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Taproot-Elongation Rates of Soybean Cultivars in the Glasshouse and Their Relation to Field Rooting Depth¹

Deeper rooting should improve soybean [Glycine max (L.) Merr.] water stress avoidance by increasing water uptake from deep soil strata. Because taproots are initially the deepest segment of soybean root systems, soybean genotypes with rapidly elongating taproots may have deeper root systems than genotypes with slowly elongating taproots. Taproot-elongation rates of 105 soybean cultivars from Maturity Groups I, II, and III were measured in a glasshouse using clear plastic tubes filled with vermiculite and then inclined 15°. Taproot-elongation rates within a maturity group differed among cultivars by as much as 1.3 cm/day. Seed weight within a seedlot and seed source also influenced the taproot-elongation rate of a cultivar. Selected Maturity Group II cultivars with either faster or slower taproot-elongation rates in the glasshouse were grown in the Ames rhizotron and in field plots. Excavation of rhizotron compartments 49 days after planting revealed that roots of faster elongators were 10 cm deeper than those of slower elongators. In field plots 56 days after planting, the faster elongating group was 9 cm deeper and had more roots at 150 cm than did the slower elongating group. The two groups did not differ significantly, however, 72 days after planting. The group of cultivars with faster elongation rates also depleted soil-water below 120 cm slightly more than did the slower group.

Key Words: Glycine max (L.) Merr. • Roots • Water uptake • Water stress • Rooting depth • Selection criteria • Seed weight • Seed source

² Former graduate research assistant (now plant physiologist, USDA/ARS, Iowa State Univ., Ames, IA 5001-11), professor (now professor, Texas Tech Univ., Lubbock, TX 79409), and professor, Agron. Dep., Iowa State Univ., Ames, IA 50011.

Received for publication May 9, 1983.

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