Abstracts by Reed senior thesis students working under David Dalton's supervision, 1996/7.
Effects of Seasonality and Position in the Canopy on the Antioxidant Defense Systems of Douglas-fir (Pseudotsuga menziesii) and Grand fir (Abies grandis)
Eliza C. Gould
Photooxidation and oxidative stress in plants are the result of various environmental conditions that limit the rate of photosynthesis. Water stress and extreme temperature variations in combination with intense light are among the factors that lead to photooxidative stress. Photodamage to lipid membranes and various proteins by activated forms of oxygen is kept at a minimum by antioxidant defenses. Enzymatic components of the antioxidant defense system include ascorbate peroxidase which scavenges H2O2 and glutathione reductase which regenerates reduced ascorbate in the ascorbate-glutathione pathway. Seasonal variations in antioxidants have been documented in conifers such asPinus sylvestris, Pinus strobus, and Picea abies. There is a maximal metabolite (ascorbate and glutathione) concentration and increased enzyme activity (ascorbate peroxidase and glutathione reductase) during the winter.
Old growth canopies in the Pacific Northwest have not been intensely assessed as far as photooxidation and their corresponding antioxidant defense mechanism. Little research has addressed the potential difference position of foliage in the canopy may have on photoactivity as it is related to photooxidation and environmental stresses. The Wind River Canopy Crane in Carson, Washington provides the opportunity to accurately assess photooxidative stress throughout the various strata of an old growth Doulgas-fir canopy. A project design has been submitted and approved by the Wind River Canopy Crane Research Facility. The intentions of this study will be to determine 1) if there are seasonal variations of antioxidants in Douglas-fir (Pseudotsuga menziesi) and grand fir (Abies grandis), 2) if there is a difference between needles at the top versus needles embedded in and at lower levels of the canopy, and 3) if there is any significant difference between the two species for these two parameters. Such a study might help in the gradual understanding and appreciation of how changes in metabolic characteristics and enzymatic activities are associated with the position in the canopy of a Douglas-fir old growth forest.
Effects of Water Stress on Nitrogen Fixation by Lupinus nanus
Jason S. Julian
Environmental conditions can significantly alter the nitrogen-fixing capabilities of leguminous root nodules. Of special interest are the negative effects caused by poor water relations. In this set of experiments, the effects of various water conditions on the nitrogen-fixing capabilities of the legume species Lupinus nanus is to be tested. Plants kept in the Reed College greenhouse will be monitored under experimental flood and drought conditions produced through the addition or withholding of irrigation. Water potential of the plants will be used as a measure of water-stress severity. The water potential of leaves and root nodules will be measured using a pressure chamber and specific nitrogenase activity (SNA) will be measured by acetylene reduction assay. Nodule total protein and the specific activities of ascorbate peroxidase, glutathione reductase, and superoxide dismutase will also be analyzed to determine effects of water stress on antioxidant processes in nodules. The data collected should give insight into the physiology of nitrogen fixation by this species as well as a better understanding of its limitations by and adaptations to water stress.
Associative Nitrogen Fixation byAmmophila arenaria (European Beach Grass) in Oregon Coastal Dunes
Suzanne Fusaro
The question of nitrogen cycling in coastal Oregon sand dunes is of particular importance in the foredunes, a severely nitrogen deficient environment colonized almost exclusively by European beachgrass, Ammophila arenaria. Associations with rhizosphere bacteria such as Azotobacter and Bacillus have previously been discovered in populations of Ammophila in Europe, suggesting the importance of associative nitrogen fixation in the mineral nutrition of this, and possibly other, dune species. Recently discovered endorhizospheric associations, such as occurs with diazotrophic bacteria (Acetobacter and Azospirillum) and various grasses (maize, sugarcane, wheat and sorghum) suggest another model in which Ammophila might benefit from bacterial nitrogen fixation. Endogenous associations provide bacteria with carbon sources that may be otherwise absent from the soil; thus, if such associations could be discovered in fordune grasses, they may help explain how diazotrophic bacteria cope with soil carbon deficiencies in foredunes. This study will investigate the role and nature of associative nitrogen fixation in Oregon coastal dunes where, as of yet, little work of this type has been conducted. Questions of interest include a) does nitrogen fixation occur in Oregon coastal dunes in association with Ammophila arenaria, and b) if so, does the associated bacteria colonize the endorhizosphere or the rhizosphere alone? This study also asks c) under what environmental conditions do bacteria fix nitrogen in Ammophila arenaria, and d) do other foredune species exhibit similar tendencies? The results of this study will hopefully contribute to the understanding of nutrient cycling in carbon and nitrogen poor environments via plant associations with diazotrophic bacteria.
Bionic Bacteria: Effects of Concentrations of Antioxidants on Nitrogen-Fixing Azorhizobium and Soybean Bacteroids in Flow Chamber Reactions
Emily Ross
Aerobic organisms must use oxygen in order to meet the energy demands of metabolism. With this comes the risk of oxidative damage that occurs via the formation of activated forms of oxygen such as hydrogen peroxide and superoxide radicals. Plants have a variety of measures to defend against active forms of oxygen, most notably ascorbate peroxidase, ascorbic acid, glutathione reductase, and glutathione. Plants that fix nitrogen are especially vulnerable to oxidative damage due to strong reducing conditions required to fix N2 and to the tendency of leghemoglobin (Lb) to autooxidize and produce active oxygen species. Given that activated forms of oxygen are highly destructive in N2-fixing plants, it is important to investigate how antioxidants and Lb play a role in the protection of N2 fixation. In order to achieve this, this study will examine the performance of N-fixing organisms (either non-symbiotic nitrogen-fixing Azorhizobium or bacteroids (Bradyrhizobium japonicum) from soybean) in a flow through in vitro reconstitution system containing Lb or myoglobin (Mb) and various concentrations of antioxidants and antioxidant enzymes. The relative oxygenation of Mb or Lb (and rates of O2 consumption) will be determined spectrophotometrically and nitrogen fixation rates will be determined using the acetylene reduction method. We hope to find that increased levels of antioxidants play a considerable role in the protection of nitrogen fixation.
Degradation of 2,4-D by Soil Microflora
Karen C. Wolma
The biodegradability of 2,4-dichlorophenoxyacetic acid (2,4-D), a commonly used herbicide, is well documented. Many bacteria have been isolated that can degrade 2,4-D within a matter of a few weeks. This research thus seeks to examine three hypotheses concerning 2,4-D degradation. First, because Nitrosomonas europaea, an ammonia-oxidizing soil bacterium, is known to have a range of substrates, it will be used to test the degradability of 2,4-D by the enzymes which catalyze the ammonia-oxidation process. This cometabolism of 2,4-D by Nitrosomonas should prove that 2,4-D is used as a substrate by the enzymes which oxidize ammonia to nitrite, suggesting that nitrifying bacteria might be useful in bioremediation. 2,4- D degradation will be monitored by gas chromatography-mass spectrometry. Second, soil that has been contaminated with 2,4-D will be tested for enhanced populations of indigenous 2,4- D-degrading microorganisms. Three types of selective media (soil extract agar, acid agar, and starch casein agar) will be used to look for enhanced populations of bacteria, fungi, and actinomycetes, respectively. Third, if Nitrosomonas proves capable of degrading 2,4 -D in culture, then its capacity for degrading 2,4 -D in contaminated soils will be examined.
Back to :
[David Dalton's Home Page]
or [Reed College Home Page]