Reconsidering the rhizotoxicity of hydroxyl, sulphate, and fluoride complexes of aluminium

Recent years have witnessed some convergence of opinion regarding the identity of the rhizotoxic species of aluminium. AlO4Al12(OH)24(H2O)127+(Al13) and Al3+ are almost certainly toxic, but no rhizotoxicity has been detected for Al-SO4(AlSO4+ and Al(SO4)2-) or Al-F (e.g. AIF2+ 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 phenomena, such as Al stimulation of growth, unexplained by the hypothesis that Al-OH is toxic. It is concluded, therefore, that Al-OH is not toxic at achievable activities. The previous failure to detect toxicity from Al-SO4 and Al-F also may have been a consequence of the low activities used in the experiments. New experiments again failed to reveal a toxicity for Al-SO4, but do indicate that AlF2+ and AlF2+ are toxic. Several alternative hypotheses for the apparent toxicity of Al-F were considered and rejected. Consequently, this report concludes that the following Al species are toxic to wheat roots in the following order: Al13>Al3+>AlF2+>AlF2+. The correlation between toxicity and charge is discussed in light of the fact that the binding strength of cations to plasma membrane surfaces generally increases with charge.