Al³⁺-Ca²⁺ interactions in aluminum rhizotoxicity

Abstract

The cation Al³⁺ is toxic to plants at micromolar concentrations and can severely inhibit root growth in solution experiments.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.