AL3-CA2+ INTERACTIONS IN ALUMINUM RHIZOTOXICITY .2. EVALUATING THE CA2+-DISPLACEMENT HYPOTHESIS()

Several mineral rhizotoxicities, including those induced by Al3+, H+, and Na+, can be relieved by elevated Ca2+ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca2+ from tran sport channels or surface ligands that must be occupied by Ca2+ in ord er for root elongation to occur. In this study with wheat (Triticum ae stivum. L.) seedlings, we have determined, in the case of Al3+, that ( i) Ca2+, Mg2+, and Sr2+ are equally ameliorative, (ii) that root elong ation does not increase as Ca2+ replaces Mg2+ or Sr2+ in the rooting m edia, and (iii) that rhizotoxicity is a function solely of Al3+ activi ty at the root-cell membrane surface as computed by a Gouy-Chapman-Ste rn model. The rhizotoxicity was indifferent to the computed membrane-s urface Ca2+ activity. The rhizotoxicity induced by high levels of tris (ethylenediamine)cobaltic ion (TEC3+), in contrast to Al3+, was specif ically relieved by Ca2+ at the membrane surface. The rhizotoxicity ind uced by H+ exhibited a weak specific response to Ca2+ at the membrane surface. We conclude that the Ca2+-displacement hypothesis fails in th e case of Al3+ rhizotoxicity and that amelioration by cations (includi ng monovalent cations) occurs because of decreased membrane-surface ne gativity and the consequent decrease in the membrane-surface activity of Al3+. However, TEC3+, but not Al3+, may be toxic because it inhibit s Ca2+ uptake. The nature of the specific H+-Ca2+ interaction is uncer tain.