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   2516篇
  免费   183篇
  国内免费   112篇
  2023年   15篇
  2022年   26篇
  2021年   33篇
  2020年   42篇
  2019年   60篇
  2018年   51篇
  2017年   81篇
  2016年   100篇
  2015年   74篇
  2014年   128篇
  2013年   143篇
  2012年   81篇
  2011年   148篇
  2010年   62篇
  2009年   160篇
  2008年   144篇
  2007年   139篇
  2006年   125篇
  2005年   124篇
  2004年   111篇
  2003年   77篇
  2002年   66篇
  2001年   30篇
  2000年   53篇
  1999年   54篇
  1998年   56篇
  1997年   39篇
  1996年   56篇
  1995年   62篇
  1994年   49篇
  1993年   43篇
  1992年   38篇
  1991年   40篇
  1990年   34篇
  1989年   41篇
  1988年   34篇
  1987年   18篇
  1986年   24篇
  1985年   26篇
  1984年   28篇
  1983年   13篇
  1982年   22篇
  1981年   10篇
  1980年   11篇
  1979年   7篇
  1978年   5篇
  1977年   4篇
  1975年   6篇
  1973年   9篇
  1971年   3篇
排序方式: 共有2811条查询结果,搜索用时 15 毫秒
71.
The proton pumping ATPase in the plasma membrane of Elodea canadensis is believed to play a major role in inorganic carbon acquisition. To investigate potentially different carbon uptake strategies within the same plant, plasma membrane H+-ATPase distribution and polar current patterns were investigated in Elodea leaves and stems. Specific activity of plasma membrane H+-ATPase in leaf microsomal fractions was tenfold higher than in stem derived microsomes. Probing western blots with a monoclonal antibody specific for plasma membrane H+-ATPase, yielded strongly visible double bands at 100 kDa in leaf microsome preparations, whereas little antigen was detected in analogous stem microsome preparations. Using the same plasma membrane H+-ATPase specific antibody on tissue sections, the enzyme was found almost exclusively localized at the border of cells at the lower leaf surface. A positive ion current leaving the lower leaf surface was measured, using a vibrating probe device. Part of this current entered the upper leaf surface and part of it the internodes of the stem. The experimental results support the view, that Elodea leaves have different means of inorganic carbon uptake than stem internodes.  相似文献   
72.
Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (), hydrogen isotope ratio (D) and N-levels, and with altitude were assessed. In the samples, where values of above 15 indicate predominant performance of C3 photosynthesis, there were slight tendencies of increasing , D and N-levels with increasing altitude and of increasing with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of with altitude in the samples, where values of below 10 indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C3-CAM transitions in C3/CAM intermediate species, low availability of water is the most important.  相似文献   
73.
 CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) is one of the four known nickel enzymes. It is a bifunctional protein that catalyzes the oxidation of CO to CO2 at a nickel iron-sulfur cluster (Cluster C) and a remarkable condensation reaction between a methyl group (donated from a methylated corrinoid iron-sulfur protein), carbon monoxide, and coenzyme A to form acetyl-CoA at a separate nickel iron-sulfur cluster (Cluster A). This review focuses on the current understanding of the structure and function of Cluster A and on related model chemistry. It describes studies that uncovered the first example of a biological organometallic reaction sequence. The mechanism of acetyl-CoA synthesis includes enzymebound methylnickel, iron-carbonyl, and acylmetal intermediates. Discovery of the methylnickel species constituted the first example of an alkylnickel species in biology and unveiled a new biological role for nickel. Received: 10 April 1996 / Accepted: 4 July 1996  相似文献   
74.
Abstract. We describe an approach for developing a Dynamic Global Vegetation Model (DGVM) that accounts for transient changes in vegetation distribution over a decadal time scale. The DGVM structure is based on a linkage between an equilibrium global vegetation model and smaller scale ecosystem dynamics modules that simulate the rate of vegetation change. Vegetation change is classified into four basic types, based largely on the projected change in above-ground biomass of the vegetation. These four types of change are: (1) dieback of forest, shrubland or grassland; (2) successional replacement within forest, shrubland or grassland; (3) invasion of forest, shrubland or grassland; (4) change in tree/grass ratio. We then propose an approach in which the appropriate ecosystem dynamics module for each type of change is applied and the grid cells of the global model updated accordingly. An approach for accounting for fire, as an example of a disturbance which may strongly influence the rate and spatial pattern of forest dieback, is incorporated. We also discuss data needs for the development, calibration and validation of the model.  相似文献   
75.
76.
Detached leaves of Bryophyllum fedtschenkoi Hamet et Perrier kept in normal air show a single period of net CO2 fixation on transfer to constant darkness at temperatures in the range 0–25 °C. The duration of this initial fixation period is largely independent of temperature in the range 5–20 °C, but lengthens very markedly at temperatures below 4 °C, and is reduced at temperatures above 25 °C. The onset of net fixation of CO2 on transfer of leaves to constant darkness is immediate at low temperatures, but is delayed as the temperature is increased. The ambient temperature also determines whether or not a circadian rhythm of CO2 exchange occurs. The rhythm begins to appear at about 20 °C, is most evident at 30 °C and becomes less distinct at 35 °C. The occurrence of a distinct circadian rhythm in CO2 output at 30° C in the absence of a detectable rhythm in PEPCase kinase activity shows that the kinase rhythm is not a mandatory requirement for the rhythm of PEPCase activity. However, when it occurs, the kinase rhythm undoubtedly amplifies the PEPCase rhythm.Abbreviation PEPCase phosphoenolpyruvate carboxylase We thank the Agricultural and Food Research Council for financial support for this work.  相似文献   
77.
78.
Abstract: Substrate utilization of microbial cells extracted from soil with a 0.85% aqueous sodium chloride solution, was determined to estimate effects on soil microorganisms at the community level with microtiter plates (Biolog GN®) containing 95 different sources of organic carbon. A consistent pattern of utilized substrates was obtained after 24 h of microtiter plate incubation at 28°C. The absorbance values (OD590) obtained from a microtiter plate reader after background correction were transformed by using the average absorbance values of oxidized substrates as a threshold to distinguish between well utilized and poorly or non-utilized substrates and thereby reduce variances between replicates. Doubling times of the extracted soil microorganisms in the microtiter plates were tested with 12 substrates and ranged from 1.96 h to 3.23 h, depending on the carbon source. The carbon source utilization assay was used to assess the effects of soil inoculation with Corynebacterium glutamicum with and without a genetically engineered plasmid (pUN1; 6.3 kb), which encoded for the synthesis of the mammalian protease inhibiting peptide, aprotinin. Additionally, aprotinin itself was added at two concentrations to soil samples. An identical decrease in the number of carbon sources utilized, especially carbohydrates, occurred upon soil inoculation with both C. glutamicum strains after inoculation with 106 cells g−1 soil. This effect was only detectable during the first three weeks of incubation, as long as cell numbers of C. glutamicum (pUN1) were above 105 cfu g−1. Soil amendment with aprotinin resulted in utilization of additional substrates, most of them carbohydrates. With 0.1 mg aprotinin g−1 soil this stimulation lasted 2 days and with 10 mg g−1 it lasted for 7 days.  相似文献   
79.
Carbon and nitrogen cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide, methane and nitrous oxide emission rates and pore-water profiles of CO2, ammonium and nitrate. A comparison between chamber measured carbon dioxide fluxes and those based on CO2 pore-water gradients using Fick's First law indicates that apparent diffusion coefficients are 2 to 28 times higher than bulk sediment diffusion coefficients based on molecular diffusion. Seasonal changes in gaseous carbon fluxes or CO2 pore water concentrations cannot be used directly, or in a simple way, to determine seasonal rates of mineralization, because of marked seasonal changes in pore-water storage and exchange parameters.The annual amount of carbon delivered to the sediment is 42 mol m–2, of which about 42% becomes buried, the remaining being emitted as methane (7%) or carbon dioxide (50%). Each year about 2.6 mol N m–2 of particulate nitrogen reaches the sediment; 1.1 mol m–2 is buried and 1.6 mol m–2 is mineralized to ammonium. Only 0.42 mol m–2 yr–1 of the ammonium produced escapes from the sediments, the remaining being first nitrified (1.2 mol m–2 yr–1) and then denitrified (1.7 mol m–2 yr–1). Simple calculations indicate that intertidal sediments may account for about 14% and 30% of the total estuarine retention of nitrogen and carbon, respectively.  相似文献   
80.
 The relationship between stand biomass production, and tree age and size is generally a curve with a maximum. To understand why wood production decreases in the final stages of stand development, the influence of increasing tree size on foliage chemical composition and substrate requirement for foliage construction in terms of glucose [CC, g glucose (g dry mass) –  1] was investigated in the evergreen conifer Picea abies (L.) Karst. Because it was already known that irradiance affects both foliage morphology and chemistry in this species, and it was expected that the foliage in large overstory trees would intercept on average more light than that in saplings in understory, irradiance was measured in the sampling locations and included in the statistical models. CC of needles increased with increasing total tree height (TH) and was independent of relative irradiance. A major reason for increasing CC with increasing TH was a greater proportion of carbon-rich lignin in the needles in large trees. However, lignin did not fully account for the observed changes in CC, and it was necessary to assume that certain other carbon-rich secondary metabolites such as terpenes also accumulate in the foliage of large trees. Enhanced requirements for needle mechanical strength as evidenced by greater lignin concentrations in large trees were attributed to increased water limitations with increasing tree height. Because water relations may also control the sink capacities for assimilate usage, apart from the mechanical requirements, they may provide an explanation for the accumulation of other energetically expensive compounds in the needles as well. Biomass partitioning within the shoot was another foliar parameter modified in response to increasing tree size. The proportion of shoot axes, which serve to provide needles with mechanical support and to supply them with water, decreased with increasing TH. This may limit water availability in the needles, and/or manifest a lower water requirement of the needles containing proportionally more supporting and storage substances, and consequently, less physiologically active compounds such as proteins. Probably the same factors which caused CC of the needles to depend on TH, were also responsible for greater CC of the shoot axes in larger trees. These results collectively suggest that increasingly more adverse water relations with increasing tree size may provide a mechanistic explanation for the decline in foliar biomass and its functional activity during stand ageing. Received: 9 April 1996 / Accepted: 14 January 1997  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