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   2037篇
  免费   121篇
  2158篇
  2022年   16篇
  2021年   25篇
  2020年   9篇
  2019年   15篇
  2018年   27篇
  2017年   22篇
  2016年   47篇
  2015年   82篇
  2014年   72篇
  2013年   102篇
  2012年   138篇
  2011年   117篇
  2010年   90篇
  2009年   79篇
  2008年   133篇
  2007年   115篇
  2006年   121篇
  2005年   113篇
  2004年   105篇
  2003年   110篇
  2002年   90篇
  2001年   48篇
  2000年   22篇
  1999年   38篇
  1998年   32篇
  1997年   22篇
  1996年   23篇
  1995年   25篇
  1994年   23篇
  1993年   27篇
  1992年   22篇
  1991年   30篇
  1990年   16篇
  1989年   10篇
  1988年   20篇
  1987年   16篇
  1986年   14篇
  1985年   15篇
  1984年   8篇
  1983年   10篇
  1982年   11篇
  1981年   13篇
  1980年   4篇
  1979年   10篇
  1978年   11篇
  1977年   7篇
  1976年   11篇
  1973年   4篇
  1968年   4篇
  1959年   3篇
排序方式: 共有2158条查询结果,搜索用时 15 毫秒
1.
Abstract The sequence of the cohesive ends of actinophage RP3 DNA has been determined. As with all other phages of Gram-positive bacteria that have been studied sofar, RP3 DNA has 3'-protruding ends. A shuttle cosmid has been constructed containing this cos area which can be efficiently transduced by phage RP3 to host cells of Streptomyces rimosus .  相似文献   
2.
Although the strain composition of mixed cultures may hugely affect production of various fermented foods, such as e.g. cheese, tools for investigating it have so far been limited.  相似文献   
3.
4.
Three of the nitrilase isoenzymes of Arabidopsis thaliana (L.) Heynh. are located on chromosome III in tandem and these genes (NIT2/NIT1/NIT3 in the 5′→3′ direction) encode highly similar polypeptides. Copy DNAs encompassing the entire coding sequences for all three nitrilases were expressed in Escherichia coli as fusion proteins containing a C-terminal hexahistidine extension. All three nitrilases were obtained as enzymatically active proteins, and their characteristics were determined, including a detailed comparative analysis of their substrate preferences. All three nitrilases converted indole-3-acetonitrile (IAN) to indole-3-acetic acid (IAA), albeit, compared to the most effective substrates found, phenylpropionitrile (PPN), allylcyanide, (phenylthio)acetonitrile and (methylthio)acetonitrile, with low affinity and velocity. The preferred substrates are either naturally occurring substrates, which may originate from glucosinolate breakdown, or they are close relatives of these. Thus, a major function of NIT1, NIT2 and NIT3 is assigned to be the conversion to carboxylic acids of nitriles from glucosinolate turnover or degradation. While all nitrilases exhibit a similar pH optimum around neutral, and NIT1 and NIT3 exhibit a similar temperature optimum around 30 °C independent of the substrate analyzed (IAN, PPN), NIT2 showed a remarkably different temperature optimum for IAN (15 °C) and PPN (35–40 °C). A potential role for NIT2 in breaking seed dormancy in A. thaliana by low temperatures (stratification), however, was ruled out, although NIT2 was the predominantly expressed nitrilase isoform in developing embryos and in germinating seeds, as judged from an analysis of β-glucuronidase reporter gene expression under the control of the promoters of the four isogenes. It is possible that NIT2 is involved in supplying IAA during seed development rather than during stratification. Received: 13 May 2000 / Accepted: 14 August 2000  相似文献   
5.
6.
It has been suggested that desert vegetation will show the strongest response to rising atmospheric carbon dioxide due to strong water limitations in these systems that may be ameliorated by both photosynthetic enhancements and reductions in stomatal conductance. Here, we report the long‐term effect of 55 Pa atmospheric CO2 on photosynthesis and stomatal conductance for three Mojave Desert shrubs of differing leaf phenology (Ambrosia dumosa—drought‐deciduous, Krameria erecta—winter‐deciduous, Larrea tridentata—evergreen). The shrubs were growing in an undisturbed ecosystem fumigated using FACE technology and were measured over a four‐year period that included both above and below‐average precipitation. Daily integrated photosynthesis (Aday) was significantly enhanced by elevated CO2 for all three species, although Krameria erecta showed the greatest enhancements (63% vs. 32% for the other species) enhancements were constant throughout the entire measurement period. Only one species, Larrea tridentata, decreased stomatal conductance by 25–50% in response to elevated CO2, and then only at the onset of the summer dry season and following late summer convective precipitation. Similarly, reductions in the maximum carboxylation rate of Rubisco were limited to Larrea during spring. These results suggest that the elevated CO2 response of desert vegetation is a function of complex interactions between species functional types and prevailing environmental conditions. Elevated CO2 did not extend the active growing season into the summer dry season because of overall negligible stomatal conductance responses that did not result in significant water conservation. Overall, we expect the greatest response of desert vegetation during years with above‐average precipitation when the active growing season is not limited to ~ 2 months and, consequently, the effects of increased photosynthesis can accumulate over a biologically significant time period.  相似文献   
7.
Three new strains of eubacterial hyperthermophiles were isolated from continental solfataric springs at Lac Abbé (Djibouti, Africa). Due to their morphology, lipids, and RNA polymerases they belong to the genus Thermotoga. Strains LA4 and LA10 are closely related to Thermotoga neapolitana found up to now only in the marine environment. Strain LA 3 differs from Thermotoga maritima and Thermotoga neapolitana in significant physiological and molecular properties. It is described as the new species Thermotoga thermarum.  相似文献   
8.
    
Quantifying successful ecological spillover from marine protected areas (MPAs) is challenging yet crucial for conservation planning. The queen conch (Lobatus gigas) supports an iconic Bahamian fishery, but populations are declining. Here we provide evidence for MPAs as a solution: showing that a well-enforced MPA supplies ecological spillover through larval supply. Dive surveys throughout the Exuma Cays, including a centrally-located MPA, provided information on abundance, size, and age. Data showed higher-adult abundance within the MPA and positive associations between enforcement and conch size and age. A biophysical model estimated that MPA larvae settled in unprotected areas, and that MPA larval sources included unprotected sites with densities too low for reproduction. The MPA is currently sustaining nearby populations, yet its future is in jeopardy without upstream larval sources. Our results and consultations with stakeholders regarding management of the Bahamian conch fishery support creating a network of MPAs that exchange larvae for a sustainable future.  相似文献   
9.
10.
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