Aluminium and pH effects on dry bean root growth in nutrient solution

Abstract

Despite considerable interest in the phytotoxic nature of soluble aluminium (Al), the relationships between Al species and observed phytotoxicity are not fully understood. A solution culture technique was employed to study the effects of Al species, at varying pH levels, on the root growth of the dry bean (Phaseolus vulgaris L. cv. ‘Kaimberg’). Increasing the solution pH from 4,2 to 4,8 resulted in a significant reduction in the measured monomeric Al concentration. A highly significant (R ²=0,958) relationship between measured monomeric Al concentration and relative root length (RRL) was obtained. Poor relationships between RRL and the activities of individual monomeric Al species were observed over the imposed pH range. Regression analysis of the sum of activities of monomeric Al species (Σa _{mono Al}=Al³⁺+AlOH²⁺+Al(OH)₂ ⁺+AlSO₄ ⁺) and RRL resulted in a highly significant coefficient of determination (R ²=0,928). The regression of RRL versus the sum of the activities of Al³⁺ and hydroxy-Al species (Σ Al³⁺+AlOH²⁺+Al(OH)₂ ⁺) resulted in the improvement of the coefficient of determination (R ²=0,996), thereby implicating these species in an observed phytotoxic response. Threshold values of Σa_{mono Al} and the sum of the activities of Al³⁺ and hydroxy-Al species were found to be 10,2 and 9,0 μM respectively for a 10% reduction in root growth.