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排序方式: 共有2119条查询结果,搜索用时 468 毫秒
71.
72.
One-day rate measurements for estimating net nitrification potential in humid forest soils 总被引:2,自引:0,他引:2
Donald S. Ross Guinevere Fredriksen Austin E. Jamison Beverley C. Wemple Scott W. Bailey James B. Shanley Gregory B. Lawrence 《Forest Ecology and Management》2006,230(1-3):91-95
Measurements of net nitrification rates in forest soils have usually been performed by extended sample incubation (2–8 weeks), either in the field or in the lab. Because of disturbance effects, these measurements are only estimates of nitrification potential and shorter incubations may suffice. In three separate studies of northeastern USA forest soil surface horizons, we found that laboratory nitrification rates measured over 1 day related well to those measured over 4 weeks. Soil samples of Oa or A horizons were mixed by hand and the initial extraction of subsamples, using 2 mol L−1 KCl, occurred in the field as soon as feasible after sampling. Soils were kept near field temperature and subsampled again the following day in the laboratory. Rates measured by this method were about three times higher than the 4-week rates. Variability in measured rates was similar over either incubation period. Because NO3− concentrations were usually quite low in the field, average rates from 10 research watersheds could be estimated with only a single, 1-day extraction. Methodological studies showed that the concentration of NH4+ increased slowly during contact time with the KCl extractant and, thus, this contact time should be kept similar during the procedure. This method allows a large number of samples to be rapidly assessed. 相似文献
73.
We analyzed processes limiting photosynthesis in two-year-old, container-grown Pinus pinaster Ait. seedlings subjected to phosphorus (P) deficiency. After withholding P for 3 months, seedlings were supplied P at four relative addition rates (0, 0.005, 0.01 and 0.02 day(-1)) in a nutrient recycling system. At Weeks 12 and 22, responses of photosynthesis to CO(2) and irradiance were measured and the following parameters derived: maximal velocity of carboxylation by Rubisco, V(m); apparent quantum efficiency of electron transport, alpha maximal electron transport rate, J(m); stomatal conductance and relative stomatal limitation of photosynthesis. At Week 22, these measurements were combined with concurrent measurements of chlorophyll fluorescence to determine the quantum yield of PSII, and a theoretical partitioning of total light-driven linear electron flow between fractions used to regenerate carboxylated and oxygenated RuBP. After 12 weeks of treatment, needle P concentrations ranged from 0.04 to 0.15 x 10(-2) g g(DW) (-1), and then remained constant until Week 22. Values of J(m), alpha and V(m) increased with increasing needle P concentration (from 30 to 133 &mgr;mol m(-2) s(-1), 0.02 to 0.25 mol mol(-1) and 13 to 78 &mgr;mol CO(2) m(-2) s(-1) at the lowest and highest needle P concentrations, respectively). Under ambient conditions, net assimilation rates in P-deficient seedlings were limited by V(m) under saturating irradiance, and by J(m) under limiting irradiance, but not by triose-P regeneration. There was no detectable change in the partitioning of total light-driven linear electron flow between the fractions used for carboxylation and oxygenation. Predawn photochemical efficiency of PSII was significantly reduced in seedlings with low P concentrations. Although stomatal conductance tended to decrease with decreasing needle P concentration, relative stomatal limitation was not significantly affected. At Week 22, there was an attenuation of the effects of P nutrition on V(m) and an increase in alpha and J(m) that was probably related to cessation of growth and the seasonal decline in natural irradiance. 相似文献
74.
Phytopathogenic fungi associated with the bark beetles Tomicus piniperda and Orthotomicus erosus were isolated in various pine forests of Tunisia. Tomicus piniperda and its galleries yielded Leptographium wingfieldii, Ophiostoma minus, and Ophiostoma ips. Ophiostoma minus was the most frequent species associated with T. piniperda, in both the attacking and the emerging beetles. It was collected from most investigated forests, whereas O. ips and L. wingfieldii were obtained only from forests located in Central and Northern Tunisia. Frequencies of association with T. piniperda were always low, reaching 11.1% only once, for O. ips. Ophiostoma ips was the only blue stain fungus associated with O. erosus and its galleries. It was found in all the localities, but at a low and variable frequency, exceeding 15% very rarely. The virulence of 16 fungal isolates was tested by single inoculations into Pinus halepensis (Aleppo pine) at two localities. Differences were detected among species; L. wingfieldii was the most virulent and O. minus the least virulent species in terms of phloem reaction zone formation and fungal growth in the phloem. In a separate experiment, mass inoculations (400 and 800 inoculations per m2 of bole on 1‐m high belts) were performed with two isolates of L. wingfieldii on Aleppo pine and on Pinus brutia (Brutia pine). Three months later, measurements of sapwood status (% of conductive transversal section) and of its specific hydraulic conductivity, as well as of the phloem reaction zone lengths, did not show any isolate or density effect. At these experimental sites, Brutia pines appeared significantly more susceptible than Aleppo pines. 相似文献
75.
76.
77.
Ben C Maddison John Spiropoulos Christopher M Vickery Richard Lockey Jonathan P Owen Keith Bishop Claire A Baker Kevin C Gough 《Veterinary research》2015,46(1)
Ovine scrapie can be transmitted via environmental reservoirs. A pool of ovine scrapie isolates were incubated on soil for one day or thirteen months and eluted prion was used to challenge tg338 mice transgenic for ovine PrP. After one-day incubation on soil, two PrPSc phenotypes were present: G338 or Apl338ii. Thirteen months later some divergent PrPSc phenotypes were seen: a mixture of Apl338ii with either G338 or P338, and a completely novel PrPSc deposition, designated Cag338. The data show that prolonged ageing of scrapie prions within an environmental matrix may result in changes in the dominant PrPSc biological/biochemical properties. 相似文献
78.
79.
Impact of source tissue and ex vivo expansion on the characterization of goat mesenchymal stem cells
Background
There is considerable interest in using goats as models for genetically engineering dairy animals and also for using stem cells as therapeutics for bone and cartilage repair. Mesenchymal stem cells (MSCs) have been isolated and characterized from various species, but are poorly characterized in goats.Results
Goat MSCs isolated from bone marrow (BM-MSCs) and adipose tissue (ASCs) have the ability to undergo osteogenic, adipogenic and chondrogenic differentiation. Cytochemical staining and gene expression analysis show that ASCs have a greater capacity for adipogenic differentiation compared to BM-MSCs and fibroblasts. Different methods of inducing adipogenesis also affect the extent and profile of adipogenic differentiation in MSCs. Goat fibroblasts were not capable of osteogenesis, hence distinguishing them from the MSCs. Goat MSCs and fibroblasts express CD90, CD105, CD73 but not CD45, and exhibit cytoplasmic localization of OCT4 protein. Goat MSCs can be stably transfected by Nucleofection, but, as evidenced by colony-forming efficiency (CFE), yield significantly different levels of progenitor cells that are robust enough to proliferate into colonies of integrants following G418 selection. BM-MSCs expanded over increasing passages in vitro maintained karyotypic stability up to 20 passages in culture, exhibited an increase in adipogenic differentiation and CFE, but showed altered morphology and amenability to genetic modification by selection.Conclusions
Our findings provide characterization information on goat MSCs, and show that there can be significant differences between MSCs isolated from different tissues and from within the same tissue. Fibroblasts do not exhibit trilineage differentiation potential at the same capacity as MSCs, making it a more reliable method for distinguishing MSCs from fibroblasts, compared to cell surface marker expression.Electronic supplementary material
The online version of this article (doi:10.1186/2049-1891-6-1) contains supplementary material, which is available to authorized users. 相似文献80.
Amara-Dali WB Lesieur P Artzner F Karray N Attia H Ollivon M 《Journal of agricultural and food chemistry》2007,55(12):4741-4751
Crystallization and melting properties of triacylglycerols (TGs) in anhydrous goat's milk fat (AGMF) are investigated by X-ray diffraction as a function of temperature (XRDT) coupled with high-sensitivity differential scanning calorimetry (DSC), using synchrotron radiation and Microcalix. The polymorphic behavior of AGMF was monitored by varying the cooling rates between 5 and 1 degrees C/min from 45 to -20 degrees C with their subsequent melting at 1 degrees C/min. Quenching of AGMF at -20 degrees C was also examined to determine the metastable polymorphic form of AGMF. At intermediate cooling rates, TGs in AGMF crystallize, from about 18 degrees C in two different lamellar structures with triple chain length 3Lalpha stacking of 72 A and a double chain length 2Lalpha stacking of 48 A, which are correlated to two overlapped exothermic peaks recorded by DSC. A reversible transition sub alpha <--> alpha was observed. Subsequent heating at 1 degrees C/min shows numerous structural rearrangements before final melting. At fast cooling of AGMF (5 degrees C/min), similar unstable crystalline varieties are formed while three endotherms are recorded. Several new unstable lamellar structures are observed after quenching. All of these data are compared to those previously reported at slow cooling (0.1 degrees C/min) showing a relative stability of the structures formed. In spite of general similitude, the thermal and structural behavior of the goat's milk is more complex than that of the cow's milk. 相似文献