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   1070篇
  免费   67篇
  2023年   9篇
  2022年   10篇
  2021年   25篇
  2020年   21篇
  2019年   18篇
  2018年   26篇
  2017年   28篇
  2016年   38篇
  2015年   51篇
  2014年   61篇
  2013年   80篇
  2012年   86篇
  2011年   84篇
  2010年   54篇
  2009年   37篇
  2008年   55篇
  2007年   59篇
  2006年   40篇
  2005年   46篇
  2004年   42篇
  2003年   26篇
  2002年   15篇
  2001年   9篇
  2000年   5篇
  1999年   8篇
  1997年   6篇
  1996年   5篇
  1995年   7篇
  1991年   6篇
  1990年   7篇
  1989年   5篇
  1988年   5篇
  1986年   9篇
  1985年   11篇
  1984年   8篇
  1983年   7篇
  1982年   5篇
  1980年   8篇
  1979年   7篇
  1977年   5篇
  1976年   7篇
  1975年   6篇
  1974年   7篇
  1973年   8篇
  1972年   7篇
  1970年   16篇
  1969年   9篇
  1968年   6篇
  1967年   4篇
  1966年   5篇
排序方式: 共有1137条查询结果,搜索用时 15 毫秒
41.
A series of 13 phosphonium salts on the basis of pyridoxine derivatives were synthesized and their antibacterial activity against clinically relevant strains was tested in vitro. All compounds were almost inactive against gram-negative bacteria and exhibited structure-dependent activity against gram-positive bacteria. A crucial role of ketal protection group in phosphonium salts for their antibacterial properties was demonstrated. Among synthesized compounds 5,6-bis[triphenylphosphonio(methyl)]-2,2,8-trimethyl-4H-[1,3]dioxino[4,5-c]pyridine dichloride (compound 20) was found to be the most effective towards Staphylococcus aureus and Staphylococcus epidermidis strains (MIC 5 μg/ml). The mechanism of antibacterial activity of this compound probably involves cell penetration and interaction with genomic and plasmid DNA.  相似文献   
42.
A wide range of pests and diseases attacks larch, and over 3 years in an orchard in north-western Romania, the most frequently occurring were larch needle disease (Hypodermella laricis), the case-bearer (Coleophora laricella) and woolly adelgid (Adelges laricis) insects. Their attack on seven larch clones obtained through selection from different geographic populations from Romania was studied. To evaluate the attack intensity, four branches per tree were analysed in the upper, middle and lower third of the crowns in all cardinal directions. According to the position in the crown, a high attack intensity by H. laricis was recorded on the branches in the north direction. A high intensity of C. laricella attack was registered on the upper third of the crown, especially in the east, west and south directions. A positive relationship was identified between the damaged sprouts caused by the C. laricella and A. laricis attack. The differences among the reaction of the clones and the heritability coefficients illustrated that the response to the attacks had a strong genetic determinism, with resistance being clearly influenced by genotype. In addition to the identification of gene sources that are useful for larch breeding, this study offers technical information about the potential to develop efficient treatments based on the attacks on the tree crowns.  相似文献   
43.

Purpose

In this two-part paper (Background and Initial Assumptions (part 1) and Results of Survey Research (part 2)), we present surveys whose main objective is to determine whether, and to what extent, the life cycle assessment (LCA) technique is used for the identification and assessment of environmental aspects in environmental management systems (EMS) and whether there are any differences in this respect between the companies and countries analysed.

Methods

The survey research was carried out using the computer assisted self-administered interviewing method among selected Polish, German and Swedish organisations which implement EMS in accordance with the requirements of ISO 14001 and/or the EMAS regulation.

Results

The organisations investigated, regardless of their country, are dominated by qualitative and semi-quantitative techniques of assessment and identification of environmental aspects. LCA was used sporadically, although some differences can be observed between the countries analysed.

Conclusions

The environmental managers accustomed to traditional qualitative and semi-quantitative solutions have not been given preparation to enable them to understand and adopt different approaches such as LCA. On the other hand, representatives of the organisations investigated declared that they were ready to accept an even longer timescale for the identification and assessment processes relating to environmental aspects, which represents a potential opportunity for LCA. The more precise understanding and definition of environmental problems that are precisely defined in LCA would represent a novelty for environmental managers. In practice, environmental problems are defined in a general sense and rather ambiguously, as this level of detail is sufficient in the context of qualitative and semi-quantitative techniques commonly used for the identification and assessment of environmental aspects.  相似文献   
44.
45.
46.
The availability of data for reference values in cerebrospinal fluid for healthy humans is limited due to obvious practical and ethical issues. The variability of reported values for metabolites in human cerebrospinal fluid is quite large. Dogs present great similarities with humans, including in cases of central nervous system pathologies. The paper presents the first study on healthy dog cerebrospinal fluid metabolomic profile using 1H NMR spectroscopy. A number of 13 metabolites have been identified and quantified from cerebrospinal fluid collected from a group of 10 mix breed healthy dogs. The biological variability as resulting from the relative standard deviation of the physiological concentrations of the identified metabolites had a mean of 18.20% (range between 9.3% and 44.8%). The reported concentrations for metabolites may be used as normal reference values. The homogeneity of the obtained results and the low biologic variability show that the 1H NMR analysis of the dog’s cerebrospinal fluid is reliable in designing and interpreting clinical and therapeutic trials in dogs with central nervous system pathologies.  相似文献   
47.
48.
Responses to human cytomegalovirus (HCMV) infection are largely individual and cell type specific. We investigated molecular profiles in 2 primary cell cultures of human fibroblasts, which are highly or marginally sensitive to HCMV infection, respectively. We screened expression of genes and microRNAs (miRs) at the early (3 hours) stage of infection. To assess molecular pathway activation profiles, we applied bioinformatic algorithms OncoFinder and MiRImpact. In both cell types, pathway regulation properties at mRNA and miR levels were markedly different. Surprisingly, in the infected highly sensitive cells, we observed a “freeze” of miR expression profiles compared to uninfected controls. Our results evidence that in the sensitive cells, HCMV blocks intracellular regulation of microRNA expression already at the earliest stage of infection. These data suggest somewhat new functions for HCMV products and demonstrate dependence of miR expression arrest on the host-encoded factors.  相似文献   
49.
The multidisciplinarity of integrative taxonomy is particularly useful for clarifying the systematics of speciose groups that are poorly differentiated morphologically, and this approach can also illuminate their evolutionary history and biogeography. Here, we utilize it to examine the systematics and taxonomy of a newly recognized amphipod species, Gammarus hamaticornis n. sp., which belongs to a highly diverse genus of endemic freshwater crustaceans that show very limited morphological differentiation. Since this species is endemic to northern Dobrogea, a region at the northwestern Black Sea coast devoid of permafrost during the Last Glacial Maximum, we hypothesized that it survived in situ during the Quaternary climatic oscillations. We first examined the phylogenetic position of Gammarus hamaticornis n. sp. within the genus and then compared its morphology, phylogeography, distribution, and climatic niche with that of its sister species. Results indicate that G. hamaticornis n. sp. is most closely related to its widely distributed northern neighbor, G. kischineffensis, and a remarkable agreement was observed among morphological, multilocus coalescent and climatic analyses which supported the distinctiveness of both taxa. These apparently diverged during the Pliocene from a common ancestor that likely colonized freshwaters from the adjacent brackish basins of the shrinking Paratethys. The results indicate that G. hamaticornis n. sp. has persisted throughout the Pleistocene in northern Dobrogea, a hitherto hypothesized refugium confirmed for the first time with molecular genetic data. Due to its narrow geographical range, rarity in the local communities and highly fluctuating nature of the streams it inhabits, this species should be in the focus of future conservation priorities.  相似文献   
50.
Grain size and weight are important components of a suite of yield‐related traits in crops. Here, we showed that the CRISPR‐Cas9 gene editing of TaGW7, a homolog of rice OsGW7 encoding a TONNEAU1‐recruiting motif (TRM) protein, affects grain shape and weight in allohexaploid wheat. By editing the TaGW7 homoeologs in the B and D genomes, we showed that mutations in either of the two or both genomes increased the grain width and weight but reduced the grain length. The effect sizes of mutations in the TaGW7 gene homoeologs coincided with the relative levels of their expression in the B and D genomes. The effects of gene editing on grain morphology and weight traits were dosage dependent with the double‐copy mutant showing larger effect than the respective single copy mutants. The TaGW7‐centered gene co‐expression network indicated that this gene is involved in the pathways regulating cell division and organ growth, also confirmed by the cellular co‐localization of TaGW7 with α‐ and β‐tubulin proteins, the building blocks of microtubule arrays. The analyses of exome capture data in tetraploid domesticated and wild emmer, and hexaploid wheat revealed the loss of diversity around TaGW7‐associated with domestication selection, suggesting that TaGW7 is likely to play an important role in the evolution of yield component traits in wheat. Our study showed how integrating CRISPR‐Cas9 system with cross‐species comparison can help to uncover the function of a gene fixed in wheat for allelic variants targeted by domestication selection and select targets for engineering new gene variants for crop improvement.  相似文献   
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