Abstract
The cation Al3+ is toxic to plants at micromolar concentrations and can severely inhibit root growth in solution experiments.Footnote 1 The inhibition of Ca2+ uptake in roots by Al3+ has been proposed as a possible mechanism for Al3+ toxicity, and in this study the hypothesis was tested directly. Root growth and Ca2+ uptake were measured in 5-d-old seedlings of wheat (Triticum aestivum L. Thell) during exposure to Al3+ in a low-Ca2+ basal medium, and to Al3+ in the presence of added cations. Uptake of Ca2+ in whole roots and translocation to the shoot were measured using 45Ca2+, and localized measurements of net Ca2+ flux were also made at the root apex using the technique of microelectrode ion-flux estimation. Treatment with 2.64 μM AlCl3 in 226 μM CaCl2, at pH 4.5, severely inhibited root growth without affecting Ca2+ uptake. Addition of 30 mM Na2+, 3 mM Mg2+ or 50 μM tris(ethylenediamine)cobalt(III) to this Al3+ treatment restored root growth but significantly reduced Ca2+ uptake measured over the entire root system and at the root apex. The Al3+ and Ca2+ concentrations were adjusted so that the activities of the Al3+ and Ca2+ ions were constant in all solutions (1.5 μM and 200 μM, respectively). Root growth can be severely inhibited by Al3+ concentrations that do not affect Ca2+ uptake, while the addition of ameliorating cations depresses Ca2+ uptake. These results argue against the hypothesis that Al3+ inhibits root growth by reducing Ca2+ uptake.
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Notes
Trivalent aluminum hydrolyzes in solution, and, apart from the Al3+ ion, which dominates speciation below pH 5.0, various mononuclear and polynuclear hydroxy-Al species can also occur (Kinraide 1991). Accumulating evidence suggests that Al3+ is the rhizotoxic species under the experimental conditions used in the present study (Kinraide 1991; Kinraide et al. 1992).
Abbreviations
- {Xn+}E :
-
activity of the cation Xn+ in the bathing solution
- [Xn+]E :
-
concentration of cation Xn+ in the bathing solution
- RRL:
-
relative root length
- TEC3+ :
-
tris(ethylenediamine)cobalt(III).
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The authors are grateful to Dr. R. J. Reid for his helpful discussions. This work was funded by U.S. Department of Agriculture/National Research Initiative Competitive Grant No. 91-37100-6630 to L.V.K.
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Ryan, P.R., Kinraide, T.B. & Kochian, L.V. Al3+-Ca2+ interactions in aluminum rhizotoxicity. Planta 192, 98–103 (1993). https://doi.org/10.1007/BF00198698
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DOI: https://doi.org/10.1007/BF00198698