ROOT CHARACTERISTICS ASSOCIATED WITH DIVERGENT SELECTION FOR SEEDLING ALUMINUM TOLERANCE IN SOYBEAN

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