Correlation of shoot and root growth and its role in selecting for aluminum tolerance in soybean

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Abstract
Aluminum‐tolerant soybean cultivars are needed for deeper rooting and increased drought tolerance in acid subsoils. A major limitation in the development of such cultivars is Al‐screening methodology. Shoot growth is often used to infer root growth, but the genotypic relationships between root and shoot growth of older soybean plants have not been evaluated. Our objectives were (i) to test the hypothesis that shoot growth is a reliable indicator of acid soil (Al) tolerance in soybean, and (ii) to determine the relative Al tolerances of selected soybean genotypes. Nine genotypes were evaluated for Al tolerance by growing them for 37 days in greenhouse pots of unlimed (pH 4.3) and limed (pH 5.3) Tatum subsoil. Aluminum tolerance was determined by root and shoot growth and plant symptoms. Aluminum tolerance was detected using both shoot and root growth, and agreement between these two selection criteria was good. Genotypic correlations between root and shoot growth for unlimed soil, for limed soil, and for growth on unlimed soil expressed as a percentage of that on limed soil were 0.89, 0.86 and 0.93, respectively. Results support the conclusion that shoot growth can be used effectively to screen for acid soil (Al) tolerance in soybean. PI 416937 and Perry cultivars were classified as most tolerant, but the former was more susceptible to petiole collapse than the latter, suggesting that their tolerance mechanisms may be different. Forrest, Bossier, and Chief were the least tolerant when grown in the Tatum subsoil at pH 4.3. When plants were grown on unlimed soil, the first trifoliolates of the Al‐tolerant, symptom‐free Perry contained significantly lower concentrations of Al and Fe than did those of PI 416937 which showed petiole collapse. First trifoliolates of Al‐tolerant Perry were also significantly lower in Al and Fe than those of Al‐sensitive Chief, which showed severe leaf distortion and stunting on the unlimed soil. We conclude that variation for Al tolerance exists in soybean and that this variability may be sufficient to warrant breeding attention.