Abstract
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 transport channels or surface ligands that must be occupied by Ca2+ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al3+, that (i) Ca2+, Mg2+, and Sr2+ are equally ameliorative, (ii) that root elongation does not increase as Ca2+ replaces Mg2+ or Sr2+ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al3+ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca2+ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC3+), in contrast to Al3+, was specifically relieved by Ca2+ at the membrane surface. The rhizotoxicity induced by H+ exhibited a weak specific response to Ca2+ at the membrane surface. We conclude that the Ca2+-displacement hypothesis fails in the case of Al3+ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al3+. However, TEC3+, but not Al3+, may be toxic because it inhibits Ca2+ uptake. The nature of the specific H+-Ca2+ interaction is uncertain.
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Abbreviations
- {Al3+ }0 :
-
chemical activity of Al3+ at the root-cell membrane surface
- {Al3+ }E :
-
chemical activity of Al3+ in the external rooting medium
- E0 :
-
electrical potential at the root-cell membrane surface
- HXM2+ :
-
hexamethonium ion
- TEC3+ :
-
tris(ethylenediamine)cobaltic ion
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Kinraide, T.B., Ryan, P.R. & Kochian, L.V. Al3+-Ca2+ interactions in aluminum rhizotoxicity. Planta 192, 104–109 (1993). https://doi.org/10.1007/BF00198699
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DOI: https://doi.org/10.1007/BF00198699