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排序方式: 共有526条查询结果,搜索用时 734 毫秒
121.
Effects of aspen (Populus tremuloides Michx.) canopy removal and root trenching on understory vegetation were examined at Parkland and Boreal sites in Alberta, Canada to provide a better understanding of the ecological basis of agroforestry systems suitable for north temperate and boreal areas. The greatest changes in understory production (ANPP) were in response to canopy removal with less consistent changes from reduction of root effects. Root trenching did not influence ANPP at the parkland site, but did at the boreal site where it interacted with canopy removal. During severe drought, ANPP under a full canopy at the parkland site increased and may relate to paradoxical understory resource increases during drought. At the parkland site there was a shift away from forb and shrub production under a full canopy towards a greater proportion of graminoids with complete or partial canopy removal. At the boreal site, partial canopy removal resulted in more shrubs and greater production from graminoid species relative to complete overstory removal but only with root trenching using a barrier. Trends in the relative yields of understory species and the cover of plant groups generally followed the patterns observed in ANPP. Leaf litter decreased with the level of canopy removal and may have factored in greater moisture conservation under aspen. Understory production gains with a partial canopy may be attributed to favourable microclimatic conditions of a closed forest retained by the partial aspen cover, while simultaneously increasing PAR penetration. Soil resources were important to ANPP and cover, particularly among forbs, however, when root trenching was significant, it generally coincided with increases in PAR. 相似文献
122.
Effects of three aspen (Populus tremuloides Michx.) canopy removal treatments and root trenching on understory microenvironment and moisture were tested at Parkland
and Boreal sites in Alberta, Canada. Aspen canopies moderated air temperature by reducing maximums and increasing minimums,
and increased the frost-free period in the understory by reducing radiative frosts. When daily differences were found among
canopy treatments, maximum absolute humidity was greater with complete canopy removal. Maximum daily relative humidity was
greater in openings at night than with either full or partial canopy cover. Predictably, increasing aspen cover reduced PAR
reaching the understory. Soil moisture response was highly variable, changing with site, aspen density and precipitation patterns,
but there were only marginal differences due to root trenching. In the Parkland site, soil moisture conservation from aspen
canopy and leaf litter effects were masked by tree uptake in most periods, but a net increase in soil water (+5.2%) was observed
during drought. Soil and microclimatic conditions in thinned aspen stands suggest potentially favourable production benefits
from developing and adopting agroforestry systems in these northern ecosystems. 相似文献
123.
Vicente AR Ortugno C Rosli H Powell AL Greve LC Labavitch JM 《Journal of agricultural and food chemistry》2007,55(10):4125-4130
Softening and pathogen susceptibility are the major factors limiting the marketing of blueberries as fresh fruits, and these traits are associated with fruit cell wall structure. However, few studies that characterize wall modifications occurring during development and ripening have been reported for this fruit. In this study the ripening-associated modifications of blueberry fruit cell walls (composition, pectin and hemicellulose solubilization, and depolymerization) at five stages of ripeness have been analyzed. Xylose was found to be the most abundant noncellulosic neutral sugar associated with fruit walls, and the observed high Xyl/Glc ratio suggested that xylans, which are usually a minor hemicellulosic fruit wall component, are abundant in blueberry. The pectic matrix showed increased solubilization at early and intermediate stages of ripening, but no changes were detected in late ripening. Furthermore, little reduction in pectin polymer size occurred during blueberry ripening. In contrast, hemicellulose levels decreased as ripening progressed, and a clear depolymerization of these components was observed. A model for cell wall degradation in this fruit is discussed. 相似文献
124.
Mellor PJ Haugland S Murphy S Smith KC Holloway A Archer J Powell RM Polton GA Tasker S McCormick D Tempest ME McNeil PE Scase TJ Knott CD Bonfanti U Villiers EJ Argyle DJ Herrtage ME Day MJ 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》2006,20(6):1376-1383
BACKGROUND: Myeloma-related disorders (MRD) are rare neoplasms of plasma cells. Published case reports describe a diversity of clinical presentations with confusing terminology and diagnostic criteria as a consequence of the assumption that MRD in cats are analogous to those in dogs or humans. OBJECTIVE: The aim of the study was to describe clinical, clinicopathologic and imaging findings, response to treatment, survival and possible associations with other diseases or vaccination in a large case series. A priori hypotheses were that cats with MRD commonly present with extramedullary involvement and uncommonly have radiographic bone lesions, in contrast to human patients. ANIMALS: Twenty-four cats with MRD confirmed by cytology or histopathology and immunohistochemistry. METHOD: A multicenter retrospective study was performed. RESULTS: Two types of clinical presentation were observed. The first group (n = 17) had neoplasia involving abdominal organs, bone marrow, or both. All developed systemic clinical signs and paraproteinemia. Five of 7 cats that received chemotherapy improved clinically or had decreased serum globulin concentration (median survival, 12.3 months; range, 8.5-22 months). The second group comprised 7 cats with skin masses, 2 of which were paraproteinemic and developed rapidly worsening systemic signs. In cats without systemic signs, excision of the skin masses appeared to be associated with prolonged survival (up to 2.4 years). Cats with MRD commonly presented with extramedullary involvement (67%), versus humans with MRD (5%) (P < .001), and uncommonly presented with radiographic bone lesions (8%) versus humans with MRD (80%) (P < .001). CONCLUSIONS: Radiographic bone lesions are uncommon in cats with MRD and extramedullary presentation is common, relative to human myeloma. 相似文献
125.
Osterloh JM Yang J Rooney TM Fox AN Adalbert R Powell EH Sheehan AE Avery MA Hackett R Logan MA MacDonald JM Ziegenfuss JS Milde S Hou YJ Nathan C Ding A Brown RH Conforti L Coleman M Tessier-Lavigne M Züchner S Freeman MR 《Science (New York, N.Y.)》2012,337(6093):481-484
Axonal and synaptic degeneration is a hallmark of peripheral neuropathy, brain injury, and neurodegenerative disease. Axonal degeneration has been proposed to be mediated by an active autodestruction program, akin to apoptotic cell death; however, loss-of-function mutations capable of potently blocking axon self-destruction have not been described. Here, we show that loss of the Drosophila Toll receptor adaptor dSarm (sterile α/Armadillo/Toll-Interleukin receptor homology domain protein) cell-autonomously suppresses Wallerian degeneration for weeks after axotomy. Severed mouse Sarm1 null axons exhibit remarkable long-term survival both in vivo and in vitro, indicating that Sarm1 prodegenerative signaling is conserved in mammals. Our results provide direct evidence that axons actively promote their own destruction after injury and identify dSarm/Sarm1 as a member of an ancient axon death signaling pathway. 相似文献
126.
An investigation of the influence of soil properties on microbial community dynamics in soil on sub-Antarctic Macquarie Island found that both carbon and nitrogen were important factors in determining soil microbial community structure. The phylogenetic diversity of soil microbial communities in hydrocarbon contaminated and non-contaminated sites was compared to the diversity of hydrocarbon-degrading genes and soil physicochemical characteristics. Genes involved in hydrocarbon degradation including alkane mono-oxygenase, catchecol-2,3-dioxygenase and naphthalene dioxygenase were found throughout the study sites. Terminal restriction fragment length polymorphism analysis of the 16S rRNA and alkB genes found that the patterns of diversity of these two genes were only correlated with each other where measurable levels of hydrocarbons were detected. We found that different sections of the microbial community are affected by different environmental factors depending on whether hydrocarbons were present. The overall microbial community structure as measured by the 16S rRNA gene was most influenced by the presence of carbon both as total organic carbon and as petroleum hydrocarbons. The alkane-degrading community was also influenced by carbon. Where hydrocarbons were present petroleum hydrocarbon concentration as well as the form and concentration of nitrogen present also influenced the alkane-degrading community. This level of complexity in the microbial community dynamics suggests that it is unlikely that one single environmental factor is responsible for structuring microbial communities. 相似文献
127.
Michael Jeger Ziyang Chen Eleanor Cunningham George Martin Glen Powell 《European journal of plant pathology / European Foundation for Plant Pathology》2012,133(1):3-23
Natural enemies have long been used in biological control programs to mitigate the damage caused by herbivory. Many herbivorous
insect species also act as plant virus vectors, enabling virus transmission from plant to plant and hence disease development
in a plant population. Whilst an intuitive assumption would be to expect a decrease in vector numbers to lead to subsequent
reductions in virus transmission, recent evidence suggests that introduction of natural enemies (parasitoids and predators)
may in some cases increase plant virus transmission while at the same time reducing vector numbers. In this paper we review
the evidence for plant-virus-vector-natural enemy interactions, the signalling mechanisms involved and their implications
for virus transmission, and show how a modelling approach can assist in identifying the key parameters and relationships involved
in determining the disease outcome. A mathematical model linking the population dynamics of a vector-parasitoid system with
virus transmission was used to investigate the effects of virus inoculation and acquisition rates, parasitoid attack rate
and vector aggregation on disease dynamics across a wide range of parameter value combinations. Virus spread was found to
increase with enhanced inoculation, acquisition and parasitoid attack rate but decrease with high levels of vector aggregation. 相似文献
128.
Conway Ll. Powell 《Soil biology & biochemistry》1980,12(3):247-250
A method to assess the mycorrhizal inoculum infectivity of soils is described. Mature pasture soils in the North Island, New Zealand, had 6–19 mycorrhizal propagules g?1 soil. Of 31 samples taken from extensive areas of eroded soil around Gisborne and Masterton 22 had fewer than 1.0 mycorrhizal propagule g?1 soil, and 13 of these soils had less than 0.2 propagules g?1. In a pot trial, mycorrhizal inoculation of white clover plants increased shoot growth in seven eroded soils, by 1–12-fold. 相似文献
129.
Parnell NK Powell LL Hohenhaus AE Patnaik AK Peterson ME 《Journal of the American Veterinary Medical Association》1999,214(8):1208-11, 1200
Hypoadrenocorticism secondary to bilateral lymphomatous invasion of adrenocortical tissue was diagnosed in 2 adult castrated domestic shorthair cats. In both cats, findings during initial physical examination (ie, lethargy, anorexia, weight loss, and severe weakness) were consistent with severe hypoadrenocorticism. Hyperkalemia, hyponatremia, and azotemia were also detected, which were consistent with hypoadrenocorticism. Hypoadrenocorticism was documented on the basis of finding low baseline and ACTH-stimulated serum cortisol concentrations. The cats initially responded well to treatment, but both owners chose to have the cat euthanatized. Multicentric lymphoma with infiltration and destruction of the adrenal glands was confirmed during necropsy. 相似文献
130.
Dickinson PJ Anderson PJ Williams DC Powell HC Shelton GD Morris JG LeCouteur RA 《American journal of veterinary research》2004,65(5):671-680
OBJECTIVE: To determine the neurologic effects of reduced intake of phenylalanine and tyrosine in black-haired cats. ANIMALS: 53 specific pathogen-free black domestic shorthair cats. PROCEDURE: Cats were fed purified diets containing various concentrations of phenylalanine and tyrosine for < or = 9 months. Blood samples were obtained every 2 months for evaluation of serum aromatic amino acid concentrations. Cats were monitored for changes in hair color and neurologic or behavioral abnormalities. Three cats with neurologic deficits underwent clinical and electrophysiologic investigation; muscle and nerve biopsy specimens were also obtained from these cats. RESULTS: After 6 months, neurologic and behavioral abnormalities including vocalization and abnormal posture and gait were observed in cats that had received diets containing < 16 g of total aromatic amino acid/kg of diet. Electrophysiologic data and results of microscopic examination of muscle and nerve biopsy specimens from 3 cats with neurologic signs were consistent with sensory neuropathy with primary axonal degeneration. Changes in hair color were detected in cats from all groups receiving < 16 g of phenylalanine plus tyrosine/kg of diet. CONCLUSIONS AND CLINICAL RELEVANCE: Findings suggested that chronic dietary restriction of phenylalanine and tyrosine in cats may result in a predominantly sensory neuropathy. In cats, the long-term nutritional requirement for phenylalanine and tyrosine appears to be greater for normal neurologic function than that required in short-term growth experiments. Official present-day recommendations for dietary phenylalanine and tyrosine in cats may be insufficient to support normal long-term neurologic function. 相似文献