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

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+ >Cl+ ). 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.