Aluminum enhancement of plant growth in acid rooting media. A case of reciprocal alleviation of toxicity by two toxic cations

Abstract

The generally rhizotoxic ion Al³⁺ often enhances root growth at low concentrations. The hypothesis that Al³⁺ enhances growth by relieving H⁺ toxicity was tested with wheat seedlings (Triticum aestivum L.). Growth enhancement by Al³⁺ only occurred under acidic conditions that reduced root elongation. Al³⁺ increased cell membrane electrical polarity and stimulated H⁺ extrusion. Previous investigations have shown that Al³⁺ decreases solute leakage at low _p H and that the alleviation of H⁺ toxicity by cations appears to be a general phenomenon with effectiveness dependent upon charge (C³⁺ >C²⁺ >C^l+ ). Alleviation of one cation toxicity by another toxic cation appears to be reciprocal so that Al³⁺ toxicity is relieved by H⁺ . It has been argued previously that this latter phenomenon accounts for the apparent toxicity of ALOH²⁺ and Al(OH)⁺₂ . Reduction of cell-surface electrical potential by the ameliorative cation may reduce the cell-surface activity of the toxic cation.