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   44873篇
  免费   2896篇
  国内免费   15篇
  2023年   219篇
  2022年   582篇
  2021年   1024篇
  2020年   606篇
  2019年   782篇
  2018年   1102篇
  2017年   920篇
  2016年   1522篇
  2015年   2311篇
  2014年   2516篇
  2013年   3421篇
  2012年   3902篇
  2011年   3758篇
  2010年   2273篇
  2009年   2031篇
  2008年   2810篇
  2007年   2715篇
  2006年   2464篇
  2005年   2182篇
  2004年   2099篇
  2003年   1966篇
  2002年   1810篇
  2001年   351篇
  2000年   251篇
  1999年   342篇
  1998年   434篇
  1997年   293篇
  1996年   285篇
  1995年   257篇
  1994年   241篇
  1993年   240篇
  1992年   160篇
  1991年   167篇
  1990年   168篇
  1989年   124篇
  1988年   118篇
  1987年   99篇
  1986年   77篇
  1985年   112篇
  1984年   108篇
  1983年   77篇
  1982年   95篇
  1981年   87篇
  1980年   81篇
  1979年   76篇
  1978年   41篇
  1977年   54篇
  1976年   36篇
  1975年   38篇
  1973年   43篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
973.
The ionome is the elemental composition of a living organism, its tissues, cells or cell compartments. The ionomes of roots, stems and leaves of 14 native Brazilian forest species were characterised to examine the relationships between plant and organ ionomes and the phylogenetic and ecological affiliations of species. The null hypothesis that ionomes of Brazilian forest species and their organs do not differ was tested. Concentrations of mineral nutrients in roots, stems and leaves were determined for 14 Brazilian forest species, representing seven angiosperm orders, grown hydroponically in a complete nutrient solution. The 14 species could be differentiated by their ionomes and the partitioning of mineral nutrients between organs. The ionomic differences between the 14 species did not reflect their phylogenetic relationships or successional ecology. Differences between shoot ionomes and root ionomes were greater than differences in the ionome of an organ when compared among genotypes. In conclusion, differences in ionomes of species and their organs reflect a combination of ancient phylogenetic and recent environmental adaptations.  相似文献   
974.
975.
Cotton (Gossypium hirsutum L.) is a key fiber crop of great commercial importance. Numerous phytopathogens decimate crop production by causing various diseases. During July-August 2018, leaf spot symptoms were recurrently observed on cotton leaves in Rahim Yar Khan, Pakistan and adjacent areas. Infected leaf samples were collected and plated on potato dextrose agar (PDA) media. Causal agent of cotton leaf spot was isolated, characterized and identified as Aspergillus tubingensis based on morphological and microscopic observations. Conclusive identification of pathogen was done on the comparative molecular analysis of CaM and β-tubulin gene sequences. BLAST analysis of both sequenced genes showed 99% similarity with A. tubingensis. Koch’s postulates were followed to confirm the pathogenicity of the isolated fungus. Healthy plants were inoculated with fungus and similar disease symptoms were observed. Fungus was re-isolated and identified to be identical to the inoculated fungus. To our knowledge, this is the first report describing the involvement of A. tubingensis in causing leaf spot disease of cotton in Pakistan and around the world.  相似文献   
976.
High CO2 concentrations stimulate net photosynthesis by increasing CO2 substrate availability for Rubisco, simultaneously suppressing photorespiration. Previously, we reported that silencing the chloroplast vesiculation (cv) gene in rice increased source fitness, through the maintenance of chloroplast stability and the expression of photorespiration-associated genes. Because high atmospheric CO2 conditions diminished photorespiration, we tested whether CV silencing might be a viable strategy to improve the effects of high CO2 on grain yield and N assimilation in rice. Under elevated CO2, OsCV expression was induced, and OsCV was targeted to peroxisomes where it facilitated the removal of OsPEX11-1 from the peroxisome and delivered it to the vacuole for degradation. This process correlated well with the reduction in the number of peroxisomes, the decreased catalase activity and the increased H2O2 content in wild-type plants under elevated CO2. At elevated CO2, CV-silenced rice plants maintained peroxisome proliferation and photorespiration and displayed higher N assimilation than wild-type plants. This was supported by higher activity of enzymes involved in NO3 and NH4+ assimilation and higher total and seed protein contents. Co-immunoprecipitation of OsCV-interacting proteins suggested that, similar to its role in chloroplast protein turnover, OsCV acted as a scaffold, binding peroxisomal proteins.  相似文献   
977.
The aim of the present study was to simultaneously assess several potential predictors of outcome (co-morbidity, previous and in-hospital treatment, radiologic Brixia score) in patients with COVID-19.This retrospective cohort study included 258 consecutive patients with confirmed COVID-19 admitted to a medical ward at Montichiari Hospital, Brescia, Italy from February 28th to April 30rd, 2020. Patients had SARS-CoV-2 related pneumonia with respiratory failure, and were treated with hydroxychloroquine and lopinavir plus ritonavir. In some patients, additional treatment with tocilizumab, dexamethasone and enoxaparin was adopted. Outcomes (death or recovery) were assessed at the end of the discharge period or at the end of the follow-up (August 2020).During hospitalization, 59 patients died, while 6 died after discharge. The following variables were demonstrated to be associated with a worse prognosis: Radiologic Brixia score higher than 8, presence at baseline of hypertension, diabetes, chronic obstructive pulmonary disease, heart disease, cancer, previous treatment with ACE-inhibitors or anti-platelet drugs. Anticoagulant treatment during hospital admission with enoxaparin at a dose higher than 4000 U once daily was associated with a better prognosis.In conclusion, our study demonstrates that some co-morbidities and cardiovascular risk factors may affect prognosis. The radiologic Brixia score may be a useful tool to stratify the risk of death at baseline. Anticoagulant treatment with enoxaparin might be associated to a clinical benefit in terms of survival in patients with COVID-19.  相似文献   
978.
Biological Trace Element Research - Silicon is a trace element found mainly in plant-based food and proposed to be beneficial for bone health. Urinary excretion of Si has been shown to be a...  相似文献   
979.
980.
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