全文获取类型
收费全文 | 2353篇 |
免费 | 238篇 |
国内免费 | 2篇 |
出版年
2023年 | 8篇 |
2022年 | 14篇 |
2021年 | 30篇 |
2020年 | 25篇 |
2019年 | 19篇 |
2018年 | 39篇 |
2017年 | 35篇 |
2016年 | 59篇 |
2015年 | 107篇 |
2014年 | 131篇 |
2013年 | 130篇 |
2012年 | 176篇 |
2011年 | 158篇 |
2010年 | 104篇 |
2009年 | 103篇 |
2008年 | 149篇 |
2007年 | 153篇 |
2006年 | 180篇 |
2005年 | 147篇 |
2004年 | 122篇 |
2003年 | 124篇 |
2002年 | 137篇 |
2001年 | 25篇 |
2000年 | 20篇 |
1999年 | 24篇 |
1998年 | 33篇 |
1997年 | 20篇 |
1996年 | 30篇 |
1995年 | 27篇 |
1994年 | 32篇 |
1993年 | 19篇 |
1992年 | 27篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 21篇 |
1988年 | 15篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 15篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1971年 | 3篇 |
1968年 | 3篇 |
排序方式: 共有2593条查询结果,搜索用时 31 毫秒
991.
992.
Solitary revertants which have been observed on single mutant tree individuals have up to now been believed to be grow-through
cells belonging to the rootstock on which they are commonly grafted. In this study three different phenotypically visible
mutants revealing revertant shoots on the same tree were chosen for genetic analysis. The mutant Quercus robur L. ‘argenteomarginata’ was grafted on a normal rootstock, an individual of Carpinus betulus L. var. quercifolia Desf. as well as an individual of Picea glauca (Moench) Voss. ‘conica’ are supposed to have grown from seeds. By means of a highly specific M13 PCR fingerprinting technique
the mutant and revertant tissues were analysed in comparison to different individuals of each of the species. With the grafted
mutant, cambium tissue of the rootstock was also investigated. Whereas conspecific individuals could be clearly distinguished
from each other, mutant and revertant tissues revealed the same banding patterns for each of the three trees. In case of the
grafted mutant, the fingerprint obtained from cambium tissue of the rootstock was clearly different from the pattern of mutant
and revertant tissue. Results demonstrate the potential of the tool for genetic differentiation between individuals of three
tree species hence in the case of the grafted mutant, the hypothesis that the observed reversion is caused by a grow-through
of the rootstock is rejected. Furthermore, identical fingerprints of mutant and revertant tissue support identical genetic
background of the tissues excluding the gene(s) responsible of the mutation. Possible causes of mutations and reversions regarding
the three mutant trees are discussed.
Received: 15 September 1997 / Accepted: 24 November 1997 相似文献
993.
Soil warming alters microbial substrate use in alpine soils 总被引:2,自引:0,他引:2
Kathrin Streit Frank Hagedorn David Hiltbrunner Magdalena Portmann Matthias Saurer Nina Buchmann Birgit Wild Andreas Richter Sonja Wipf Rolf T. W. Siegwolf 《Global Change Biology》2014,20(4):1327-1338
Will warming lead to an increased use of older soil organic carbon (SOC) by microbial communities, thereby inducing C losses from C‐rich alpine soils? We studied soil microbial community composition, activity, and substrate use after 3 and 4 years of soil warming (+4 °C, 2007–2010) at the alpine treeline in Switzerland. The warming experiment was nested in a free air CO2 enrichment experiment using depleted 13CO2 (δ13C = ?30‰, 2001–2009). We traced this depleted 13C label in phospholipid fatty acids (PLFA) of the organic layer (0–5 cm soil depth) and in C mineralized from root‐free soils to distinguish substrate ages used by soil microorganisms: fixed before 2001 (‘old’), from 2001 to 2009 (‘new’) or in 2010 (‘recent’). Warming induced a sustained stimulation of soil respiration (+38%) without decline in mineralizable SOC. PLFA concentrations did not reveal changes in microbial community composition due to soil warming, but soil microbial metabolic activity was stimulated (+66%). Warming decreased the amount of new and recent C in the fungal biomarker 18:2ω6,9 and the amount of new C mineralized from root‐free soils, implying a shift in microbial substrate use toward a greater use of old SOC. This shift in substrate use could indicate an imbalance between C inputs and outputs, which could eventually decrease SOC storage in this alpine ecosystem. 相似文献
994.
Marion Liberloo Martin Lukac Carlo Calfapietra Marcel R. Hoosbeek Birgit Gielen Franco Miglietta Giuseppe E. Scarascia-Mugnozza Reinhart Ceulemans 《The New phytologist》2009,182(2):331-346
A poplar short rotation coppice (SRC) grown for the production of bioenergy can combine carbon (C) storage with fossil fuel substitution. Here, we summarize the responses of a poplar ( Populus ) plantation to 6 yr of free air CO2 enrichment (POP/EUROFACE consisting of two rotation cycles). We show that a poplar plantation growing in nonlimiting light, nutrient and water conditions will significantly increase its productivity in elevated CO2 concentrations ([CO2 ]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO2 ]. Sufficient nutrient availability, increased nitrogen use efficiency (NUE) and the large sink capacity of poplars contributed to the sustained increase in C uptake over 6 yr. Additional C taken up in high [CO2 ] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO2 ] to above-ground pools, as fine root biomass declined and its [CO2 ] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO2 ] during the 6 yr experiment. However, elevated [CO2 ] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO2 ] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production. 相似文献
995.
Lipid-mediated introduction of hepatitis B virus capsids into nonsusceptible cells allows highly efficient replication and facilitates the study of early infection events 总被引:3,自引:0,他引:3 下载免费PDF全文
The hepatitis B virus (HBV) is an enveloped DNA virus which is highly infectious in vivo. In vitro, only primary hepatocytes of humans and Tupaia belangeri or the novel HepaRG cell line are susceptible to HBV, but infection is inefficient and study of early infection events in single cells is unsatisfactory. Since hepatoma cells replicate the virus efficiently after transfection, this limited infection efficiency must be related to the initial entry phase. Here, we describe the lipid-based delivery of HBV capsids into nonsusceptible cells, circumventing the natural entry pathway. Successful infection was monitored by observing the emergence of the nuclear viral covalently closed circular DNA and the production of progeny virus and subviral particles. Lipid-mediated transfer initiated productive infection that was at least 100-fold more effective than infection of permissive cell cultures. High-dose capsid transfer showed that the uptake was not receptor limited and allowed the intracellular transport of capsids and genomes to be examined microscopically. The addition of inhibitors confirmed an entry pathway by fusion of the lipid with the plasma membrane. By indirect immune fluorescence and native fluorescence in situ hybridization, we followed the pathway of capsids and viral genomes in individual cells. We observed an active microtubule-dependent capsid transfer to the nucleus and a subsequent release of the viral genomes exclusively into the karyoplasm. Lipid-mediated transfer of viral capsids thus appears to allow efficient introduction of genetic information into target cells, facilitating studies of early infection events which are otherwise impeded by the small number of viruses entering the cell. 相似文献
996.
Involvement of cinnamyl-alcohol dehydrogenase in the control of lignin formation in Sorghum bicolor L. Moench 总被引:16,自引:0,他引:16
Christian Pillonel Marcel M. Mulder Jaap J. Boon Birgit Forster Andres Binder 《Planta》1991,185(4):538-544
The lignin structure and enzyme activities of normal and brown-midrib (BMR-6) mutant lines of Sorghum bicolor have been compared to identify the enzyme(s) involved in the reduction of the lignin content of the mutant. The results indicate that cinnamyl-alcohol dehydrogenase (CAD) and caffeic acid O-methyltransferase are depressed in the BMR-6 line, whereas the structural modifications correspond only to a reduction of CAD activity. Apparently, the change in the Sorghum lignin content, caused by depression of CAD activity, is accompanied by the incorporation of cinnamaldehydes into the core lignin.Abbreviations CAD
cinnamyl-alcohol dehydrogenase
- HPLC
high-performance liquid chromatography
- m/z
mass number
- OMT
caffeic acid O-methyltransferase 相似文献
997.
998.
干旱区胡杨叶片含水量和叶绿素含量特征 总被引:2,自引:0,他引:2
在阿拉干样区内,距河道不同距离选取40棵胡杨树(Populus euphratica)采样,使用相关分析方法探讨胡杨叶片相对含水量(fuel moisture content,FMC)、等效水厚度(equiv-alent water thickness,EWT)、叶绿素含量随距河道距离的空间差异,以及随地面温度的变化特征,并讨论了FMC、EWT和叶绿素含量之间的相关性。结果表明:FMC、EWT、叶绿素含量与河道距离有极显著的相关关系(P<0.01)。距河道越远,3项指标均随距河道距离的增加极显著的减少,其中EWT与距河道距离的相关性最大(-0.577)。温度变化对FMC和EWT均有极显著的影响(P<0.01),但对叶绿素含量影响不大(P>0.05)。因FMC、EWT的物理含义不同,它们的平均值在胡杨树个体尺度/水平上极显著相关,但在单片叶子尺度上FMC和EWT之间无显著相关(P>0.05),而胡杨树个体平均和单片叶子的EWT与叶绿素含量均极显著相关(P<0.01)。由于EWT与距河道距离和叶绿素含量的相关性极显著,而且对温度变化不敏感,因此,具有很好的开发应用潜力,建议将EWT作为干旱区河岸生态系统植被含水量遥感监测的一个指标。 相似文献
999.
Morgane Algrain Elise Hennebert Philip Bertemes Ruddy Wattiez Patrick Flammang Birgit Lengerer 《Open biology》2022,12(8)
Sea stars adhere strongly but temporarily to underwater substrata via the secretion of a blend of proteins, forming an adhesive footprint that they leave on the surface after detachment. Their tube feet enclose a duo-gland adhesive system comprising two types of adhesive cells, contributing different layers of the footprint and de-adhesive cells. In this study, we characterized the catalogue of sea star footprint proteins (Sfps) in the species Asterias rubens to gain insights in their potential function. We identified 16 Sfps and mapped their expression to type 1 and/or type 2 adhesive cells or to de-adhesive cells by double fluorescent in situ hybridization. Based on their cellular expression pattern and their conserved functional domains, we propose that the identified Sfps serve different functions during attachment, with two Sfps coupling to the surface, six providing cohesive strength and the rest forming a binding matrix. Immunolabelling of footprints with antibodies directed against one protein of each category confirmed these roles. A de-adhesive gland cell-specific astacin-like proteinase presumably weakens the bond between the adhesive material and the tube foot surface during detachment. Overall, we provide a model for temporary adhesion in sea stars, including a comprehensive list of the proteins involved. 相似文献
1000.
The anoxygenic phototroph Rhodobacter sphaeroides uses 3-hydroxypropionate as a sole carbon source for growth. Previously, we showed that the gene (RSP_1434) known as acuI, which encodes a protein of the medium-chain dehydrogenase/reductase (MDR) superfamily, was involved in 3-hydroxypropionate assimilation via the reductive conversion to propionyl-coenzyme A (CoA). Based on these results, we speculated that acuI encoded acrylyl-CoA reductase. In this work, we characterize the in vitro enzyme activity of purified, recombinant AcuI using a coupled spectrophotometric assay. AcuI from R. sphaeroides catalyzes the NADPH-dependent acrylyl-CoA reduction to produce propionyl-CoA. Two other members of the MDR012 family within the MDR superfamily, the products of SPO_1914 from Ruegeria pomeroyi and yhdH from Escherichia coli, were shown to also be part of this new class of NADPH-dependent acrylyl-CoA reductases. The activities of the three enzymes were characterized by an extremely low Km for acrylyl-CoA (<3 μM) and turnover numbers of 45 to 80 s−1. These homodimeric enzymes were highly specific for NADPH (Km = 18 to 33 μM), with catalytic efficiencies of more than 10-fold higher for NADPH than for NADH. The introduction of codon-optimized SPO_1914 or yhdH into a ΔacuI::kan mutant of R. sphaeroides on a plasmid complemented 3-hydroxypropionate-dependent growth. However, in their native hosts, SPO_1914 and yhdH are believed to function in the metabolism of substrates other than 3-hydroxypropionate, where acrylyl-CoA is an intermediate. Complementation of the ΔacuI::kan mutant phenotype by crotonyl-CoA carboxylase/reductase from R. sphaeroides was attributed to the fact that the enzyme also uses acrylyl-CoA as a substrate. 相似文献