Free metal ion availability is a major factor for tolerance and growth in Physcomitrella patens

A decreased ratio of gametophyte to protonema growth (G:P) was observed as a first reaction to even very low metal levels; G:P measurements can therefore be used as sensitive stress indicators in P. patens. Though all metals caused inhibiting effects with all anions, ethylenediaminetetraacetic acid chelates showed up to three orders of magnitude less toxic. Using Visual MINTEQ, we modeled the water solubility of zinc almost perfectly but achieved less accurate results for copper. For both metals, water solubility was rather under- than overestimated, indicating that adsorption to the agar played only a minor role in controlling metal solubility. Free metal cations were useful to explain growth inhibition which could not be fully explained by total metal concentration, calculated or experimentally determined water solubility. Especially the low toxicity of ethylenediaminetetraacetic acid chelates could be explained satisfactory by shielding of the metal ions.