Cation amelioration of aluminum toxicity in wheat

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practical interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing 0.4 millimolar CaCl(2) at pH 4.3 variously supplemented with AlCl(3) and additional amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 {Al(3+)}/({Al(3+)} + K(m) + alpha{C}(beta)) where {Al(3+)} is the activity of Al(3+) expressed in micromolar, {C} is the activity of the added cation expressed in millimolar, and K(m) (= 1.2 micromolar) is the {Al(3+)} required for 50% inhibition in the absence of added salt. For Ca(2+), Mg(2+), and Na(+) the values of alpha were 2.4, 1.6, and 0.011, respectively, and the values for beta were 1.5, 1.5, and 1.8, respectively. With regard to relative ameliorative effectiveness, Ca(2+) > Mg(2+) approximately Sr(2+) >> K(+) approximately Na(+). Other cations were tested, but La(3+), Sc(3+), Li(+), Rb(+), and Cs(+) were toxic at potentially ameliorative levels. The salt amelioration is not solely attributable to reductions in {Al(3+)} caused by increases in ionic strength. Competition between the cation and Al for external binding sites may account for most of the amelioration.