelgrin Uri 
Published: December 2021

The principal accomplishment of the research supported by BARD was progress toward a comprehensive view of cell-surface electrical effects (both in cell walls [CWs] and at plasma membrane [PM] surfaces) upon ion uptake, intoxication, and amelioration. The research confirmed that electrostatic models (e.g., Gouy-Chapman-Stern [G-C-S]), with parameter values contributed by us, successfully predict ion behavior at cell surfaces. Specific research objectives 1. To characterize the sorption of selected heavy metals (Cu, Zn, Pb, Cd) to the root PM in the presence of other cations and organic ligands (citric and humic acids). 2. To compute the parameters of a G-C-S model for heavy-metal sorption to the root PM. 3. To characterize the accumulation of selected heavy metals in various plant parts. 4. To determine whether model-computed ion binding or ion activities at root PM surfaces predict heavy-metal accumulation in whole roots, root tips, or plant shoots. 5. To determine whether measured ion binding by protoplast-free roots (i.e., root CWs) predicts heavy-metal accumulation in whole roots, root tips, or plant shoots. 6. To correlate growth inhibition, and other toxic responses, with the measured and computed factors mentioned above. 7. To determine whether genotypic differences in heavy-metal accumulation and toxic responses correlate with genotypic differences in parameters of the G-C-S model. Of the original objectives, all except for objective 7 were met. Work performed to meet the other objectives, and necessitated on the basis of experimental findings, took the time that would have been required to meet objective 7. In addition, work with Pb was unsuccessful due to experimental complications and work on Cd is still in progress. On the other hand, the uptake and toxicity of the anion, selenate was characterized with respect to electrostatic effects and the influences of metal cations. In addition, the project included more theoretical work, supported by experimentation, than was originally planned. This included transmembrane ion fluxes considered in terms of PM-surface electrical potentials and the influence of CWs upon ion concentrations at PM surfaces. A important feature of the biogeochemistry of trace elements in the rhizosphere is the interaction between plant-root surfaces and the ions present in the soil solution. The ions, especially the cations, of the soil solution may be accumulated in the aqueous phases of cell surfaces external to the PMs, sometimes referred to as the "water free space" and the "Donnan free space". In addition, ions may bind to the CW components or to the PM surface with variable binding strength. Accumulation at the cell surface often leads to accumulation in other plant parts with implications for the safety and quality of foods. A G-C-S model for PMs and a Donnan-plus-binding model for CWs were used successfully to compute electrical potentials, ion binding, and ion concentration at root-cell surfaces. With these electrical potentials, corresponding values for ion activities may be computed that are at least proportional to actual values also. The computed cell-surface ion activities predict and explain ion uptake, intoxication, and amelioration of intoxication much more accurately than ion activities in the bulk-phase rooting medium.