Apparent phytotoxicity of mononuclear hydroxy‐aluminum to four dicotyledonous species

It has long been assumed that Al 3+ is an important rhizotoxic ion in acid soils around the world, but the toxicity of Al 3+ relative to mononuclear hydroxy‐Al [AlOH 2+ and Al(OH) + 2 ] has been examined in detail only for an Al‐sensitive wheat variety ( Triticum aestivum L. cv. Tyler). That plant appears to be sensitive to Al 3+ but not to AlOH 2+ and Al(OH) + 2 . New experiments, and reanalyses of previously published experiments, provide evidence that dicotyledonous species may be sensitive to mononuclear hydroxy‐Al and that Al 3+ may be nontoxic, or less toxic, to those plants. Despite these consistently measured differences between wheat and the dicotyledons, the determination of relative toxicities (Al 3+ vs mononuclear hydroxy‐Al) may be an intractable problem. Because of hydrolysis equilibria, (AlOH 2+ ) and (Al(OH) + 2 ) are equivalent to (Al 3+ )k 1 (H + ) −1 and (l 3+ )k 2 (H + ) −2 , respectively, in which k 1 and k 2 are the first and second hydrolysis constants (braces denote activities). Thus, any expression of root elongation as a function of mononuclear hydroxy‐Al can be alternatively expressed as a function of (Al 3+ ) and (H + ). Toxicity attributed to mononuclear hydroxy‐Al may actually be Al 3+ toxicity that increases as pH rises (i.e. Al 3+ toxicity ameliorated by H + ).