Root Characteristics Associated with Divergent Selection for Seedling Aluminum Tolerance in Soybean

Soybean ( Glycine max [L.] Merr.) roots are susceptible to A1 toxicity. This study tested the hypothesis that seedling characteristics could serve as selection criteria for plant tolerance to A1. Phenotypic‐recurrent, divergent selection programs for seedling tolerance to A1 were completed within two broad‐base soybean populations based on a laboratory screen. Genotypes differing in seedling tolerance to A1, rate of electron release (and, H + release) from the root surface, and root thickness (g cm ‒1 ) were identified. Root penetration from a soil horizon with low A1 into soils with high A1 was selected to measure A1 tolerance for greenhouse‐grown plants. Selection pressure for seedling root growth under A1 within these two populations evolved a number of apparent mechanisms affecting seedling root growth in solutions containing A1. Selecting genotypes for differences in A1 seedling tolerance gave inconsistent associated responses for root penetration into soils with high A1. High electron release rates and small roots (and seeds) were associated with selection for low A1 seedling tolerance. Selecting genotypes for differences in electron release rate or for differences in root thickness did not create an associated response in root penetration into soils with high A1. Preconditioning of seedling roots to A1 increased tolerance to A1 proportionately in tolerant and nontolerant selections, when tolerant and nontolerant selections were grown in different A1 concentrations selected to yield ≈75% reduction in root growth. Genetic variability available was not such as has been associated with the inducible A1‐binding proteins proposed for wheat ( Triticum aestium L.). New genetic sources for A1 tolerance and information on the nature of this tolerance are needed to develop selection criteria for A1 tolerance.