Root elongation of soybean genotypes in response to acidity constraints ina subsurface solution compartment

Aluminum-tolerant germplasm Is needed to overcome subsurface acidity constr aints to root growth and plant access to water and nutrients. Root elongati on of four soybean [Glycine max (L.) Merr.] genotypes exposed to varying co ncentrations of Al, H, and Ca were compared in two experiments using a vert ically split root system. Roots extending from a limed surface soil compart ment grew for 12 d into a subsurface compartment with nutrient solution tre atments. In Exp. 1 root growth for cv. Ransom and Plant Introduction 416937 (PI) were compared in solutions with factorial combinations of pH (4.2, 5. 2) and Al (0, 7.5, 15 mu M) with Ca maintained at 10 mM. In Exp. 2 soybean line N93-S-179 (N93), PI, and cultivars Ransom and Young were compared in s olutions with factorial combinations of Ca (2 and 10 mM) and Al (7.5 and 15 mu M) maintained at pH 4.6. Ransom and PI had similar responses in tap and lateral root elongation to solution pH and Al treatments in Exp. 1, but me an tap root length of Ransom in the subsurface compartment exceeded that of PI by 22%. Aluminum inhibited the length of lateral roots more than tap ro ots in both experiments. Molar activity ratios between Ca and Al3+ {Ca/Al3} accounted for most of the differences in root elongation response among s olution treatments in Exp. 2. A 50% reduction in relative length of tap roo ts for all genotypes occurred with a {Ca/Al3+} value of 891. Values of {Ca/ Al3+} for 50% reductions in relative length of lateral roots differed among genotypes and were 1.6 to 3.5 times greater than for tap roots. On the bas is of the {Ca/Al3+} indices for lateral root length, line N93 and Ransom ex hibited greater tolerance to subsurface solution Al than PI and Young.