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   4280篇
  免费   582篇
  2021年   39篇
  2018年   38篇
  2016年   57篇
  2015年   113篇
  2014年   103篇
  2013年   131篇
  2012年   195篇
  2011年   164篇
  2010年   112篇
  2009年   104篇
  2008年   147篇
  2007年   177篇
  2006年   138篇
  2005年   114篇
  2004年   157篇
  2003年   141篇
  2002年   135篇
  2001年   147篇
  2000年   141篇
  1999年   151篇
  1998年   50篇
  1997年   48篇
  1996年   54篇
  1995年   47篇
  1994年   44篇
  1993年   40篇
  1992年   97篇
  1991年   115篇
  1990年   123篇
  1989年   101篇
  1988年   114篇
  1987年   97篇
  1986年   98篇
  1985年   102篇
  1984年   87篇
  1983年   85篇
  1982年   74篇
  1981年   59篇
  1980年   55篇
  1979年   92篇
  1978年   59篇
  1977年   59篇
  1976年   51篇
  1975年   62篇
  1974年   50篇
  1973年   58篇
  1972年   37篇
  1971年   39篇
  1969年   40篇
  1967年   37篇
排序方式: 共有4862条查询结果,搜索用时 15 毫秒
991.
Methylphosphonate (MP) oligodeoxynucleotides (MPOs) are metabolically stable analogs of conventional DNA containing a methyl group in place of one of the non-bonding phosphoryl oxygens. All 16 possible chiral R(P) MP dinucleotides were synthesized and derivatized for automated oligonucleotide synthesis. These dimer synthons can be used to prepare (i) all-MP linked oligonucleotides having defined R(P) chirality at every other position (R(P) chirally enriched MPOs) or (ii) alternating R(P) MP/phosphodiester backbone oligonucleotides, depending on the composition of the 3'-coupling group. Chirally pure dimer synthons were also prepared with 2'-O-methyl sugar modifications. Oligonucleotides prepared with these R(P) chiral methylphosphonate linkage synthons bind RNA with significantly higher affinity than racemic MPOs.  相似文献   
992.
Novel Arabidopsis mutants with lowered levels of endogenous abscisic acid (ABA) were isolated. These were selected in a screen for germination in the presence of the gibberellin biosynthesis inhibitor paclobutrazol. Another mutant was isolated in a screen for NaCl tolerance. The ABA-deficiency was caused by two monogenic, recessive mutations, aba2 and aba3 , that were both located on chromosome 1. The mutants showed a phenotype that is known to be characteristic for ABA-deficiency: a reduced seed dormancy and excessive water loss, leading to a wilty phenotype. Double mutant analysis, combining different aba mutations, indicated the leaky nature of the mutations.  相似文献   
993.
994.
Identifying the physiological and genetic basis of stress tolerance in plants has proven to be critical to understanding adaptation in both agricultural and natural systems. However, many discoveries were initially made in the controlled conditions of greenhouses or laboratories, not in the field. To test the comparability of drought responses across field and greenhouse environments, we undertook three independent experiments using the switchgrass reference genotype Alamo AP13. We analyzed physiological and gene expression variation across four locations, two sampling times, and three years. Relatively similar physiological responses and expression coefficients of variation across experiments masked highly dissimilar gene expression responses to drought. Critically, a drought experiment utilizing small pots in the greenhouse elicited nearly identical physiological changes as an experiment conducted in the field, but an order of magnitude more differentially expressed genes. However, we were able to define a suite of several hundred genes that were differentially expressed across all experiments. This list was strongly enriched in photosynthesis, water status, and reactive oxygen species responsive genes. The strong across-experiment correlations between physiological plasticity—but not differential gene expression—highlight the complex and diverse genetic mechanisms that can produce phenotypically similar responses to various soil water deficits.Crop productivity and wild plant distributions are governed by the availability of soil moisture (Axelrod, 1972; Boyer, 1982; Ciais et al., 2005). The impact of drought and soil water deficit in agriculture is estimated to be the largest abiotic determinant of yield (Boyer, 1982; Araus et al., 2002), while drought is also considered a primary cause of speciation and adaptation in nature (Stebbins, 1952). Dehydration avoidance and other drought adaptive strategies permit plants to survive or maintain growth during periodic droughts (Blum, 1996; Chaves et al., 2003; Chaves and Oliveira, 2004). Specifically, phenotypic plasticity of stomatal conductance, water foraging, and growth traits (among many others) may effectively maintain homeostasis of leaf water potential despite soil water deficits.Leaf water potential is a bellwether of the physiological impact of water deficit (Jones, 2007). Under drought, decreasing water availability results in reduced leaf water potentials and a sequence of physiological responses including reduced photosynthesis, growth rate, and ultimately, fitness (Taiz and Zeiger, 2014). Plants therefore seek to maintain homeostasis of leaf water potential, with the highest (least negative) values supporting the most efficient functioning of photosynthesis and other metabolic processes in most species (Lawlor and Fock, 1978; Turner and Begg, 1981; Kramer and Boyer, 1995; Cornic and Massacci, 1996; Jones, 2007). Plants that exhibit dehydration avoidance strategies compensate for soil water deficit through phenotypic plasticity of gene expression (Verslues et al., 2006; DesMarais and Juenger, 2010; DesMarais et al., 2013; Lovell et al., 2015) and downstream physiological phenotypes (Levitt, 1980), among others.To understand plant stress responses, it is critical to determine the physiological and genetic underpinnings of drought adaptation in both field and laboratory conditions (Travers et al., 2007; Gaudin et al., 2013). A common finding among such studies is that physiological and gene expression responses to drought vary considerably depending on the severity and temporal dynamics of drying soil (Chaves et al., 2003; Barker et al., 2005; Malmberg et al., 2005; Mittler, 2006; Mishra et al., 2012). Natural soil moisture variation, which has shaped adaptive responses to drought in wild populations, is not necessarily recapitulated by controlled (often, “shock”) laboratory experiments. For example, single abiotic stresses rarely occur in isolation in the field (Mittler, 2006). Instead, wild and crop plants respond to the combination of diverse stressors such as drought, heat, and salinity, simultaneously and at both molecular (e.g. Rizhsky et al., 2002; Rizhsky et al., 2004; Suzuki et al., 2005) and physiological (e.g. Heyne and Brunson, 1940; Craufurd and Peacock, 1993; Machado and Paulsen, 2001) levels. Therefore, inquiries into evolved plant stress responses are perhaps best served by experimental conditions that emulate selective agents in the field. Given that the extent and severity of stress causes qualitatively different physiological responses, it is not surprising that several studies have found relatively weak genetic correlations between laboratory phenotypes and those collected in the field (e.g. Weinig et al., 2002; Malmberg et al., 2005; Anderson et al., 2011; Mishra et al., 2012).Soil properties and biota can also affect plant growth and physiology (Meisner et al., 2013; Schweitzer et al., 2014), which may be exacerbated by contrasts between growth in potting mix or in native soil (Rowe et al., 2007; Heinze et al., 2016). The observation that field-grown plants have different root systems and greater total water storage than those in greenhouse pots is of particular importance to water relations (Poorter et al., 2012a). Short-term drought stress in the field may be buffered by access to larger volumes of soil and more complex root-soil-water dynamics, conditions poorly represented in most controlled settings.The field of experimental design has been fundamentally shaped by a central problem of biology: that it is notoriously difficult to control environmental factors in the field (Jones, 2013). A classic solution is to increase biological replication, but this is generally not feasible with costly and time-sensitive physiological and genetic assays (Poorter et al., 2012b; Marchand et al., 2013). Despite these difficulties, understanding the effects of drought in field conditions is necessary because it is in these settings that yield is impacted and selection is acting to shape adaptive responses to stress. Here, we determine how the interplay between drought severity, planting condition (e.g. field, potted, greenhouse) and sampling timing impacts physiological and genomic responses to drought in the C4 perennial grass, Panicum virgatum (switchgrass). To accomplish this, we used observations collected from clonally replicated individuals of the “AP13” switchgrass genotype (derived from the Alamo cultivar), which is the genome reference for this important biofuel crop and dominant member of mesic tall grass prairie ecosystems. The Alamo cultivar is a southern lowland accession that has high vigor and performance across a variety of climatic conditions. Replicates were grown in three separate soil moisture manipulation experiments with distinct rooting environments: in medium sized pots in a greenhouse, in large containers in a field setting, and in native soil under rainout shelters. In all three of these experiments, we collected leaf-level physiological and whole-genome gene expression data from droughted and control plants.Combined, the three experiments represent contrasts in drought experimental manipulations (i.e. the extent, timing, and duration of drought), plant characteristics (i.e. age, maturity, and size), and broadly fit with the concepts of best practice for physiological analysis of drought responses (Poorter et al., 2012b). Contrasting these experimental design considerations allows us to address how edaphic and climactic conditions impact links between gene expression and physiological phenotypic plasticity. Specifically, we assessed three fundamental questions pertaining to physiological genomics in the field: (1) How consistent is phenotypic plasticity to drought across experiments? (2) Which soil moisture deficit responses vary across sites, years, and timing of sampling? (3) How does plasticity of physiological and gene expression phenotypes covary within and across experiments? To assess these questions, we tested how leaf physiology and whole-genome gene expression responded to the effects of drought treatments, leaf water potential, and sampling time (midday and predawn). These analyses permitted inference of the number, relative effect size, and identity of differentially expressed (plastic) genes. Overall, our results suggested that differences in leaf water potential and diurnal patterns were the major drivers of gene expression variation. Furthermore, we observed consistent physiological plasticity across greenhouse dry-down and field precipitation manipulation experiments, but extreme variability in the number of differentially expressed genes.  相似文献   
995.
996.
The boom of massive parallel sequencing (MPS) technology and its applications in conservation of natural and managed populations brings new opportunities and challenges to meet the scientific questions that can be addressed. Genomic conservation offers a wide range of approaches and analytical techniques, with their respective strengths and weaknesses that rely on several implicit assumptions. However, finding the most suitable approaches and analysis regarding our scientific question are often difficult and time‐consuming. To address this gap, a recent workshop entitled ‘ConGen 2015’ was held at Montana University in order to bring together the knowledge accumulated in this field and to provide training in conceptual and practical aspects of data analysis applied to the field of conservation and evolutionary genomics. Here, we summarize the expertise yield by each instructor that has led us to consider the importance of keeping in mind the scientific question from sampling to management practices along with the selection of appropriate genomics tools and bioinformatics challenges.  相似文献   
997.
Persistent staphylococcal infections often involve surface‐associated communities called biofilms. Staphylococcus aureus biofilm development is mediated by the co‐ordinated production of the biofilm matrix, which can be composed of polysaccharides, extracellular DNA (eDNA) and proteins including amyloid fibers. The nature of the interactions between matrix components, and how these interactions contribute to the formation of matrix, remain unclear. Here we show that the presence of eDNA in S. aureus biofilms promotes the formation of amyloid fibers. Conditions or mutants that do not generate eDNA result in lack of amyloids during biofilm growth despite the amyloidogeneic subunits, phenol soluble modulin peptides, being produced. In vitro studies revealed that the presence of DNA promotes amyloid formation by PSM peptides. Thus, this work exposes a previously unacknowledged interaction between biofilm matrix components that furthers our understanding of functional amyloid formation and S. aureus biofilm biology.  相似文献   
998.
The ‘hydraulic vulnerability segmentation’ hypothesis predicts that expendable distal organs are more susceptible to water stress‐induced embolism than the main stem of the plant. In the current work, we present the first in vivo visualization of this phenomenon. In two separate experiments, using magnetic resonance imaging or synchrotron‐based microcomputed tomography, grapevines (Vitis vinifera) were dehydrated while simultaneously scanning the main stems and petioles for the occurrence of emboli at different xylem pressures (Ψx). Magnetic resonance imaging revealed that 50% of the conductive xylem area of the petioles was embolized at a Ψx of ?1.54 MPa, whereas the stems did not reach similar losses until ?1.9 MPa. Microcomputed tomography confirmed these findings, showing that approximately half the vessels in the petioles were embolized at a Ψx of ?1.6 MPa, whereas only few were embolized in the stems. Petioles were shown to be more resistant to water stress‐induced embolism than previously measured with invasive hydraulic methods. The results provide the first direct evidence for the hydraulic vulnerability segmentation hypothesis and highlight its importance in grapevine responses to severe water stress. Additionally, these data suggest that air entry through the petiole into the stem is unlikely in grapevines during drought.  相似文献   
999.
Flowing blood exerts a frictional force, fluid shear stress (FSS), on the endothelial cells that line the blood and lymphatic vessels. The magnitude, pulsatility, and directional characteristics of FSS are constantly sensed by the endothelium. Sustained increases or decreases in FSS induce vessel remodeling to maintain proper perfusion of tissue. In this review, we discuss these mechanisms and their relevance to physiology and disease, and propose a model for how information from different mechanosensors might be integrated to govern remodeling.  相似文献   
1000.
Understanding population structure is important for guiding ongoing conservation and restoration efforts. The greater sage-grouse (Centrocercus urophasianus) is a species of concern distributed across 1.2 million km2 of western North America. We genotyped 1499 greater sage-grouse from 297 leks across Montana, North Dakota and South Dakota using a 15 locus microsatellite panel, then examined spatial autocorrelation, spatial principal components analysis, and hierarchical Bayesian clustering to identify population structure. Our results show that at distances of up to ~240 km individuals exhibit greater genetic similarity than expected by chance, suggesting that the cumulative effect of short-range dispersal translates to long-range connectivity. We found two levels of hierarchical genetic subpopulation structure. These subpopulations occupy significantly different elevations and are surrounded by divergent vegetative communities with different dominant subspecies of sagebrush, each with its own chemical defense against herbivory. We propose five management groups reflective of genetic subpopulation structure. These genetic groups are largely synonymous with existing priority areas for conservation. On average, 85.8 % of individuals within each conservation priority area assign to a distinct subpopulation. Our results largely support existing management decisions regarding subpopulation boundaries.  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