首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   94篇
  免费   6篇
  国内免费   1篇
  2020年   1篇
  2019年   1篇
  2014年   7篇
  2013年   2篇
  2012年   4篇
  2011年   5篇
  2010年   3篇
  2009年   2篇
  2008年   5篇
  2007年   3篇
  2006年   5篇
  2005年   3篇
  2004年   1篇
  2003年   2篇
  2002年   3篇
  2000年   3篇
  1999年   3篇
  1998年   9篇
  1995年   2篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1990年   2篇
  1983年   3篇
  1982年   1篇
  1981年   4篇
  1980年   1篇
  1977年   2篇
  1976年   2篇
  1959年   2篇
  1958年   5篇
  1957年   2篇
  1955年   2篇
  1954年   1篇
  1953年   1篇
  1952年   2篇
  1951年   3篇
排序方式: 共有101条查询结果,搜索用时 312 毫秒
51.
Hybridization is a significant threat for endangered species and could potentially even lead to their extinction. This concern applies to the globally vulnerable Greater Spotted Eagle Aquila clanga, a species that co‐occurs, and potentially interbreeds, with the more common Lesser Spotted Eagle Aquila pomarina in a vast area of Eastern Europe. We applied single nucleotide polymorphism (SNP) and microsatellite markers in order to study hybridization and introgression in 14 European spotted eagle populations. We detected hybridization and/or introgression in all studied sympatric populations. In most regions, hybridization took place prevalently between A. pomarina males and A. clanga females, with introgression to the more common A. pomarina. However, such a pattern was not as obvious in regions where A. clanga is still numerous. In the course of 16 years of genetic monitoring of a mixed population in Estonia, we observed the abandonment of A. clanga breeding territories and the replacement of A. clanga pairs by A. pomarina, whereby on several occasions hybridization was an intermediate step before the disappearance of A. clanga. Although the total number of Estonian A. clanga × A. pomarina pairs was twice as high as that of A. clanga pairs, the number of pairs recorded yearly were approximately equal, which suggests a higher turnover rate in interbreeding pairs. This study shows that interspecific introgressive hybridization occurs rather frequently in a hybrid zone at least 1700‐km wide: it poses an additional threat for the vulnerable A. clanga, and may contribute to the extinction of its populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 725–736.  相似文献   
52.
It has been forecast that the challenge of meeting increased food demand and protecting environmental quality will be won or lost in maize, rice and wheat cropping systems, and that the problem of environmental nitrogen enrichment is most likely to be solved by substituting synthetic nitrogen fertilizers by the creation of cereal crops that are able to fix nitrogen symbiotically as legumes do. In legumes, rhizobia present intracellularly in membrane-bound vesicular compartments in the cytoplasm of nodule cells fix nitrogen endosymbiotically. Within these symbiosomes, membrane-bound vesicular compartments, rhizobia are supplied with energy derived from plant photosynthates and in return supply the plant with biologically fixed nitrogen, usually as ammonia. This minimizes or eliminates the need for inputs of synthetic nitrogen fertilizers. Recently we have demonstrated, using novel inoculation conditions with very low numbers of bacteria, that cells of root meristems of maize, rice, wheat and other major non-legume crops, such as oilseed rape and tomato, can be intracellularly colonized by the non-rhizobial, non-nodulating, nitrogen fixing bacterium, Gluconacetobacter diazotrophicus that naturally occurs in sugarcane. G. diazotrophicus expressing nitrogen fixing (nifH) genes is present in symbiosome-like compartments in the cytoplasm of cells of the root meristems of the target cereals and non-legume crop species, somewhat similar to the intracellular symbiosome colonization of legume nodule cells by rhizobia. To obtain an indication of the likelihood of adequate growth and yield, of maize for example, with reduced inputs of synthetic nitrogen fertilizers, we are currently determining the extent to which nitrogen fixation, as assessed using various methods, is correlated with the extent of systemic intracellular colonization by G. diazotrophicus, with minimal or zero inputs.  相似文献   
53.
54.
The two main allelic variants of the Atlantic cod (Gadus morhua L.) pantophysin I (Pan I) locus have different frequencies within different cod stocks. The Dra I polymorphism which distinguishes the two alleles can thus be used for discrimination of coastal and offshore cod populations. We present a new method for Pan I genotyping using fluorescent allele‐specific duplex polymerase chain reaction (PCR). This method is more rapid, reliable and cost‐effective than the previously published method and it is not affected by DNA source and quality. This improvement is important for studies demanding high throughput and accuracy of Pan I genotyping  相似文献   
55.
Scientific evidence supports the use of common reference intervals (RIs) for many general chemistry analytes, in particular those with sound calibration and traceability in place. Already the Nordic countries and United Kingdom have largely achieved harmonised RIs. Following a series of workshops organised by the Australasian Association of Clinical Biochemists (AACB) between 2012 and 2014 at which an evidence-based approach for determination of common intervals was developed, pathology organisations in Australia and New Zealand have reached a scientific consensus on what adult and paediatric intervals we should use across Australasia. The aim of this report is to describe the processes that the AACB and the Royal College of Pathologists of Australasia have taken towards recommending the implementation of a first panel of common RIs for use in Australasia.  相似文献   
56.
A further increase in nitrogen (N) intensive biomass supplies to substitute fossil carbon sources implies inclusion of additional reactive nitrogen (Nr) into the biosphere. A Danish model study compared low‐intensity managed seminatural beech forest and a winter wheat system with respect to N losses and greenhouse gas (GHG) emissions. Losses of reactive N to air and groundwater per unit of energy produced were four to six times higher for the winter wheat system. The energy efficiency was an order of magnitude higher in the forest system, whereas the related GHG emission reduction by fossil coal substitution differed by <25%. The question is whether a low or a high intensity of cultivation yields the best overall ecosystem service performance? Given the detrimental effect of excess reactive N on natural ecosystems, we suggest that bioenergy production from unfertilized forest with seminatural structure and function should be preferred over N‐intensive crop production.  相似文献   
57.
We present an analysis of direct land use change (dLUC) resulting from the conversion of semiarid woodlands in Brazil and India to Jatropha curcas, a perennial biofuel crop. The sites examined include prosopis woodlands, managed for woodfuel production under periodic coppicing, in southern India, and unmanaged caatinga woodlands in the Brazilian state of Minas Gerais. The jatropha plantations under consideration include pruned and unpruned stands and ranged from 2 to 4 years of age. Stocks of carbon in aboveground (AG) pools, including woody biomass, coarse debris, leaf litter, and herbaceous matter, as well as soil organic carbon (SOC) were evaluated. The jatropha plantations store 8–10 tons of carbon per hectare (t C ha?1) in AG biomass and litter when managed with regular pruning in both India and Brazil. Unpruned trees, only examined in Brazil, store less biomass (and carbon), accumulating just 3 t C ha?1 in AG pools. The two woodlands that were replaced with jatropha show substantial differences in carbon pools: prosopis contains ~11 t C ha?1 in AG stocks of carbon, which was very close to the jatropha stand which replaced it. In contrast, caatinga stores ~35 t C ha?1 in AG biomass. Moreover, no change in SOC was detected in land that was converted from Prosopis to jatropha. As a result, there is no detectable change in AG carbon stocks at the sites in South India where jatropha replaced prosopis woodlands. In contrast, large losses of AG carbon were detected in Central Brazil where jatropha replaced native caatinga woodlands. These losses represent a carbon debt that would take 10–20 years to repay.  相似文献   
58.
59.
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号