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   4644篇
  免费   327篇
  国内免费   2篇
  2023年   19篇
  2022年   63篇
  2021年   117篇
  2020年   84篇
  2019年   97篇
  2018年   133篇
  2017年   132篇
  2016年   177篇
  2015年   235篇
  2014年   264篇
  2013年   328篇
  2012年   407篇
  2011年   365篇
  2010年   231篇
  2009年   191篇
  2008年   275篇
  2007年   262篇
  2006年   222篇
  2005年   206篇
  2004年   179篇
  2003年   197篇
  2002年   177篇
  2001年   57篇
  2000年   42篇
  1999年   43篇
  1998年   45篇
  1997年   33篇
  1996年   31篇
  1995年   27篇
  1994年   23篇
  1993年   27篇
  1992年   25篇
  1991年   26篇
  1990年   26篇
  1989年   16篇
  1988年   9篇
  1987年   8篇
  1986年   20篇
  1985年   15篇
  1984年   19篇
  1983年   16篇
  1982年   8篇
  1981年   8篇
  1980年   11篇
  1978年   5篇
  1977年   6篇
  1974年   5篇
  1973年   6篇
  1939年   6篇
  1938年   11篇
排序方式: 共有4973条查询结果,搜索用时 31 毫秒
991.
992.
The last few decades have seen a growing number of species invasions globally, including many insect species. In drosophilids, there are several examples of successful invasions, i.e. Zaprionus indianus and Drosophila subobscura some decades ago, but the most recent and prominent example is the invasion of Europe and North America by the pest species, Drosophila suzukii. During the invasive process, species often encounter diverse environmental conditions that they must respond to, either through rapid genetic adaptive shifts or phenotypic plasticity, or by some combination of both. Consequently, invasive species constitute powerful models for investigating various questions related to the adaptive processes that underpin successful invasions. In this paper, we highlight how Drosophila have been and remain a valuable model group for understanding these underlying adaptive processes, and how they enable insight into key questions in invasion biology, including how quickly adaptive responses can occur when species are faced with new environmental conditions.  相似文献   
993.
Based on enzyme activity assays and metabolic responses to waterlogging of the legume Lotus japonicus, it was previously suggested that, during hypoxia, the tricarboxylic acid cycle switches to a noncyclic operation mode. Hypotheses were postulated to explain the alternative metabolic pathways involved, but as yet, a direct analysis of the relative redistribution of label through the corresponding pathways was not made. Here, we describe the use of stable isotope-labeling experiments for studying metabolism under hypoxia using wild-type roots of the crop legume soybean (Glycine max). [13C]Pyruvate labeling was performed to compare metabolism through the tricarboxylic acid cycle, fermentation, alanine metabolism, and the γ-aminobutyric acid shunt, while [13C]glutamate and [15N]ammonium labeling were performed to address the metabolism via glutamate to succinate. Following these labelings, the time course for the redistribution of the 13C/15N label throughout the metabolic network was evaluated with gas chromatography-time of flight-mass spectrometry. Our combined labeling data suggest the inhibition of the tricarboxylic acid cycle enzyme succinate dehydrogenase, also known as complex II of the mitochondrial electron transport chain, providing support for the bifurcation of the cycle and the down-regulation of the rate of respiration measured during hypoxic stress. Moreover, up-regulation of the γ-aminobutyric acid shunt and alanine metabolism explained the accumulation of succinate and alanine during hypoxia.Plants are sessile, unable to relocate when exposed to diverse environmental and seasonal stimuli, and hence must be able to respond rapidly to survive stress conditions. Flooding or waterlogging of the soil is a common environmental condition that can greatly affect crop production and quality by blocking the entry of oxygen into the soil so that roots and other belowground organs cannot maintain respiration. In recent decades, the number of extreme floodings has strongly increased, which is especially tragic because most arable land worldwide is located in regions that are threatened by regular flooding events (Voesenek and Bailey-Serres, 2015).In plant heterotrophic tissues, respiratory metabolism is composed of various pathways, including glycolysis, the mitochondrial tricarboxylic acid cycle, and the mitochondrial electron transport chain. Under normal conditions, the conversion of Glc to pyruvate in the cytosol involves an initial input of ATP and produces the reduced cofactor NADH. The reactions of the tricarboxylic acid cycle occur within the mitochondrial matrix and lead to the complete oxidation of pyruvate, moving electrons from organic acids to the oxidized redox cofactors NAD+ and FAD, forming the reducing equivalents NADH and FADH2 and concomitantly releasing carbon dioxide (Tovar-Méndez et al., 2003; Millar et al., 2011). Finally, the reduced cofactors generated during glycolysis and the tricarboxylic acid cycle are subsequently oxidized by the mitochondrial electron transport chain to fuel ATP synthesis by a process known as oxidative phosphorylation (Fernie et al., 2004; Plaxton and Podesta, 2006). The tricarboxylic acid cycle turnover rate depends greatly on the rate of NADH reoxidation by the mitochondrial electron transport chain and on the cellular rate of ATP utilization (Plaxton and Podesta, 2006). Besides supporting ATP synthesis, the reactions of the tricarboxylic acid cycle also contribute to the production of key metabolic intermediates for use in many other fundamental biosynthetic processes elsewhere in the cell (Fernie et al., 2004; Sweetlove et al., 2010; van Dongen et al., 2011; Araújo et al., 2012). Nevertheless, the control and regulation of the carbon flux through the tricarboxylic acid cycle are still poorly understood in plants, and noncyclic modes have been described to operate under certain circumstances (Rocha et al., 2010; Sweetlove et al., 2010; Araújo et al., 2012).Upon hypoxia, respiratory energy (ATP) production via oxidative phosphorylation by the mitochondrial electron transport chain goes down. To compensate for this, the glycolytic flux increases and Glc is consumed faster in an attempt to produce ATP via the glycolytic pathway, a process known as the Pasteur effect. To survive short-term hypoxia during flooding or waterlogging, plants must generate sufficient ATP and regenerate NADP+ and NAD+, which are required for glycolysis (Narsai et al., 2011; van Dongen et al., 2011). In addition to the accumulation of ethanol and lactate in oxygen-deprived plant tissues, metabolites such as Ala, succinate, and γ-aminobutyric acid (GABA) have also been shown to accumulate (Sousa and Sodek, 2003; Kreuzwieser et al., 2009; van Dongen et al., 2009; Rocha et al., 2010; Zabalza et al., 2011), although hardly anything is known about the fate of these products of hypoxic metabolism. However, the relative abundance of these products of hypoxic metabolism varies between plant species, genotypes, and tissues and can change throughout the course of oxygen limitation stress as well (Narsai et al., 2011).A model describing metabolic changes during hypoxia has been described previously for waterlogged roots of the highly flood-tolerant model crop legume Lotus japonicus (Rocha et al., 2010): upon waterlogging, the rate of pyruvate production is enhanced due to the activation of glycolysis (Pasteur effect) and the concomitant production of ATP via substrate-level phosphorylation. At the same time, the fermentation pathway is activated with the accumulation of lactate via lactate dehydrogenase and ethanol via two subsequent reactions catalyzed by pyruvate decarboxylase and alcohol dehydrogenase (Tadege et al., 1999). The amount of pyruvate produced can be reduced via alanine aminotransferease (AlaAT), which catalyzes the reversible reaction interconverting pyruvate and Glu to Ala and 2-oxoglutarate (2OG). Concomitantly, 2OG was suggested to reenter the tricarboxylic acid cycle to be used to produce another ATP and also succinate, which accumulates in the cell (Rocha et al., 2010). This Ala pathway provides a means for the role of Ala accumulation during hypoxia via reorganization of the tricarboxylic acid cycle. Furthermore, given that the use of this strategy prevents pyruvate accumulation, the continued operation of glycolysis during waterlogging can occur.It should be noted, however, that measurements of metabolite levels alone do not provide information about the actual activity of the metabolic pathways involved. Furthermore, the previous studies did not reveal which enzymes of the tricarboxylic acid cycle change their activity that leads to reorganization of the tricarboxylic acid cycle. To overcome this, analysis of metabolism using isotope-labeled substrates has proven to be essential for understanding the control and regulation of metabolic networks, and it has often been observed that significant changes in carbon flow are sometimes associated with only small adjustments in metabolite abundance (Schwender et al., 2004; Ratcliffe and Shachar-Hill, 2006). Metabolomics studies that require extensive metabolite labeling utilize uniformly labeled stable isotope tracers. Alternatively, detailed analysis of central carbon metabolism can make use of positional labeling as well. Following the extraction of labeled metabolites, the 13C label redistribution is measured usually with NMR or gas chromatography-mass spectrometry methods (Jorge et al., 2015). Schwender and Ohlrogge (2002) used both labeling approaches to investigate embryo development in Brassica napus seeds. While uniformly labeled [13C6]Glc and [13C12]Suc were applied to determine the metabolic flux through the major pathways of carbon metabolism, positionally labeled [1,2-13C]Glc was used to specifically outline the glycolytic/oxidative pentose phosphate pathway network during embryo development (Schwender and Ohlrogge, 2002). Gas chromatography-mass spectrometry analysis was used in this study to evaluate the 13C enrichment and isotopomer composition. In earlier studies of hypoxic metabolism, positionally labeled [1-13C]Glc was used to specifically investigate energy metabolism and pH regulation in hypoxic maize (Zea mays) root tips (Roberts et al., 1992; Edwards et al., 1998).In this study, we performed stable isotope labeling experiments using wild-type soybean (Glycine max) roots in order to better understand the dynamics of metabolism in operation in plant cells under hypoxic conditions. For this, we used fully labeled 13C and 15N tracers rather than positional labeling, as this allowed us to cover a broad view of the central carbon and nitrogen metabolic network. The labeling pattern of metabolites was subsequently measured with gas chromatography-time of flight-mass spectrometry (GC-TOF-MS). Our studies confirm the activity of Ala metabolism while revealing the parallel activity of the GABA shunt. The results provide evidence that the bifurcation of the tricarboxylic acid cycle results from the inhibition of the tricarboxylic acid cycle enzyme succinate dehydrogenase (SDH), also known as complex II of the mitochondrial electron transport chain (mETC).  相似文献   
994.
995.
The influence of the environment on the geographical variation of morphological traits has been recognized in a number of taxa. Pecari tajacu and Tayassu pecari are ideal models to investigate intraspecific geographic variation in skull because of their wide and heterogeneous geographical distribution in South America. We used geometric morphometric procedures to examine the geographical variation in skull shape of 294 adult specimens of these species from 134 localities. We quantified to what extent skull shape variation was explained by environment, skull size and geographical space using variation partitioning analysis. We detected a strong pattern of geographic variation for P. tajacu skull shape, but not for T. pecari. The environment seems to be the major selective force that drives skull shape variation in both species. Nevertheless, other spatially structured processes (e.g. genetic drift, gene flow) might also have affected variation in the skull shape of the more widespread species P. tajacu. Allometric relationships might reflect the biomechanical constraints that are thought to be strong enough to limit size‐related changes in T. pecari skull shape.  相似文献   
996.
997.
998.
999.
Vegetative propagation of superior conifer trees can be achieved, e.g., through rooted cuttings or rooted microshoots, the latter predominantly through in vitro tissue culture. Both techniques are used to achieve rapid multiplication of trees with favorable genetic combinations and to capture a large proportion of the genetic diversity in a single generation cycle. However, adventitious rooting of shoots (cuttings) is often not efficient due to various problems, such as scarcity of roots and cessation of their growth, both of which limit the application of vegetative propagation in some conifer species. Many factors are involved in the adventitious rooting of shoots, including physical and chemical ones, such as plant growth regulators, carbohydrates, light quality, temperature and rooting substrates, or media [reviewed by Ragonezi et al. (Trees 24(6):975–992, 2010)]. The focus of this review is on biological factors, such as inoculations with Agrobacterium rhizogenes, plant-growth promoting rhizobacteria and other endophytes, and mycorrhizal fungi, which were found to stimulate adventitious rooting. These microorganisms could contribute not only to adventitious root development but also to help in protecting conifer plants against pathogenic microorganisms, facilitate acclimation and transplanting, and contribute to more sustainable, chemical-free forests.  相似文献   
1000.
When newly created habitats are initially colonized by genotypes with rapid population growth rates, later arriving colonists may be prevented from establishing. Although these priority effects have been documented in multiple systems, their duration may be influenced by the diversity of the founding population. We conducted a large‐scale field manipulation to investigate how initial clonal diversity influences temporal and landscape patterns of genetic structure in a developing metapopulation. Six genotypes of obligately asexual Daphnia pulex were stocked alone (no clonal diversity) or in combination (‘high’ clonal diversity) into newly created experimental woodland ponds. We also measured the population growth rate of all clones in the laboratory when raised on higher‐quality and lower‐quality resources. Our predictions were that in the 3 years following stocking, clonally diverse populations would be more likely to persist than nonclonally diverse populations and exhibit evidence for persistent founder effects. We expected that faster growing clones would be found in more pools and comprise a greater proportion of individuals genotyped from the landscape. Genetic composition, both locally and regionally, changed significantly following stocking. Six of 27 populations exhibited evidence for persistent founder effects, and populations stocked with ‘high’ clonal diversity were more likely to exhibit these effects than nonclonally diverse populations. Performance in the laboratory was not predictive of clonal persistence or overall dominance in the field. Hence, we conclude that although laboratory estimates of fitness did not fully explain metapopulation genetic structure, initial clonal diversity did enhance D. pulex population establishment and persistence in this system.  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