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   1468篇
  免费   86篇
  国内免费   8篇
  2024年   1篇
  2023年   10篇
  2022年   23篇
  2021年   46篇
  2020年   25篇
  2019年   32篇
  2018年   50篇
  2017年   40篇
  2016年   55篇
  2015年   83篇
  2014年   106篇
  2013年   94篇
  2012年   137篇
  2011年   120篇
  2010年   91篇
  2009年   63篇
  2008年   76篇
  2007年   80篇
  2006年   70篇
  2005年   65篇
  2004年   41篇
  2003年   40篇
  2002年   36篇
  2001年   28篇
  2000年   36篇
  1999年   19篇
  1998年   12篇
  1997年   9篇
  1996年   14篇
  1995年   5篇
  1994年   2篇
  1992年   6篇
  1991年   7篇
  1990年   6篇
  1989年   3篇
  1988年   6篇
  1987年   3篇
  1986年   1篇
  1984年   2篇
  1983年   2篇
  1981年   2篇
  1980年   3篇
  1977年   2篇
  1975年   2篇
  1974年   2篇
  1973年   1篇
  1972年   1篇
  1968年   2篇
  1922年   1篇
  1899年   1篇
排序方式: 共有1562条查询结果,搜索用时 31 毫秒
951.
952.
  相似文献   
953.
An improved laboratory bioassay was used to characterize an oviposition stimulant from the surface of sweet potato Ipomoea batatas (L.) Lam. storage roots for the sweetpotato weevil, Cylas formicarius elegantulus (Summers). Filter paper discs impregnated with a methylene chloride surface extract of sweet potato storage roots induced significantly (p<0.05) higher oviposition on root cores than those treated with solvent only. Significantly higher oviposition was also observed in the nonpolar fractions, especially one that contains a tentatively identified triterpenoid present in susceptible cultivars.
Caractérisation d'une substance extraite de la surface des racines tubéreuses d'Ipomoea batatas induisant la ponte de Cylas formicarius elegantulus
Résumé La caractérisation des substances extraites de la surface des racines tubéreuses d'I. batatas Lam et stimulant la ponte de C. formicarious elegantulus Summers, a été effectuée à partir d'une technique améliorée. Différents substrats ont été essayés: extraits au chlorure de méthylène ou péridermes intacts de cultivars résistants ou sensibles, racines tubéreuses après élimination du périderme, morceaux de Solanum tuberosum ou de papier filtre présentés au milieu de plaques à 24 plots pour culture de tissus. Le meilleur substrat s'est révélé être de petits disques de papier filtre (diamètre 0,4 cm), fixés au milieu d'un morceau de racine avec encore un peu de périderme sur lequel le coléoptère pouvait pondre. Cette méthode a été utilisée dans les tests ultérieurs pour caractériser la nature du stimulant.Des disques de papier filtre inhibés d'extraits au chlorure de méthylène du contenu des structures superficielles de racines tubéreuses de cultivars sensibles induisent une ponte significativement plus importante (p<0,05) sur les morceaux de racines par comparaison avec ceux traités au chlorure de méthylène pur. Une ponte significativement plus importante a aussi été obtenue avec des fractions non-polaires, particulièrement celles qui contiennent un triterpènoïde de cultivars dont l'identification a été tentée.
  相似文献   
954.
Abstract

Surveys were conducted between the years of 2005 and 2006 at several locations in the northern, central and southern parts of West Malaysia to study the polymorphism of Ralstonia solanacearum strains. These sites included vegetables and farms with known hosts of the pathogen, such as banana, tomato, eggplant, chili and tobacco. Samples were collected from the suspected wilted plants and weeds, including soil and water samples, in selected areas. The bacterium was isolated in all samples using semi-selective tetrazolium chloride medium (TZC). The bacteria strains were detected by using the BIOLOG identification system and were confirmed by nested-PCR. Fatty Acid Methyl Esters (FAME) profiling was performed to determine polymorphism among 58 bacterial isolates. The results showed that the fatty acid composition varied for all R. solanacearum isolates. Grouping of R. solanacearum isolates by fatty acid composition suggested that the existence of distinct groups that were significantly related to host of bacteria but low correlation between fatty acid profiles and biovar or sampling site was detected. A unique FAME profile was found among the strains that have been isolated from banana.  相似文献   
955.
Salmonella enterica is one of the most important bacterial enteric pathogens worldwide. However, little is known about its distribution and diversity in the environment. The present study explored the diversity of 104 strains of Salmonella enterica isolated over 2 years from 12 coastal waterways in central California. Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing were used to probe species diversity. Seventy-four PFGE patterns and 38 sequence types (STs) were found, including 18 newly described STs. Nineteen of 25 PFGE patterns were indistinguishable from those of clinical isolates in PulseNet. The most common ST was consistent with S. enterica serovar Typhimurium, and other frequently detected STs were associated with the serovars Heidelberg and Enteritidis; all of these serovars are important etiologies of salmonellosis. An investigation into S. enterica biogeography was conducted at the level of ST and subspecies. At the ST and subspecies level, we found a taxon-time relationship but no taxon-area or taxon-environmental distance relationships. STs collected during wet versus dry conditions tended to be more similar; however, STs collected from waterways adjacent to watersheds with similar land covers did not tend to be similar. The results suggest that the lack of dispersal limitation may be an important factor affecting the diversity of S. enterica in the region.  相似文献   
956.
TGFBI, a transforming growth factor β-induced extracellular matrix protein, circulates at a level of ~ 300 ng/ml in humans and modulates several integrin-mediated cellular functions. The protein contains an N-terminal EMI domain, four consecutive FAS1 domains, and the RGD motif. Each FAS1 domain and the RGD motif have been known to interact with avb3 integrin. Here, we found that the binding affinity (Kd) of TGFBI for αvβ3 integrin was approximately 3.8 × 10? 8 M, a value ~ 2300-fold higher than that of a single FAS1 domain, and demonstrated that this greater affinity was due to the cooperative action of the four FAS1 domains and the RGD motif. Moreover, TGFBI exhibited more potent anti-angiogenic and anti-tumorigenic activities, even at a 100-fold lower molar dose than the reported effective dose of the FAS1 domain. Finally, our data showed that TGFBI specifically targeted the tumor vasculature and accumulated at the tumor site. Collectively, our results support the theory that TGFBI acts as a potent endogenous anti-tumor and anti-angiogenic molecule by targeting αvβ3 integrin, and highlights the importance of physiological circulating TGFBI levels in inhibiting tumor growth.  相似文献   
957.
A catalytic single chain variable antibody (scFv), 3D8 scFv, which has DNase activities, was functionally expressed in Nicotiana tabacum. The subcellular localization of the GFP-fused 3D8 indicated that the 3D8 protein was expressed in the cytosol of the N. tabacum protoplasts. Progenies of the transgenic tobacco plants exhibited complete resistance against two single stranded (ss) DNA geminiviruses, including the Beet curly top virus and the Beet severe curly top virus, without viral accumulation or disease symptoms. We presented a novel strategy for targeting the viral DNA itself in a sequence non-specific manner, rather than the viral proteins or RNAs, in order to generate virus-resistant transgenic plants. No noticeable adverse effects on the growth and reproduction of the transgenic plants were observed. Our results demonstrated that targeting viral DNA is an effective strategy for protecting plants from ssDNA viruses.  相似文献   
958.
To investigate the factors associated with compliance with recommendations regarding liver cancer screening intervals and methods among individuals at high-risk for liver cancer in the Republic of Korea. We used data from the fourth Korean National Health and Nutrition Examination Survey (KNHANES IV), a representative cross-sectional nationwide survey conducted between 2007 and 2009. The liver cancer screening rate and factors associated with compliance with recommended screening intervals (6 months) and methods (both abdominal ultrasonography and serum alpha-fetoprotein testing) among individuals at high risk for liver cancer such as hepatitis B virus (HBV) carriers were investigated. Out of 24,871 KNHANES IV participants, 604 HBV carriers aged ≥20 years were included in our analysis. 39.6% of our study sample reported attending liver cancer screening at least once in their lifetime, 12.3% had attended within the previous 6 months, and 14.6% were screened using both recommended methods. Older age was associated with increased compliance with screening intervals (P-trend 0.011) and methods (40–49 year: OR = 3.25, 95% CI: 1.62–6.51; 50–59 years: OR = 3.09, 95% CI: 1.44–6.66; 60–69 years: OR = 3.17, 95% CI: 1.28–7.82). Unawareness of HBV infection status was negatively related to compliance with screening intervals and methods (OR = 0.30, 95% CI: 0.17–0.53; OR = 0.45, 95% CI: 0.26–0.79). Female sex (OR = 0.45, 95% CI: 0.25–0.78), lower household income (P-trend 0.011), and routine and manual occupations (OR = 0.46, 95% CI: 0.22–0.97) were associated with decreased compliance with screening methods. The liver cancer screening rate among high-risk individuals is much less suboptimal. Considering that those unaware of their HBV infection status got regular and complete liver cancer screening much less often, efforts should be made not only to decrease sociodemographic disparities, but also to better identify the high-risk population.  相似文献   
959.
Modifiable lifestyle-related factors such as smoking and alcohol drinking are associated with cognitive impairment in the elderly population but the relationships have shown various results. To evaluate the relationship of alcohol drinking and smoking in the early 60 s with the risk of developing incident cognitive impairment. In 1999, we evaluated cognitive function, smoking, and drinking status in 3,174 inhabitants aged 60–64 years in a rural area of Korea, with a follow-up assessment of cognitive function 7 years later. A total of 1,810 individuals who did not show cognitive impairment at baseline were included. A stratified analysis was applied to evaluate how smoking and alcohol drinking affected the risk of developing cognitive impairment based on gender. Current smokers showed a higher risk for developing cognitive impairment than did never smokers (odds ratio [OR], 1.53; 95% confidence interval [CI], 1.09–2.15). The OR for female current smokers compared with never smokers was 1.62 (95% CI, 1.05–2.52), and smokers with higher pack-years were more likely to develop cognitive impairment than never smokers, showing a dose–response relationship (P for trend = 0.004). Frequent alcohol consumption increased the risk of developing cognitive impairment (OR, 1.68; 95% CI, 1.01–2.78), and a dose–response relationship was observed among male subjects (P for trend = 0.044). Infrequent drinking in females decreased the odds of developing cognitive impairment (OR, 0.67; 95% CI, 0.42–1.00), whereas frequent drinking tended to increase the odds, although this trend was not significant, suggesting a U-shaped relationship. Although the sample was small for some analyses, especially in female, our data suggest that smoking and drinking in the early 60 s are associated with a risk of developing cognitive impairment, and this relationship is characterized by gender differences.  相似文献   
960.

Aim

The aims of the current study were to assess the validity of the fecal occult blood test (FOBT) in an organized screening setting in Korea and to determine factors associated with FOBT validity, such as screening round, age group, and anatomical location of the cancer.

Methods

Study participants were those who were 50 years and older who received an FOBT through the National Cancer Screening Program between 2004 and 2007. Colorectal cancer diagnoses were ascertained through linkage with the Korean National Cancer Incidence Database. The positivity rate, colorectal cancer detection rate, interval cancer rate, sensitivity, specificity, and positive predictive value of the FOBT were calculated.

Results

A total of 2,193,093 tests were included in the analysis. Overall, the sensitivity of the FOBT for colorectal cancer was 59.7% for the first round and 56.1% for the subsequent round. Sensitivity was highest for distal colon cancer (65.9%) in the first round, and for rectal cancer (58.4%) for the subsequent round. The sensitivity and positive predictive value of the FOBT generally improved between 2004 and 2008.

Conclusions

The FOBT showed reasonable validity in an organized screening setting, and the validity of the FOBT varied by screening round, anatomical location, and screening year.  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