Mechanism of silicon induced alleviation of aluminum phytotoxicity
Complexation of aluminum (Al) with silicon (Si) has been attributed as a mechanism of Si induced alleviation of Al toxicity to plants. Effects of various concentrations of Si (0, 534, 1068, or 1602 mu-M) on root growth of two sorghum cultivar seedlings (NB9040 and Martin) was evaluated in nutrient solutions containing 0 or 296 mu-M Al. Three experiments were conducted with variation in the sequence of pH adjustment of the solution, and addition of Al and Si stock solutions. In solutions containing 296 mu-M Al with no Si, growth of sorghum seedlings was significantly decreased as compared to that of the seedlings grown in solutions containing 0 Al. Root growth of seedlings of both cultivars in solutions containing 296 mu-M Al improved significantly with increasing concentrations of Si, but only when Al was added to the basal nutrient solution following the addition of Si without lowering the solution pH. Addition of Si to the basal nutrient solution increased the pH gtoreq 10.0, which facilitated precipitation of Al, thus decreasing its toxicity. If the solution pH was lowered to 4.0 following the addition of Si, prior to the addition of Al, the toxicity of Al was least influenced by the presence of Si in solution within the range of 534 to 1602 mu-M. Silicon-induced alleviation of Al toxicity appears to be due to pH effects and not the direct effects of Si on Al in solutions.