Date of Award

12-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Division of Agriculture (SAFES)

Committee Member

Dr. Sruthi Narayanan, Committee Chair

Committee Member

Dr. Benjamin Fallen

Committee Member

Dr. Juan Carlos Melgar

Abstract

Drought stress has been identified as the major environmental factor limiting soybean (Glycine max L. Merr.) yield in the United States and other parts of the world. Water use efficiency (WUE) that results in greater yield per unit of rainfall is an important parameter in determining crop yields in rain-fed production systems, and is often related with crop drought tolerance. Even though roots are major plant organs that perceive and respond to drought stress, their utility in improving soybean yield and WUE under different environmental and management conditions are largely unclear. The objectives of this research were to evaluate soybean genotypes for root morphology, hardpan penetrability, WUE, and yield, and to determine whether root traits are related with any above-ground trait related with productivity. Two independent controlled-environmental experiments were conducted to evaluate 49 genotypes for root morphological traits and root penetrability of synthetic hardpans (penetration resistance, 1.5 MPa at 30°C) in 2016 and 2017. Significant genetic variability was observed for root traits among the 49 genotypes tested, genotypes that penetrated the synthetic hardpan were identified. From this experiment 10 genotypes were selected based on varying root morphological traits. The 10 selected genotypes were then evaluated in field trials at two locations in South Carolina (Florence and Pendleton) during the 2017 cropping season. The lines were evaluated for yield and root morphological traits under irrigated and non-irrigated conditions. Another controlled-environmental experiment was conducted in 2018 to further test those 10 genotypes for WUE and penetrated root length (PRL), along with root morphological traits. Shoot dry weight and chlorophyll index (easily selectable traits, often related with productivity) were positively related with total root length, surface area, and volume, and fine root length. Seed size was not correlated with any root traits indicating that large seeds may not always produce large root systems. In the field study, the slow wilting lines NTCPR94-5157 and N09-13890 had equal or greater yield than the checks - cultivar NC-Raleigh and elite South Carolina breeding line SC07-1518RR, under irrigated and non-irrigated conditions. The high yielding genotypes NTCPR94-5157, N09-13890, and SC07-1518RR also exhibited root parsimony (reduced root development) in the field environment though they had the inherent ability to produce prolific root systems as shown by the controlled environmental experiments. Our results support the recent hypothesis in literature that reduced root development would have an adaptational advantage to improve crop yield under high input field conditions. The high yielding genotypes NTCPR94-5157, N09-13890, NC-Raleigh, and SC07-1518RR, and cultivar Boggs (intermediate in yield), possessed high WUE and had increased root penetrability of hardpans characterized by PRL. These genotypes (NTCPR94-5157, N09-13890, NC-Raleigh, SC07-1518RR, and Boggs) offer useful genetic materials for improving yield, drought tolerance, and/or hardpan penetrability in soybean breeding programs.

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