Degree Type


Date of Award


Degree Name

Doctor of Philosophy




Soybean Glycine max (L.) Merr. genotypes with rapidly elongating taproots may attain greater rooting depths and may be more drought avoidant than soybean genotypes with taproots that elongate more slowly. The objectives of this study were to improve and standardize a glasshouse procedure for measuring soybean taproot elongation rates, and to determine whether soybean cultivars with widely differing taproot elongation rates have different patterns of root growth and water extraction in the field;Taproot elongation rates of 105 soybean cultivars were measured by growing individual plants in a glasshouse in acrylic plastic tubes filled with vermiculite and slanted at 15(DEGREES) from the vertical. There were significant differences in taproot elongation rate among cultivars. In general, the procedure developed for measuring taproot elongation rates yielded reasonably consistent and reproducible rankings of cultivars;Seed weight, seed source and quality, planting depth, and seedling damage affected taproot elongation rate. Plants grown from heavier seed had faster taproot elongation rates than those grown from lighter seed, at least within cultivars. Cultivar differences, however, were not related to seed weight. Older or poorer quality seed produced plants with slower taproot elongation rates. Deeper planting slowed rate of taproot elongation. Lastly, seedling damage treatments, consisting of removal of various plant parts, decreased taproot elongation rates;Eight soybean cultivars, four with relatively fast rate of elongation and four with relatively slow rates, were grown in field plots at Castana, Iowa. Rooting depth and relative root length density with depth were determined from 10 cm diameter cores using the core-break method. Soil-water-use with depth was determined using a neutron soil moisture probe. Generally, cultivars with relatively fast taproot elongation had deeper maximum rooting depths and greater relative root length densities at 100, 150 and 200 cm depths than did those in the slow elongating group. Additionally, cultivars with faster elongation rates obtained a larger percentage of their water from below 120 cm.



Digital Repository @ Iowa State University,

Copyright Owner

Thomas Charles Kaspar



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109 pages