全文获取类型
收费全文 | 3324篇 |
免费 | 224篇 |
国内免费 | 1篇 |
专业分类
3549篇 |
出版年
2021年 | 27篇 |
2019年 | 22篇 |
2018年 | 29篇 |
2016年 | 50篇 |
2015年 | 71篇 |
2014年 | 83篇 |
2013年 | 145篇 |
2012年 | 167篇 |
2011年 | 157篇 |
2010年 | 97篇 |
2009年 | 86篇 |
2008年 | 172篇 |
2007年 | 151篇 |
2006年 | 151篇 |
2005年 | 127篇 |
2004年 | 129篇 |
2003年 | 137篇 |
2002年 | 130篇 |
2001年 | 32篇 |
2000年 | 27篇 |
1999年 | 36篇 |
1998年 | 38篇 |
1997年 | 39篇 |
1996年 | 34篇 |
1995年 | 54篇 |
1994年 | 31篇 |
1993年 | 44篇 |
1992年 | 22篇 |
1991年 | 20篇 |
1990年 | 24篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1985年 | 16篇 |
1984年 | 30篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 38篇 |
1980年 | 22篇 |
1979年 | 23篇 |
1978年 | 41篇 |
1974年 | 15篇 |
1973年 | 27篇 |
1971年 | 15篇 |
1966年 | 22篇 |
1965年 | 18篇 |
1964年 | 19篇 |
1960年 | 16篇 |
1956年 | 24篇 |
1939年 | 18篇 |
排序方式: 共有3549条查询结果,搜索用时 0 毫秒
121.
Improvement of cowpea productivity by rhizobial and mycorrhizal inoculation in Burkina Faso 总被引:1,自引:0,他引:1
Hadou Haro Kadidia B. Sanon Christine Le Roux Robin Duponnois Alfred S. Traoré 《Symbiosis (Philadelphia, Pa.)》2018,74(2):107-120
Cowpea is one of the most important food legume crops in Burkina Faso. It is able to associate with arbuscular mycorrhizal fungi (AMF) and rhizobia. This dual symbiosis improves nitrogen and phosphorus nutrient uptake in cowpea. As the application of exotic inoculants frequently lacks positive responses in field experiments, this study set out to select well-adapted native symbiotic rhizobial and AMF strains. Soil samples were collected from six study sites in three different climatic zones of Burkina Faso to investigate their native symbiotic strains. Soil-extraction of native spores led to the identification of four AMF genera (Scutellospora, Gigaspora, Glomus and Entrophospora) by morpho-anatomical characterization. The two most effective cowpea fungal strains were selected after spore isolation from field-collected soils, multiplication on maize roots and inoculation on cowpea seedlings in a greenhouse experiment. Cowpea-nodulating rhizobial strains were trapped in the greenhouse by planting cowpea seeds in collected soil samples and the strains were characterized using molecular methods. This characterization led to the rhizobial isolates being classified in four clusters on the phylogenetic tree (using the Maximum-Likelihood Phylogenies method). All strains belonged to the Bradyrhizobium genus and most of them were included in the B. japonicum branch. Some groups were clearly distinct species already identified and may be new species. The two most effective strains for cowpea yield improvement in the field were selected after cowpea inoculation in a greenhouse experiment. The inoculation design in the field experiment consisted of four single inoculation treatments, either rhizobial or mycorrhizal, along with four dual inoculations, one treatment with chemical fertilizers, and one uninoculated control. The results showed that cowpea productivity was significatively improved by dual inoculation with native rhizobial and mycorrhizal strains, reaching the same level as the application of commonly used chemical fertilizers [Nitrogen, Phosphorus and Potassium fertilizers (NPK)]. In addition, dual inoculation resulted in the highest iron content in cowpea leaves. 相似文献
122.
123.
The discovery and characterization of a vibration response in a black lipid bilayer membrane is the topic of this paper. An electrical vibration response is obtained when the membrane is under voltage clamp and a weaker, but significant, response is obtained under current clamp. The effect arises from an induced variation in the membrane capacitance. It is further shown that the capacitance variation arises from a change in the membrane area as the membrane undergoes drumhead vibration. Possible physiological significance in mechanoreception is discussed. 相似文献
124.
Dr. Sanjida Mughal Abdullatif A. Al-Bader Alfred Cuschieri Bassim Kharbat 《Cell and tissue research》1987,250(2):349-354
Summary HeLa cells in a monolayer culture were synchronized to S, G2 and mitotic phases by use of excess (2.5 mM) deoxythymidine double-block technique. The localizations of Ca++-activated adenosine triphosphatase (ATPase) at different phases of the cell cycle were studied using light and electron-microscopic histochemical techniques, and microphotometric comparisons of the densities of reaction products. Enzyme reaction product was always localized in the endoplasmic reticulum, nuclear membrane, mitochondria and Golgi apparatus, but there were qualitative and quantitative differences related to the phases of the cell cycle. In S phase the activity was mainly concentrated in a perinuclear area of the cytoplasm whereas in G2 and mitosis the activity was scattered throughout the cell. The total activity per cell was maximal in G2, was less in S phase and least in mitosis. Activity in the mitochondria and endoplasmic reticulum was distinctly less in mitosis than in other phases of the cell cycle. The mitochondrial ATPase differed from the ATPase at other sites in ion dependence and sensitivity to oligomycin. The results suggest that there may be several distinct ATPases in proliferating cells. 相似文献
125.
Summary We first present two simple dimeric models of cotransport that may account for all of the kinetics of Na++-d-glucose cotransport published so far in the small intestine. Both the sigmoidicity in the Na++ activation of transport (positive cooperativity) and the upward deviations from linearity in the Eadie-Hofstee plots relative to glucose concentrations (negative cooperativity) can be rationalized within the concept of allosteric kinetic mechanisms corresponding to either of two models involving sequential or mixed concerted and sequential conformational changes. Such models also allow for 2 Na++ 1 S and 1 Na++ 1 S stoichiometries of cotransport at low and high substrate concentrations, respectively, and for partial inhibition by inhibitors or substrate analogues. Moreover, it is shown that the dimeric models may present physiological advantages over the seemingly admitted hypothesis of two different cotransporters in that tissue. We next address the reevaluation of Na++-d-glucose cotransport kinetics in rabbit intestinal brush border membrane vesicles using stable membrane preparations, a dynamic approach with the Fast Sampling Rapid Filtration Apparatus (FSRFA), and both nonlinear regression and statistical analyses. Under different conditions of temperatures, Na++ concentrations, and membrane potentials clamped using two different techniques, we demonstrate that our data can be fully accounted for by the presence of only one carrier in rabbit jejunal brush border membranes since transport kinetics relative to glucose concentrations satisfy simple Michaelis-Menten kinetics. Although supporting a monomeric structure of the cotransporter, such a conclusion would conflict with previous kinetic data and more recent studies implying a polymeric structure of the carrier protein. We thus consider a number of alternatives trying to reconcile the observation of Michaelis-Menten kinetics with allosteric mechanisms of cotransport associated with both positive and negative cooperativities for Na++ and glucose binding, respectively. Such models, implying energy storage and release steps through conformational changes associated with ligand binding to an allosteric protein, provide a rational hypothesis to understand the long-time debated question of energy transduction from the Na++ electrochemical gradient to the transporter.This research was supported by grant MT-7607 from the Medical Research Council of Canada. One of the authors (A.B.) was supported by a scholarship from the Fonds de la Recherche en Santé du Québec and C. C. was supported by a fellowship from the GRTM. The technical assistance of Mrs. C. Leroy has been greatly appreciated. The authors also thank D.D. Maenz and C. Malo for insightful discussions and C. Gauthier for the art work. 相似文献
126.
Ten Cryptococcus strains were screened for phytase activity, of which the Cryptococcus laurentii ABO 510 strain showed the highest level of activity. The cell wall-associated enzyme displayed temperature and pH optima of 62 degrees C and 5.0, respectively. The enzyme was thermostable at 70 degrees C, with a loss of 40% of its original activity after 3 h. The enzyme was active on a broad range of substrates, including ATP, D-glucose 6-phosphate, D-fructose 1,6-diphosphate and p-nitrophenyl phosphate (p-NPP), but its preferred substrate was phytic acid (K(m) of 21 microM). The enzyme activity was completely inhibited by 0.5 mM inorganic phosphate or 5 mM phytic acid, and moderately inhibited in the presence of Hg(2+), Zn(2+), Cd(2+) and Ca(2+). These characteristics suggest that the Cry. laurentii ABO 510 phytase may be considered for application as an animal feed additive to assist in the hydrolysis of phytate complexes to improve the bioavailability of phosphorus in plant feedstuff. 相似文献
127.
Tiffert Y Franz-Wachtel M Fladerer C Nordheim A Reuther J Wohlleben W Mast Y 《Applied microbiology and biotechnology》2011,89(4):1149-1159
GlnR is the global regulator of nitrogen assimilation in Streptomyces coelicolor M145 and other actinobacteria. Two-dimensional polyacrylamide gel electrophoresis analyses were performed to identify new
GlnR target genes by proteomic comparison of wild-type S. coelicolor M145 and a ΔglnR mutant. Fifty proteins were found to be differentially regulated between S. coelicolor M145 and the ΔglnR mutant. These spots were identified by nanoHPLC–ESI-MS/MS and classified according to their cellular role. Most of the identified
proteins are involved in amino acid biosynthesis and in carbon metabolism, demonstrating that the role of GlnR is not restricted
to nitrogen metabolism. Thus, GlnR is supposed to play an important role in the global metabolic control of S. coelicolor M145. 相似文献
128.
Barjaktarović Z Nordheim A Lamkemeyer T Fladerer C Madlung J Hampp R 《Journal of experimental botany》2007,58(15-16):4357-4363
In previous studies it has been shown that callus cell cultures of Arabidopsis thaliana respond to changes in gravitational field strengths by altered gene expression. In this study an investigation was carried out into how different g conditions affect the proteome of such cells. For this purpose, callus cells were exposed to 8 g (centrifugation) and simulated microgravity (2-D clinorotation: fast rotating clinostat, yielding 0.0016 g at maximum; and 3-D random positioning) for up to 16 h. Extracts containing total soluble protein were subjected to 2-D SDS-PAGE. Image analysis of Sypro Ruby-stained gels showed that approximately 28 spots reproducibly and significantly (P <0.05) changed in amount after 2 h of hypergravity (18 up- and 10 down-regulated). These spots were analysed by electrospray ionization tandem mass spectrometry (ESI-MS/MS). In the case of 2-D clinorotation, 19 proteins changed in a manner similar to hypergravity, while random positioning affected only eight spots. Identified proteins were mainly stress related, and are involved in detoxification of reactive oxygen species, signalling, and calcium binding. Surprisingly, centrifugation and clinorotation showed homologies which were not detected for random positioning. The data indicate that simulation of weightlessness is different between clinorotation and random positioning. 相似文献
129.
Boron deficiency causes accumulation of chlorogenic acid and caffeoyl polyamine conjugates in tobacco leaves 总被引:5,自引:0,他引:5
Camacho-Cristóbal JJ Lunar L Lafont F Baumert A González-Fontes A 《Journal of plant physiology》2004,161(7):879-881
The effects of boron (B) deficiency on carbohydrate concentrations and the pattern of phenolic compounds were studied in leaves of tobacco plants (Nicotiana tabacum L.). Plants grown under B deficiency showed a notable increase in leaf carbohydrates and total phenolic compounds when compared to controls. The qualitative composition of phenolics was analyzed by HPLC-mass spectrometry. The level of caffeate conjugates (i.e., chlorogenic acid) increased in B-deficient plants. In addition, the accumulation of two caffeic acid amides (N-caffeoylputrescine and putative dicaffeoylspermidine) was observed. 相似文献
130.
J Salvador Meza Marc F Schetelig C Silvia Zepeda-Cisneros Alfred M Handler 《BMC genetics》2014,15(Z2):S4