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   4504篇
  免费   241篇
  国内免费   270篇
  2023年   68篇
  2022年   74篇
  2021年   104篇
  2020年   126篇
  2019年   138篇
  2018年   91篇
  2017年   107篇
  2016年   115篇
  2015年   128篇
  2014年   216篇
  2013年   301篇
  2012年   178篇
  2011年   224篇
  2010年   153篇
  2009年   170篇
  2008年   223篇
  2007年   222篇
  2006年   202篇
  2005年   179篇
  2004年   177篇
  2003年   155篇
  2002年   129篇
  2001年   124篇
  2000年   114篇
  1999年   121篇
  1998年   77篇
  1997年   70篇
  1996年   73篇
  1995年   81篇
  1994年   75篇
  1993年   63篇
  1992年   54篇
  1991年   51篇
  1990年   39篇
  1989年   44篇
  1988年   42篇
  1987年   43篇
  1986年   50篇
  1985年   51篇
  1984年   33篇
  1983年   35篇
  1982年   44篇
  1981年   25篇
  1980年   43篇
  1979年   31篇
  1978年   34篇
  1977年   20篇
  1976年   18篇
  1975年   24篇
  1972年   16篇
排序方式: 共有5015条查询结果,搜索用时 15 毫秒
991.
Kunduru is an important Anatolian landrace having peculiar traits that are appreciated by farmers and breeders. 33 accessions known as Kunduru collected by ICARDA from six geographical provinces of Turkey, were used to study the phenotypic and genotypic intra-diversity. Kunduru landraces exhibited high intra-diversity for most of the studied morphological traits. GPC (12.10–14.90%), vitreousness (75–100%), TKW (31.80–56.70 g), YP (4.70–8.00 ppm), b*-value (14.30–19.50), ash content (1.60–2.0%) and gluten strength (14–60 ml) showed marked variations. Gliadin and glutenin banding patterns showed high polymorphism. 65 alleles were detected with 14 SSR markers, giving a mean of 6.77 alleles per locus. The average PIC value was 0.44 and ranged from 0.11 to 0.70. The average genetic distance between pairs of landraces was 0.47 and ranged between 0.11 and 0.72. This study showed that Kunduru landraces maintains high allelic variation. PCoA indicated that eco-geographical variables have a significant effect on SSR diversity as well as morphological traits. Many of the landraces studied are in danger of disappearing from the local farmers' fields; this study demonstrates the importance of maintaining and conserving this precious genetic resources.  相似文献   
992.
The benzomorphan scaffold has great potential as lead structure and the nature of the N-substituent is able to influence affinity, potency, and efficacy at all three opioid receptors. Building upon these considerations, we synthesized a new series of LP1 analogues by introducing naphthyl or heteroaromatic rings in propanamide side chain of its N-substituent (915). In vitro competition-binding assays in HEK293 cells stably expressing MOR, DOR or KOR showed that in compound 9 the 1-naphthyl ring led to the retention of MOR affinity (KiMOR = 38 ± 4 nM) displaying good selectivity versus DOR and KOR. In the electrically stimulated GPI, compound 9 was inactive as agonist but produced an antagonist potency value (pA2) of 8.6 in presence of MOR agonist DAMGO. Moreover, subcutaneously administered it antagonized the antinociceptive effects of morphine with an AD50 = 2.0 mg/kg in mouse-tail flick test. Modeling studies on MOR revealed that compound 9 fit very well in the binding pocket but in a different way in respect to the agonist LP1. Probably the replacement of its N-substituent on the III, IV and V TM domains reflects an antagonist behavior. Therefore, compound 9 could represent a potential lead to further develop antagonists as valid therapeutic agents and useful pharmacological tools to study opioid receptor function.  相似文献   
993.
Heart failure with preserved ejection fraction (HFpEF) is a growing healthcare burden worldwide and its prevalence is increasing. Diagnosing HFpEF is challenging and relies upon the presence of symptoms and/or signs of heart failure, preserved left ventricular systolic function, and evidence of diastolic dysfunction. Current diagnostic algorithms mainly rely on echocardiography (E/e’) and biomarkers (NT-proBNP). However, only a minority of patients with HFpEF are identified, and especially HFpEF patients at an early stage of the disease are easily missed. We propose to incorporate invasive stress testing, by means of right heart catheterisation at rest and during exercise, and accurate assessment of right ventricular function, by means of cardiac magnetic resonance imaging. These additions to the current diagnostic work-up will improve diagnostic sensitivity and accurate staging of HFpEF patients.  相似文献   
994.
WONG  M. Y. 《Biometrika》1989,76(1):141-148
  相似文献   
995.
.Using fixed sporozoites in a 3-layer immunofluorescence assay (TLIFA), class-specific, parasite-specific antibody responses in chicks to single-pulse infection with Eimeria tenella have been studied in gut contents and bile as well as plasma and feces. After infection with 103 oocysts, IgA antibody was first detected in the duodenal lumen, then in bile, plasma, cecum, and the distal small intestine. The kinetics of the bile IgA response correlated with that in plasma and peaked 9 days post-infection (d.p.i.); IgM was detected in gut contents and bile as well as plasma, and IgG was occasionally detected in gut contents, especially in the duodenum. In some experiments, IgA was detected in gut contents and bile to at least 21 d.p.i. Infection with 103 oocysts resulted in an earlier and increased response and relatively high IgG titers in cecal contents. Coproantibody was detected inconsistently and at low titer. When sporozoites that excysted in vitro were incubated in specific, antibody-positive (9 d.p.i.) cecal contents, some complement-mediated IgG-associated anti-sporozoite effects were observed; however, the major effect of cecal contents and the only effect of bile was a non-lethal agglutination of living sporozoites. By fractionation of cecal contents and imtnunoblotting this was confirmed to be IgA mediated; IgA antibodies in cecal contents and bile after infection were shown to bind to sporozoite membrane antigens by surface fluorescence as well as agglutination. Agglutination detected anti-sporozoite antibody in gut contents and bile up to 21 d.p.i., peaking between 7 and 13 d.p.i., corresponding with TLIFA results. The immunofluorescence studies reveal the extremely labile nature of the sporozoite surface antigens and support the hypothesis that naturally elaborated IgA antibodies are involved in the modulation of complexed sporozoite surface antigens.  相似文献   
996.
Spatial pattern and ecological analysis   总被引:65,自引:0,他引:65  
  相似文献   
997.
杭州湾、钱塘江流域沿岸外环境的肉毒梭菌分布很广。调查22个点检出阳性21个点。外环境各种样品的总检出率为75.95%,上海地区的检出率最低(29.41%),其它地区间的检出率无显著差异。绍兴市区生产的酱豆面制品肉毒梭菌的检出率为20%。检出的所有菌株几乎均为无毒株,其中2株弱毒株其毒力弱到无法定型。生化试验结果显示,被试大多数菌株的生化特性与A、B、F型的PL株相似,但有13株菌株的个别生化反应与试验菌株的总体不同。  相似文献   
998.
More extensive genetic tests have been performed on a series of 832 X-ray-induced specific-locus mutations in the ad-3 region of a 2-component heterokaryon (H-12) of Neurospora crassa, reported earlier (Webber and de Serres 1965). Using a new tester strains and techniques for performing large-scale genetic tests (heterokaryon, dikaryon and trikaryon) to characterize ad-3 mutants induced in 2-component heterokaryons, new data have been obtained on this sample of X-ray-induced ad-3 mutants. These new data show that unexpectedly high frequencies of both single-locus (gene/point) mutations and multilocus deletions in the ad-3 region have additional, but separate, sites of resessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory and classical models of chromosome structure during interphase. Current models of interphase chromosome structure in higher eukaryotes as revealed by chromosome “painting” offer a possible explanation of the Neurospora data.  相似文献   
999.
1000.
Summary In a pot experiment with soils of Alfisol, Entisol, and Inceptisol orders, the relative yield of Egyptian clover (Trifolium alexandrinum L.) was significantly correlated with Morgan's reagent (N NaOAc+HOAc, pH 4.8)—extractable soil S (r=0.88), plant S (r=0.82), and plant N/S ratio (r=−0.77) suggesting suitability of these tests for diagnosing S deficiency. Total plant S lower than 0.21 per cent, plant N/S ratio wider than 17, and extractable soil S lower than 10 ppm were indicative of S deficiency, and were suggested therefore to be critical limits for these tests. Nitrogen and S in plant proteins were in near constant ratio of 16 and were significantly correlated (r=0.99). Sixty one per cent of 250 surface soil samples had less than 10 ppm extractable S and hence were deficient in S, suggesting a widespread S deficiency in soils under study. Extractable soil S in all soil series was significantly correlated with electrical conductivity and alkaline KMnO4-extractable N, but not with pH, organic C, and CaCO3.  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