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   311篇
  免费   17篇
  2023年   4篇
  2022年   7篇
  2021年   11篇
  2020年   5篇
  2019年   11篇
  2018年   15篇
  2017年   10篇
  2016年   25篇
  2015年   16篇
  2014年   26篇
  2013年   29篇
  2012年   36篇
  2011年   19篇
  2010年   15篇
  2009年   8篇
  2008年   9篇
  2007年   21篇
  2006年   13篇
  2005年   8篇
  2004年   6篇
  2003年   7篇
  2002年   4篇
  2001年   1篇
  2000年   1篇
  1999年   2篇
  1998年   1篇
  1997年   3篇
  1992年   1篇
  1991年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   2篇
  1981年   1篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1972年   1篇
  1967年   1篇
  1966年   1篇
  1952年   1篇
排序方式: 共有328条查询结果,搜索用时 15 毫秒
51.
52.
Mate value discrepancy (MVD) between heterosexual partners is an important factor influencing relationship satisfaction which, in turn, has an effect on the quality and the stability of the relationship. Therefore, partners’ involvement in mate retention behaviours, such as controlling behaviours, can be related to MVD and our study aims to determine whether such an association exists. In order to do so, we analysed female perception of MVD and their opinion regarding the intensity of controlling behaviours performed by themselves as well as their romantic partners. Female perception of the intensity of controlling behaviours performed by both partners was the highest in couples where a woman assesses her own mate value (MV) as higher than her partner’s MV and significantly different than in relationships where male MV exceeded those of the female. Our study also indicates that MVD should be taken into account when analysing sex differences in intensities of mate retention behaviours. Finally, we provide evidence supporting the significance of the relationship length for controlling behaviour intensity. Findings are discussed within an evolutionary perspective.  相似文献   
53.
Common reed (Phragmites australis (Cav.) Trin. ex Steud.) plants were harvested from four natural water ecosystems of the Bogdanka river catchment (Poznań, Poland) four times throughout the 2014 vegetative season. Over the year, average metal contents followed different decreasing trends according to the analyzed tissue: Zn > Cu ≈ Pb > Cd (rhizomes) and Zn > Pb > Cu > Cd (leaves), and mean translocation ratios (leaves/rhizomes) were found as follows: 0.93, 0.70, 0.65, 0.40 for Zn, Pb, Cd and Cu, respectively. Metal content increased gradually during the growing season, and in the case of Cu, Cd and Pb exceeded the upper limit of average concentration detected in plants from natural ecosystems. However, the content of salicylic acid did not follow the increase of metal accumulation. In rhizomes, the highest production of the metabolite was observed in May and reached 324 ng g?1 fresh weight (FW) (mean value). Afterwards, a significant drop to 50 ng g?1 FW was observed. Simultaneously, the highest values of total salicylic acid in P. australis leaves were observed in July and accompanied the intensive development of the aboveground biomass of the plant (11.3 µg g?1 FW–mean value). Subsequently, its content in leaves showed a significant decrease down to 2.1 µg g?1 FW in November. Multivariate regression analysis revealed significant interactions between analyzed metals influencing the plant response to metal-derived stress. Cu and Zn showed antagonistic properties considering their uptake and the induction of salicylic acid biosynthesis, whereas non-essential metals (Pb and Cd) acted similarly and stimulated the formation of salicylic acid glucoside.  相似文献   
54.
Amphibians are good indicators of human impact, declining steadily worldwide. We explored the relationships between the ponds and nearby landscape parameters and population size of the Agile Frog (Rana dalmatina), estimated from the number of egg masses, in a cultural landscape within the central section of Târnava Mare Basin, Romania. Forty-three permanent ponds were surveyed in a 2600 km2 area. The average number of egg masses per pond was 211.13 (SD = 426.41). The egg mass number was significantly and positively related to the emergent aquatic macrophyte cover (its effect peaks at around 50%) and the green connecting corridors between the ponds and forests, and negatively related to the extent of nearby urban areas. The proximity of the forest (positive effect) and the presence of high traffic roads (negative effect) were highly correlated with green corridors and further eliminated from the model due to multicollinearity. Both these variables had significant effects when incorporated in univariate models and multivariate models without green corridors. Since a large part of our study area was currently declared as Natura 2000 site, there is an increased need for management proposals and conservation applications for biodiversity, including amphibians. Rana dalmatina is an important species for monitoring because it is common in the studied area and is suited for short surveys.  相似文献   
55.
56.
The phenomenon of cross-resistance allows plants to acquire resistance to a broad range of stresses after previous exposure to one specific factor. Although this stress–response relationship has been known for decades, the sequence of events that underpin cross-resistance remains unknown. Our experiments revealed that susceptible potato (Solanum tuberosum L. cv. Bintje) undergoing aluminum (Al) stress at the root level showed enhanced defense responses correlated with reduced disease symptoms after leaf inoculation with Phytophthora infestans. The protection capacity of Al to subsequent stress was associated with the local accumulation of H2O2 in roots and systemic activation of salicylic acid (SA) and nitric oxide (NO) dependent pathways. The most crucial Al-mediated changes involved coding of NO message in an enhanced S-nitrosothiol formation in leaves tuned with an abundant SNOs accumulation in the main vein of leaves. Al-induced distal NO generation was correlated with the overexpression of PR-2 and PR-3 at both mRNA and protein activity levels. In turn, after contact with a pathogen we observed early up-regulation of SA-mediated defense genes, e.g. PR1, PR-2, PR-3 and PAL, and subsequent disease limitation. Taken together Al exposure induced distal changes in the biochemical stress imprint, facilitating more effective responses to a subsequent pathogen attack.  相似文献   
57.
Wall teichoic acid (WTA) was isolated from Enterococcus faecium strain U0317 and structurally characterized using 1H, 13C, and 31P NMR spectroscopy, including two-dimensional COSY, TOCSY, ROESY, HMQC, and HMBC experiments. Further compositional determination was undertaken using classical chemical methods and HF treatment followed by GLC and GLC–MS analyses. The repeating unit of WTA consisted of two residues of 2-acetamido-2-deoxy-d-galactose, glycerol (Gro), and phosphate, and has the structure shown below:→6)-α-d-GalpNAc-(1→3)-β-d-GalpNAc-(1→2)-Gro-(3→P→  相似文献   
58.
The lipopolysaccharide (LPS) of the bacterium Pectobacterium atrosepticum SCRI 1039 was hydrolyzed and the products were separated. A study of the obtained O-polysaccharide by means of chemical methods, GLC, GLC–MS, and NMR spectroscopy allowed us to identify a branched polymer with a pentasaccharide repeating unit of the structure shown below, in which the fucose residue was partially O-acetylated at C-2, C-3 or C-4.  相似文献   
59.
Laminin-associated polypeptide 2 (LAP2) proteins are alternatively spliced products of a single gene; they belong to the LEM domain family and, in mammals, locate to the nuclear envelope (NE) and nuclear lamina. Isoforms lacking the transmembrane domain also locate to the nucleoplasm. We used new specific antibodies against the N-terminal domain of Xenopus LAP2 to perform immunoprecipitation, identification and localization studies during Xenopus development. By immunoprecipitation and mass spectrometry (LC/MS/MS), we identified the embryonic isoform XLAP2??, which was downregulated during development similarly to XLAP2??. Embryonic isoforms XLAP2?? and XLAP2?? were located in close association with chromatin up to the blastula stage. Later in development, both embryonic isoforms and the adult isoform XLAP2?? were localized in a similar way at the NE. All isoforms colocalized with lamin B2/B3 during development, whereas XLAP2?? was colocalized with lamin B2 and apparently with the F/G repeat nucleoporins throughout the cell cycle in adult tissues and culture cells. XLAP2?? was localized in clusters on chromatin, both at the NE and inside the nucleus. Embryonic isoforms were also localized in clusters at the NE of oocytes. Our results suggest that XLAP2 isoforms participate in the maintenance and anchoring of chromatin domains to the NE and in the formation of lamin B microdomains.  相似文献   
60.
The globally widespread genus Nymphaea exhibits a wide range of morphological and taxonomical diversity. The intrusion of a cultivated variety by progressive propagation and its affect on aquatic habitat is demonstrated in this case study. We have studied the genetic diversity, population, and stand structure of the neophyte Nymphaea × ‘Panama Pacific’ as well as other species found in Lake Hévíz and dikes nearby using inter-simple sequence repeat (ISSR) markers. The ISSR assay revealed a low genetic variability for the small populations of Nymphaea caerulea, Nymphaea lotus var. thermalis, and a medium level for Nymphaea alba, Nymphaea rubra var. longiflora, and Nymphaea × ‘Panama Pacific’. The evolutionary genetic status of individuals found in the overlapping cultivation area of Nymphaea × ‘Panama Pacific’ and N. caerulea was affirmed to be of hybrid origin by reticulate network analysis and with morphological parameters. The Bayesian analysis of hybrid classes and the segregation of the ISSR markers also confirmed the hybrid origin of the individuals in question and revealed that they are falling into F2 or latter genotype frequency classes, indicating the viability and fertility of the hybrids. The set of analyzed species by phylogenetic network analysis of ISSR data has been divided into three major groups according to their evolutionary patterns (subg. Barachyceras, Lotos, and Nymphaea). Our results are in accordance with these three major subgenera within Nymphaea.  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