Master of Science

Demand for potassium (K) increases with increasing soybean yield. Little research has been conducted on soybean response to K on coastal plain and piedmont soils of Virginia, especially in double-crop systems. Nineteen full-season and 14 double-crop soybean experiments were conducted in 2013 and 2014 in Virginia and northeastern North Carolina to determine full-season and double-crop soybean (with wheat straw remaining or removed) response to soil test K and K fertilizer application rates. Field moist, air dried, and oven dry soil test K extraction techniques were also compared to predict responsiveness of sites to K applications. Potassium fertilizer increased yield in five of 19 full-season experiments and one of 14 double-crop experiments. Full-season soybean yield plateaued at 88% relative yield and soil test K value of 38.8 mg K kg-1. Full-season plant K critical concentrations were 18.2 g K kg-1 for V5 and 24.6 g K kg-1 for R2. Although critical concentrations could not be determined for double-crop soybean, V5 and R2 concentrations ranged from 17.6 to 35.6 g K kg-1 or 13.2 to 28.1 K kg-1, respectively, most of which were within or above accepted sufficiency levels. Eight of 13 sites resulted in greater soil K concentrations when alternative soil drying methods were compared to air-dry methods. However, differences were not consistent and no single method was superior for these soils. More data is needed for double-crop soybean systems due to lack of response and lack of low soil test K sites in these experiments. / Master of Science

Effect of irradiance on both initial and total ribulose bisphosphate carboxylase (RuBP Case) activities was investigated in soybean (Glycine max cv. Will) leaves. Initial RuBP Case activity (IA), present in a leaf extract prepared at 0°C and assayed without prior activation, was used as an estimate of in vivo activity. Total activity (TA) was measured in leaf extracts after full activation in high levels of CO₂ and Mg²⁺. Observed large variations in RuBP Case IA and TA in preliminary study, which prompted the necessity for optimizing conditions for extracting and assaying the enzyme in soybean. In a series of experiments optimal assay pH, Ru3P levels, Mg⁺ levels, and CO₂ levels were determined. Stability of RuBP Case in leaves and extracts was also investigated; observed storage of leaf tissue or extract at 0°C maintained stability of RuBP Case activation state for hours. Modified assay procedure was used to measure Ru3P Case IA and TA in leaves of soybean plants grown in 4 different irradiances: 20%, 53%, 70% and 100% full sun (FS). TA expressed on a leaf area basis was significantly reduced with lower irradiance, the result of a reduction in leaf volume. Percent activation (%act, IA/TA) was similar for the higher irradiance treatments (approx. 94%), but lower (74%) in leaves of the 20% FS grown-plants. However, percent activation in the 20% and 100% FS grown-plants was similar when leaves were exposed briefly to similar light conditions. Concluded %act to be dependent on leaf irradiance level at the time of leaf removal and extraction and not influenced by irradiance level during growth. Diurnal response of RuBP Case IA and TA also investigated in soybean plants growing in field, greenhouse and growth chamber. In all 3 environments, both IA and TA increased with increasing irradiance. Diurnal changes in TA were not due to changes in level of Fraction 1 protein (RuBP Case). Concluded light may also have a role in increasing TA as well as IA of RuBP Case. / Master of Science

This study was conducted to evaluate the effect of leaflet shape on yield and other characters of soybeans, <i>Glycine max.</i> (L.) Merrill, by using narrow and broad leaflet isolines. The breeding procedure employed developed pairs of isolines whose genetic backgrounds were a random recombination of the narrow and broad leaflet genotypes of their parents. Results obtained from 1976 to 1978 indicated that the narrow leaflet isolines on the average are similar to the broad leaflet ones in seed yield. The narrow leaflet isolines were earlier, but the average difference vas no more than 1.7 days. They also were consistently 1.5 to 3.0 cm shorter in all three years. The difference in lodging between leaflet types was always very small. The differences in these agronomic traits, though significant in several instances, do not imply any practical advantage of either leaflet type. Data from six pairs of isolines sampled in 1978 showed that leaflet shape had a significant effect on seed size, but not on seeds per plant, pods per plant, and seeds per pod. Narrow and broad leaflet isolines showed similar oil and protein contents. The narrow leaflet isolines of the same six pairs exhibited less leaf area per plant, less leaf weight per plant, lower leaf:stem ratio, and higher seed:stem ratio than the broad leaflet ones. The difference in specific leaf weight was non-significant. None of these traits was associated with yield per plant. / M. S.

Master of Science / Department of Agronomy / William T. Schapaugh Jr / Fusarium virguliforme, the soilborne fungus which causes sudden death syndrome (SDS) of soybean, and Heterodera glycines Ichinohe, soybean cyst nematode (SCN), are two economically important pathogens in the Midwest. The pathogens are often found together in soybean (Glycine max (L.) Merr.) fields. This study was conducted to determine the effect of soybean genotype, F. virguliforme populations, and H. glycines populations have upon yield and to examine the interaction between the two pathogens. In 2008 and 2009, four genotypes with different levels of resistance to SDS and H. glycines were planted at seven environments. F. virguliforme and H. glycines soil populations were quantified at planting, midseason, and harvest. At the end of the growing season, area under the disease progress curves of SDS, F. virguliforme root populations, and H. glycines reproductive indices were determined and plots harvested for seed yield. Soil populations of F. virguliforme and H. glycines at planting, midseason, and harvest varied across environments. Within environments, generally, they were not significantly different. Seed yield varied within and across environments. As disease pressure increased, the performance of resistant genotypes increased compared to susceptible genotypes. Genotypes resistant to SDS yielded higher than susceptible genotypes. There were negative correlations between yield and disease rating and F. virguliforme root populations. F. virguliforme soil populations and H. glycines populations at planting were positively correlated. It is important to manage both SDS and H. glycines in fields with a history of the two diseases. This can be achieved through genetic resistance. Information in this study will improve decisions regarding genotype selection to minimize losses to SDS and H. glycines.

No description available.

A field lysimeter experiment was conducted on a sandy loam soil during the 1990 and 1991 growing seasons. The experiment tested the effects of different watertables on soybean yields, and on moisture distribution and nitrogen concentration of the soil profile. The watertable depths were 40, 60, 80, and 100 centimeters (cm). / Yields were measured in terms of number of beans per plant, number of pods per plant, number of beans per pod, and seed protein content at harvest. / Soil samples collected at depths of 30 and 70 cm from the soil surface were analyzed for moisture content and NO$ sb3 sp-$-N and NH$ sb4 sp+$-N concentrations. / The experimental results showed that controlled watertable management increased the yield and decreased soil NO$ sb3 sp-$-N levels. The best results from the watertables tested were found to be at 60 and 80 cm. This is suggested as the range of watertable depths that should be maintained for optimum soybean production.

Soybean (Glycine max (L.) Merr.) is a subtropical legume that requires root zone temperatures (RZTs) in the 25 to 30$ sp circ$C range for optimal symbiotic activity. The inability of soybean to adapt to cool soil conditions limits its development and yield in short season areas. In particular, nodulation and N$ sb2$ fixation by this subtropical crop species is sensitive to cool (RZT). The objectives of this thesis were to determine whether or not PGPR could be used to help overcome the low RZT inhibition of soybean nodulation, to improve soybean nitrogen fixation and yield under field conditions and to determine the methods by which such increases occurred. The work reported in this thesis has demonstrated that PGPR can increase early season nodulation and total seasonal nitrogen fixation and yield of soybean growing in an area with cool spring soils. The ability of PGPR to stimulate soybean nodulation and growth was shown to be related to their ability to colonize soybean roots, and this was shown to be related to RZT. All steps in early nodulation were stimulated by the presence of PGPR. The beneficial effects of PGPR are exerted through a diffusible molecule excreted into the growth medium. The addition of genistein, a plant-to-bacteria signal molecule already shown to stimulate soybean N$ sb2$ fixation at low RZT, plus PGPR causes increases in soybean nodulation, N$ sb2$ fixation, and growth that were greater than those caused by the addition of PGPR alone, but only at 25 and 17.5$ sp circ$C, and not at 15$ sp circ$C RZT.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 171-174). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 456-600). Also available on the Internet.