| A realistic strategy to overcome subsoil acidity constraints to root growth and associated drought stress is the development of Al-tolerant germplasm. Root growth of four soybean (Glycine max (L.) Merr.) genotypes were compared in three greenhouse and phytotron experiments using a vertical split-root system. Roots extending from a limed and fertilized surface soil compartment grew for 12 days in solutions with factorial combinations of H, Al, Ca, and Mg concentrations in a subsurface solution compartment. In the first experiment root growth for cultivar Ransom and plant introduction PI 416937 (PI) were compared in solution with combinations of pH (4.2, 5.2) and Al (0, 7.5, {dollar}rm 15mu M){dollar} under a constant supply of 10mM Ca. In the second experiment, root elongation for genotypes N93-S-179 (N93), PI, Ransom and Young were compared in subsurface treatment combinations of Ca (2 and 10mM) and Al (7.5 and {dollar}rm 15mu M){dollar} with pH maintained at 4.6. In the third experiment, root growth of genotypes N93 and Young were compared in solution treatments maintained at pH 4.6 with either variable Mg supply (0, 2 and 10 mM) and no Al, or the combination of Ca (2 and 10mM) and Mg (0, 2 and 10mM) in the presence of 15 M Al). Aluminum inhibited the length of lateral roots more than tap and other root classes in all genotypes. Concentrations of 7.5 and {dollar}rm 15mu M{dollar} Al inhibited tap root elongation only at pH 4.2, whereas the same Al concentrations reduced length of laterals on tap roots at both pH 4.2 and 5.2. Molar activity ratios between Ca and {dollar}rm Alsp{lcub}3+{rcub} {lcub}Ca/Alsp{lcub}3+{rcub}{rcub}{dollar} among solutions with variable Ca and Al levels were good predictors of both tap and lateral root elongation response. A 50% reduction in tap root elongation occurred for all genotypes when the solution {dollar}rm{lcub} Ca/Alsp{lcub}3+{rcub}{rcub}{dollar} was reduced to 891. Values of the {dollar}rm{lcub} Ca/Alsp{lcub}3+{rcub}{rcub}{dollar} index for similar reductions in length of lateral roots differed among genotypes and were two or three times greater than for tap roots. Genotypes N93 and Ransom maintained a higher percentage of their maximum root length at lower solution {dollar}rm{lcub} Ca/Alsp{lcub}3+{rcub}{rcub}{dollar} values than genotypes PI and Young. Magnesium promoted tap and lateral root elongation both in the presence and absence of solution Al. Root elongation response to Mg in the absence of Al was attributed to both the alleviation of {dollar}rm Hsp+{dollar} toxicity in the subsurface solutions and the need for an external supply of Mg near actively growing root tips to overcome limited basipetal transport of this nutrient within plant roots. |
