Seedling validation of acid soil tolerance of lucerne populations selected in solution culture high in aluminium

This study tested the hypothesis that lucerne (Medicago sativa L.) populations selected in solution culture high in aluminium (Al) would increase seedling root growth when grown in an acid soil high in exchangeable Al. Root growth of six elite populations (Aurora C(2), UQL-1 C(2), T02-011 C(1), T02-011 C(2), A513 C(3) and Sardi 7 C(2)) selected in high-Al solution culture (SHASC) was compared with that of corresponding parent as well as the Georgia acid soil-tolerant populations in an acid soil in pots grown for 8 days under controlled environmental conditions. Lime was added to the soil to provide contrasts in the severity of stress imposed by low pH and high Al. Averaged across six SHASC populations, total root length increased 19% at pH 4.34 in CaCl(2) (35% exchangeable Al) and 26% at pH 5.26 (< 1% exchangeable Al) compared with the control populations. At all pH levels SHASC populations showed increased tap root length, total root length (includes lateral roots), root weight and root surface area, but decreased average root diameter compared with the six control populations. A large amount of variability was observed both between and within lucerne populations with three SHASC populations (Aurora C(2), UQL-1 C(2) and Sardi 7 C(2)) exhibiting increased root growth at lower pH levels, but little increase in root length at higher pH, consistent with increased tolerance to Al toxicity. This was in contrast to three other SHASC populations (T02-011 C(1), T02-011 C(2) and A513 C(3)), which exhibited increased root length at all pH levels, consistent with increased seedling vigour. The Sardi 7 C(2) population exhibited the greatest increase in tap root growth with tap root length increasing by 40 and 30% at pH 4.34 and 4.48, respectively, compared with its parent population Sardi 7. This study provides evidence that seedlings of lucerne populations selected in high-Al solution culture can confer significantly improved root and shoot growth in acid soil. It is recommended that such screening be incorporated into lucerne breeding programs to reduce costs in space and time.