RECONSIDERING THE RHIZOTOXICITY OF HYDROXYL, SULFATE, AND FLUORIDE COMPLEXES OF ALUMINUM

Recent years have witnessed some convergence of opinion regarding the identity of the rhizotoxic species of aluminium, AlO4Al12(OH)(24)(H2O) (12)(7+) (Al-13) and Al3+ are almost certainly toxic, but no rhizotoxi city has been detected for Al-SO4 (AlSO4+ and Al(SO4)(2)(-)) or Al-F ( e.g. AlF2+ and AlF2+). The status of Al-OH (e.g. AlOH2+ and Al(OH)(2)( +)) is uncertain because experimental results often appear to indicate Al-OH toxicity, In this article it is argued that this appearance of toxicity is the consequence of the relief of Al3+ toxicity by H+ (and vice versa), Furthermore, this view provides an explanation for phenom ena, such as Al stimulation of growth, unexplained by the hypothesis t hat Al-OH is toxic, It is concluded, therefore, that Al-OH is not toxi c at achievable activities, The previous failure to detect toxicity fr om Al-SO4 and Al-F also may have been a consequence of the low activit ies used in the experiments. New experiments again failed to reveal a toxicity for Al-SO4, but do indicate that AlF2+ and AlF2+ are toxic, S everal alternative hypotheses for the apparent toxicity of Al-F were c onsidered and rejected, Consequently, this report concludes that the f ollowing Al species are toxic to wheat roots in the following order: A l-13 > Al3+ > AlF2+ > AlF2+. The correlation between toxicity and char ge is discussed in light of the fact that the binding strength of cati ons to plasma membrane surfaces generally increases with charge.