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   8902篇
  免费   668篇
  国内免费   678篇
  10248篇
  2024年   19篇
  2023年   114篇
  2022年   282篇
  2021年   494篇
  2020年   293篇
  2019年   371篇
  2018年   378篇
  2017年   301篇
  2016年   368篇
  2015年   567篇
  2014年   687篇
  2013年   703篇
  2012年   834篇
  2011年   729篇
  2010年   431篇
  2009年   406篇
  2008年   479篇
  2007年   387篇
  2006年   302篇
  2005年   253篇
  2004年   194篇
  2003年   188篇
  2002年   160篇
  2001年   145篇
  2000年   129篇
  1999年   106篇
  1998年   71篇
  1997年   84篇
  1996年   81篇
  1995年   64篇
  1994年   48篇
  1993年   45篇
  1992年   63篇
  1991年   49篇
  1990年   49篇
  1989年   33篇
  1988年   39篇
  1987年   26篇
  1986年   23篇
  1985年   36篇
  1984年   33篇
  1983年   14篇
  1982年   15篇
  1981年   8篇
  1978年   11篇
  1974年   11篇
  1973年   7篇
  1971年   9篇
  1967年   7篇
  1966年   9篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.

Key message

Overexpression of CsHis in tobacco promoted chromatin condensation, but did not affect the phenotype. It also conferred tolerance to low-temperature, high-salinity, ABA, drought and oxidative stress in transgenic tobacco.

Abstract

H1 histone, as a major structural protein of higher-order chromatin, is associated with stress responses in plants. Here, we describe the functions of the Camellia sinensis H1 Histone gene (CsHis) to illustrate its roles in plant responses to stresses. Subcellular localization and prokaryotic expression assays showed that the CsHis protein is localized in the nucleus, and its molecular size is approximately 22.5 kD. The expression levels of CsHis in C. sinensis leaves under various conditions were investigated by qRT-PCR, and the results indicated that CsHis was strongly induced by various abiotic stresses such as low-temperature, high-salinity, ABA, drought and oxidative stress. Overexpression of CsHis in tobacco (Nicotiana tabacum) promoted chromatin condensation, while there were almost no changes in the growth and development of transgenic tobacco plants. Phylogenetic analysis showed that CsHis belongs to the H1C and H1D variants of H1 histones, which are stress-induced variants and not the key variants required for growth and development. Stress tolerance analysis indicated that the transgenic tobacco plants exhibited higher tolerance than the WT plants upon exposure to various abiotic stresses; the transgenic plants displayed reduced wilting and senescence and exhibited greater net photosynthetic rate (Pn), stomatal conductance (Gs) and maximal photochemical efficiency (Fv/Fm) values. All the above results suggest that CsHis is a stress-induced gene and that its overexpression improves the tolerance to various abiotic stresses in the transgenic tobacco plants, possibly through the maintenance of photosynthetic efficiency.  相似文献   
102.

Purpose

To explore the prevalence rates and predictors of somatic symptoms among child and adolescent survivors with probable posttraumatic stress disorder (PTSD) after an earthquake.

Methods

A total of 3053 students from 21 primary and secondary schools in Baoxing County were administered the Patient Health Questionnaire-13 (PHQ-13), a short version of PHQ-15 without the two items about sexuality and menstruation, the Children''s Revised Impact of Event Scale (CRIES), and the self-made Earthquake-Related Experience Questionnaire 3 months after the Lushan earthquake.

Results

Among child and adolescent survivors, the prevalence rates of all somatic symptoms were higher in the probable PTSD group compared with the controls. The most frequent somatic symptoms were trouble sleeping (83.2%), feeling tired or having low energy (74.4%), stomach pain (63.2%), dizziness (58.1%), and headache (57.7%) in the probable PTSD group. Older age, having lost family members, having witnessed someone get seriously injured, and having witnessed someone get buried were predictors for somatic symptoms among child and adolescent survivors with probable PTSD.

Conclusions

Somatic symptoms among child and adolescent earthquake survivors with probable PTSD in schools were common, and predictors of these somatic symptoms were identified. These findings may help those providing psychological health programs to find the child and adolescent students with probable PTSD who are at high risk of somatic symptoms in schools after an earthquake in China.  相似文献   
103.
104.
Sun  Fan  Yang  WanChun  Du  ChaoLing  Chen  YangXi  FU  TianYi  Shi  DaNing 《Plasmonics (Norwell, Mass.)》2020,15(4):949-955
Plasmonics - Both the excitation wavelength and aspect ratio AR are key parameters controlling the refractive index sensitivity performances of plasmonic sensors of individual Au nanoplate dimmers....  相似文献   
105.
In this study, we report three novel naturally occurring compounds, blapsins A (1) and B (2), and blapsamide (3) from the ethanol extract of the stink beetle, Blaps japanensis. The structures of these compounds were determined using spectroscopic methods. Compound 3 is a phenolic compound bearing a formamido group in the structure. Functional studies revealed that compounds 1 and 2 potently inhibited 14-3-3 protein-protein interactions (PPIs) with IC(50) values of 9.2 and 10.0 μM as determined by an ELISA assay, and 2.0 and 2.5 μM in an FP assay, respectively. These compounds represent the first example of natural small-molecule 14-3-3 inhibitors.  相似文献   
106.
WB Yang  QL Liang  ZJ Ye  CM Niu  WL Ma  XZ Xiong  RH Du  Q Zhou  JC Zhang  HZ Shi 《PloS one》2012,7(7):e40450
The objective of the present study was to investigate the presence of interleukin (IL)-27 in pleural effusions and to evaluate the diagnostic significance of pleural IL-27. The concentrations of IL-27 were determined in pleural fluids and sera from 68 patients with tuberculous pleural effusion, 63 malignant pleural effusion, 22 infectious pleural effusion, and 21 transudative pleural effusion. Flow cytometry was used to identify which pleural cell types expressed IL-27. It was found that the concentrations of pleural IL-27 in tuberculous group were significantly higher than those in malignant, infectious, and transudative groups, respectively. Pleural CD4(+) T cells, CD8(+) T cells, NK cells, NKT cells, B cells, monocytes, macrophages, and mesothelial cells might be the cell sources for IL-27. IL-27 levels could be used for diagnostic purpose for tuberculous pleural effusion, with the cut off value of 1,007 ng/L, IL-27 had a sensitivity of 92.7% and specificity of 99.1% for differential diagnosing tuberculous pleural effusion from non-tuberculous pleural effusions. Therefore, compared to non-tuberculous pleural effusions, IL-27 appeared to be increased in tuberculous pleural effusion. IL-27 in pleural fluid is a sensitive and specific biomarker for the differential diagnosing tuberculous pleural effusion from pleural effusions with the other causes.  相似文献   
107.
While sulfur dioxide (SO2) has been previously known for its toxicological effects, it is now known to be produced endogenously in mammals from sulfur-containing amino acid l-cysteine. l-cysteine is catalyzed by cysteine dioxygenase (CDO) to l-cysteinesulfinate, which converts to β-sulfinylpyruvate through transamination by aspartate aminotransferase (AAT), and finally spontaneously decomposes to pyruvate and SO2. The present study explored endogenous SO2 production, and AAT and CDO distribution in different rat tissue. SO2 content was highest in stomach, followed by tissues in the right ventricle, left ventricle, cerebral gray matter, pancreas, lung, cerebral white matter, renal medulla, spleen, renal cortex and liver. AAT activity and AAT1 mRNA expression were highest in the left ventricle, while AAT1 protein expression was highest in the right ventricle. AAT2 and CDO mRNA expressions were both highest in liver tissue. AAT2 protein expression was highest in the renal medulla, but CDO protein expression was highest in liver tissue. In all tissues, AAT1 and AAT2 were mainly distributed in the cytoplasm rather than the nucleus. These observed differences among tissues endogenously generating SO2 and associated enzymes are important in implicating the discovery of SO2 as a novel endogenous signaling molecule.  相似文献   
108.
Song Y  Du B  Zhou T  Han B  Yu F  Yang R  Hu X  Ni Y  Li Q 《Carbohydrate research》2011,(2):958-310
In this work, response surface methodology was used to determine optimum conditions for extraction of polysaccharides from defatted peanut cake. A central composite design including independent variables, such as extraction temperature (x1), extraction time (x2), and ethanol concentration (x3) was used. Selected response which evaluates the extraction process was polysaccharide yield, and the second-order model obtained for polysaccharide yield revealed coefficient of determination of 97.81%. The independent variable with the largest effect on response was ethanol concentration (x3). The optimum extraction conditions were found to be extraction temperature 48.7 °C, extraction time 1.52 h, and ethanol concentration of 61.9% (v/v), respectively. Under these conditions, the extraction efficiency of polysaccharide can increase to 25.89%. The results of structural analysis showed that the main composition of defatted peanut cake polysaccharide was α-galactose.  相似文献   
109.
冀东地区西伯利亚白刺开发利用价值研究   总被引:1,自引:0,他引:1  
西伯利亚白刺是华北沿海地区仅有的一个白剌种,在冀东滨海地区有一定的野生群落分布。研究结果表明,西伯利亚白刺具有极强的耐盐能力,0—50cm土体平均全盐含量最高达到1.97%,极端最高值达到3.99%。其果汁含有非常丰富的维生素C,高达444.07mg/100g,仅次于酸枣,排在各类蔬菜、水果的第二位,是开发功能饮料的上好原料。果实成熟后的嫩茎叶含粗蛋白28.80%,粗纤维8.86%,粗脂肪2.24%,是优质的饲料。种子脂肪含量17.2%,其中油酸含量25.02%,亚油酸含量69.02%,是生产优质保健食用油的好原料。所以,西伯利亚白刺是开发利用重盐碱地和沿海滩涂的优良植物,极具开发价值。  相似文献   
110.
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