25 results on '"Hill, Walter A."'
Search Results
2. Temporal Variation in the Importance of a Dominant Consumer to Stream Nutrient Cycling.
- Author
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Griffiths, Natalie and Hill, Walter
- Subjects
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NUTRIENT cycles , *ECOSYSTEMS , *EXCRETION , *BIODEGRADATION , *ORGANIC compounds , *AMMONIUM , *PHOSPHORUS compounds - Abstract
Animal excretion can be a significant nutrient flux within ecosystems, where it supports primary production and facilitates microbial decomposition of organic matter. The effects of excretory products on nutrient cycling have been documented for various species and ecosystems, but temporal variation in these processes is poorly understood. We examined variation in excretion rates of a dominant grazing snail, Elimia clavaeformis, and its contribution to nutrient cycling, over the course of 14 months in a well-studied, low-nutrient stream (Walker Branch, east Tennessee, USA). Biomass-specific excretion rates of ammonium varied over twofold during the study, coinciding with seasonal changes in food availability (measured as gross primary production) and water temperature (multiple linear regression, R = 0.57, P = 0.053). The contribution of ammonium excretion to nutrient cycling varied with seasonal changes in both biological (that is, nutrient uptake rate) and physical (that is, stream flow) variables. On average, ammonium excretion accounted for 58% of stream water ammonium concentrations, 26% of whole-stream nitrogen demand, and 66% of autotrophic nitrogen uptake. Phosphorus excretion by Elimia was contrastingly low throughout the year, supplying only 1% of total dissolved phosphorus concentrations. The high average N:P ratio (89:1) of snail excretion likely exacerbated phosphorus limitation in Walker Branch. To fully characterize animal excretion rates and effects on ecosystem processes, multiple measurements through time are necessary, especially in ecosystems that experience strong seasonality. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
3. Resource synergy in stream periphyton communities.
- Author
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Hill, Walter R., Roberts, Brian J., Francoeur, Steven N., and Fanta, Shari E.
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PERIPHYTON , *PRIMARY productivity (Biology) , *AQUATIC invertebrates , *PHOTOSYNTHESIS , *PHOSPHORUS - Abstract
Light and nutrients play pivotal roles in determining the growth of autotrophs, yet the potential for synergistic interactions between the two resources in algal communities is poorly understood, especially in stream ecosystems. In this study, light and phosphorus were manipulated in large experimental streams to examine resource colimitation and synergy in stream periphyton. Whole-stream metabolism was simultaneously limited by light and phosphorus. Increasing the supply of either light or phosphorus resulted in significant increases in primary production and the transformation of the streams from heterotrophy to autotrophy. Resource-driven changes in periphyton community structure occurred in concert with changes in production. Algal assemblages in highly shaded streams were composed primarily of small diatoms such as Achnanthidium minutissima, whereas larger diatoms such as Melosira varians predominated at higher irradiances. Phosphorus enrichment had relatively little effect on assemblage structure, but it did substantially diminish the abundance of Meridion circulare, a diatom whose mucilaginous colonies were conspicuously abundant in phosphorus-poor, high-light streams. Bacterial biomass declined relative to algal biomass with increases in primary productivity, regardless of whether the increases were caused by light or phosphorus. Synergistic effects on primary production appeared to occur because the availability of one resource facilitated the utilization of the other. Light increased the abundance of large diatoms, which are known to convert high concentrations of nutrients into primary production more effectively than smaller taxa. Phosphorus enrichment led to the replacement of Meridion circulare by non-mucilaginous taxa in phosphorus-enriched streams, and we hypothesize that this change enabled more efficient use of light in photosynthesis. Higher ratios of chlorophyll a : biomass in phosphorus-enriched streams may have also led to more efficient photon capture and higher photosynthetic rates. Synthesis. Our results underscore the potential for resource colimitation, even in habitats where a single resource is as strongly limiting as is light in shaded streams. The capacity of autotrophic communities to respond to more than one limiting resource suggests that prevailing single-resource models of ecosystem productivity are overly simplistic. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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4. Light, nutrients, and herbivore growth in oligotrophic streams.
- Author
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Hill, Walter R., Smith, John G., and Stewart, Arthur J.
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AQUATIC ecology , *HERBIVORES , *PERIPHYTON , *AQUATIC invertebrates , *GROWTH rate , *WHITE oak , *LIGHT , *PHOSPHORUS - Abstract
The light nutrient hypothesis posits that herbivore growth is increasingly constrained by low food quality as the ratio of light to nutrients increases in aquatic ecosystems. We tested predictions of this hypothesis by examining the effects of large seasonal cycles in light and nutrients on the mineral content of periphyton and the growth rate of a dominant herbivore (the snail Elimia clavaeformis) in two oligotrophic streams. Streambed irradiances in White Oak Creek and Walker Branch (eastern Tennessee, USA) varied dramatically on a seasonal basis due to leaf phenology in the surrounding deciduous forests and seasonal changes in sun angle. Concentrations of dissolved nutrients varied inversely with light, causing light: nitrate and light: phosphate to range almost 100-fold over the course of any individual year. Periphyton nitrogen and phosphorus concentrations were much lower than the concentrations of these elements in snails, and they bottomed out in early spring when streambed irradiances were highest. Snail growth, however, peaked in early spring when light: nutrient ratios were highest and periphyton nutrient concentrations were lowest. Growth was linearly related to primary production (accounting for up to 85% of growth variance in individual years), which in turn was driven by seasonal variation in light. Conceptual models of herbivore growth indicate that growth should initially increase as increasing light levels stimulate primary production, but then level off, and then decrease as the negative effects of decreasing algal nutrient content override the positive effects of increased food production. Our results showed no, evidence of an inflection point where increasing ratios of light to nutrients negatively affected growth. Snail growth in these intensively grazed streams is probably unaffected by periphyton nutrient content because exploitative competition for food reduces growth rates to levels where the demand for nitrogen and phosphorus is small enough to be satisfied by even low levels of these nutrients in periphyton. Competition for limited food resources in habitats where herbivore densities are uncontrolled by predation or other mortality factors should strongly influence the potential for herbivores to be limited by mineral deficits in their food. [ABSTRACT FROM AUTHOR]
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- 2010
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5. Feeding at different plankton densities alters invasive bighead carp ( Hypophthalmichthys nobilis) growth and zooplankton species composition.
- Author
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Cooke, Sandra L., Hill, Walter R., and Meyer, Kevin P.
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HYPOPHTHALMICHTHYS , *BIGHEAD carp , *CARP , *INTRODUCED species , *MICROCYSTIS , *PLANKTON , *AQUATIC biology ,GREAT Lakes (Africa) - Abstract
Invasive Asian carps Hypophthalmichthys spp. are an ecological threat to non-native aquatic ecosystems throughout the world, and are poised to enter the Laurentian Great Lakes. Little is known about how these filter-feeding planktivores grow and impact zooplankton communities in mesotrophic to oligotrophic systems like the Great Lakes. Our purpose was to determine how different plankton densities affect bighead carp H. nobilis biomass and how bighead carp affect zooplankton species composition. We conducted a 37-day indoor mesocosm experiment (volume = 678 l) with high and low plankton treatments (zooplankton dry mass ≈ 1,900 and 700 μg l−1; chlorophyll a = 25 and 14 μg l−1, respectively) in the presence and absence of juvenile bighead carp (mean = 5.0 g, 8.5 cm). Carp lost weight in the low plankton treatment and gained weight in the high plankton treatment, suggesting that food availability may be a limiting factor to bighead carp growth in regions of low plankton densities. In the presence of carp, zooplankton shifted from Daphnia to copepod dominance, while in the absence of carp, Daphnia remained dominant. Chydorids and ostracods increased in the presence of carp, but only in the low plankton treatment, suggesting that the impact of bighead carp on zooplankton species composition may vary with zooplankton density. Chlorophyll was higher in the absence of carp than in the presence. Chlorophyll and zooplankton densities in many Great Lakes ecosystems are substantially lower than our low treatment conditions, and thus our results suggest that Asian carp establishment in these regions may be unlikely. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
6. Growth dilution in multilevel food chains
- Author
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Herendeen, Robert A. and Hill, Walter R.
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FOOD chains , *BIOACCUMULATION , *ECOLOGICAL niche , *BIOLOGICAL productivity - Abstract
Microalgae can absorb contaminants from the aqueous environment, and harvesting microalgae has been proposed as a method to purify water. However, rapid growth of microalgae (stimulated by increased light, for example) results in lowered tissue concentration of contaminant. This reduction has been observed to propagate to herbivores. Here we investigate (with simulation and supporting analytical argument) the propagation of growth dilution in all trophic levels of a food chain. We are concerned with concentration as well as overall mass of contaminant in each level, for different functional relationships between levels. We find that transient (i.e., prompt) growth dilution occurs for all levels. However, the new steady state concentrations can increase or decrease, depending on functional relationships (e.g., ratio versus prey dependence). These results, which have implications for pollution control, call for experimental testing. [Copyright &y& Elsevier]
- Published
- 2004
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7. STREAM ECOSYSTEM RESPONSES TO FOREST LEAF EMERGENCE IN SPRING.
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Hill, Walter P., Mulholland, Patrick J., and Marzolf, Erich R.
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RIVER ecology ,LEAF growth - Abstract
Examines the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Light; Temperature and discharge; Nutrients; Periphyton biomass and pigments; Primary producers; Herbivores.
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- 2001
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8. Light limitation in a stream ecosystem: Responses by primary producers and consumers.
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Hill, Walter R. and Ryon, Michael G.
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PERIPHYTON , *SNAILS , *SHADES & shadows - Abstract
Examines the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in a Tennessee forest. Effects of light limitation and grazing; Shade adaptation; Snail growth versus irradiance; Snail growth; Lipid accumulation; Photosynthesis-irradiance curves.
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- 1995
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9. OUTSOURCING CONSEQUENCES.
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Hill, Walter
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LETTERS to the editor , *CONTRACTING out - Abstract
A letter to the editor is presented in response to the article "The Data Doughnut and the Software Hole" in the June 2006 issue of "The Profession."
- Published
- 2006
10. RETHINKING THE ECONOMICS OF WAR: THE INTERSECTION OF NEED, CREED, AND GREED.
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Hill, Walter W.
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CIVIL war , *NONFICTION , *ECONOMICS - Abstract
The article reviews the book "Rethinking the Economics of War: The Intersection of Need, Creed, and Greed," edited by Cynthia J. Arnson and I. William Zartman.
- Published
- 2009
11. THE GLOBAL COLD WAR.
- Author
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Hill, Walter
- Subjects
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COLD War, 1945-1991 , *NONFICTION - Abstract
The article reviews the book "THE GLOBAL COLD WAR," by Odd Arne Westad.
- Published
- 2008
12. Polling and Prediction in the 2016 Presidential Election.
- Author
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Valentino, Nicholas A., King, John Leslie, and Hill, Walter W.
- Subjects
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UNITED States presidential election, 2016 , *EXIT polling (Elections) , *SAMPLING (Process) , *EXPERIMENTAL design , *MATHEMATICAL models - Abstract
In the wake of experts' failure to predict the outcome of the 2016 presidential election, a rigorous analysis of what went right and wrong is needed to improve future polling. Despite claims that "data is dead," low-tech factors such as sampling errors and inaccurate likely-voter models were probably most responsible. [ABSTRACT FROM PUBLISHER]
- Published
- 2017
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13. Changes in carbon stable isotope ratios during periphyton development.
- Author
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Hill, Walter R. and Middleton, R. Gerry
- Subjects
- *
CARBON isotopes , *STABLE isotopes , *PERIPHYTON , *CHLOROPHYLL , *FOOD chains , *BIOMASS , *INORGANIC compounds - Abstract
Stable isotopes are widely used to infer trophic relationships with little attention paid to temporal variability at the base of the food web. We examined changes in the carbon-stable isotope composition during periphyton development, sampling periphyton that accumulated on ceramic tiles at four stream sites over a 2-month period. Periphyton 13C rose and fell in general concordance with rising and falling biomass at all four sites, resulting in significant correlations between periphyton δ13C and chlorophyll a (Chl a). Mean δ13C values at one site rose from -26% to -20% in 2 weeks, falling back to -24% the next week after a large scouring spate. Periphyton 13C also underwent a smaller, longer-term increase that correlated with a gradual rise in stream temperature. Multiple regression analysis with both Chl a and temperature as independent variables accounted for up to 88% of the temporal variability in δ13C, with Chl a the largest source of variability. Water velocity, measured on each sampling occasion, was unrelated to temporal changes in 13C. Depletion of inorganic carbon within the periphyton matrix is the probable cause of increasing 13C in periphyton as biomass develops. Rising δ13C values during periphyton biomass development suggest the possibility of carbon-limited periphyton growth, even in alkaline waters. The strong link between biomass and periphyton 13C helps explain the large range of δ13C reported for periphyton in streams, where temporal and spatial variability in periphyton biomass are notorious. [ABSTRACT FROM AUTHOR]
- Published
- 2006
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14. Multiple Scales of Temporal Variability in Ecosystem Metabolism Rates: Results from 2 Years of Continuous Monitoring in a Forested Headwater Stream.
- Author
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Roberts, Brian J., Mulholland, Patrick J., and Hill, Walter R.
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BIOCHEMISTRY , *BIOTIC communities , *ORGANIC compounds , *PRIMARY productivity (Biology) , *FORESTS & forestry , *RIPARIAN plants , *ECOLOGY , *METABOLISM , *STREAM plants - Abstract
Headwater streams are key sites of nutrient and organic matter processing and retention, but little is known about temporal variability in gross primary production (GPP) and ecosystem respiration (ER) rates as a result of the short duration of most metabolism measurements in lotic ecosystems. We examined temporal variability and controls on ecosystem metabolism by measuring daily rates continuously for 2 years in Walker Branch, a first-order deciduous forest stream. Four important scales of temporal variability in ecosystem metabolism rates were identified: (1) seasonal, (2) day-to-day, (3) episodic (storm-related), and (4) inter-annual. Seasonal patterns were largely controlled by the leaf phenology and productivity of the deciduous riparian forest. Walker Branch was strongly net heterotrophic throughout the year with the exception of the open-canopy spring when GPP and ER rates were co-equal. Day-to-day variability in weather conditions influenced light reaching the streambed, resulting in high day-to-day variability in GPP particularly during spring (daily light levels explained 84% of the variance in daily GPP in April). Episodic storms depressed GPP for several days in spring, but increased GPP in autumn by removing leaves shading the streambed. Storms depressed ER initially, but then stimulated ER to 2–3 times pre-storm levels for several days. Walker Branch was strongly net heterotrophic in both years of the study, with annual GPP being similar (488 and 519 g O2 m−2 y−1 or 183 and 195 g C m−2 y−1) but annual ER being higher in 2004 than 2005 (−1,645 vs. −1,292 g O2 m−2 y−1 or −617 and −485 g C m−2 y−1). Inter-annual variability in ecosystem metabolism (assessed by comparing 2004 and 2005 rates with previous measurements) was the result of the storm frequency and timing and the size of the spring macroalgal bloom. Changes in local climate can have substantial impacts on stream ecosystem metabolism rates and ultimately influence the carbon source and sink properties of these important ecosystems. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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15. Influence of Microgravity Environment on Root Growth, Soluble Sugars, and Starch Concentration of Sweetpotato Stem Cuttings.
- Author
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Mortley, Desmond G., Bonsi, Conrad K., Hill, Walter A., Morris, Carlton E., Williams, Carol S., Davis, Ceyla F., Williams, John W., Levine, Lanfang H., Petersen, Barbara V., and Wheeler, Raymond M.
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PLANT growth , *PLANT development , *SWEETPOTATO whitefly , *PLANT roots , *PLANT physiology , *REDUCED gravity environments - Abstract
Because sweetpotato [Ipomoea batatas (L.) Lam.] stem cuttings regenerate very easily and quickly, a study of their early growth and development in microgravity could be useful to an understanding of morphological changes that might occur under such conditions for crops that are propagated vegetatively. An experiment was conducted aboard a U.S. Space Shuttle to investigate the impact of microgravity on root growth, distribution of amyloplasts in the root cells, and on the concentration of soluble sugars and starch in the stems of sweetpotatoes. Twelve stem cuttings of 'Whatley/Loretan' sweetpotato (5 cm long) with three to four nodes were grown in each of two plant growth units filled with a nutrient agarose medium impregnated with a half-strength Hoagland solution. One plant growth unit was flown on Space Shuttle Colombia for 5 days, whereas the other remained on the ground as a control. The cuttings were received within 2 h postflight and, along with ground controls, processed in ≈45 min. Adventitious roots were counted, measured, and fixed for electron microscopy and stems frozen for starch and sugar assays. Air samples were collected from the headspace of each plant growth unit for postflight determination of carbon dioxide, oxygen, and ethylene levels. All stem cuttings produced adventitious roots and growth was quite vigorous in both ground-based and flight samples and, except for a slight browning of some root tips in the flight samples, all stem cuttings appeared normal. The roots on the flight cuttings tended to grow in random directions. Also, stem cuttings grown in microgravity had more roots and greater total root length than ground-based controls. Amyloplasts in root cap cells of ground-based controls were evenly sedimented toward one end compared with a more random distribution in the flight samples. The concentration of soluble sugars, glucose, fructose, and sucrose and total starch concentration were all substantially greater in the stems of flight samples than those found in the ground-based samples. Carbon dioxide levels were 50% greater and oxygen marginally lower in the flight plants, whereas ethylene levels were similar and averaged less than 10 nL·L-1. Despite the greater accumulation of carbohydrates in the stems, and greater root growth in the flight cuttings, overall results showed minimal differences in cell development between space flight and ground-based tissues. This suggests that the space flight environment did not adversely impact sweetpotato metabolism and that vegetative cuttings should be an acceptable approach for propagating sweetpotato plants for space applications. [ABSTRACT FROM AUTHOR]
- Published
- 2008
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16. LETTERS.
- Author
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Sumner, Gordon, Haap, Michael, Mahler, Mike, D'Andries, Michael, Hill, Walter, and Beck, John
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SLEEP deprivation & health , *CAVALRY - Published
- 2022
17. Book reviews: International relations.
- Author
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Hill, Walter W.
- Subjects
- FUTURE'S Back, The (Book)
- Abstract
Reviews the book `The Future's Back: Nuclear Rivalry, Deterrence Theory, and Crisis Stability After the Cold War,' by Frank P. Harvey.
- Published
- 1998
18. Failure Analysis under Electric Lights: Growth and Yield of Sweetpotato in Response to 14 Days of Prolonged Darkness.
- Author
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Mortley, Desmond G., Hileman, Douglas R., Bonsi, Conrad K., Hill, Walter A., and Morris, Carlton E.
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SWEET potato yield , *PLANT growth , *EFFECT of light on plants , *GENOTYPES , *PHOTOSYNTHESIS - Abstract
Two sweetpotato [Ipomoea batatas (L.) Lam] genotypes (TU-82-155 and NCC-58) were grown hydroponically and subjected to a temporary loss of lighting in the form of 14 days of prolonged darkness compared with a lighted control under standard daily light periods to determine the impact on growth responses and storage root yield. Vine cuttings of both genotypes were grown in rectangular channels. At 65 days after planting, lights were turned off in the treatment chambers and replaced by a single incandescent lamp, providing between 7 and 10 µmol-m-2·s-1 photosynthetic photon flux (PPF) for 18 hours, and the temperature lowered from 28/22 °C light/dark, to a constant 20 °C. Plants remained under these conditions for 14 days after which the original light level was restored. Growth chamber conditions predark included, a PPF mean provided by 400-W metal halide lamps, of 600 ± 25 pimol·m-2-s-1, an 18-hour light/6-hour dark cycle and a relative humidity of 70% ± 5%. The nutrient solution used was a modified half-Hoagland with pH and electrical conductivity (EC) maintained between 5.5-6.0 and 1000-1200 µS·cm-1, respectively, and was adjusted weekly. Storage root number and fresh weight were similar regardless of treatments. Plants exposed to prolonged darkness produced 10.5% and 25% lower fibrous root fresh and dry mass, respectively, but similar foliage yield and harvest index (HI). 'NCC-58' produced an average of 31% greater storage root yield than that of 'TU-82-155' but the number of storage roots as well as % dry matter (%DM) were similar. 'NCC-58' also produced 31% greater fibrous root dry weight, whereas 'TU-82-155' produced a 44% greater HI. The significant interaction between prolonged darkness and cultivars for %DM of the storage roots showed that DM for 'TU-82-155' was 18.4% under prolonged darkness and 17.9% in the light. That for 'NCC-58' was 16.4% under prolonged darkness compared with 19.4% (14.8% greater) for plants that were not subjected to prolonged darkness. The evidence that there were no adverse impacts on storage root yield following the exposure to prolonged darkness suggests that the detrimental effects were below the detectable limits for these cultivars in response to the short perturbation in the available light and that sweetpotatoes would be hardy under short-term failure situations. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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19. Letters.
- Author
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SWINDELLS, JOHN E., TUCKER, JAMES, STRANG, MICHAEL, MARX, LOWELL E., HUISKING, PETER V., CAINE, BRUCE T. "WOODY", FANO, MARTIN, HILL, WALTER J., and MAIER, JOHN
- Subjects
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MILITARY science , *INFANTRY ,UNITED States National Guard - Abstract
Several letters to the editor are presented in response to articles in previous issues including "Megacity Warfare: Taking Urban Combat to a Whole New Level," by William Matthews in the March 2015 issue, "The Forward Edge," by Edwin L. Kennedy Jr. in the February 2015 issue, and "Thinking Matters: The Difference Between Training and Education," by Keith H. Ferguson in the March 2015 issue.
- Published
- 2015
20. Potential Use of DNA Barcodes in Regulatory Science: Applications of the Regulatory Fish Encyclopedia.
- Author
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Yancy, Haile F., Zemlark, Tyler S., Mason, Jacquline A., Washington, Jewell D., Tenge, Bradley J., Nguyen, Ngoc-Lan T., Barnett, James D., Savary, Warren E., Hill, Walter E., Moore, Michelle M., Fry, Frederick S., Randolph, Spring C., Rogers, Patricia L., and Hebert, Paul D. N.
- Subjects
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BAR codes , *DNA , *FISHES , *FRAUD - Abstract
The use of a DNA-based identification system (DNA barcoding) founded on the mitochondrial gene cytochrome c oxidase subunit I (COl) was investigated for updating the U.S. Food and Drug Administration Regulatory Fish Encyclopedia (RFE; http://www.cfsan.fda.gov/∼frf/rfe0.html). The RFE is a compilation of data used to identify fish species. It was compiled to help regulators identify species substitution that could result in potential adverse health consequences or could be a source of economic fraud. For each of many aquatic species commonly sold in the United States, the RFE includes high-resolution photographs of whole fish and their marketed product forms and species-specific biochemical patterns for authenticated fish species. These patterns currently include data from isoelectric focusing studies. In this article, we describe the generation of DNA barcodes for 172 individual authenticated fish representing 72 species from 27 families contained in the RFE. These barcode sequences can be used as an additional identification resource. In a blind study, 60 unknown fish muscle samples were barcoded, and the results were compared with the RFE barcode reference library. All 60 samples were correctly identified to species based on the barcoding data. Our study indicates that DNA barcoding can be a powerful tool for species identification and has broad potential applications. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
21. The Real-Time Path of Translation Factor IF3 onto and off the Ribosome
- Author
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Fabbretti, Attilio, Pon, Cynthia L., Hennelly, Scott P., Hill, Walter E., Lodmell, J. Stephen, and Gualerzi, Claudio O.
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- *
RIBOSOMES , *BACTERIAL proteins , *BIOMOLECULES , *CELL metabolism - Abstract
Summary: Translation initiation factor IF3 is an essential bacterial protein, consisting of two domains (IF3C and IF3N) separated by a linker, which interferes with ribosomal subunit association, promotes codon-anticodon interaction in the P site, and ensures translation initiation fidelity. Using time-resolved chemical probing, we followed the dynamic binding path of IF3 on the 30S subunit and its release upon 30S-50S association. During binding, IF3 first contacts the platform (near G700) of the 30S subunit with the C domain and then the P-decoding region (near A790) with its N domain. At equilibrium, attained within less than a second, both sites are protected, but before reaching binding equilibrium, IF3 causes additional transient perturbations of both the platform edge and the solvent side of the subunit. Upon 30S-50S association, IF3 dissociates concomitantly with the establishment of the 30S-50S bridges, following the reverse path of its binding with the IF3N-A790 interaction being lost before the IF3C-G700 interaction. [Copyright &y& Elsevier]
- Published
- 2007
- Full Text
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22. Interaction of Thiostrepton and Elongation Factor-G with the Ribosomal Protein L11-binding Domain.
- Author
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Bowen, William S., Van Dyke, Natalya, Murgola, Emanuel J., Lodmell, J. Stephen, and Hill, Walter E.
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- *
RIBOSOMES , *PROTEINS , *RNA , *GENETIC translation , *BIOCHEMISTRY - Abstract
Ribosomal protein L11 and the L11 binding region of ribosomal RNA constitute an important domain involved in active functions of the ribosome during translation. We studied the effects of L11 knock-out and truncation mutations on the structure of the rRNA in this region and on its interactions with a translation elongation factor and the antibiotic thiostrepton. The results indicated that the structure of the L11-binding rRNA becomes conformationally flexible when ribosomes lack the entire L11 protein, but not when the C-terminal domain is present on ribosomes. Probing wild type and mutant ribosomes in the presence of the antibiotic thiostrepton and elongation factor-G (EF-G) rigorously localized the binding cleft of thiostrepton and suggested a role for the rRNA in the L11-binding domain in modulating factor binding. Our results also provide evidence that the structure of the rRNA stabilized by the C-terminal domain of L11 is necessary to stabilize EF-G binding in the post-translocation state, and thiostrepton may modulate this structure in a manner that interferes with the ribosome-EF-G interaction. The implications for recent models of thiostrepton activity and factor interactions are discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2005
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23. Influence of Harvest Intervals on Growth Responses and Fatty Acid Content of Purslane (Portulaca oleracea).
- Author
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Mortley, Desmond G., Jun-Hyun Oh, Johnson, Damicca S., Bonsi, Conrad K., and Hill, Walter A.
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- *
GREENHOUSES , *BIOMASS , *FATTY acids , *PORTULACA , *GAS chromatography - Abstract
A greenhouse study was conducted to evaluate the influence of harvest intervals on biomass yield and omega fatty acids of 'Golden purslane' (Portulaca oleracea). Nutrients were supplied as a modified full-strength Hoagland solution two to three times weekly. Plants were harvested sequentially at 20, 40, and 60 days after transplanting (DAT) corresponding to 42, 63, and 84 days after sowing. Fatty acids were determined using a gas chromatography-mass spectrometry. Harvest intervals significantly influenced foliage fresh and dry weight, leaf number and plant height, and root length and fresh weight and were greatest at 60 DAT. Fatty acid analysis verified the presence of myristate, palmitate, linoleate, and linolenate at 20 DAT and in all three harvests, whereas stearate and oleate were detected only in the last two harvests (40 and 60 DAT). Linoleate, palminate, and linolenate were the most abundant fatty acids in purslane with levels in excess of 300 mg⋅kg-1. Those for myristate, stearate, and oleate were in excess of 200 mg⋅kg-1. The ratio of omega-6/omega-3 ranged from 0.44 for Harvest 1 to 1.1 for Harvest 3, whereas ratios for harvest intervals two and three were equal to or greater than the recommended daily human requirement. Results showed qualitative and quantitative differences of harvest intervals of purslane, suggesting that an optimal ratio of omega-6 to omega-3 fatty acids can be achieved ≈20 DAT. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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24. Influence of Daily Light Period and Irradiance on Yield and Leaf Elemental Concentration of Hydroponically Grown Sweetpotato.
- Author
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Mortley, Desmond G., Burrell, Stephanie, Bonsi, Conrad K., Hill, Walter A., and Morris, Carlton E.
- Subjects
- *
EFFECT of light on plants , *HYDROPONICS , *SWEET potatoes , *GROWTH cabinets & rooms , *ROOT growth , *NUTRIENT film culture ,LEAF growth - Abstract
Growth chamber experiments were conducted to evaluate the effect of irradiance and daily light period on storage root yield and leaf elemental concentration of two sweetpotato cultivars grown hydroponically by use of the nutrient film technique (NFT). Stem cuttings (15 cm) of cv. Whatley/Loretan and Georgia Jet were grown in N FT channels (0.15 x 0.15 x 1.2 m) in reach-in growth chambers under light period/irradiance combinations of 18 h: 300 µmol·m-2·s-1 or 9 h: 600 µmol·m-2 ·s-1photosynthetic photon flux. Temperature was 28/22 °C fight/dark with a relative humidity of 70% ± 5%. Storage root and foliage yields were greater in both cultivars exposed to a longer daily light period and lower irradiance. The main effect of cultivar indicated that storage root yield was significantly greater among plants of 'Whatley/Loretan' compared with that of 'Georgia Jet', whereas foliage yield was similar between cultivars. Leaves of plants grown under longer daily light period and lower irradiance had significantly lower concentrations of all elements, nitrogen, phosphorus, potassium, calcium, magnesium, manganese, iron, calcium, boron, and zinc, except for calcium, manganese, and boron. There were no significant differences in leaf elemental concentration between cultivars. Thus, a longer daily light and lower irradiance enhanced biomass production of sweetpotato but reduced leaf elemental concentration probably because of a "dilution" effect. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
25. Letters.
- Author
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Campbell, Robert Charles, Robertson, Christopher H., Kohn, Steve, and Hill, Walter J.
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COMMAND of troops - Abstract
Several letters to the editor are presented in response to articles in previous issues including "Train, Advise, Assist' Brigades: Milley's New Vision for Ongoing Mission" in December 2016 issue; "Army Receiving Its First AMPV" in December issue; and one related to military strategic leadership.
- Published
- 2017
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