The amelioration of aluminium toxicity by silicon in wheat (Triticum aestivum L.):: malate exudation as evidence for an in planta mechanism

Two wheat (Triticum aestivum L.) cultivars, one aluminium tolerant (Atlas 66) and one sensitive (Scout 66), were grown in a continuous-flow culture system (less than or equal to pH 5.0) containing aluminium (0-100 mu M) and silicon (0-2000 mu M) in factorial combination. Treatment with silicon resulted in a highly significant amelioration of aluminium toxicity as assessed by root growth in both cultivars. Amelioration was influenced by wheat cultivar and silicon concentration, as 2000 mu M silicon significantly ameliorated the toxic effects of 100 mu M aluminium in Atlas 66, and only 5 mu M silicon alleviated the effect of 1.5 mu M aluminium on Scout 66. Nutrient medium pH was critical, as an amelioration by silicon was apparent only at pH > 4.2 for Atlas 66, and at pH > 4.6 for Scout 66. Silicon neither reduced levels of toxic aluminium species in the growth solutions, nor the amount of aluminium taken up by roots. In experiments to assess exudation of malate by Atlas 66 roots treated with 100 mu M aluminium, the presence of 2000 mu M silicon (pH 4.6) was found to have a negligible effect on exudation. In contrast, citrate, a known aluminium chelator, reduced aluminium-induced exudation of malate at 5-40 mu M and completely inhibited it at 100 mu M citrate. The results indicate that silicon does not reduce aluminium phytotoxicity as a result of aluminium/silicon interactions in the external media, and that the mechanism of amelioration has an in planta component.