Graduate Thesis Or Dissertation
 

Investigations into the physiology of nitrogen fixation : Part I. Nickel metabolism of plants and bacteria : Part II. Peroxide scavenging in soybean root nodules

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  • PART I. The concentration of nickel in some soils may be insufficient to meet the requirements of enzymes such as urease in soybeans and hydrogenase in Rhizobium. Nickel availability in soils was examined by measuring microbial urease activity in soils which were treated with glucose and urea. Supplemental nickel was also added to some soil samples. In one low-nickel soil (total Ni 13 ppm) urease activity increased 150% in response to additional nickel, while other soils (total Ni 22-3491 ppm) failed to respond to nickel. However, additional nickel did stimulate urease activity up to 109% in 3 out of 10 soils to which purified CaCO 3 was added. Nickel fertilization increased leaf urease and nodule hydrogenase activities of soybeans grown in low-nickel soil. The nickel requirements of red alder (Alnus rubra) were examined in purified solution culture. Nickeldeficient plants had substantially lower urease activity in leaves. Urease was found to be produced by free-living Rhizobium and by bacteroids isolated from soybean nodules. Nickel-deficient cultures had much lower urease activity, and reduced growth and protein concentration when grown with urea as the sole nitrogen source. Urease from B.japonicum was partially purified and characterized. PART II. The critical problem of oxygen toxicity for nitrogen-fixing organisms may be related to damage caused by oxygen radicals and peroxides. An enzymatic mechanism is described for removal of peroxides in root nodules of soybean (Glycine max). The system utilizes ascorbate as an antioxidant and glutathione as a reductant to regenerate ascorbate from dehydroascorbate. The enzymes involved are ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Glutathione peroxidase was not detected.
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