Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognised as rhizotoxic for about 100 years but there is no consensus on its mode of action. We contend that the primary mechanism of rhizotoxicity of many metal cations is non-specific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands - especially carboxylate ligands of the cell wall pectic matrix. Specifically, we propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations, such as Al(3+) and Hg(2+) (amongst others), which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations, such as Ca(2+) , Na(+) , Mn(2+) , and Zn(2+) , which bind weakly to hard ligands bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl(+) , Ag(+) , Cs(+) , and Cu(2+) ) are sometimes more toxic than expected through binding to hard ligands. The data presented here demonstrate the importance of 'strength of binding to hard ligands' in influencing a range of important physiological processes within roots through non-specific mechanisms. Environ Toxicol Chem © 2013 SETAC.