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   3017篇
  免费   166篇
  国内免费   3篇
  3186篇
  2023年   8篇
  2022年   16篇
  2021年   32篇
  2020年   34篇
  2019年   32篇
  2018年   64篇
  2017年   61篇
  2016年   87篇
  2015年   128篇
  2014年   142篇
  2013年   183篇
  2012年   209篇
  2011年   214篇
  2010年   136篇
  2009年   121篇
  2008年   199篇
  2007年   225篇
  2006年   179篇
  2005年   178篇
  2004年   192篇
  2003年   186篇
  2002年   164篇
  2001年   31篇
  2000年   18篇
  1999年   23篇
  1998年   34篇
  1997年   23篇
  1996年   20篇
  1995年   11篇
  1994年   12篇
  1993年   13篇
  1992年   15篇
  1991年   7篇
  1990年   11篇
  1989年   11篇
  1988年   9篇
  1987年   9篇
  1985年   16篇
  1984年   11篇
  1983年   10篇
  1982年   15篇
  1981年   10篇
  1980年   8篇
  1979年   8篇
  1977年   17篇
  1975年   5篇
  1974年   5篇
  1973年   6篇
  1970年   6篇
  1967年   4篇
排序方式: 共有3186条查询结果,搜索用时 0 毫秒
101.
Leptosphaeria maculans and L. biglobosa are closely related sibling fungal pathogens that cause phoma leaf spotting, stem canker (blackleg) and stem necrosis of oilseed rape (Brassica napus). The disease is distributed worldwide, and it is one of the main causes of considerable decrease in seed yield and quality. Information about the time of ascospore release at a particular location provides important data for decision making in plant protection, thereby enabling fungicides to be used only when necessary and at optimal times and doses. Although the pathogens have been studied very extensively, the effect of climate change on the frequencies and distributions of their aerially dispersed primary inoculum has not been reported to date. We have collected a large dataset of spore counts from Poznan, located in central-west part of Poland, and studied the relationships between climate and the daily concentrations of airborne propagules over a period of 17 years (1998–2014). The average air temperature and precipitation for the time of development of pseudothecia and ascospore release (July–November), increased during the years under study at the rates of 0.1 °C and 6.3 mm per year. The day of the year (DOY) for the first detection of spores, as well as the date with maximum of spores, shifted from 270 to 248 DOY, and from 315 to 265 DOY, respectively. The acceleration of the former parameter by 22 days and the latter by 50 days has great influence on the severity of stem canker of oilseed rape.  相似文献   
102.
A total of 45 racing pigeons were genotyped using PCR-RFLP method. PCR product of the LDH-A gene was amplified according to the Long-PCR procedure. The amplification products were digested with restriction enzymes. PCR-RFLPs for two restriction enzymes, HaeIII and NlaIV, were observed. Two pairs of alleles LDH-A(A) and LDH-A(B) for LDH-A-NlaIV polymorphism and LDH-A(C) and LDH-A(D) for LDH-A-HaeIII polymorphism were detected in the homozygous and heterozygous states. Frequencies of alleles were as follows: A - 0.622, B - 0.378 and C - 0.256, D - 0.744.  相似文献   
103.
Summary Differences have been noted in metabolic activity of bacteria isolated from water, mud and from Canadian pondweed (Elodea canadensis).The epiphytic bacteria showed the greatest metabolic activity in every instance, benthic bacteria were least active and the water isolates were intermediate.Non-chromogenic bacteria were metabolically more active than the chromogens.Casamino acids proved to be the most readily oxidized substrate among the compounds studied. Glucose, succinate, fumarate and gluconate were readily used by most epiphytic and planktonic bacteria. Acetate was intermediate and fructose was the least suitable substrate for these organisms. Casamino acids proved to be the best substrate for all three groups, although the benthic bacteria were less active with this substrate than the former two groups of bacteria. Gluconate, succinate, pyruvate and acetate were intermediate, and glucose and fructose were least suitable for most of the benthic bacteria.  相似文献   
104.
Kraft pulps, prepared from softwoods, and small chips of birch wood were treated with heme and tert-butyl hydroperoxide in aqueous solutions at reflux temperature. Analyses of treated pulps showed decreases in kappa number (a measure of lignin content) from about 36 to less than 2, with concomitant increases in brightness (80% increase in the better samples). Analyses of treated wood chips revealed selective delignification and removal of hemicelluloses. After 48 h of treatment, lignin losses from the wood chips approached 40%, and xylose/mannose (hemicellulose) losses approached 70%, while glucose (cellulose) losses were less than 10%. Examination of delignified chips by transmission electron microscopy showed that the removal of lignin occurred in a manner virtually indistinguishable from that seen after decay by white rot fungi. Various metalloporphyrins, which act as biomimetic catalysts, were compared to horseradish peroxidase and fungal manganese peroxidase in their abilities to oxidize syringaldazine in an organic solvent, dioxane. The metalloporphyrins and peroxidases behaved similarly, and it appeared that the activities of the peroxidases resulted from the extraction of heme into the organic phase, rather than from the activities of the enzymes themselves. We concluded that heme-tert-butyl hydroperoxide systems in the absence of a protein carrier mimic the decay of lignified tissues by white rot fungi.  相似文献   
105.

Background

The relationship between lower urinary tract symptoms (LUTS) and common mental health disorders such as depression and anxiety in men remains unclear. Inflammation has recently been identified as an independent risk factor for LUTS and depression. This study aimed to assess the association between depression, anxiety and LUTS, and the moderating influence of systemic inflammation, in the presence of other biopsychosocial confounders.

Methods

Participants were randomly-selected from urban, community-dwelling males aged 35–80 years at recruitment (n = 1195; sample response rate:67.8%). Of these, 730 men who attended baseline (2002–5) and follow-up clinic visits (2007–10), with complete outcome measures, and without prostate or bladder cancer and/or surgery, neurodegenerative conditions, or antipsychotic medications use, were selected for the present study. Unadjusted and multi-adjusted regression models of incident storage and voiding LUTS and incident depression and anxiety were combined with serum inflammatory markers (high-sensitive C-reactive protein (hsCRP), tumor necrosis factor-alpha (TNF-α), interleukin–6 (IL–6), myeloperoxidase (MPO), soluble e-selectin (e-Sel)) and socio-demographic, lifestyle, and health-related factors. Hierarchical multiple regression was used to assessed the moderating effect of inflammatory markers.

Results

The incidence of storage, voiding LUTS, depression and anxiety was 16.3% (n = 108), 12.1% (n = 88), 14.5% (n = 108), and 12.2% (n = 107). Regression models demonstrated that men with depression and anxiety at baseline were more likely to have incident storage, but not voiding LUTS (OR: 1.26, 99%CI: 1.01–4.02; and OR:1.74; 99%CI:1.05–2.21, respectively). Men with anxiety and storage LUTS at baseline were more likely to have incident depression (OR: 2.77, 99%CI: 1.65–7.89; and OR:1.45; 99%CI:1.05–2.36, respectively), while men with depression and voiding LUTS were more likely to have anxiety at follow-up (OR: 5.06, 99%CI: 2.81–9.11; and OR:2.40; 99%CI:1.16–4.98, respectively). CRP, TNF-α, and e-Sel were found to have significant moderating effects on the development of storage LUTS (1.06, 0.91–1.96, R2 change: 12.7%), depression (1.17, 1.01–1.54, R2 change: 9.8%), and anxiety (1.35, 1.03–1.76, R2 change: 10.6%), respectively.

Conclusions

There is a bidirectional relationship between storage, but not voiding, LUTS and both depression and anxiety. We observed variable moderation effects for selected inflammatory markers on the development of depression, anxiety and storage LUTS.  相似文献   
106.
The cytotoxic T lymphocyte antigen-4 (CTLA-4)-blocking antibody ipilimumab induces immune-mediated long-term control of metastatic melanoma in a fraction of patients. Although ipilimumab undoubtedly exerts its therapeutic effects via immunostimulation, thus far clinically useful, immunologically relevant biomarkers that predict treatment efficiency have been elusive. Here, we show that neutralization of IL-2 or blocking the α and β subunits of the IL-2 receptor (CD25 and CD122, respectively) abolished the antitumor effects and the accompanying improvement of the ratio of intratumoral T effector versus regulatory cells (Tregs), which were otherwise induced by CTLA-4 blockade in preclinical mouse models. CTLA-4 blockade led to the reduction of a suppressive CD4+ T cell subset expressing Lag3, ICOS, IL-10 and Egr2 with a concomitant rise in IL-2-producing effector cells that lost FoxP3 expression and accumulated in regressing tumors. While recombinant IL-2 improved the therapeutic efficacy of CTLA-4 blockade, the decoy IL-2 receptor α (IL-2Rα, sCD25) inhibited the anticancer effects of CTLA-4 blockade. In 262 metastatic melanoma patients receiving ipilimumab, baseline serum concentrations of sCD25 represented an independent indicator of overall survival, with high levels predicting resistance to therapy. Altogether, these results unravel a role for IL-2 and IL-2 receptors in the anticancer activity of CTLA-4 blockade. Importantly, our study provides the first immunologically relevant biomarker, namely elevated serum sCD25, that predicts resistance to CTLA-4 blockade in patients with melanoma.  相似文献   
107.
Mass spectrometry and a time-course cell lysis method were used to study proteins involved in perchlorate and chlorate metabolism in pure bacterial cultures and environmental samples. The bacterial cultures used included Dechlorosoma sp. KJ, Dechloromonas hortensis, Pseudomonas chloritidismutans ASK-1, and Pseudomonas stutzeri. The environmental samples included an anaerobic sludge enrichment culture from a sewage treatment plant, a sample of a biomass-covered activated carbon matrix from a bioreactor used for treating perchlorate-contaminated drinking water, and a waste water effluent sample from a paper mill. The approach focused on detection of perchlorate (and chlorate) reductase and chlorite dismutase proteins, which are the two central enzymes in the perchlorate (or chlorate) reduction pathways. In addition, acetate-metabolizing enzymes in pure bacterial samples and housekeeping proteins from perchlorate (or chlorate)-reducing microorganisms in environmental samples were also identified.  相似文献   
108.
INTRODUCTION, MATERIAL AND METHODS: Visfatin is a cytokine, mainly expressed in visceral fat, that exerts insulin-mimicking effects in rodents through activation of an insulin receptor, although the binding-site is distinct from that of insulin. However, the mechanisms that regulate visfatin synthesis are still not fully understood. In particular, it is not clear whether short-term glucose-induced hyperglycaemia and hyperinsulinaemia as well as a glucocorticoid-induced increase in insulin resistance are reflected in appreciable alterations in serum visfatin levels in humans. In order to investigate this we measured serum visfatin, glucose and insulin concentrations during a 75.0 gram oral glucose tolerance test (OGTT) [Study 1], as well as before and after oral administration of dexamethasone [Study 2]. Study 1 included 17 subjects (2 males), aged 35.7 +/- 15.6 (mean +/- SD) years of BMI 35.2 +/- 9.3 kg/m(2). Blood samples were taken before (0 minutes) and at 60 and 120 minutes after glucose administration. Study 2 included 20 subjects (4 males, 5 subjects with type 2 diabetes), aged 42.1 +/- 17.2 years of BMI 36.7 +/- 8.38 kg/m(2) who underwent screening for Cushing's disease/syndrome. Dexamethasone was administered at a dose of 0.5 mg every 6 hours for 48 hours. Fasting serum concentrations of visfatin, glucose and insulin were assessed before (D0) and after 48 hours of dexamethasone administration (D2). Insulin resistance was assessed according to the HOMA method in non-diabetic individuals (n = 15). RESULTS: In Study 1 two subjects were found to have impaired glucose tolerance and one subject was found to have diabetes mellitus. Glucose administration resulted in a highly significant increase in insulin (from 11.4 +/- 7.2 microU/mL at 0 min to 98.9 +/- 68.6 microU/mL at 60 min and 72.6 +/- 45.1 microU/mL at 120 minute of OGTT, p < 0.001 for 60 and 120 minutes in comparison to baseline). However, there was no change in serum visfatin concentrations (84.6 +/- 11.6 ng/mL at 0 minutes, 82.6 +/- 12.7 ng/mL at 60 minutes and 81.1 +/- 14.5 ng/mL at 120 minutes of OGTT, p = ns). All subjects in Study 2 achieved suppression of cortisol concentrations below 50 nmo/l. Dexamethasone administration resulted in an increase in fasting insulin (from 11.5 +/- 6.9 to 16.9 +/- 7.6 microU/mL; p = 0.011) and an increase in HOMA (from 2.73 +/- 1.74 to 4.02 +/- 2.27; p = 0.015), albeit without a significant change in serum visfatin concentrations (61.1 +/- 19.8 vs. 68.3 +/- 19.4 ng/mL, p = ns). In neither Study 1 nor Study 2 was there any significant correlation between serum visfatin and age, BMI or HOMA. CONCLUSIONS: There is a striking difference between the marked rise in insulin concentrations and the lack of change in visfatin concentrations during the oral glucose tolerance test. This implies that it is highly unlikely that visfatin is involved in the short-term regulation of glucose homeostasis in human subjects. Dexamethasone administration (4 mg/48 hours) induces an increase in insulin resistance, although without significant change in serum visfatin concentrations. Therefore in contrast to the in vitro data, short term glucocorticoid administration does not result in appreciable changes in serum levels of this adipocytokine. Furthermore, the results of our study do not support the notion that glucocorticoid-induced insulin resistance is likely to be related to changes in serum concentrations of visfatin.  相似文献   
109.
Disturbance is claimed to be one of the most important triggers of biological invasions. There is a lack of data about disturbance impacts on the youngest life stage of invasive trees and shrubs. Thus, we aimed to assess the role of disturbances in shaping responses of natural regeneration of three model invasive species—Prunus serotina Ehrh., Quercus rubra L., and Robinia pseudoacacia L.—to disturbances in forest plant communities. Our study was conducted over 3 years on 372 study plots (100 m2), across nine types of temperate forests in Wielkopolski National Park (Poland). Disturbance was assessed using ecological indicator values for disturbance severity and frequency. Our study revealed the high importance of disturbance on species composition of understory vegetation. We also found relationships between ecological success (density and biomass of natural regeneration) of invaders and disturbance indices. These models were statistically significant but their effect sizes were low. Due to the low effect sizes, we can state only limited conclusions about impact of disturbance on ecological success of invasive species natural regeneration. The results suggest that for seedlings (up to 50 cm height—threshold between understory and shrub layer) disturbance, a leading factor in biological invasions of numerous taxa, has a small role in this case. Thus, we may assume that their ecological success is connected with stochastic processes in populations of the invader’s seedlings, rather than with stochastic release from competition caused by disturbances.  相似文献   
110.
Abstract

The cytotoxicity of anti-PAI-5 hexadecanucleotides (phosphodiesters and phosphorothioates) and their conjugates with lipophilic alcohols was tested in EA.hy 926 hybrid endothelial cells. Some cytotoxicity was found for cholesteryl and bornyl conjugates at concentrations higher than those used for antisense inhibition experiments.  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