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Relationships between Root Length of Soybean and Calculated Activities of Aluminum Monomers in Nutrient Solution¹

ABSTRACT

Aluminum species composition in solution is largely dependent on solution pH. Effects of Al on root length of soybean [Glycine max (L.) Merr.] were studied in solution culture using an incomplete factorial design comprising varying pH levels of 4.0, 4.2, 4.4, 4.6, 4.8, and 5.0 and added Al concentrations of 10, 15, 20, 25, 30 and 40 µM. These treatments caused calculated sum of activities of monomeric Al species ( a_{Al mono}) in solution to vary from 4.4 to 33.8 µM. The range in calculated activities of each of the Al monomers was 0.4 to 16.1 µM for Al³⁺, 0.3 to 2.8 µM for Al(OH)²⁺, 2.5 to 19.3 µM for Al(OH)⁺₂, 0.1 to 4.4 µM for AlSO⁺₄, and 0.02 to 0.22 µM for Al(OH)⁰₃. At a given concentration of added Al an increase in pH was associated with a decrease in calculated a_{Al mono} in solution and an increase in root length. Relative root length of soybean was highly correlated with calculated a_{Al mono} in solution (R² = 0.910). Among the individual Al monomers, relative root length was most highly correlated with calculated activity of Al(OH)²⁺ (R² = 0.870), followed by AlSO⁺₄ (R² = 0.674), Al(OH)⁺₂ (R² = 0.621), and Al³⁺ (R² = 0.590). No significant correlation was found between root length and calculated activity of Al(OH)⁰₃. Reinterpretation of data from other studies with soybean, subterranean clover (Trifolium subterraneum L.), alfalfa (Medicago sativa L.), and sunflower (Helianthus annuus L.) also revealed that root growth was most highly correlated with activity of either Al(OH)²⁺ or Al(OH)⁺₂. In a majority of cases the relationship between root growth and activity of Al³⁺ was relatively poor.

¹ Contribution from the Dep. of Agriculture, Univ. of Queensland, St. Lucia. Australia 4067.

² Postdoctoral Research Fellow, Associate Professor of Plant Nutrition, Professor of Agriculture, and Senior Lecturer in Crop Agronomy, respectively. Present address of senior author is Dep. of Agronomy and Soil Science, Univ. of New England, Armidale, Australia 2351.

Received for publication September 23, 1985.