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

The generally rhizotoxic ion Al 3+ often enhances root growth at low concentrations. The hypothesis that Al 3+ enhances growth by relieving H + toxicity was tested with wheat seedlings ( Triticum aestivum L.). Growth enhancement by Al 3+ only occurred under acidic conditions that reduced root elongation. Al 3+ increased cell membrane electrical polarity and stimulated H + extrusion. Previous investigations have shown that Al 3+ 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 (C 3+ >C 2+ >C l+ ). Alleviation of one cation toxicity by another toxic cation appears to be reciprocal so that Al 3+ toxicity is relieved by H + . It has been argued previously that this latter phenomenon accounts for the apparent toxicity of ALOH 2+ and Al(OH) + 2 . Reduction of cell‐surface electrical potential by the ameliorative cation may reduce the cell‐surface activity of the toxic cation.