Taproot-elongation rates of soybean cultivars in the glasshouse and their relation to field rooting depth

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 taprootelongation 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. Additional index words: Glycine max (L.) Merr., Roots, Water uptake, Water stress, Rooting depth, Selection criteria, Seed weight, Seed source. ENLARGING the volume of soil penetrated by a soybean [Glycine max (L.) Merr.] root system would increase available water to the plant and should delay or moderate water stress. When the soybean is planted in rows of 100 cm or less, lateral roots from one row soon compete with roots from adjacent rows for water and nutrients (B6hm, 1977). Thus, abreedin~ program designed to increase soybean rooting deptla may improve soybean water stress avoidance. Before such a breeding program would be practical, soybean characteristics necessary to produce a deep root s.ystern must be identified; and standardized screening procedures should be developed. One characteristic that may influence rooting depth is taproot-elongation rate. Plant species, such as alfalfa (Medicago sativa L.), that have dominant taproots are usually deeply rooted and drought avoiders (Weaver, 1926). Soybean root systems, however, are usually categorized as weakly taprooted or diffuse. Nonetheless, taproot-elongation rate initially determines soybean rooting depth. Therefore, a soybean genotype with a dominant, rapidly elongating taproot may have a deeper root system and better water availabifity than a genotype with a weak, slow-growing taproot. A preliminary glasshouse study has indicated that ~ Journal Paper no. J-10814 of the Iowa Agric. and Home Econ. Exp. Stn., Ames; Project 2354. Received 9 May 1983. ~Former graduate research assistant (now plant physiologist, USDA/ARS] Iowa State Univ., Ames, IA 5001-1), professor (now professor, Texas Tech Univ., Lubbock, TX 79409), and professor, Agron. Dep., Iowa State Univ., Ames, IA 50011. taproot-elongation rates of soybean cultivars differ significantly (Taylor et al., 1978). Unfortunately, glasshouse evaluations of plant characteristics are not always valid indicators of field performance. Furthermore, root characteristics rarely have been used successfully as selection criteria in crop improvement programs. Therefore, glasshouse trials were used to identify soybean cultivars with widely differing rates of taproot elongation. Selected cultivars were then examined in field plots to determine if their taprootelongation rates in the glasshouse were related to their field rooting patterns. MATERIALS AND METHODS Glasshouse Experiments The technique used for evaluating taproot-elongation rates in the glasshouse was a modification of that used by Taylor et al. (1978). Soybean plants were grown in 200 clear, acrylic-plastic tubes (185 cm long, 7 cm i.d.) filled with medium-grade horticultural vermiculite. All tubes were mounted in four metal racks covered with 2.5-cm expanded polystyrene sheets to prevent drastic temperature fluctuations in the vermiculite and to avoid exposure of roots to direct sunlight. A trough filled with gravel at the bottom of each rack allowed drainage from the open-ended tubes, and kept the vermiculite in place. A bank of four 40-W fluorescent lamps and six 100-W incandescent lamps mounted on the top of each rack supplemented daylight and maintained a 15.5-h photoperiod. To begin each trial, two weighed (_+ 1 mg) seed were planted 3 cm deep in moist vermiculite in each tube. After germination, plants were thinned to one/tube. Plants were watered daily with a modified, double-strength Hoagland’s solution (Epstein, 1972) containing chelated iron (4 mg Fe/L). Because the tubes were inclined 15° from vertical, taproots that normally grow downward were forced to grow along the inside surface of the tubes and could be seen easily. When a taproot of any of the 200 plants in a trial reached the bottom of its tube, the trial was terminated; and taproot length was measured from seed depth to apex position. Elongation rates were calculated by dividing taproot length by days after planting. Each trial consisted of four randomized, complete blocks. The 50 tubes within each block were randomly assigned to a maximum of 48 different cultivars, plus two plantings of ’Wayne’ that served as a standard. In 1979, seed of 105 soybean cultivars were obtained from the Soybean Germplasm Collection at Urbana, Ill., and from local sources. Because only 48 cultivars could be evaluated concurrently, the 105 cultivars were divided into maturity groups. Each maturity group was evaluated in s.eparate trials; therefore, comparison of cultivars among maturity groups is inappropriate. S.eed obtained in 1979 were used in the rhizotron study and in the first four trials, including two trials of Maturity Group II cultivars and one trial each of Maturity Group I and III cultivars. In 1980, seed of all cultivars were increased in the field at Ames, Iowa; and this seed was used in one trial for each maturity group, in the seed weight experiment, and in the 1981 field experiment. Data for